Last month we looked at how variable-frequency drives (vfd's) impact power quality of the building electrical system by creating harmonic voltage distortion. These devices can also cause problems for motors due to load-side power quality impacts.
In early PWM drives the switching frequency was on the order of 2,000 cycles per second (Hz). Newer, high-speed switching devices called integrated gate bipolar transistors (IGBT), capable of switching high voltages and currents more rapidly, have permitted the switching frequency (also called the carrier frequency) to exceed 20 kHz. "High carrier frequency" vfd's have been popular in the hvac market because increasing the switching frequency pushes the frequency of motor noise generated by the vfd above the audible range. However, the use of IGBTs has created problems for the motor stemming from their fast turn-on and the resulting short rise time of the voltage pulse.