PWM AC Chopper Control of Single-Phase Induction Motor for Variable-Speed Fan Application Deniz Yildirim
Murat Bilgic
Istanbul Technical University Faculty of Electrical and Electronics Engineering Department of Electrical Engineering 34469 Istanbul, Turkey Email:
[email protected]
Istanbul Technical University Faculty of Electrical and Electronics Engineering Department of Electrical Engineering 34469 Istanbul, Turkey Email:
[email protected]
torque
Abstract—This work presents a variable speed control method for fan applications. A pulse-width-modulated (PWM) AC chopper changes the effective value of the supply voltage applied to a single-phase induction motor. This variable supply voltage gives the ability to control the speed of the motor. Harmonics generated by the speed control unit are filtered by an input filter according to EN 61000-3-2 limits for harmonic current emissions standards. Experimental results on a 230V, 210W in-line centrifugal fan for exhaust ventilation application shows that PWM AC chopper is simple and cost effective control method compared to other methods in terms of simplicity and input harmonic content.
operating point
V2 motor V3
fan
I. I NTRODUCTION Fractional horse power permanent-split-capacitor (PSC) single-phase induction motors are widely used in blower and fan applications. These motors mostly have external rotor types resulting in compact structure and installation easiness of fan propeller to the rotating part of the motor. When rated voltage is applied, fan motors typically run at a constant speed determined by the capacitance value that gives optimum running performance with a proper starting torque. Depending on the application area, variable air flow rate, therefore, a variable speed operation may be advantageous in terms of energy efficiency [1]. In a typical application where low air flow rate is required, some form of mechanical flow reducers are employed while motor is running at rated speed. This type of control is not energy efficient since motor is consuming rated power even though lower operating speed may give the same air flow resulting in smaller power consumption. Several methods exist for variable speed operation of a single-phase induction motor. Considering simplicity and low cost, most common type is the control of applied voltage to the motor. Typical torque-speed characteristics of an induction motor with variable voltage along with the fan load curve are shown in Fig. 1 indicating the adjustable speed operation. The voltage applied to the motor can be varied by an autotransformer or a tapped winding arrangement [2]. Since tap changing is performed by a mechanical switch and considering size and weight of the transformer, this type of voltage control may not be favorable. Another approach is to employ an AC chopper using triac semiconductor switches.
k,(((
V1
V1>V2>V3
speed n1
ns
Fig. 1. Torque-speed characteristics of an induction motor and fan load with variable voltage.
The triac is turned on at a desired phase angle allowing some portions of the supply voltage be applied to the main and auxiliary windings of the motor [3], [4]. In another method, only main winding voltage is varied by a triac AC chopper while keeping the auxiliary winding voltage constant at rated value [5]. Phase control method results in discontinuous input current waveform which consists of higher order odd harmonics of supply frequency. Multi-speed operation at twothirds and three-thirds of the supply frequency is proposed in [6] using a triac bridge where the current waveform contains odd and even harmonics as well as subharmonics of supply frequency which exceeds the harmonic limit standards. AC choppers employing integral-cycle control method uses certain number of complete cycles be applied to load followed by certain number of zero voltage periods [7]. Subharmonics of the supply frequency also occur in this type of control which is very difficult to filter. Operation of pulse-width-modulated (PWM) AC choppers at high chopping frequencies will result in harmonics appearing at higher frequencies where small sized filters can easily eliminate them. A four-quadrant high-frequency AC chopper operation is given in [8] where chopper feeds an inductorcapacitor load. In another application, PWM AC chopper is used to control the speed of a universal motor [9] and a single-phase induction motor [10] at a switching frequency
of 1.8kHz. II. S YSTEM D ESCRIPTION Typical application for a variable speed fan operation employing PWM AC chopper is illustrated in Fig. 2 where closed loop operation permits to keep the temperature of a process at desired level by adjusting the rate of air flow. The input filter removes high frequency switching present in the input line current.
(a) BDTX−200B
120
PWM AC chopper
supply voltage
control signal
100
A 80
motor and fan sensor
gating circuit
controller
input power (W)
input filter
sensor gain
60
40
B: variable speed control
-
+ reference
Fig. 2.
