Abstract A two-level PWM based switching power amplifier for active magnetic bearings with current-loop, which can be used in either voltage-controlled mode ...
A PWM Based Switching Power Amplifier for Active Magnetic Bearings* Zhu Changsheng, Mao Zhiwei
Department of Electrical Engineering, College of Electrical Engineering Zhejiang University, Hangzhou 310027. China
is bigger than the linear power amplifier, it can be reduced by properly using of the pulse modulation technique. The switching power amplifiers are widely using in all AMB systems. A two-level PWM based switching power amplifier with current-loop, which can be used in either voltagecontrolled mode or current-controlled mode was proposed, designed and debugged in this paper. The characteristics of the switching power amplifier, such as the static inputoutput characteristic, dynamic frequency response characteristics, and current harmonic distortion, were measured, and the effects of some parameters on the dynamic characteristics were dealt with. The effectiveness of the switching power amplifier in the AMB system was shown in a rotor system.
Abstract A two-level PWM based switching power amplifier for active magnetic bearings with current-loop, which can be used in either voltage-controlled mode or current-controlled mode, was proposed, designed and debugged in this paper. The characteristics of the switching power amplifier with the maximum output current 8 A, such as the static input-output characteristic, frequency response characteristics and current harmonic distortion, were measured. The power amplifier was successfully used to suspend a rotor system. It is shown the proposed two-level PWM switching power amplifier can meet the requirements of the amplifiers of industrial active magnetic bearings. Index: switching power amplifier, PWM. active magnetic bearing.
Active magnetic bearings(AMB), which use the controllable attractive force of electromagnets to maintain a rotating ferromagnetic body contact-free in its nominal position, have seriously developed in the rotating machinery, provide numerous advantages compared with the conventional bearings, such as: no contact, no wear, no lubrication, long life and controllable dynamic characteristics etc. However, the AMB system is inherently unstable, a position control loop is necessary to stabilize this system in which the position controller gets the information of the actual rotor position from the transducers and controls the coil current of the magnets by means of power amplifier. The power amplifier, a bridge between the control signal and the applied current in the coil, is one of the main components of an AMB system and has a great effect on the dynamic characteristics of the AMB-rotor system. According to its operating principles, the power amplifiers are classified into linear analog power amplifier and switching power amplifier. The linear power amplifier was used in the early development of AMBs of a small power AMBs, because it can be easily realized and controlled. It has some advantages against the switching power amplifier, for example, less current ripple and noise, greater bandwidth. The main disadvantage of the linear power amplifier is the large power losses or very inefficient, especially at a large power rating. In the switching power amplifier, high voltage and high current is present in the power transistors only during the short periodic. The power loss is greatly reduced. Though the current harmonic distortion
II. TWO-LEVEL PWM SWITCHING POWER AMPLIFIER In generally, the simple switching power amplifier is composed of drive circuit and power main circuit. The switching power amplifier with inner feedback loop is composed of inner controller, drive circuit, power main circuit, and feedback circuit, as shown in Figure 1. The main purpose of adding the inner feedback loop is to improve the characteristics of the power amplifier.
(a) simple switching power amplifier _..
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(b) switching power amplifier with inner feedback loop Fig 1. Basic structure of a switching power amplifier The drive circuit, including the pulse modulator and signal amplifier is to produce pulse signal with variable duty-cycle according to the control signal from the inner controller and the position controller, and to amplify the pulse signal to a certain voltage to drive the transistor switches in the power main circuit. The power main circuit realizes the amplification function of the power. The feedback circuit, including the transducers and related measurement circuit, is used to get the feedback information that may be voltage, current or flux in the
(R104129), Key Project of Education Department
* This project is supported by the Nature Science Foundation of Zhejiang Province of Zhejiang Province and National Science Foundation of China (1 033203 0), respectively.
bearing. The function the inner controller is to produce the inner control signal to adjust the amplifier's characteristics according to the inner feedback information. The pulse modulator, power main circuit and feedback circuit are key components in the switching power amplifier.
