Two different methods of PWM control of 3-phase bridge inverter, namely the regular and the optimal PWM switching strategies, have been implemented by microprocessors for use in variable-speed induction motor drives. Microprocessors permit implementation of novel PWM switching strategies which would be very difficult, or perhaps impossible, to realise by means of analog methods. A particular example is the optimal PWM strategy, which is not suitable for nalog implementation.The use of microprocessors also results in considerable flexibility in the control of induction motor drives, since only simple software alterations are needed when the motor characteristics are to change. This is in contrast to analog methods which require parts of the hardware to be modified, adjusted and recalibrated in order to cater for different characteristics of a new motor.tow particular versions of the regular-sampled PWM strategy have been implemented with frequency ratios (F/f) of 18 and 30 (F carrier frequency, f modulating frequency). These two versions resulted in harmonics up to the 17th and 29th respectively, to have small or negligible magnitudes compared with the fundamental magnitude of the motor line voltages. However, harmonics higher than the 17th in the first version, and the 29th in the second version are present with considerable magnitudes. Specially-designed LC filters are therefore needed to prevent these harmonics from reaching the motor terminals, when the inductance of the motor is not Sufficiently large to smooth their effects on the motor current waveform. The voltage/frequency ratio is kept constant over the full range of inverter output frequencies to maintain a constant flux inside the motor The two particular implementations of the optimal PWM strategy have been achieved with 17 and 29 pulses per output cycle. They result in voltage harmonics up to the 23rd and 41st, respectively, to have negligible magnitudes throughout the operating range of the inverter. The implementation method employed for the optimal PWM strategy enables a good three-phase balance to be maintained when the inverter voltage and frequency are both varied. The hardware developed for this switching strategy offers considerable flexibility since it can be employed to implement other optimal strategies based on different performance criteria. The implementation of another optimal PWM strategy, such as that based on the minimisation of the RMS ripple current, does not involve any hardware modifications but only requires changing the contents of the look-up table where NR values are stored. This flexibility was demonstrated by implementing the combined regular-optimal strategy on the same hardware. th regular-sampled and optimal PWM switching strategies, the -phase balance has been achieved simply by software without ing complex hardware circuitry. mbined regular-optimal PWM strategy results in negligibly small e harmonics up to the 74th for the output frequency range O to and up to the 41st for the output frequency range 20 to 50 Hz. agnitudesof the fundamental voltage component produced by the ar switching strategy at different inverter output frequencies wer than th ose produced by the optimal strategy. omparison in terms of harmonic spectra between the regular and 1 strategies on the basis of both the computer simulation and mental results indicate that the spectra produced by the optimal egy is superior to that produced by the regular strategy. er, the combined regular-optimal strategy provides a good omise and can be said to be superior to both of the above gies when the full output frequency range 0-50 Hz is considered. The regular switching strategy is easier to implement than the optimal strategy since it does not need a PLL to control the inverter output frequency. The facility for varying the inverter frequency is included in the analytic expression describing the widths of the regular sampled PWM pulses. Furthermore, the phase balance is achieved more easily in the regular switching strategy (as described in Chapter 3) without requiring the extra timers and software routines necessitated by the optimal strategy. The regular strategy is therefore cheaper to implement than the optimal strategy and requires less hardware and software. However, the much superior performance offered by the optimal strategy makes it a preferred choice in induction motor drive applications which do not involve very low speed operation.The combined Regular-Optimal strategy is the best choice for use in icroprocessor-based PWM inverter drives since it solves the problems encountered by the optimal strategy at low output frequencies and retains the high performance of the optimal strategy at high output frequencies.