Module #4 Motor Controls - Idaho State University

Energy Systems Engineering Technology

College of Technology Motors and Controls Module # 4 Motor Controls

Document Intent: The intent of this document is to provide an example of how a subject matter expert might teach Motor Controls. This approach is what Idaho State University College of Technology is using to teach its Energy Systems Instrumentation and Control curriculum for Motor Controls. The approach is based on a Systematic Approach to Training where training is developed and delivered in a two step process. This document depicts the two step approach with knowledge objectives being presented first followed by skill objectives. Step one teaches essential knowledge objectives to prepare students for the application of that knowledge. Step two is to let students apply what they have learned with actual hands on experiences in a controlled laboratory setting. Examples used are equivalent to equipment and resources available to instructional staff members at Idaho State University. Fundamentals of Motor Controls Introduction: This module covers fundamental aspects of Motor Controls as essential knowledge necessary to perform work safely according to national and local standards on or around electrical power sources that are associated with motors and controls. Students will be taught the fundamentals of Motor Controls using classroom instruction, demonstration, and laboratory exercises to demonstrate knowledge and skill mastery of Motor Controls. Completion of this module will

Motor Controls Module

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Energy Systems Engineering Technology allow students to demonstrate mastery of knowledge and skill objectives by completing a series of tasks demonstrating safe work practices on or around electrical power sources.

References This document includes knowledge and skill sections with objectives, information, and examples of how Motors and Control could be taught in a vocational or industry setting. This document has been developed by Idaho State University’s College of Technology. Reference material used includes information from: American Technical Publication – Electrical Motor Controls for Integrated Systems, Third Edition, by Gary J. Rockis and Glen A. Mazur, ISBN 0-8269-1207-9 (Chapters 8, 10, 17, & 18) National Electrical Code® International Electrical Code® Series, NFPA 70TM, NEC 2008, ISBN-13: 978-087765790-3


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Energy Systems Engineering Technology STEP ONE Motor Controls Course Knowledge Objectives Knowledge Terminal Objective (KTO)

KTO 3. 1.

ANALYZE Motor Controls to compare advantages and disadvantages to ensure they are correctly selected for applications according to manufacturing specifications and electrical requirements

Knowledge Enabling Objectives (KEO)

Knowledge Enabling Objectives (Chapter 8 Contactors and Motor Starters): KEO 4.1.

DESCRIBE the differences between Manual and Automatic Motor Control.

KEO 4.2.

DESCRIBE the differences between Manual Contactors and Motor Starters

KEO 4.3.

DESCRIBE what Manual Contactors are and how they are used in the control of electrical loads to include: Double-Break Contacts, Three Phase Manual Contactors, and Contact Construction.

KEO 4.4.

DESCRIBE what a Manual Starter consists of and how they provide motor control and what protection they provide motors.

KEO 4.5.

DESCRIBE the three stages a motor goes through during normal operation and how they are protected against a potential overload condition to include: Motor Overload Conditions, Melting Alloy Overloads, Heating Coils, and Resetting Overload Devices.


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Energy Systems Engineering Technology KEO 4.6.

EXPLAIN how Electronic Overloads provide protection against changes in current and temperature.

KEO 4.7.

EXPALIN the criteria for Selecting AC Manual Motor Starters to include: Phasing, Enclosures, and Manual Starter Applications.

KEO 4.8.

DESCRIBE the principles of operation of Magnetic Contactors to include: Magnetic Contactor Construction, Magnetic Contactor Wiring, Control Circuit Wiring, and Control Circuit Voltage.

KEO 4.9.

DESCRIBE the principles of operation of AC and DC Controllers to include: AC Arc Suppression (AC and DC), Arc Chutes, and DC Magnetic Blowout Coils.

KEO 4.10.

DESCRIBE the basic Contact Construction criteria associated with Contacts to include: Single and Double Breaking Contacts, and General-Purpose AC/DC Contactor Sizes and Ratings.

KEO 4.11.

DESCRIBE principles of operation for a Magnetic Motor Starter to include how it provides Overload Protection including: Melting Alloy Overload Relays, Magnetic Overload Relays, Bimetallic Overload Relays, Trip Indicators, and Overload Current Transformers.

KEO 4.12.

DESCRIBE how Overload Heater Sizing and Selection is accomplished to include: Full-Load Current, Service Factor, Ambient Temperature, Manufacture Heater Selection Charts, Checking Selections, and Ambient Temperature Compensation.

KEO 4.13.

DESCRIBE what Inherent Motor Protection consist of and their principles of operation to include: Bimetallic Thermo-Discs, and Thermistor Overload Devices.

KEO 4.14.

EXPLAIN how Electronic Overload Protection is incorporated into Contactor and Magnetic Motor Modifications to expand their capabilities to include: Additional Electrical Contacts, Power Poles, Pneumatic Timers, Transient Suppression Modules, and Control Circuit Fuse Holders.


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DESCRIBE Troubleshooting Contactors and Motor Starter techniques and procedures.

KEO 4.16.

DESCRIBE how a Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a technician.

KEO 4.17.

DESCRIBE role Motor Drives provide in the control of Contactors and Motor Starters to include: Motor Drive Programming and Programming Overload Protection.

Knowledge Enabling Objectives (Chapter 10 Reversing Motor Circuits): KEO 4.18.

EXPLAIN the concepts associated with how to reverse the direction of motors to include: AC Three Phase, AC Single Phase / Capacitor Start, and DC Motors.

KEO 4.19.

EXPLAIN the concepts and needs associated with mechanical interlocking devices used to protect motors.

Knowledge Enabling Objectives (Chapter 17 Reducing-Voltage Starting): KEO 4.20.

DESCRIBE the role Reduced Voltage Starting and Silicon-Controlled Rectifiers provide in motor controls.

Knowledge Enabling Objectives (Chapter 18 Accelerating and Decelerating Methods: KEO 4.21.

EXPLAIN the concepts associated Braking and Plugging as they apply to motors.

KEO 4.22.

EXPLAIN the concepts associated Motor Drive Stopping to include: Stopping without Applied Force from the drive, and Stopping using Reduced Voltage as the motor Decelerates.

KEO 4.23.

EXPLAIN the concepts of multiple poles in the acceleration, deceleration control of AC Motors.


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Motor Controls

Knowledge Enabling Objectives (Chapter 8 Contactors and Motor Starters): KEO 4.1. DESCRIBE the differences between Manual and Automatic Motor Control. 

Manual Starting involves a mechanical device (like a switch) that is used to start and stop a motor. This is performed by a human given the responsibility to ensure that a motor has either started or stopped in order to control the process or product output driven by a motor. The motor is essentially controlled by someone telling someone else the action to be taken.



Automatic Starting involves mechanical controls that receive a feedback signal from electrical devices that not only start and stop a motor, they also provide a host of control options such as safety of the equipment or personnel, control of the speed at which the motor is to run, and the direction it is to rotate. The monitor load and torque to provide safe and efficient use of the motor to control the process or product output load for a given motor.

KEO 4.2. DESCRIBE the differences between Manual Contactors and Motor Starters 

Manual Contactors are devices that utilize a pushbutton operated by hand to energize or de-energize the load connected to it. Manual Contactors manually opens or closes contacts in an electrical circuit. Manual Contactors cannot be used to start or stop motors as they do not provide overload protection as it is not built into the Manual Contactor. Manual Contactors are generally used with lighting circuits, and resistive loads such as heaters or large lamp loads. A fuse or circuit breaker may be provided in the same enclosure with a Manual Contactor. Manual Contactors typically are energized by a separate electrical signal with a magnetic coil that closes a set of electrical contacts providing power to the intended electrical load.


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Energy Systems Engineering Technology o The below picture depicts a typical Manual Contactor showing where three phase power is utilized for a load via a coil activated by a smaller control voltage:

Figure 8-3 page 174 o In the picture above, the load power comes in the top three connections of the contactor and the coil (two small screws in the lower portion of the contactor) closes the contactor, which then applies the load power o the load from the bottom three connections of the contactor. 

Motor Starters utilize Contacts that when they are closed either by a manual pushbutton to the contactor coil or by a control signal from a device. Motor Starters will start and stop a motor providing overload protection.