It is shown that both the hysteresis and the samplehold have significant shortcomings in application. The hysteresis amplifier suffers from short-pulse susceptibility leading to low efficiency or output device failure. The sample-hold one provides excellent short-pulse immunity but produces fairly severe harmonic distortion and deadband, especially at low signal amplitudes. The minimum pulse width combines the advantages of both the hysteresis and the sample-hold, but the control signals are much complex. The PWM is most common as the PWM technique has been thoroughly studied and widely implemented successfully in power electronic systems. The PWM of the semi-bridge power amplifier may be based on two-level or three-level scheme. For the twolevel scheme, the output voltage of the power amplifier is either positive DC-link voltage or negative one. The main drawback is that the current harmonic distortions are related with DC-link voltage. By varying the duty-cycle between both levels, the average output current is controlled. The two-level PWM scheme can be easily realized. It is shown that the two-level semi-bridge amplifier is enough for the AMB application. For the three-level scheme, the output voltage of the power amplifier is not only positive and negative DC-link voltage, but also zero voltage. The main advantages over the two-level scheme are large bandwidth and small current harmonic distortion. Figure 3(a) shows a conventional two-level PWM scheme. In this PWM scheme, the duty-cycle between positive and negative pulses determined by the crossing points between the command voltage Ur and the triangle reference voltage is proportional to the voltage U, . The switching frequency of the power switchers is constant, and equal to the frequency of the triangle reference
A. Main Circuit
There are two basic power main circuits of the switching power amplifiers: the monopole(or semi-bridge type) and dipole(or full-bridge type). The full-bridge type has small current ripple than the semi-bridge type, and can change the direction of the output current, but more expensive and larger size than the semi-bridge type. Since a standard AMB magnet requires only two quadrants control, i.e., the output voltage of the power amplifier should be modulable between a negative and positive limit and the current between zero and the maximum. Negative currents are undesirable. It means that it is not necessary to change the current direction in the coil of the AMB. The semi-bridge type is enough for the AMB power amplifier. This may greatly simplify the structure of the power amplifier and reduce the cost, which is very appropriate for the application of the AMB
Fig.2 Principle schematic of a semi-bridge power main circuit of switching power amplifier
Figure 2 shows the principle schematic of a semibridge power main circuit of switching power amplifier working in two-guadrant PWM mode. Most industrial AMB amplifiers are built based on this configuration. It consists of two transistor switches(TI and T2), such as MOSFET, IGBT etc, a capacitor and two free-wheel diodes(DI and D2), which form the H bridge structure. The two transistors form one circuit that allows the coil current to be increased. The two free-wheel passive diodes form the other circuit that allows the coil current to be decreased when the transistors are in the "off' state. Only the two transistor switches have to be controlled by the drive circuit. This guarantees the current direction in the bearing's coil. B. PWM Modulation
There are a number of the modulation techniques in the switching power amplifier: such as pulse width modulation (PWM), current hysteresis, sample-hold, and minimum pulse width[14 .
Fig.3 Schematic of a two-level PWM scheme In this paper, we are focus on the two-level semi-
bridge PWM switching power amplifier. The operation states of the two-level semi-bridge PWM switching power amplifier are divided into charge state and discharge state. (a) Charge state When the control voltage from the
controller makes the switches TI and T2 close, as shown in Figure 4(a), a positive voltage with the amplitude of Vlj, is applied to the bearing coil and the coil current increases. The coil current after the switches TI and T2 close is
JKIdC)= ' 2VI)('le
Figure 5 shows the power main circuit of the twolevel PWM switching power amplifier based on the MOSFET. It has two P mode MOSFETs and one N mode MOSFET, shearing the same driver circuit, which simplifies the structure of the power amplifier and improves its reliability. The power main circuit can also be used in the three-level scheme.
where Vdc is the DC supply voltage of the power amplifier, VD is the voltage on the switches when the switches are close, r= LI R is a time constant of the circuit, L and R are the inductance and the resistance of the coil, respectively. IL , is the initiation current in the coil at closing the switchers. As r is much larger than the switching period, i(t) can be simplified as a linear function and its slope is (V
C. Drive Circuit The drive circuit, especially the PWM modulator, is an important part of the switching power amplifier. Different PWM modulators, such as the two-level or three-level scheme, will result in different amplifier's characteristics, The PWM modulator can be realized in two ways: hardware method and software method. The hardware method, which is based on the special PWM module, such as TL494, to generate PWM wave, as shown in Figure 6(a), can be integrated with the inner controller, inner feedback loop, power main circuit etc as an independent switching power amplifier. This kind of switching power amplifier done not use the computer and has more widely application, but it is difficult to adjust the PWM scheme.
(a) charge state
~T2 (b) discharge state Fig. 4 The operation states of the two-level switching power amplifier
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(b) Discharge state When the control voltage from the controller makes the switches TI and T2 open, as ~ - aLI' shown incic~Figure 3(c), the circuit output negative voltage + 'V)I
Figur 6(b), the PM we is p
where V, is the voltage on the diodes. Similarly, i(t) also can be treat as a linear function with a slope of
(asrdcptuende ) switching power amplifier
mehd In ths way it is eas to adut th-PM scem
By change the duration of charge state and discharge state in every period, the average current can be 9