Modern age induction motors (IM) are operated with voltage source inverters (VSIs) for better control
and improved dynamic performance. In high-power IM drives, the energy loss on account of switching
is quite high. Hence, for reliable operation of the inverter, the switching frequency is kept low. In high-
speed motor drives, the maximum modulation frequency is quite high. Hence, in both the cases, the
ratio of switching frequency to modulation frequency, i.e. the pulse number P, is quite low. Motor drives
operating at such low pulse numbers suffer from high total harmonic distortion (THD) in line current
and pulsating torque. This work investigates pulse width modulation (PWM) techniques to reduce THD
and/or pulsating torque in such IM drives.
This work studies minimization of line current THD when the PWM waveform has only a few switching angles per quarter (say 2 to 7). Two types of PWM waveforms (i.e. type A and type B), having all waveform symmetries preserved, are considered for study. Space vector based analysis of the optimal type-A and type-B waveforms brings out the sequences in which the voltage vectors are applied over a sector in these methods. This study leads to the determination of optimal vector sequences and also helps in implementation of the optimal PWM in a simplified manner.
Two optimal PWM techniques are proposed to minimize pulsating torque in IM drives operating at low pulse number. The proposed methods are shown to be better than sine-triangle PWM and selective harmonic elimination PWM. These methods are also extended to a neutral-point-clamped three-level inverter fed induction motor drive.
While methods to analyze current ripple and torque ripple are well established in high switching frequency drives, this work attempts the same for low pulse number operations. A method to predict the current and torque harmonic spectra for such drives based on PWM waveforms is proposed.
Closed loop control of motor drives is also challenging at low pulse numbers. An improved flux estimation method is proposed for such drives. This is used to implement sensor-less vector control of drives, operating at low switching frequencies in the range of 250 Hz to 500 Hz. All concepts proposed are validated through simulations and experiments on a 3.7 kW IM drive.
Avanish Tripathi received the B.Tech. degree in Electrical Engineering from IIT (BHU), Varanasi in 2009. He has completed his M.S. and Ph.D. at Indian Institute of Science, Bangalore, in the Department of Electrical Engineering. Currently, he is working with Delta Power Solutions India Pvt. Ltd., Bangalore, India. His area of interest includes pulse width modulation techniques, electric machine drives, modelling and control of voltage source inverters and optimization techniques.