Shaotang Chen

Source of induction motor bearing currents caused by PWM inverters

The recent increase of bearing damage in induction machines driven by transistorized inverters has spurred activity on possible causes related to PWM inverters. This paper looks into a typical power conversion system from this perspective. It identifies the existence of the common mode voltages produced in all types of converters. A hypothesis is then proposed to explain the bearing current problem. In particular, it is shown that in an inverter-motor system the common mode voltages generated by a PWM inverter, which are attributed to PWM switching harmonics, couple through parasitic capacitances from stator windings to the rotor body and then returns through the motor bearings to the commonly grounded stator case as a closed loop circuit. The hypothesis is verified by experimental measurement of common mode coupling currents and true bearing currents in a specially modified induction motor. Solutions are then provided to suppress the bearing currents.

Circulating type motor bearing current in inverter drives

To facilitate understanding of the differences between the two types of inverter-induced bearing currents which appear in induction motor drives, a review of the previous noncirculating type of bearing current theory is given. A brief introduction to the concept of coupling current in drive systems follows. This article then proposes an explanation to the generation of a circulating-type of bearing current in inverter-fed motor drives. The theory is backed up by the experimental measurement of bearing currents. Finally, a solution to these bearing currents, of both circulating and noncirculating types, is also proposed.

A novel soft-switched PWM inverter for AC motor drives

A novel soft-switched inverter topology is derived from the passively clamped quasi-resonant link (PCQRL) circuit. By introducing magnetic coupling between the two resonant inductors, the number of auxiliary switches can be reduced from two to one, and only a single magnetic core is required for the resonant DC link. An analysis of this novel PCQRL topology with coupled inductors is presented to reveal the various soft-switching characteristics. In comparison with the conventional passively clamped, continuously resonant DC link inverter, this soft-switched inverter can reduce voltage stresses from more than 2 per unit (pu) to 1.1-1.3 pu. It can also provide soft-switched pulse-width modulated (PWM) operation. Simulations and experiments are performed to backup the analysis.

Design and implementation of a passively clamped quasi resonant DC link inverter

The passively clamped quasi resonant DC link (PCQRL) inverter is a novel soft switched inverter topology proposed to solve two major problems associated with the conventional passively clamped resonant DC link inverter: the high clamp factor problem and the lack of PWM capability. This paper deals with design and control of a three phase PCQRL inverter. Major design issues are discussed. Successful operation of this novel PCQRL inverter based on a prototype design is demonstrated. The results show that this novel converter can reduce the clamp factor from the value of 2.0 p.u. obtained in previous circuits to around 1.25 and add the PWM capabilities to the conventional passive clamp resonant DC link. Several results are presented to illustrate the performance of the proposed inverter topology.

Soft-switched inverter for electric vehicle drives

A new type of soft-switched inverter which has particular potential for battery powered applications such as electric vehicle drives is proposed. The topology is derived from the passively clamped quasi resonant link (PCQRL) circuit. By introducing magnetic coupling between the two resonant inductors, the auxiliary switches can be reduced from two to one and only a single magnetic core is required for the resonant DC link. An analysis of this novel PCQRL topology with coupled inductors is presented to reveal the various soft switching characteristics. In comparison with the conventional passively clamped continuously resonant DC link inverter, this novel soft switched inverter can reduce voltage stresses from more than 2 p.u. to 1.1-1.3 p.u. It can also provide soft-switched PWM operation. Simulation and experiment are performed to backup the analysis.<<ETX>>

Design and experiments of a novel axial flux circumferential current permanent magnet (AFCC) machine with radial airgap

In this paper, an improvement to the axial flux, circumferential current (AFCC) machine is presented by introducing a radial airgap instead of an axial airgap. Sizing equations and three-dimensional finite element analyses are presented to predict the flux distribution, inductance, torque capability and other performances of this novel machine. Time-domain simulations are also performed to confirm the analysis and operation with a power converter. Design guidelines have been summarized for the quest for machine topologies with high power density or high torque capability. A prototype of the machine had been designed, built and tested with a closed-loop drive system based on DSP technology. The test results of the system are compared with the estimations. It is found that the machine offers larger torque capability in comparison with traditional commercial induction machines and brushless permanent magnet machines.

Bearing currents and shaft voltages of an induction motor under hard and soft switching inverter excitation

Bearing currents and shaft voltages of an induction motor are measured under hard and soft switching inverter excitation. The objective is to investigate whether the soft switching technologies can provide solutions for reducing the bearing currents and shaft voltages. Two of the prevailing soft switching inverters, the resonant DC link (RDCL) inverter and the quasi resonant DC link (QRDCL) inverter, are tested. The results are compared with those obtained using the conventional hard switching inverter. To ensure objective comparisons between the soft and hard switching inverters, all inverters were configured identically and drive the same induction motor under the same operating conditions when the test data is collected. An insightful explanation of the experimental results is also provided to help understand the mechanisms of bearing currents and shaft voltages produced in inverter drives. Consistency between the bearing current theory and the experimental results has been demonstrated. Conclusions are then drawn regarding the effectiveness of the soft switching technologies on the solution to the bearing currents and shaft voltages problems.