Colloquium

Investigations on Pulse Width Modulation Techniques for Split-Phase Induction Motor Drives

Speaker: LALGY GOPI . of Ph.D. (Engg) in Electrical Engineering under Electrical Engineering

Date/Time: Jul 26 / 14:30:00

Location: Multi Media Class Room (MMCR), EE Department Team Link

Research Supervisor: Narayanan G

Abstract:
A split-phase induction motor (SPIM) has two sets of identical three-phase windings separated by 30° in space, typically fed by two- level voltage-source inverters. This configuration reduces the voltage rating of the windings and the DC-link voltage of the inverters. SPIMs are advantageous for high-power variable-speed applications due to reduced torque pulsations, lower voltage-rated power converters, higher torque per ampere, and increased reliability. However, they may require more semiconductor switches and circuitry. Although the winding structure avoids harmonic torques of the order 6k (k=1, 3, 5…), small voltages of harmonic order 6h±1 (h=1,3,5…) can cause large currents and increase copper loss. Properly designed pulse width modulation (PWM) techniques can improve SPIM drive performance by reducing stator copper loss and pulsating torque. In triangle comparison (TC) based PWM techniques for SPIM drives, two sets of modulating signals, which are phase-shifted by 30° from each other, are compared against a common carrier wave. In contrast, space vector (SV) based PWM techniques use the reference voltage vector to calculate the dwell times of each voltage vector, which are then applied in a specific sequence. TC-based PWM techniques are renowned for their simplicity in implementation, whereas SV-based techniques offer improved performance but come with increased computational complexity. While the unified understanding of these two types of PWM techniques is well established for three-phase induction motors, this thesis aims to enhance the unified understanding between TC-based and SV-based PWM techniques for SPIM drives. Several SV-based PWM techniques have been introduced for SPIM drives to enhance harmonic performance. Four-dimensional 24-sector SVPWM techniques outperform 12-sector methods but are more complex to implement. This thesis performs a per-phase analysis of these techniques to develop an efficient implementation scheme. It evaluates and analyses the switching cycle averaged pole voltages to determine the common mode voltage (CMV) for each inverter, showing that CMV relates to the fundamental voltage of different phases in different sectors. This leads to understanding the offset voltages needed for the two sets of three-phase sinusoidal modulating signals to generate equivalent signals for the two inverters driving the SPIM. Adding the derived zero-sequence signal to the sinusoidal modulating signals results in a computationally efficient implementation method. Equivalent carrier waves are derived based on the switching sequences, requiring discontinuous carrier waves for improved harmonic performance. This carrier-based implementation significantly reduces computation time on a TMS320F28377S DSP platform. Experimental results of stator current waveforms under steady-state and various dynamic conditions from a 6kW, 200V, 50Hz SPIM drive are presented. A comparative study based on the stator flux ripple analysis is carried out to evaluate the stator current harmonic distortion and rms torque ripple. The comparison of analytically evaluated torque ripple factor, simulated instantaneous torque ripple and harmonic spectra of torque are presented to validate the performance of different PWM techniques. The proposed discontinuous PWM method offers lower total harmonic distortion (THD) of stator current at high speeds. The equivalent modulating signals of the discontinuous PWM technique helped to evaluate the switching loss of semiconductor devices used. A comparison based on switching loss factor is carried to demonstrate the reduced switching loss due to this PWM technique. The discontinuous PWM technique is shown to have reduced switching losses at high power factors than the continuous PWM techniques with significantly low computational effort. An indirect field-oriented control scheme has also been presented to demonstrate the dynamic performance of the proposed implementation scheme. Two advanced bus-clamping pulse width modulation (ABCPWM) techniques are proposed in this thesis for SPIM drives to enhance their performance further. These techniques employ special switching sequences which apply the null vector once and one of the active vectors twice in each sub-cycle. Stator flux ripple-based analysis brings out the superior performance of the proposed special sequences over the conventional sequences at high modulation indices. Switching loss factor-based analysis shows that the inverter switching loss gets significantly reduced with the proposed PWM techniques at high power factors. Simulations and experiments on a 6kW, 200V, 50Hz SPIM drive show that the THD of stator current is reduced significantly at high speeds by the proposed PWM techniques at the same average switching frequency. In particular, one of the proposed techniques improves the no-load stator current THD by 32% at rated frequency, compared to a state-of-the-art SVPWM technique.

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