SUMMARY:    

Manual Starting involves a mechanical device (like a switch) that is used to start and stop a motor. Automat Starting involves mechanical controls that receive a feedback signal from electrical devices to start and stop a motor. Manual Contactors are devices that utilize a pushbutton operated by hand to energize or de-energize the load connected to it and cannot be used to start or stop motors as they do not provide overload protection. Motor Starters will start and stop a motor and include overload protection.


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

DESCRIBE what Manual Contactors are and how they are used in the control of electrical loads to include: Double-Break Contacts, Three Phase Manual Contactors, and Contact Construction.

Double-Break Contacts can act a direct controller for an electrical load. Double-Break Contacts break and electrical circuit in two power lines. The below picture illustrates how this is done:

Figure 8-4 page 174 Double-Break Contacts allow devices to be designed that have a higher contact rating (current rating) in a smaller space than devices with a single-break contact. The can be so constructed to provide Normally Open (NO) or Normally Closed (NC) contact. They are manually opened or closed by the push button device.


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Energy Systems Engineering Technology 

Three Phase Manual Contactors are similar to a Double-Break Contacts except that they open and close three sets of contacts for a three phase system. Three Phase Manual Contactors are similar to a disconnect device where the mechanical linkage consistently and quickly makes or breaks the three phase load circuit. o The below picture illustrates a Three Phase Manual Contactors:

Figure 8-5 page 175 The illustration above is for Three Phase Power loads. These devices can also purchased to open and close single or dual voltage sources by design. The advantage of these manual contactors is that the movable contacts have no physical connection to external electrical wires. The movable contacts move into arc hoods and bridge the gap between as set of fixed contacts to make or break the circuit. All physical electrical connections are made indirectly to the fixed contacts, normally through saddle clamps. 

Contact Construction is improved today from the knife switches made from soft copper. Today most contacts are made of a Low-Resistance Silver Alloy. Silver is alloyed (mixed) with cadmium or cadmium oxide to make an arc-resistant material which has


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Energy Systems Engineering Technology good conductivity (low resistance). In addition, the Silver Alloy has good mechanical strength, enabling it to endure the continual wear encountered by many openings and closings. Another advantage is that the oxide forming on the metal is an excellent conductor of electricity, even when the contacts appear dull or tarnished, they are still cable of normal operation. Manual Contactors are used to directly control power circuits and an understanding of wiring diagrams is required to make changes in power circuits. The below two diagrams illustrates a wiring diagram showing the connection on an installation or is component devices and parts used in a single control of a heating element and dual control for a high and lower heat control option:

Figure 8-7 page 176


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Figure 8-8 page 176 In the second diagram showing dual heat level controls, Mechanical Interlocks are utilized such that only one set of set of contacts are forced open to allow closing of a second set of contacts. This is provided by a separate relay activating a set of NO and NC contacts so that the load is distributed to the correct resistive circuit to provide either low or high heat demand. This separate relay uses a coil that when it is energized it allows the normal status of internal contacts to open or close when energized.


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DESCRIBE what a Manual Starter consists of and how they provide motor control and what protection they provide motors.

Manual Starters consist of a Contactor with an added Overload Protection Device. Manual Starters are used only for Motor Starters for Electrical Motor Circuits. The primary difference between a Manual Contactor and a Manual Starter is the addition of overload devices as depicted in the below picture:

Figure 8-10 page 176 

The overload protection MUST be added because the National Electrical Code® (NEC®) requires that a control device shall not only turn ON or OFF a motor, it should also protect the motor from destroying itself under an overloaded situation, such as a lock rotor.



A Locked Rotor is a condition created when a motor is loaded so heavily that the motor shaft cannot rotate. This condition draws excessive current and heat that can cause a motor to burn up if not disconnected from the applied line voltage. Overload devices sense the excessive current and open the circuit to protect the motor.


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Energy Systems Engineering Technology SUMMARY:  Double-Break Contacts break and electrical circuit in two power lines.  Three Phase Manual Contactors are similar to a Double-Break Contacts except that they open and close three sets of contacts for a three phase system.  Manual Contactors are used to directly control power circuits and not for motor cicuits.  Manual Starters consist of a Contactor with an added Overload Protection Device and are used for motors.  Motor Overload protection MUST be added because the National Electrical Code® (NEC®) requires that a motor control device shall not only turn ON or OFF a motor, it should also protect the motor from destroying itself under an overloaded situation.  Overload Devices sense the excessive current and open the circuit to protect the motor. KEO 4.5.

DESCRIBE the three stages a motor goes through during normal operation and how they are protected against a potential overload condition to include: Motor Overload Conditions, Melting Alloy Overloads, Heating Coils, and Resetting Overload Devices.

The Three Stages A Motor Goes Through are: Resting, Starting, and Operating Under Load as illustrated in the below picture:

Figure 8-11 page 177 Motor Controls Module

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A motor at Rest requires no current as the motor circuit is open. A motor that is Starting draws a tremendous amount of Inrush Current (normally 6 – 8 times the motor running current as the motor ramps up to its speed. A motor Operating Under Load is operating at its expected full load current. Fuses, Breakers, or Heaters as Overload Protective Devices must be rated at a sufficiently high ampere rating to avoid the immediate opening of the circuit caused by Inrush Current. Breaker and Fuses Overload Devices are designed to allow the Inrush Current to pass through them momentarily while the motor is starting and reaching its operating (under load) current limit. Fuses Overload Devices are termed as Slow Blow, whereas breakers are designed internally to accept excessive current before it heats up enough to trip the breaker. The National Electrical Code® addresses the types and specifications that MUST be used for motors.



Motor Overload Conditions may occur while a motor is running after it has reached its designed Operating Under Load condition. This may occur as the motor is: o o o o o o o

Forced to do more work than it was intended to do. Motor bearings failing. Lack of air flow through the motor. The insulation of its internal windings starts to age. Increased ambient temperatures. Lose electrical connections (electricity’s worst night-mare) Etc.

These conditions may not draw enough excess current to trip the protective devices; however it can increase the temperature enough to cause motor damage sufficient to burn the motor up. The most common motor overload devices are sensitive enough to trip on excessive heat and are interlocked into the motor control circuit to open the voltage source to the motor. To meet overload motor protection needs, overload relays are designed to have a time delay to allow harmless, temporary overloads without disrupting the circuit. Overload relays must also have a trip capability to open the circuit if mildly dangerous currents that could result in motor damage over a period of time. All overload relays have some means of resetting the current once the overload condition is resolved. Heat is the end product that can destroy a motor if not monitored. There are several devices used to detect a combination of heat and overload:


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Melting Alloy Overloads consist of a device to measure both overload and the temperature of the motor based on the current being drawn. Sometimes this temperature is a product of lose connections close to the overloads or at the motor junctions or the increase of current in the motor windings. Melting Alloy Overloads indirectly monitor conditions of the motor because the overload relay is normally located as some distance from the motor. The alloy used provides continuity for current to flow to the motor and when current flows, it gives off heat. This heat under normal conditions does not affect the alloy, however when over current conditions exist, excessive heat also exists. This excessive heat actually melts the alloy and breaks the current path to stop the motor. When the heat dissipates, the alloy actually sets back up to a solid state to be able to provide a current path back to the motor.



Heating Coils are sensing devices that sense heat generated by excessive current. This heat is detected by an interlock device like the Melting Alloy Overloads to stop a motor circuit that has overheated due to ambient temperature changes to the heating coil. Most manufactures rely on Eutectic Alloy in conjunction with a mechanical mechanism to activate a tripping device when an overload occurs. The Eutectic Alloy is a metal that has a fixed temperature at which it changes directly from a solid to a liquid state. The fixed state at which this changes is not affected by repeated melting and resetting. Most manufactures use a Ratchet Wheel and the Eutectic Alloy combination to activate a trip mechanism when the overload occurs. o A heater coil device is depicted in the below picture:


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The Ratchet Wheel is held firmly in the tube by the solid Eutectic Alloy. The inner shaft and ratchet wheel are locked into position by a Pawl (locking mechanism) so that the wheel cannot turn when the alloy is cool and in solid form. This Ratchet Wheel and the Eutectic Alloy combination is illustrated with how this action takes place in the picture below:


Excessive current applied to the heater coil (current running the motor) melts the Eutectic Alloy, which allows the Ratchet Wheel to turn freely to activate a trip mechanism as illustrated above. The main device in an Overload Relay is the Eutectic Alloy Tube.