A: mechanical control
B
20
0
0
100
200
300
400 500 air flow rate (m3 /h)
An experimental test is performed to compare the consumed power by a fan motor operating at various air flow rates with mechanical louver and variable speed controls. An inline centrifugal fan operated at rated speed is connected to one end of a long pipe as shown in Fig. 3a and air flow is varied by an adjustable diameter opening located at the other end. While power consumption is approximately same for all air flow rates with mechanical control, considerable energy savings occur for variable speed operation where reduction of input electrical power especially at low air flow rates is larger as is evident from Fig. 3b. PWM control technique simply chops the supply voltage at high frequencies as shown Fig. 4 where duty cycle D is defined as the ratio of on-time to total switching period. The line voltage is chopped by bidirectional switches. The change in the duty cycle of the switch changes the effective value of the load voltage and load current. The increase in duty cycle will allow the load current and load voltage to increase while decreasing the duty cycle will do the opposite. The chopped voltage can be expressed by multiplying the sinusoidal supply voltage with the switching signal d(t) as depicted in Fig. 4. Switching function d(t) can be expressed by opening the Fourier series of the pulse for one switching period as in (1) (an cos nωs t + bn sin nωs t)
(1)
n=1
where a0 is the DC component, an and bn are the Fourier coefficients, n is the harmonic order and ωs is the switching
k,(((
800
Fig. 3. (a) Experimental test setup and (b) power consumption of fan motor at various air flow rates obtained by a mechanical louver and variable speed control. Vm
2π
π
d(t)
detailed view
ωt
1 fs= Ts
DTs
Ts
III. PWM AC C HOPPER
∞ X
700
(b)
Typical schematic of a variable-speed fan application.
d(t) = a0 +
600
vs(t)
motor and fan ωt
DTs
Ts
Fig. 4.
PWM chopping of an AC sinusoidal voltage.
frequency. a0 , an and bn can be calculated using (2) to (4). ton ton = =D (2) a0 = T ton + tof f 1 an = sin(n2πD) (3) nπ 1 [1 + cos(n2πD)] (4) bn = − nπ p cn = a2n + b2n (5) The load voltage can then be calculated by multiplication of supply voltage and switching function.
vs (t)
=
Vm sin ωt
vL (t)
=
A. Realization of AC Chopper
vs (t) · d(t) = Vm sin ωt · d(t)
Fig. 6 shows the realized PWM AC Chopper with optocouplers where UC3525 is employed as a PWM generator
a0 Vm sin ωt ∞ X + [an Vm (cos nωs t · sin ωt) n=1
+
The terms in square brackets of (6) are the high frequency terms. When these are filtered, the load voltage can be expressed according to the fundamental component of supply frequency.
Cd Cf M4
C
induction motor
C
TSC428 UC3525
SW2 main winding
induction motor
C
(b) Fig. 5. A simple Single-Phase AC Chopper showing current flow path (a) when SW1 switch is ON and (b) when SW2 switch is ON.
switches in Fig. 5 can be realized by MOSFETs. Shorted gate and source terminals of two MOSFETS allows bidirectional controlled switch operation. Both MOSFETs in the switching network are floating devices, i.e., the converter stage and the control network must be isolated to drive these switches. This isolation can be obtained by either implementing a pulse transformer or an optocoupler.
k,(((
Realized PWM AC Chopper with optocouplers.
B. Input Filter Design
auxiliary winding fan
vs
LED
Va1=+15V
operating at operating frequency of 25kHz. In open-loop configuration the saw tooth waveform is compared to a reference DC voltage derived by a variable resistor. The output signal is then applied to a MOSFET driver which generates two complementary PWM signals. These signals are applied to the gates of the MOSFETs by two separate optocouplers.
SW2
SW1
Va3
Va1
Fig. 6.
fan
Va2 LED
isolated auxiliary power supply
auxiliary winding
(a)
main winding
Rd
The PWM AC Chopper is actually a Buck converter operating in AC mode. It consists of two bidirectional switches with complementary switching patterns. The upper switch (SW1) is employed for voltage chopping (Fig. 5a) and the lower switch (SW2) is used to provide an alternative path for the current of the motor when SW1 is turned off (Fig. 5b). Mechanical
main winding
induction motor auxiliary winding
M3
vs
The effective value of load voltage can now be calculated as in (8) D · Vm (8) vLrms = √ 2
vs
M2
(7)
vL (t) = a0 Vm sin ωt = D · Vm sin ωt
SW1
M1
Lf
(6)
bn Vm (sin nωs t · sin ωt)]
TLP250
=
TLP250
vL (t)
The input filter stage filters the high frequency switching harmonics from entering the utility. It is nearly always required that a filter must be added at the power input of a switching converter for improving power quality and interface issues. By attenuating the switching harmonics that are present in the converter input waveform, the input filter allows compliance with regulations that limit conducted electromagnetic interference (EMI) and harmonic issues. The PWM AC Chopper injects the pulsating current into the power source at harmonics multiples of the switching frequency fs and duty cycle (D) affects the magnitudes of these harmonics. Equations (3) and (4) show that the harmonics are a function of duty cycle (D). The input current is (t) and input voltage vs (t) have similar degrees of harmonics as it is in load voltage. Fig. 7 shows the experimental results of input voltage and input current waveforms when no input filter is employed. Corresponding FFT results are illustrated in Fig. 8a along with the computed ones using (5) in Fig. 8b. It is seen that the first harmonic starts at 25kHz which is the switching frequency
The addition of an input filter affects the dynamics of the power electronic converters, often in a manner that degrades the regulator performance. The input filter affects all transfer functions of converter. Moreover, the influence of this input filter on these transfer functions can be quite severe [5]. The Bode diagram shows an asymptotic peak occurring near the corner frequency causing the gain of the filter to go to infinity. This rise would cause extreme current peaks which would make the system worse than it was before. The output impedance of the LC filter tends to infinity at frequencies near corner frequency f0 [12]. f0 =
(a)
1 p 2π Lf Cf
(9)
Therefore, low pass input LC filter needs to be damped at the corner frequency. The damping is obtained by placing a series connected capacitor and resistor in parallel with original capacitor. Fig. 9 shows the structure of a undamped and damped LC filter and corresponding magnitude plots. The Lf Cf
vs
undamped input filter
PWM AC chopper and induction motor
Lf
vs
Cd
Cf
Rd
damped input filter
PWM AC chopper and induction motor
(a)
(b)
Undamped Input Filter
Magnitude (dB)
Fig. 7. Input waveforms at 110V, 50Hz, D = 75%; (a) input voltage (100V/div, 2.5ms/div), (b) input current (0.75A/div, 2.5ms/div).
