Minimisation of inverter-fed induction-motor losses by optimisation of PWM voltage waveforms

This thesis describes a method of minimising the total losses of an inverter-fed 3-phase squirrel-cage induction motor when the motor ,is subjected to a pulse-width modulated (PWM) voltage waveform. The inverter is supplied from a d.c. link and operates at variable frequency to provide speed control of the motor. Appropriate triggering of the inverter's six main thyristors generates pulse-width modulated voltage waveforms for application to the induction motor. The operation of an induction motor with nonsinusoidal voltage applied results in a reduction of the motor's efficiency due to the harmonics present in the waveform. The aim of the project is to minimise the total losses by obtaining optimum PWM voltage waveforms rather than by improving the design of the motor. This requires a thorough examination of motor losses. The determination of a RiM voltage waveform which may be produced by the described inverter is subjected to ccnstraints which characterise the operation of the drive system. The motor operates with constant airgap flux density throughout its speed range to obtain maximum output power at fixed per unit slip. In addition the switching frequency of the thyristors must not exceed a specified limit to avoid short circuiting of the inverter. The calculation of the motor's steady-state performances for both sine wave and PWM supplies is incorporated in a computer program. The detail of the experimental and theoretical performances are given and comparison is made between sinusoidal and PWM voltage wave systems. Good agreement is obtained between test and calculated results on an inverter-fed 7.5 kw squirrelcage induction motor. It is concluded that the degradation of motor efficiency due to the applied PWM voltage waves is mainly the result of increased copper losses, which are produced by harmonic currents. The minimisation of the losses for continuous, constant-flux, operation of the induction motor is achieved for the given constraints. It is found that the total losses can be further minimised if the d.c. link voltage is variable. This permits improved motor performance but adds complexity and cost to the d.c. link voltage supply.