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Energy Systems Engineering Technology o The following picture illustrates how a typical Manual Starter Overload Relay uses a compressed spring to push the normally closed contacts open under normal operating conditions:


The heater coil heats the Eutectic Alloy Tube when an overload occurs. The heat then melts the alloy, which allows the ratchet wheel to turn. The spring pushes the reset button up, which open the contacts to the voltage coil of the contactor and stops the motor. When the Eutectic Alloy cools and returns to its normal solid state, this contact can be reset until an overload conditions exists again.


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Resetting Overload Devices requires determining the cause of the overload and when this condition is determined and corrected, the device(s) can be reset as illustrated in the below picture:

Figure 8-16 page 180 o This same basic relay is used with all sizes of motors. The only difference is that the heater coil size is changed. For a single phase motor, one or two will be used and for a three phase motor, three will be used. The NEC® should be checked for the correct selection of the appropriate overload heater sizes. o Resetting Overload Devices requires that the technician must determine the cause of an overload before resetting an overload relay. If the cause is not determined and the overload devices have cooled enough to be reset, the motor circuit will trip again causing increased risk of extending damage to the motor. Nothing in the Overload Device Relay requires replacement because the heaters do not open like a fuse would unless they are defective and will not reset. When the overload condition is removed and the reset button is pressed after the eutectic alloy has cooled and become a solid again. Motor Controls Module

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Energy Systems Engineering Technology SUMMARY:           

The Three Stages A Motor Goes Through are: Resting, Starting, and Operating Under Load A motor at Rest requires no current as the motor circuit is open. A motor that is Starting draws a tremendous amount of Inrush Current (normally 6 – 8 times the motor running current as the motor ramps up to its speed. A motor Operating Under Load is operating at its expected full load current. Motor Overload Conditions may occur while a motor is running after it has reached its designed Operating Under Load condition. To meet overload motor protection needs, overload relays are designed to have a time delay to allow harmless, temporary overloads without disrupting the circuit. Heat is the end product that can destroy a motor if not monitored. Melting Alloy Overloads consist of a device to measure both overload and the temperature of the motor based on the current being drawn. Heating Coils are sensing devices that sense heat generated by excessive current. Excessive current applied to the heater coil (current running the motor) melts the Eutectic Alloy, which allows the Ratchet Wheel to turn freely to activate a trip mechanism. Resetting Overload Devices requires determining the cause of the overload and when this condition is determined and corrected, the device(s) can be reset.


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EXPLAIN how Electronic Overloads provide protection against changes in current and temperature.

Electronic Overloads provide protection against changes in current and temperature by a device that has built in circuitry to sense changes (increases) in motor circuit current. Today’s newer Manual Starters include Electronic Overload Protection instead of heaters. This is accomplished by the built-in circuitry sensing the motor current increase from its normal full load condition. The following picture illustrates how Electronic Overloads provide protection:

Figure 8-17 page 180 o Electronic Overloads measure the strength of the magnetic field around a wire instead of measuring current that causes heat. As current flows in a conductor, the magnetic field provides a signal that is electronically is equal to that current flowing in a conductor. The Solid State Current Monitor then provides the signal necessary to open a motor circuit when the current exceeds the motor requirements. The higher the current flowing in the wire, the stronger the magnetic field produced to activate disconnecting means opening the starter power circuit contacts. Motor Controls Module

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EXPALIN the criteria for Selecting AC Manual Motor Starters to include: Phasing, Enclosures, and Manual Starter Applications.

The Criteria for Selecting AC Manual Motor Starters includes identification of specific characteristics a starter must be able to provide for the proper operation of a motor, such as what phasing requirements are, the type of enclosure needed, and the application in which the starter is to provide. The following picture illustrates typical characteristics for determining starter selection and its size:


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Energy Systems Engineering Technology Figure 8-18 page 181 

Phasing and Enclosures are the determining factors in selecting an enclosure for single or three phase motor manual starters. The smallest size starter for a single phase motor (single pole or double pole) without overload protection is “00” and smallest size of enclosure for a three phase three pole starter with overload protection is a size “0” to a size “1”. NEC® Requires that each Ungrounded Conductor in a motor circuit (meaning the hot conductors) MUST be opened when disconnecting those conductors from the motor. For example, a Single Pole Motor has one ungrounded conductor and one grounded (neutral) conductor and so only the ungrounded (hot) conductor needs to have a disconnecting means. For a Double Pole Motors and Three Pole motors ( Single Phase and Three Phase Motors) the conductors are all ungrounded (hot) and MUST have a disconnecting means provided. The following picture illustrates how this is accomplished for a Single Pole motor with a Neutral (grounded) Conductor, A Double Pole with two (ungrounded) Hot Conductors, and a Three Pole with three (ungrounded) Hot Conductors that are protected by a fuse, or breaker device as a disconnecting means:


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Enclosures provide mechanical and electrical protection for personnel and for the starter itself. Enclosures are designed to Provide Protection in a variety of situations: Water, Dust, Oil, and Hazardous Locations. Inside the enclosure, starter wiring and physical construction are the same for any location. The NEC® defines a NEMA Type 1 Enclosure as intended for indoor use primarily to provide a degree of protection against human contact with the enclosed equipment in locations where unusual conditions do not exist.



Manual Starter Applications usually include applications such as Conveyor Systems, and drill presses similar to those depicted in the below figure:


In these types of applications, the manual starter provides the disconnecting means for ON and OFF while providing motor overload protection. Manual Starter Applications in most all cases involve the starter to be at or near the equipment being started, whereas Automatic Starter Applications are remotely located.


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SUMMARY:    

    

Electronic Overloads provide protection against changes in current and temperature by a device that has built in circuitry to sense changes (increases) in motor circuit current. Today’s newer Manual Starters include Electronic Overload Protection instead of heaters. Electronic Overloads measure the strength of the magnetic field around a wire instead of measuring current that causes heat. The Criteria for Selecting AC Manual Motor Starters includes identification of specific characteristics a starter must be able to provide for the proper operation of a motor, such as what phasing requirements are, the type of enclosure needed, and the application in which the starter is to provide. Phasing and Enclosures are the determining factors in selecting an enclosure for single or three phase motor manual starters. NEC® Requires that each Ungrounded Conductor in a motor circuit (meaning the hot conductors) MUST be opened when disconnecting those conductors from the motor. Enclosures provide mechanical and electrical protection for personnel and for the starter itself. Enclosures are designed to Provide Protection in a variety of situations: Water, Dust, Oil, and Hazardous Locations. Manual Starter Applications usually include applications such as Conveyor Systems, and drill presses. Manual Starter Applications in most all cases involve the starter to be at or near the equipment being started, whereas Automatic Starter Applications are remotely located.


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

DESCRIBE the principles of operation of Magnetic Contactors to include: Magnetic Contactor Construction, Magnetic Contactor Wiring, Control Circuit Wiring, and Control Circuit Voltage.

Magnetic Contactors include devices that may be operated manually or magnetically. They are devices for repeatedly establishing and interrupting an electrical power (ungrounded) circuit. Contactor Construction consists of the principle operating mechanism of a Solenoid Action as indicated in the picture below:


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Energy Systems Engineering Technology o Control Circuit Wiring is associated with the number of conductors used in the control circuit such as two and three wire control. In a Two Wire Control Circuit, two wires lead from the control device to the contactor or starter as illustrated in the below Two-Wire Control Schematic:


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Energy Systems Engineering Technology o A Two-Wire Control Circuit provides low voltage release, but not low voltage protection. Caution must be exercised in the use and service of Two-Wire Control Circuits. o A Three-Wire Control Circuit has three wires leading from the control device to the starter or contactor as illustrated in the below Three-Wire Control Schematic:


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Energy Systems Engineering Technology This Three-Wire Control Circuit uses a momentarily closed ON pushbutton (NC) wired in series with a momentary contact OFF pushbutton (NO) wired in parallel to a set of contacts which form a holding circuit interlock (memory). A Three-Wire Control Circuit provides low-voltage release and low-voltage protection. The coil drops out at low or no voltage and cannot be reset unless the voltage returns and the operator presses the start button. 

