The principle of pulse-width modulation (PWM), as applied to variable-speed drives (vsd's), was discussed here last month. In today's drives, insulated gate bipolar transistors (IGBTs) are used to produce high-speed pulse switching to simulate the voltage sine wave to the motor.
This switching rate is referred to as the carrier frequency. In today's drives, these frequencies can be in the range of 4 to 20 kilohertz (kHz). For instance, at 4 kHz, the transistors switch on and off 4,000 times per sec (Figure 1).