An analysis of delta modulated inverters with applications to submersible motors

Choudhury, Mohammad Ali (1988) An analysis of delta modulated inverters with applications to submersible motors. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (31MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

This thesis presents an analysis of delta modulated inverters and their applications to submersible motors. Submersible motors are induction motors with unique constructional features. These motors are required to operate in one of the most harsh environmental conditions. The operating characteristics of the submersible motor pump system require soft starting facility and variable speed operation during production. To satisfy both requirements, a delta modulated inverter supply is proposed for the operation of the submersible motor. Several delta modular schemes have been studied and the rectangular wave delta modulation technique is selected for the switching of the inverter. Rectangular wave delta modulation provides inherent constant voltage/frequency (V/f) characteristic of the supply voltage and current control of the motor. The constant V/f characteristic is provided by the low-pass filter in the feedback loop of the modulator and the current control is provided by the hysteresis window of the quantizing comparator in the feedforward path of the modulator. -- The modulator and the inverter waveform were studied using discrete Fourier transform (DFT). This is simpler than the conventional Fourier series analysis. It is suitable for on-line harmonic analysis of the modulated waveforms. It also gives the information about the sub-harmonics of the modulated waveforms. An on-line harmonic minimization technique using a tuned filter in the delta modulator has been implemented. This method uses the continuous tuning of the feedback filter in the modulator to minimize the low order harmonics of the modulated wave for variable frequency operation of the inverter. -- The steady state and starting characteristics of the submersible motor supplied from the rectangular wave delta modulated inverter have been studied using time-domain analysis. Time domain analysis of inverter-fed submersible motors was found to be more accurate and easier than the conventional frequency domain analysis. The motor was provided with ramp frequency and voltage (RFV) control to reduce its electrical and mechanical starting stresses. A semi-closed loop operation has been chosen to maintain constant slip operation of the motor. For the semi-closed loop operation, motor speed was estimated using the motor terminal voltage, current and the phase angle between them. -- The characteristics of the modulator and the motor fed from the rectangular wave delta modulated inverter were experimentally verified.

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