Control Circuit Voltage involves the use of Pushbuttons, Limit Switches, Pressure Switches, Temperature Switches, etc., to control the flow of power to the contactor/motor starter magnetic coil in control of the circuit. In most cases dealing with motor controls, the control circuit is operated at a lower voltage level that the load. This is for safety as well as efficiency. Step-Down transformers are used to reduce the line voltage to a control voltage. The control voltage can be AC or DC, and us usually a reduced AC. The following schematic illustrates how line voltage is reduced (stepped-down) to provide a separate control voltage:


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SUMMARY:     

Magnetic Contactors include devices that may be operated manually or magnetically. They are devices for repeatedly establishing and interrupting an electrical power (ungrounded) circuit. Control Circuit Wiring is associated with the number of conductors used in the control circuit such as two and three wire control. A Two-Wire Control Circuit provides low voltage release, but not low voltage protection. Caution must be exercised in the use and service of Two-Wire Control Circuits. A Three-Wire Control Circuit provides low-voltage release and low-voltage protection. Control Circuit Voltage involves the use of Pushbuttons, Limit Switches, Pressure Switches, Temperature Switches, etc., to control the flow of power to the contactor/motor starter magnetic coil in control of the circuit.


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

DESCRIBE the principles of operation of AC and DC Controllers to include: AC Arc Suppression (AC and DC), Arc Chutes, and DC Magnetic Blowout Coils.

Arc Suppression is required on contactors and motor starters. An Arc Suppressor is a device that dissipates the energy present across opening contacts. Without Arc Suppression contactors and motors would require maintenance prematurely resulting in excessive down time. A period of time (a few thousandths of a second) exist when a set of contacts is open under load which the contacts are neither fully in touch with each other, nor completely separated as illustrated in the below picture:

Figure 8-27 page 187 o As contacts continue to separate, the contact surface area decreases, increasing the electrical resistance. With full load current passing through this increased resistance, temperature rises on the contact surface and cause the contact surface to become molten and emit ions of vaporized metal into the gap. An arc is then created, which can damage the contact surface. The sooner this arc can be extinguished, the longer the life expectancy of the contacts. There are AC and DC Arcs. DC is the most difficult to extinguish because DC causes current to flow constantly across a much wider gap. DC Contactors are larger and need to function faster than AC Contactors. Motor Controls Module

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Energy Systems Engineering Technology o An Arc Chute is a device that confines, divides, and extinguishes arcs drawn between contacts opened up under load as depicted in the below picture:

Figure 8-29 page 188 o Arc Chutes employ the De-Ion Principle to extinguish the arc for each contact. Arc chutes and traps are used to confine, divide, and extinguish arcs down between contacts opened under load.


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Energy Systems Engineering Technology o DC Magnetic Blowout Coils are an Electromagnetic Blowout Coil and is referred to as a Puffer because of its blowout ability as illustrated in the below picture:

Figure 8-31 page 189 With DC Arcing, an action must be taken to quickly limit the damaging effects of the heavy circuit current arc as a sustained arc may melt the contacts, weld them together, or severely damage them The Blowout Coil (Puffer) is used to reduce the distance required to and yet quenches the arc quickly. Magnetic Blowout Coils provide this action by providing a magnetic field that blows out the arc similarly to blowing out a lit match. SUMMARY:  Arc Suppression is required on contactors and motor starters.  An Arc Suppressor is a device that dissipates the energy present across opening contacts.  Without Arc Suppression contactors and motors would require maintenance prematurely resulting in excessive down time.  DC Arcs are the most difficult to extinguish because DC causes current to flow constantly across a much wider gap. DC Contactors are larger and need to function faster than AC Contactors.  Arc Chutes employ the De-Ion Principle to extinguish the arc for each contact.  Arc chutes and traps are used to confine, divide, and extinguish arcs down between contacts opened under load.  DC Magnetic Blowout Coils are an Electromagnetic Blowout Coil and is referred to as a Puffer because of its blowout ability. Motor Controls Module

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



DESCRIBE the basic Contact Construction criteria associated with Contacts to include: Single and Double Breaking Contacts, and General-Purpose AC/DC Contactor Sizes and Ratings.

Single and Double Breaking Contacts design depends on the size, current rating, and application of the contactor being utilized. Double Breaking Contacts are usually made of Silver-Cadmium Alloy. Single Breaking Contacts in large contactors are frequently made of copper because of the low cost. o Single Breaking Contacts are designed with a wiping action to remove copper oxide film that forms on the contacts. The Wiping Action is necessary because copper oxide formed on the contacts when not in use is an insulator and must be eliminated for good circuit continuity. In most cases, the slight rubbing action and burring that occur in normal operation keeps the contact surfaces clean. Copper contacts seldom open or closed should be cleaned to reduce contact resistance as high contact resistance causes serious heating of the contacts. General-Purpose AC/DC Contactor Sizes and Ratings are according to the size and type of load by NEMA (National Electrical Manufacturing Association) and are specified in the NEC® Requirements. An example of NEMA Standard Ratings for AC and DC Contactors are shown in the below picture:


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Energy Systems Engineering Technology These ratings are for each contact and not the entire contactor. Contactor dimensions vary greatly from inches to feet in length and are based on type, size and voltage available as depicted in the picture below:

Figure 8-33 page 191 Contactors vary from inches to several feet in height. SUMMARY:  

Double Breaking Contacts are usually made of Silver-Cadmium Alloy. Single Breaking Contacts in large contactors are frequently made of copper because of the low cost.


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

Single Breaking Contacts are designed with a wiping action to remove copper oxide film that forms on the contacts. The Wiping Action is necessary because copper oxide formed on the contacts when not in use is an insulator and must be eliminated for good circuit continuity. General-Purpose AC/DC Contactor Sizes and Ratings are according to the size and type of load by NEMA (National Electrical Manufacturing Association) and are specified in the NEC® Requirements.

KEO 4.11.

DESCRIBE principles of operation for a Magnetic Motor Starter to include how it provides Overload Protection including: Melting Alloy Overload Relays, Magnetic Overload Relays, Bimetallic Overload Relays, Trip Indicators, and Overload Current Transformers.

Magnetic Motor Starters are available in sizes that can switch loads of a few amperes to several hundred amperes. An example of a magnetic motor starter with electronic overload protection is depicted below:


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Energy Systems Engineering Technology A magnetic motor starter is a contactor with overload protection added. The difference from a Manual Starter and Magnetic Motor Starter is that a Motor Starter is equipped with motor overload protection. A Magnetic Motor Starter is an electrically operated switch (contactor) that includes motor overload protection. They include overload relays that detect excessive current passing through a motor and are used to switch all types and sizes of motors.

The following pictures illustrate the principles of operation for: Magnetic Overload Relays, Bimetallic Overload relays, Trip Indicators, and Overload Current Transformers: 

Magnetic Overload Relays

Figure 8-35 page 192 Magnetic overload relays use a current coil which, at a specific over-current value, acts like a solenoid and causes a set of normally closed contacts to open. Motor Controls Module

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Bimetallic Overload relays

Figure 8-36 page 193 The warping effect of a bimetallic strip is used as a means for separating contacts.


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

Trip Indicators

Figure 8-37 page 193 Trip indicators indicate that an overload has taken place for within the device.


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Overload Current Transformers

Figure 8-38 page 194 Standard overload relays may be used on very large starters by using current transformers with specific reduction ratios.


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SUMMARY:  

   

Magnetic Motor Starters are available in sizes that can switch loads of a few amperes to several hundred amperes. A Magnetic Motor Starter is an electrically operated switch (contactor) that includes motor overload protection. They include overload relays that detect excessive current passing through a motor and are used to switch all types and sizes of motors. Magnetic overload relays use a current coil which, at a specific overcurrent value, acts like a solenoid and causes a set of normally closed contacts to open. The warping effect of a bimetallic strip is used as a means for separating contacts. Trip indicators indicate that an overload has taken place for within the device. Standard overload relays may be used on very large starters by using current transformers with specific reduction ratios.


