【正文】 ts. 外文翻譯 （ 原文 ） 2 Keywords. EMI。 bearing current. 1. Introduction With the development of power electronic devices like the insulated gate bipolar transistor (IGBT), the power MOSFET and the advances in microprocessors, the ac induction motor is being popular in variable speed drives with PWM inverter circuits. Since the rise time and fall time of the IGBT are less than 200 nanoseconds, the dissipation loss across the device bees very less and there by increases the performance of the circuit. However, due to the fast switching action of the device the dv/dt of the inverter output bees large. This high dv/dt voltage transition coupled with parasitic stray capacitance in the system causes high frequency line to ground current or mon mode current and hence bearing current。 hence the mon mode voltage applied to the load is cancelled pletely. However, this method is suitable only for low voltage/power applications due to the resistive current sensor. 外文翻譯 （ 原文 ） 4 Julian proposed a filter circuit for reducing the mon mode filter circuit is based on current sensing and it operates a fast transistor amplifier for the current pensation. Significant attenuation of the mon mode voltage method presented in this paper is different from earlier methods. Here, the phase current is sensed by the individual current transformer (CT) and is suitable for any voltage level. Since the CT is used for sensing the phase current there is no voltage drop in the sensor there by there is no reduction of terminal voltage applied to the IM. In their circuit,the primary winding of the transformer is connected in series in each of the phases. Since the circuit uses the transformer, there is voltage drop due to the phase current of the IM, hence the terminal voltage applied to the IM gets reduced. The circuit is also used to reduce the mon mode voltage and the mon mode current at the star point of the I M. But this circuit is useful for the low voltage and low power the present work,the standard LISN,the standard current probes are used and the results are pared without and with the mon mode voltage reduction circuits for the PWM inverter used for variable speed ac IM drives. Working of the circuit The block diagram of the drive and conducted emission mon mode voltage minimizing circuit is shown in figure 2. The conducted emission mon mode voltage minimizing circuit block diagram is shown in figure 3. The system works on the principle of sensing the individual phase currents using CTs with a suitable ratio. A resistor shorts the secondary of the CT. The voltage across the resistors are individually amplified and summed. The summed output is further amplified using high precision, wide band and high CMRR amplifier (make: Preston pany, USA). The output of the amplifier is fed to the PNP and NPN transistor’s bases to amplify the negative and positive side signals separately. The negative and positive amplified voltages are fed to the two optoisolator circuits separately. The optoisolator outputs are amplified by the high frequency transistors and fed to the gate of 外文翻譯 （ 原文 ） 5 corresponding IGBT. The optoisolator circuit is used to isolate the power circuit from the rest of the control circuit. Figure 2 Block diagram of drive and control circuit for minimizing of conducted emission mon mode voltage. In the pushpull amplifier, the IGBTs (the IGBTs are used in the switch mode and not in the amplifier mode) are used as the switching devices. The IGBTs are switched ON and OFF as per the CT secondary sensed and amplified voltage in the block diagram in figure 3. When the IGBTs are ON, all the high frequency voltages take the shortest path to the ground through the capacitor Cc , whereas for the low frequency supply, the capacitor Cc offers high impedance. In figure 3, the coupling capacitors Cc are connected to the inverter output lines. Note:CT = current transformer. Cc = coupling capacitor. PRESTON = High CMRR 外文翻譯 （ 原文 ） 6 diff. i/p amplifier (manufactured by M/s Preston Tnc, USA). GATE DRTVE = gate drive circuit for TGBT. NPN TRANS (+ve) = NPN transistor for amplifying the +ve signals. PNP TRANS (?ve) = PNP transistor for amplifying the ?ve signals. TNT. CTRCUTT = interface circuit. TNV. AMP = Tnverting amplifier. OPTO TSOLATOR = optoisolator chip. Figure 3 Block diagram of control circuit for minimizing conducted emission mon mode voltage. Figure 4 Modified induction motor for measurement of bearing current The coupling capacitors Cc are chosen such that their impedance is sufficiently low at EMI noise frequency. In this method, all high frequency noise voltages are filtered from the inverter output voltage. In the circuit shown in figure 3, the high frequency noise voltages at the star point of the induction motor are reduced but the carrier frequency and the envelop of the fundamental frequency voltage are not reduced at the star point of the induction motor. This has to be reduced by injecting an appropriate signal at the star point of the induction motor, which