compared with the 50Hz fundamental component. Magnitudes of multiples of this harmonic content change depending on the duty cycle (D). The fundamental component is at 50Hz
60 40 20 0 −20 −40 2 10
3
10
4
10
5
10
5
10
10
6
Magnitude (dB)
Damped Input Filter 60 40 20 0 −20 −40 2 10
3
10
4
10
Frequency (rad/sec)
10
6
(b) (a)
(b)
Fig. 9.
Fig. 8. FFT of input voltage in Fig. 7a (110V, 50Hz, D = 75%); (a) experimental (25 kHz/div), (b) calculated using cn computed from (5), n = 1 is the fundamental component at 25 kHz.
which is the supply voltage frequency. These high switching harmonics cause two main problems. The first one is that high frequency switching causes electromagnetic disturbances affecting nearby electronic equipment and the second one is that it draws harmonic currents form the power supply decreasing the power quality. International standards have been developed in order to bring some limits to electromagnetic compatibility (EMC) and power quality issues [11].
k,(((
(a) Damped LC input filter and (b) frequency response.
optimum value of damping resistor Rd is calculated based on the value of Cd using a procedure given in [12]. C. Experimental Results of the PWM AC Chopper with Damped Input Filter PWM AC chopper is operated at various duty cycle values with damped input filter. The effect of the filter can easily be seen in the input current and voltage at the supply side as shown in Fig. 10 for a duty ratio of D = 75%. FFT spectrum of these waveforms in Fig. 11 indicate that higher order harmonics are completely eliminated from the supply
(a)
(b)
(c)
(d)
(a)
Fig. 11. FFT of unfiltered (a) input current and (b) voltage of Fig. 10 (25kHz/div, supply voltage 110V, 50Hz) and FFT of (c) input current and (d) voltage with damped input filter (25Hz/div, supply voltage 220V, 50Hz). M1
Lf
M2
M3
vs
Cd
induction motor auxiliary winding
Cf
main winding
Rd
(b)
Rg1
Rg2
M4
Fig. 10. Input current, input voltage and converter input current for a duty ratio of (a) D = 75% and (b) D = 50%, 5ms/div, 400V/div, 1.5A/div.
side. Only 50Hz fundamental component is present in the supply voltage and current. As explained earlier, power circuit of AC Chopper must be isolated from driving network and this isolation was realized using optocouplers shown in Fig. 6. The optocouplers work both well in isolation and signal transfer but these isolated drivers need two isolated DC power supplies which increases complexity of the total circuit structure. A secondary PWM AC chopper circuit was also developed using a pulse transformer to isolate the two stages instead of optocouplers as illustrated in Fig 12. The advantage of a pulse transformer is that there is no need for two extra isolated power supplies. The pulse transformer has an important disadvantage in which the designer must keep in mind. A transformer is not capable of transferring DC signals, therefore the drive network must be designed to work in the duty cycle range between 0.1 and 0.9. In any case of a duty cycle of 0 or 1 would force to open both of the bidirectional switches at the same time. The input filter for this new circuit has the same filter designed for the chopper shown in Fig. 6 and PWM generating section is also the same. Fig. 13 shows the output voltage and output current for the PWM AC Chopper shown in Fig. 12 at two different duty cycle values. The chopper shows the same
k,(((
C
auxiliary power supply +15V Cs
TSC428 UC3525
Fig. 12.