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DESCRIBE how Overload Heater Sizes and Selection is accomplished to include: Full-Load Current, Service Factor, Ambient Temperature, Manufacture Heater Selection Charts, Checking Selections, and Ambient Temperature Compensation.

Overload Heater Sizes – Each motor must be sized according to its own unique operating characteristics and applications. Thermal over-load heaters are selected based on the Full-Load Current rating (FLC), Service Factor (SF), and Ambient Temperature (surrounding air temperature) on the motor when it is operating. Each motor has its own nameplate which provides the motor Class, Type, and Size of the starter as illustrated in the below picture:

Figure 8-39 page 195 Selection of Motor Overload Heaters Coils for Continuous-Duty Motors are selected from manufactures tables based on the motor nameplate full-load current for maximum protection and compliance with Section 430.32 of the NEC®. Common applications use 40oC as the ambient temperature. If the temperature is different, it must be determined in order to make the correct heater selection. A motor’s Phase, Service Factor, and Full-Load Current are determined from the motor’s nameplate information.


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Energy Systems Engineering Technology It is important to Always Refer To Manufactures Instructions on thermal overload relay selection to see if any restrictions are placed on the class of the motor starter required. The below picture illustrates an example of manufactures instructions on thermal overload relay selection detailing restrictions of classes of starters:


Full-Load Current is based on information found on the motor nameplate or in manufacturing specification sheets. Heater manufactures develop charts indicating which heaters should be used with each full-load current.



Service Factor is a number (SF) designation that represents the percentage of extra demand that can be placed on a motor for short intervals with causing motor damage. This is a multiplier that can be used to determine the SF current rating. If a motor name plate indicated an SF 1.15 and the motor was a 10 amp motor. 10 x 1.15 = 11.5 amps this motor could operate for a short time interval without causing motor damage.



Ambient Temperature is associated with thermal overload devices operating on the principle of heat. Excessive current will melt a metal alloy, produce movement in a current coil or warp a bimetallic strip to allow the device to trip. Temperature (ambient) surrounding a thermal overload relay must be considered as the relay device is sensitive to heat from any source. Overload relays usually have a rating of 400 C or 1040 F. This is a standard acceptable range for most devices and temperature above or below these ranges need to be compensated for.


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Manufacture Heater Selection Charts are provided to use in selecting proper thermal overload heaters. This information is also found within the enclosure of many motor starters. An example of a Manufacture Heater Selection Chart is shown below:

Figure 8-41 page 196


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

Ambient Temperature Compensation is required when ambient temperature increases or decreases. As ambient temperature decreases, more current is needed to trip the overload devices. An example of a Thermal Unit Selection criteria is shown below:



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Manufactures also provide charts for approximating Full-Load Current when motor nameplate information is not available as illustrated below:


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Energy Systems Engineering Technology SUMMARY:             

Each motor must be sized according to its own unique operating characteristics and applications. Thermal over-load heaters are selected based on the Full-Load Current rating (FLC), Service Factor (SF), and Ambient Temperature (surrounding air temperature) on the motor when it is operating. Each motor has its own nameplate which provides the motor Class, Type, and Size of the starter. Selection of Motor Overload Heaters Coils for Continuous-Duty Motors are selected from manufactures tables based on the motor nameplate full-load current for maximum protection and compliance with Section 430.32 of the NEC®. Common applications use 40oC as the ambient temperature. If the temperature is different, it must be determined in order to make the correct heater selection. A motor’s Phase, Service Factor, and Full-Load Current are determined from the motor’s nameplate information. It is important to Always Refer To Manufactures Instructions on thermal overload relay selection to see if any restrictions are placed on the class of the motor starter required. Full-Load Current is based on information found on the motor nameplate or in manufacturing specification sheets. Service Factor is a number (SF) designation that represents the percentage of extra demand that can be placed on a motor for short intervals with causing motor damage. Ambient Temperature is associated with thermal overload devices operating on the principle of heat. Manufacture Heater Selection Charts are provided to use in selecting proper thermal overload heaters. This information is also found within the enclosure of many motor starters. Ambient Temperature Compensation is required when ambient temperature increases or decreases. Manufactures also provide charts for approximating Full-Load Current when motor nameplate information is not available.


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DESCRIBE what Inherent Motor Protection consist of and their principles of operation to include: Bimetallic Thermo-Discs, and Thermistor Overload Devices.

Inherent Motor Protection consists of a device (or devices) located locally on the motor to provide overload protection. When motors are remotely located away from the motor control center, they may be subject to ambient conditions that can cause motor failure before the overload protection will be able to open the circuit. 

Bimetallic Thermo-Discs are normally used on small horsepower motors to directly disconnect the motor from the power circuit. When these devices reach a level of heat, they warp and open a circuit. They operate on a principle associated with how different metals when joined together will react differently as temperature is applied. This twisting or warping motion will open a contact to prevent the motor from becoming damaged. An example of a Bimetallic Thermo-Disc (usually used on small HP motors) is shown below:

[ Insert Figure 8-44 page 198 ]


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Thermistor Overload Devices are an overload device combined with a thermistor, solidstate relay, and contactor into a custom-built overload protector as illustrated below:

Figure 8-45 page 198 SUMMARY:   

Inherent Motor Protection consists of a device (or devices) located locally on the motor to provide overload protection. Bimetallic Thermo-Discs are normally used on small horsepower motors to directly disconnect the motor from the power circuit. Thermistor Overload Devices are an overload device combined with a thermistor, solidstate relay, and contactor into a custom-built overload protection device.


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EXPLAIN how Electronic Overload Protection is incorporated into Contactor and Magnetic Motor Modifications to expand their capabilities to include: Additional Electrical Contacts, Power Poles, Pneumatic Timers, Transient Suppression Modules, and Control Circuit Fuse Holders.

Contactor and Magnetic Motor Modifications expand a motor contactor starter by incorporating certain devices to the basic contactors or motor starters. These designed by the contactor or starter manufacture to be optional equipment that can be electrically and mechanically attached to a contactor or starter assemble inside of its NEMA enclosure. 

The following picture shows 5 such devices that can be utilized for added motor protection:

Figure 8-46 page 199 SUMMARY:  Contactor and Magnetic Motor Modifications expand a motor contactor starter by incorporating certain devices to the basic contactors or motor starters.  Five devices that are typically installed on motor starters are 1. Additional Electrical Contacts 2. Power Poles 3. Pneumatic Timers 4. Transient Suppression Modules 5. Control Circuit Fuse Holders


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DESCRIBE Troubleshooting Contactors and Motor Starter techniques and procedures.

SAFETY NOTE REGARDING TROUBLSHOOTING MOTOR CONTROLS: 

Safety precautions must be followed when troubleshooting motor control circuits. This is because to check control circuits effectively, power must be energized and the appropriate safety personal protective equipment and procedures must be utilized by the technician at all times when working on energized equipment.



Contactors and Motor Starters are the first devices checked by a technician when troubleshooting a circuit that does not work, or has a problem because they are the point where incoming power, load, and control circuit are connected. Basic voltage readings are taken at a contactor or motor starter to determine where a problem may be. The same basic procedure used to troubleshoot a motor starter also works for contactors because a motor starter is a contactor with added overload protection.



The Number 1 overall problem with any electrical circuit is caused by poor or loose connections. This condition generates heat that can not only destroy wiring, but also components upstream of the load and out to the load. The tightness of all terminals and bus-bar connections needs to be checked. Loose connections causes: o Overheating in power circuits, contactors, and starters o Leads to equipment failure o Creates control circuit failures o Lead to Shock Hazards o Cause Electromagnetic-Generated Interference o Etc.


