Srikanthan Sridharan with Adviser P.T. Krein
Our research goal is to enhance performance and efficiency of motor drives over a wide load and speed range. High performance reported in the literature for variable frequency VSI (voltage source inverter) drives is not usually seen for CSI (current source inverter) drives, especially in the low power range. Most past comparisons adopt high-frequency pulse width modulation methods (PWM) for VSI and a more basic 120-degree mode or selective harmonic elimination for CSI. This results in high distortion in CSI output currents and poor power factor, and causes considerable torque pulsations. In this work, duality principles between VSI and CSI drives are examined to develop control techniques for improving performance and efficiency.
The development of switching devices with increasing power-handling capacities at high switching speeds motivates better-matched comparisons. Space vector PWM is adopted for both VSI and CSI drives. CSI drives offer advantages of short-circuit current-limiting and regeneration capabilities. Also, the inherent modulation of dc input current for CSI drives results in better input bus utilization than for VSI drives. But shunt capacitors at the CSI output must be used to limit voltage spikes due to switching. This produces resonant modes caused by combinations of leakage inductance and magnetizing inductance with the capacitors. These resonances must be suppressed with active damping. A fan load has been used to demonstrate the operation of these drives at various speeds.
From Figures 4-6, torque ripple is the highest for a scalar controlled (V/f) VSI drive with slower dynamic response. However, field-oriented control (FOC)-based CSI delivers voltage and currents with low distortion and torque ripple but with larger overshoots in speed, as compared to FOC-based VSI. Results show comparable efficiency values for VSI and CSI drives, even for low-power applications, when the duality relationship between the two is exploited. Further work will include a transfer function approach to design flux and speed loops and improve the CSI dynamic performance. This research is supported by the Grainger Center for Electric Machinery and Electromechanics.
This work is funded by the Grainger Center for Electric Machinery and Electromechanics and the University of Illinois SURGE Fellowship.