Realized PWM AC chopper with pulse transformer isolation.
performance with the initial circuit given in Fig. 6. Even though a pulse transformer is employed for gate drive isolation in Fig. 12, there is still need for a DC voltage source used in the PWM generating circuitry. The DC voltage(s) can be obtained from the supply voltage by a low power (about 2W) isolated flyback converter. These power supplies are implemented into the circuits given in Figs. 6 and 12 such that both choppers work independently without the need for external DC power supplies.
(a) Fig. 14. Comparison of measured torque-speed characteristics for two different voltages obtained by PWM chopper and regulated sinusoidal voltage source.
been eliminated. It can be concluded that PWM AC chopper is simple and cost effective control method compared to other methods in terms of simplicity and input harmonic content. ACKNOWLEDGMENT The authors would like to thank Mr. Omer Faruk Bahcivan from Bahcivan Electric Motor Company for his kind support in performing this study. (b)
R EFERENCES
Fig. 13. Output voltage and output current (a) D = 1 and (b) D = 0.5, 5ms/div, 200V/div, 0.75A/div
[1] M. M. Morcos, J. A. Mowry, and A. J. Heber, “A solid-state speed controller for capacitor motors driving ventilation fans,” IEEE Transactions on Industry Applications, Vol. 30, No. 3, May-June 1994, pp. 656-664. [2] T. J. E. Miller, J, H, Gliemann, C. B. Rasmussen, D. M. Ionel, “Analysis of a tapped-winding capacitor motor,” International Conference on Electrical Machines (ICEM 98), Istanbul, Turkey, Sep. 2-4, 1998, pp. 581-585. [3] K. Sundareswaran and P. S. Manujith, “Analysis and performance evaluation of triac-voltage controlled capacitor run induction motor,” Electric Power Components and Systems, 2004, pp. 913-925. [4] C. Kim, C. Choi, D. Lee, G. Choi, S. Baek, “Torque characteristics of single phase induction motor for phase control method,” Sixth International Conference Electrical Machines and Systems, ICEMS 2003, Beijing, China, Nov 9-11, pp. 510-513. [5] M. B. M. Hamid,. “New method for speed control of single phase induction motor with improved performance”, Energy Conversion and Management, 2000, pp. 941-950. [6] A. R. Julian, R. S. Wallace and P. K. Sod, “Multi-speed control of singlephase induction motors for blower applications,” IEEE Transaction On Power Electronics, Vol. 10, No.1, 1995, pp. 72-77. [7] M. S. J. Asghar, “Smooth speed control of single-phase induction motors by integral-cycle switching,” IEEE Transactions on Energy Conversion, Vol. 14, Issue 4, Dec. 1999, pp. 1094-1099. [8] S. B. Yaakov and Y. Hadad, ”A Four Quadrants HF AC Chopper with no Deadtime”, IEEE Applied Power Electronics Conference and Exposition, APEC ’06. 19-23 March 2006. [9] H. Bodur, A. F. Bakan, and M. H. Sarul, ”Universal Motor Speed Control with Current Controlled PWM AC Chopper by using a Microcontroller”, Proceedings of IEEE International Conference on Industrial Technology, Jan. 19-22 2000, pp. 394-398. [10] N. A. Ahmed, K. Amei and M. Sakui, ”AC chopper voltage controllerfed single-phase induction motor employing symmetrical PWM control technique,” Electric Power System Research, 2000, pp. 15-25. [11] EN 61000-3-2, ”Limits for harmonic current emissions (equipment input current up to and including 16A per phase),” European Standard, 2000. [12] R. W. Erickson, ”Optimal Single Resistor Damping of Input Filter”, IEEE Applied Power Electronics Conference and Exposition, APEC ’99, Vol. 2, 14-18 March 1999, pp.1073 - 1079.
IV. M OTOR P ERFORMANCE T ESTS Final performance tests are performed on the external-rotor single-phase induction motor to determine how the torquespeed curves vary when motor is fed from a PWM AC chopper. Two reduced voltage values are used in the test. Induction motor is first tested with pure sinusoidal voltage obtained from a well regulated AC power source and then same voltage values are applied through PWM AC chopper. The resulting torque-speed curves shown in Fig. 14 are obtained along with the fan characteristic curve. Although the curves are close to each other, PWM AC fed curves are somewhat lower than the sinusoidal voltage case for speeds up to maximum torque speed. V. C ONCLUSION A 25kHz PWM AC Chopper used in a single-phase induction motor drive was designed and realized in this study for both domestic and industrial fan applications. This circuit can be used to control the motor speed according to the temperature reference. Due to high frequency switching, the PWM AC chopper does not generate low frequency harmonics which are multiples of the 50Hz component. The harmonic distortions appear at higher frequencies that are actually multiples of the switching frequency and very easy to filter. Experimental results of the damped input filter have shown that the input voltage and input current harmonic distortions have
k,(((