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To troubleshoot a motor starter, apply the following procedure (Below is a pictorial presentation of this procedure to help emphasize this 6-Step approach to troubleshooting a motor starter):



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Energy Systems Engineering Technology 1. Inspect the motor starter and overload assembly.  Service or replace motor starters that show heat damage, arcing, or wear.  Replace motor starters that show signs of burning.  Check the motor and driven load for signs of an overload or other problems. 2. Reset the overload relay if there is not sign of visual damage.  Replace the overload relay if there is visual indication of damage. 3. Observe the motor starter for several minutes if the motor starts and after resetting the overload relay (observe if the overload relay continues to open if an overload problem continues to exist. 4. Check the voltage into the starter if resetting the overload relay does not start the motor.  Check circuit voltage ahead of the starter if zero voltage is at the starter.  The voltage is acceptable if within a + or – 10% of the normal voltage necessary for the circuit. 5. Energize the starter and check the starter contacts if the voltage into the starter is present at the correct value.  Verify contacts are good.  Open the starter, turn off and lockout-tagout power to replace contacts as necessary. 6. Check the overload relay if voltage is coming out of the starter contacts.  Lockout and Tagout power if the problem is downstream of the starter and continue troubleshooting downstream.

KEO 4.16.

DESCRIBE how a Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a technician.

A Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a technician in that it states problems, possible causes, and corrective actions that may be taken. Motor Controls Module

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Energy Systems Engineering Technology An example of a troubleshooting guide is shown below:


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SUMMARY:   



Safety precautions/procedures MUST be followed when troubleshooting motor control circuits because to check control circuits effectively, power must be energized. Contactors and Motor Starters are the first devices checked by a technician when troubleshooting a circuit that does not work, or has a problem because they are the point where incoming power, load, and control circuit are connected. The Number 1 overall problem with any electrical circuit is caused by poor or loose connections. These conditions generate heat that can not only destroy wiring, but also components upstream of the load and out to the load. A Contactor and Motor Starter Troubleshooting Guide can be a useful tool for a technician in that it states problems, possible causes, and corrective actions that may be taken.


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DESCRIBE role Motor Drives provide in the control of Contactors and Motor Starters to include: Motor Drive Programming and Programming Overload Protection.

MOTOR DRIVES are being incorporated more and more and replacing many applications for motor control. A Motor Drive is an electronic device designed to control the speed of a motor. This is accomplished by using solid state components. Motor Drives may control AC or DC Motors. AC motors Drives are more common than DC Motor Drives. Other terms for AC Motor Drives are: 1. Adjustable Speed Drives 2. Variable Frequency Drives 3. Inverters The following picture illustrates both an AC and DC Motor drive showing solid-state devices used to control motor speed:


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Motor Drives are designed to control the speed of motors using solid-state components and may be AC or DC drives. They perform the same function as a Motor Starter, but can also: o o o o o o

Vary motor speed. Reverse motor direction. Provide additional protection features. Displays operating information. Interfaces with other electrical equipment Provides motor control for motors from a fractional to hundreds of HP



AC Motor Drives control motor speed by converting the incoming AC to DC and then converting the DC back to a Variable Frequency AC. A typical AC motor at 60 Hz runs at full speed, at 30 Hz runs at half speed, and at 15 Hz runs at one-quarter speed.



DC Motor Drives control motor speed by controlling and monitoring the DC Output Voltage to the motor and the Current on the motor field windings and armature windings. AC Power can also be the input power source that is then converted to DC.



AC Motor Drives have revolutionized motor control making it more efficient and cost effective. They do this by providing a pulsating DC to drive the motor and control its speed by varying the frequency to the motor. Using a microprocessor circuit located inside the drive device provides fast and reliable electronic switching.



Motor Drive Programming is essential to ensure proper motor operation. A properly programmed motor drive provides maximum system performance. An improperly programmed motor drive can cause damage to the motor, other system components and create safety hazards to the area and personnel.


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Energy Systems Engineering Technology The following picture depicts the use of SCR to convert AC to DC and controlling the level of DC voltage, and the use of IGBT (Insulated Gate Bipolar Transistors) to provide PWM (Pulse Width Modulated Inverter):


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Energy Systems Engineering Technology AC drives include a converter, DC link, and an inverter as illustrated above. o Programming is done via a Human Interface Module, which is a manually operated input control unit that includes programming keys. Programming is performed by trained technicians. The following two pictures depict a typical Human Interface Module, and levels utilized for system information and programming:



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

Overload Protection is needed when a motor starter turns on a motor to protect it while it is operating using heaters or electronic overload devices included in a typical motor starter per NEC® Requirements. Programming Overload Protection with an AC Motor Drive is programmed into the Drive using the Human Interface Module to meet NEC® Requirements as well. These requirements are specified in Article 430 of the NEC®. Part III of Article 430 covers motor overload (running) protection requirement. Motor Drives meet these requirements if they are programmed properly. Over Current and Ground-Fault protection is covered in Part IV of Article 430 as well. o Overload Protection needs to be set to open a circuit at a maximum of 115% to 1125% of the motor full-load current and is addressed in NEC Article 432.32. The percentage depends on the motor temperature rise and its service factor. o Motors marked with a service factor (SF) not less than 1.15, require maximum overload protection of 125% times the motor full-load current. o Motors with a marked temperature rise not over 400 C also require a maximum overload protection of 125% times the motor full-load current. o All Other Motors are required to be to have a maximum overload protection of 115% times the motor full-load current. The following picture illustrates how a motor name plate is read to determine its Service Factor (SF) in order to program the overload protection according to NEC® Requirements:


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Energy Systems Engineering Technology SUMMARY:     

     

MOTOR DRIVES are being incorporated more and more and replacing many applications for motor control as they cost less and are more efficient. A Motor Drive is an electronic device designed to control the speed of a motor by using solid state components. Motor Drives may control AC or DC Motors. AC motors Drives are more common than DC Motor Drives. AC Motor Drives are called Adjustable Speed Drives, Variable Frequency Drives, or Inverters. Motor Drives are designed to control the speed of motors using solid-state components and may be AC or DC drives. They perform the same function as a Motor Starter and can also be programmed to do the following: o Vary motor speed. o Reverse motor direction. o Provide additional protection features. o Displays operating information. o Interfaces with other electrical equipment o Provides motor control for motors from a fractional to hundreds of HP AC Motor Drives control motor speed by converting the incoming AC to DC and then converting the DC back to a Variable Frequency AC. DC Motor Drives control motor speed by controlling and monitoring the DC Output Voltage to the motor and the Current on the motor field windings and armature windings. AC Motor Drives have revolutionized motor control making it more efficient and cost effective. AC Motor Drives use SCRs to convert AC to DC and controlling the level of DC voltage, and the use of IGBT (Insulated Gate Bipolar Transistors) to provide PWM (Pulse Width Modulated Inverter). Programming is done via a Human Interface Module, which is a manually operated input control unit that includes programming keys and is programmed by trained technicians. Programming Overload Protection with an AC Motor Drive is programmed into the Drive using the Human Interface Module to meet NEC® Requirements: o Overload Protection needs to be set to open a circuit at a maximum of 115% to 1125% of the motor full-load current and is addressed in NEC Article 432.32. The percentage depends on the motor temperature rise and its service factor. o Motors marked with a service factor (SF) not less than 1.15, require maximum overload protection of 125% times the motor full-load current. o Motors with a marked temperature rise not over 400 C also require a maximum overload protection of 125% times the motor full-load current. o All Other Motors are required to be to have a maximum overload protection of 115% times the motor full-load current.


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Energy Systems Engineering Technology Knowledge Enabling Objectives (Chapter 10 Reversing Motor Circuits): KEO 4.18.

EXPLAIN the concepts associated with how to reverse the direction of motors to include: AC Three Phase, AC Single Phase / Capacitor Start, and DC Motors.

Reversing the direction of motors is accomplished with: Manual Reversing Starters, Drum Switches, Magnetic Reversing Starters, Programmable Logic Controllers (PLCs) or Motor Drives. The below picture depicts a typical Three Phase Manual Starter having the capability of forward or reverse starting:


Manual Starters are used in pairs to reverse DC, Single Phase AC, and Three Phase AC motors. Manual Motor Starters in pairs used to provide forward or reverse rotation uses a mechanical interlock to separate the contactors starter contacts so that the motor will only run in one direction (forward or reverse).


