Induction motor speed control using a microprocessor-based PWM inverter

An MC68000, 16-bit microprocessor system was used to generate pulse-width modulation (PWM) voltage waveforms for a three-phase inverter. An MC6840 programmable timer module (PTM) was used to give real-time PWM voltage waveforms at its three outputs. The MC68000 calculates the width of the pulses for only the first quarter cycle and sorts these into a table. The remaining pulses for the complete cycle are generated using the values of the first quarter because there are conditions of quarter and half-wave symmetry. This results in a considerable saving of microprocessing time. The well-known expressions that define the width of regular sampled PWM pulses were modified to be compatible with the timing system. A real-time method of setting the 120 degrees phase shift between the three phases of the pulses using the PTM was developed and showed a good level of accuracy. The PWM inverter was tested with passive impedance and motor loads. With an induction motor load, harmonics of the stator current and voltage of an order lower than the nineteenth and twenty-third were found to be virtually eliminated. The nineteenth and twenty-third harmonics had the value of 0.09 pu of the current, compared with 0.3 for the voltage, at a depth of modulation of unity.<
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