Advantage of PWMs over an Oscillator based Induction Motor Drive
AC Motor control requires both frequency and voltage control. The voltage must be proportional to frequency, but requires some increase over constant V/Hz at low frequency. When starting the motor, a frequency of about 2 or 3% of rated frequency is initially applied, then increased at a controlled rate.

To control both the frequency and voltage without PWM, a frequency-controlled power switching inverter must be supplied from a voltage-controlled power supply. That means coordinated control of two power conversion stages. Many VFD systems have been manufactured using several topologies including controlled-rectifier -- inverter, rectifier --chopper -- inverter and rectifier -- PWM inverter. The PWM inverter configuration has proven to be the most efficient and cost-effective system with the best performance in most respects. Therefore, that type of system is now used almost exclusively. However, variations in both the topology and the control strategy are currently in use and under development.

The control signals required from the microprocessor must ultimately generate driver signals for the power devices. Emulating the final output at a signal level does not do that.