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Energy Systems Engineering Technology The following picture illustrates how two starters are interlocked to accomplish this Three Phase AC Motor Action:


Reversing Three Phase AC Motors is a simple concept. It is accomplished by interchanging any two of the three phase main power sources to the motor. The industry standard that is most often performed so that there is consistency in wiring configurations is to interchange L1 and L3 for all three phase motors to include 3, 6, and 9 lead Wye (Y), and Delta (Δ) connected motors. This can be accomplished at the power source to the starter or at the motor interchanging T1 with T3.


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Energy Systems Engineering Technology o The picture below illustrates a wiring diagram for electrical connections necessary to reverse a Three Phase Motor using a manual reversing starter:



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Energy Systems Engineering Technology o The following picture illustrates a schematic that shows how a manual reversing starter is connected to accomplish a forward or reverse direction for a three phase AC motor:


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Energy Systems Engineering Technology o The basic concept associated with reversing a three phase motor is simply interchanging two of the three phase power leads to the motor (at starter or at motor) with changing at the starter being generally the easiest location to make the change. 

Reversing Single Phase AC Motors including a Capacitor Start is not as easy as reversing the direction of a three phase motor. To reverse the direction of a Single Phase AC Motor requires interchanging the leads of the starting or the running windings. To best way to accomplish this is to refer to the manufactures wiring diagram to determine the exact wires to interchange for reversing the single phase motor. The picture below illustrates how a manual reversing motor starter is used to change the direction of a single phase motor:


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If manufacture information is not available, an electrician/technician can measure the resistance of the start and run windings to determine which leads are which. The running winding is made of a larger (heaver) gauge wire than the starting winding, thus the running winding has a lower resistance than the start winding.



The below picture illustrates how a manual reversing motor starter is wired to allow changing of forward or reverse direction of a single phase motor:

[ Insert Figure 10-6 page 262 ] The preferred method to change the rotation of a single phase motor (or capacitor start motor) is to reverse the leads of the starting winding.


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Reversing DC Motors is accomplished by reversing the direction of DC Current Flow through the Armature of all DC Motors. The following schematics illustrate how this is accomplished using a reversing starter for: DC Series Motor, DC Shunt Motor, DC Compound Motor, and a DC Permanent-Magnet Motor: a. DC Series Motor



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Energy Systems Engineering Technology b. DC Shunt Motor



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Energy Systems Engineering Technology c. DC Compound Motor



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Energy Systems Engineering Technology d. DC Permanent-Magnet Motor


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SUMMARY:  







Reversing the direction of motors is accomplished with: Manual Reversing Starters, Drum Switches, Magnetic Reversing Starters, Programmable Logic Controllers (PLCs) or Motor Drives. Manual Starters are used in pairs to reverse DC, Single Phase AC, and Three Phase AC motors. Manual Motor Starters in pairs used to provide forward or reverse rotation, uses a mechanical interlock to separate the contactors starter contacts so that the motor will only run in one direction (forward or reverse). Reversing Three Phase AC Motors is a simple concept. It is accomplished by interchanging any two of the three phase main power sources to the motor. o The industry standard that is most often performed so that there is consistency in wiring configurations is to interchange L1 and L3 for all three phase motors to include 3, 6, and 9 lead Wye (Y), and Delta (Δ) connected motors. Reversing Single Phase AC Motors including a Capacitor Start is not as easy as reversing the direction of a three phase motor. o To reverse the direction of a Single Phase AC Motor requires interchanging the leads of the starting or the running windings. o The preferred method to change the rotation of a single phase motor (or capacitor start motor) is to reverse the leads of the starting winding. Reversing DC Motors is accomplished by reversing the direction of DC Current Flow through the Armature of all DC Motors.


Page 72


EXPLAIN the concepts and needs associated with mechanical interlocking devices used to protect motors.

Mechanical Interlocking includes devices that mechanically prevent the control circuit to only function in the control mode it has been requested to function in by an operator or by devices using automatic control logic circuitry. The following schematics illustrate how this is done: 1. Magnetic Reversing Starter controlled by forward and reverse pushbuttons:



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Energy Systems Engineering Technology 2. Magnetic Reversing Starter with secondary backup using auxiliary contacts to provide electrical interlocking:

Figure 10-17 page 267 3. Pushbutton Interlocking using both NO and NC contacts mechanically connected on each pushbutton:

Figure 10-18 page 268 4. Start/Stop/Forward/Reverse circuit with indicator lights showing the direction of rotation for a motor at a given moment: Motor Controls Module

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Figure 10-20 page 269 SUMMARY: Instructor Note: This summary addresses schematics and in preparation for this summary, the instructor could generate a few questions for each of the schematics listed below to assist in determining mastery of objective KEO 4.19: EXPLAIN the concepts and needs associated with mechanical interlocking devices used to protect motors.     

Mechanical Interlocking includes devices that mechanically prevent the control circuit to only function in the control mode it has been requested to function in by an operator or by devices using automatic control logic circuitry. Magnetic Reversing Starter controlled by forward and reverse push-buttons is an example of mechanical interlocks (Figure 10-16 page 267). Magnetic Reversing Starter with secondary backup using auxiliary contacts to provide electrical interlocking is also used to ensure mechanical interlocks (Figure 10-17 page 267). Pushbutton Interlocking using both NO and NC contacts mechanically connected on each pushbutton is another example (Figure 10-18 page 268). Start/Stop/Forward/Reverse circuit with indicator lights showing the direction of rotation for a motor at a given moment is another example (Figure 10-20 page 269).


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Energy Systems Engineering Technology Knowledge Enabling Objectives (Chapter 17 Reducing-Voltage Starting): KEO 4.20.

DESCRIBE the role Reduced Voltage Starting and Silicon-Controlled Rectifiers provide in motor controls.

Silicon-Controlled Rectifiers provide the ability for both AC and DC motors to start with Reduced-Voltage. Full-Voltage is the least expensive and most efficient means of starting small horsepower (HP) motors. Applications where large horsepower (HP) are started require Reduced-Voltage Starting to reduce interference in the power system, the load, and the electrical environment surrounding the motor. Silicon-Controlled Rectifiers provide this ability. Reduced-Voltage Starting reduces starting current as illustrated below:



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Energy Systems Engineering Technology Reduced-Voltage Starting reduces the amount of motor torque produced on a load as illustrated below:

Figure 17-2 page 486 Silicon-Controlled Rectifiers have the ability to rapidly switch heavy currents. As compared to a diode, they have the ability of added control from the gate that is not possible with a diode. The following figure depicts how solid state starters reduce inrush current, minimizes starting torque, and smoothes acceleration of a motor:


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Energy Systems Engineering Technology When the signal is applied to the gate of an SCR DC Voltage Control Circuit, the SCR is triggered ON and the anode resistance decreases sharply, such that the resulting current flow through the SCR is only limited by the resistance of the load as illustrated in the picture below:


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Energy Systems Engineering Technology SCRs may be used alone in a circuit to provide one-way current control, or may be wired in reverse-parallel circuits to control AC Line current in both directions as illustrated below:

Figure 17-22 page 503 An SCR circuit with Reverse-Parallel Wiring of SCRs provides maximum control of an AC Load 3 Phase Motor as illustrated below:


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Energy Systems Engineering Technology SUMMARY:        

Silicon-Controlled Rectifiers provide the ability for both AC and DC motors to start with Reduced-Voltage. Full-Voltage is the least expensive and most efficient means of starting small horsepower (HP) motors. Applications where large horsepower (HP) are started require Reduced-Voltage Starting to reduce interference in the power system, the load, and the electrical environment surrounding the motor. Reduced-Voltage Starting reduces starting current. Reduced-Voltage Starting reduces the amount of motor torque produced on a load. Silicon-Controlled Rectifiers have the ability to rapidly switch heavy currents and when used in solid state starters, they not only reduce inrush current, they also minimizes starting torque, and smoothes acceleration of a motor. SCRs may be used alone in a circuit to provide one-way current control, or may be wired in reverse-parallel circuits to control AC Line current in both directions An SCR circuit with Reverse-Parallel Wiring of SCRs provides maximum control of an AC Load 3 Phase Motor.

Knowledge Enabling Objectives (Chapter 18 Accelerating and Decelerating Methods): KEO 4.21.

EXPLAIN the concepts associated Braking and Plugging as they apply to motors.

Braking is used when it is necessary to stop a motor more quickly than coasting allows. Braking is accomplished by different methods, each having advantages and disadvantages. The method for Braking used depends on the application, available power, circuit requirements, cost, and desired results. Examples of Braking include braking every time a motor is stopped, in an emergency, or to slow the motor down.


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Friction Brakes are the oldest motor stopping method and are similar to automobile breaks. They are normally controlled by a solenoid device that activates the brake shoes to stop a motor. An example of Friction Brake is shown in the below picture:

Figure 18-2 page 512 Advantages of Friction Brakes are they are low initial cost and easy to maintain. The disadvantage is that they require more maintenance than other braking methods. Motor Controls Module

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Plugging is a method of Braking in which the motor connections are reversed so that the motor develops a counter-torque that acts as a braking force. The counter-torque is accomplished by reversing the motor at full speed. This Plugging method allows for a very rapid stopping via a Plugging Switch as illustrated below:

Figure 18-4 page 514 The use of the Plugging Switch allows the motor to stop and not run in the opposite direction. A Continuous Plugging uses a Plugging Switch that allows the motor plug to a stop each time the motor is stopped as illustrated in the below schematic:



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Energy Systems Engineering Technology Limitations of Plugging are that it may not be applied to all motors and or applications. Braking a motor to stop using plugging requires that the motor be a revisable motor and that it can be reversed at full speed. Even though a motor can be reversed at full speed, the damage that plugging may do may outweigh it advantages. A Single Phase Shaded Pole motor cannot be reversed at full speed and cannot have plugging used to stop it and Single Phase capacitor start motors cannot be plugged as the centrifugal switch removes the start winding when it accelerates and cannot be reversed without the start winding. Heat from plugging can be created by high current to motors and for this reason, only motors with a high service factor (SF) should be used in all cases except for emergency situations. The SP needs to be at 1.35 or greater for plugging applications. The following schematic illustrates how Plugging can be used in an emergency situation:

Figure 18-6 page 515 ] 

Electric Braking is a method where DC voltage is applied to the stationary windings of a motor after the AC voltage is removed. Electric Braking is also known as DC Injection Braking. Electric Braking can be applied to bring a motor to an immediate stop if the coasting time is unacceptable, particularly in an emergency situation.


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Energy Systems Engineering Technology o An example of how Electric Braking can be provided to a Shaded-Pole, SplitPhase, and Three Phase motors is illustrated below by applying the DC voltage to the stationary windings when AC has been removed:



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Energy Systems Engineering Technology The following schematic illustrates how Electric Braking can be applied to a Three Phase with AC removed so the motor can come to a complete stop quickly:



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Dynamic Braking of DC Motors is a method in which a motor is reconnected to act as a generator immediately after it is turned OFF. Connecting a DC motor in this way makes the motor act as a loaded generator that develops a Retarding Torque, which rapidly stops the motor. This example is illustrated in the below:



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Energy Systems Engineering Technology SUMMARY:      









Braking is used when it is necessary to stop a motor more quickly than coasting allows and is used to slow a motor or to stop a motor in an emergency situation. Friction Brakes are the oldest motor stopping method and are similar to automobile breaks. They are normally controlled by a solenoid device that activates the brake shoes to stop a motor. Advantages of Friction Brakes are they are low initial cost and easy to maintain. Disadvantage of Friction Brakes is that they require more maintenance than other braking methods. Plugging is a method of Braking in which the motor connections are reversed so that the motor develops a counter-torque that acts as a braking force. Limitations of Plugging are that it may not be applied to all motors and or applications. o Braking a motor to stop using plugging requires that the motor be a revisable motor and that it can be reversed at full speed. o Even though a motor can be reversed at full speed, the damage that plugging may do may outweigh it advantages. A Single Phase Shaded Pole motor cannot be reversed at full speed and cannot have plugging used to stop it and Single Phase capacitor start motors cannot be plugged as the centrifugal switch removes the start winding when it accelerates and cannot be reversed without the start winding. Heat from plugging can be created by high current to motors and for this reason, only motors with a high service factor (SF) should be used in all cases except for emergency situations. o The SP needs to be at 1.35 or greater for plugging applications. Electric Braking is a method where DC voltage is applied to the stationary windings of a motor after the AC voltage is removed. o Electric Braking is also known as DC Injection Braking. o Electric Braking can be applied to bring a motor to an immediate stop if the coasting time is unacceptable, particularly in an emergency situation. Dynamic Braking of DC Motors is a method in which a motor is reconnected to act as a generator immediately after it is turned OFF. o Connecting a DC motor in this way makes the motor act as a loaded generator that develops a Retarding Torque, which rapidly stops the motor.


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EXPLAIN the concepts associated Motor Drive Stopping to include: Stopping without Applied Force from the drive, and Stopping using Reduced Voltage as the motor Decelerates.

Motor Drive Stopping can be accomplished with a Motor Drive. The stopping time is programmed by setting the deceleration parameter for 1 second or less to several minutes. For fast stops, (especially with high inertia loads), a braking resistor can be added. The below picture illustrates how this can be configured:



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Energy Systems Engineering Technology Motor Drive Stopping can also be accomplished by reducing the voltage to slow down or stop the motor.

KEO 4.23.

EXPLAIN the concepts of multiple poles in the acceleration, deceleration control of AC Motors.

AC Motors are considered constant speed motors. This is because of the synchronous speed of an induction motor is based on the power supply frequency and the number of poles in the motor winding. Motors designed for 60 Hz use have synchronous speeds of 3600, 1200, 900, 514, and 450 rpm. Following formula is used for calculating the Synchronous Speed of an Induction Motor:

AC Induction Motor Speed Calculation Formula

RPM syn

RPM syn f NP

=

= = =

120 x NP

f

Where Synchronous Speed (in rpm) Supply Frequency (in cycles/sec) Number of Poles (in motor winding)

Example: What is the synchronous speed of a fourpole motor operating at 50 Hz? RPM syn

=

RPM syn

=

RPM syn

=

RPM syn

=

120 x f NP 120 x 50 4 6000_ 4 1500 rpm

Pages 535-536 NOTE: Supply frequency and number of poles are the only variables that determine the speed of an AC Motor. Unlike the speed of a DC Motor, the speed of an AC Motor should not be changed by varying the applied voltage as damage may occur to an AC Motor if the supply voltage is varied more than 10% above or below the rated nameplate voltage. This Motor Controls Module

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Energy Systems Engineering Technology is because in an Induction Motor, the starting torque and breakdown torque vary as the square of the applied voltage. Example: With 90% of rated voltage, the torque is 81% (.92 = .81 or 81% of its rated torque).

SUMMARY: 

Motor Drive Stopping can be accomplished with a Motor Drive. The stopping time is programmed by setting the deceleration parameter for 1 second or less to several minutes. For fast stops, (especially with high inertia loads), a braking resistor can be added.



Motor Drive Stopping can also be accomplished by reducing the voltage to slow down or stop the motor.



AC Motors are considered constant speed motors. o This is because of the synchronous speed of an induction motor is based on the power supply frequency and the number of poles in the motor winding.



The formula used for calculating the Synchronous Speed of an Induction Motor: RPM syn = 120 x NP

f

NOTE: Supply frequency and number of poles are the only variables that determine the speed of an AC Motor. Unlike the speed of a DC Motor, the speed of an AC Motor should not be changed by varying the applied voltage as damage may occur to an AC Motor if the supply voltage is varied more than 10% above or below the rated nameplate voltage.


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STEP TWO

Motor Controls Skill/Performance Objectives

Skill Knowledge Introduction: Below are the skill knowledge objectives. How these objectives are performed depend on equipment and laboratory resources available. With each skill objective it is assumed that a set of standard test equipment and tools be provided and safety procedures be implemented during each tasked being performed.   







Design, install and test a standard three wire motor control system Demonstrate the following means of Motor Starting Across the Line Starting Reduced Voltage Starting Demonstrate the installation and testing of the following protections Overcurrent Protection Overload Protection Over voltage Under current Phase differential Demonstrate the following means of stopping a Motor Coasting Electrical Braking Mechanical Braking Demonstrate the ability to design, install and test these motor controls Speed Control Reversing Jogging Using a PLC or relays design and functionally test motor sequence control


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