AC variable frequency drives (VFDs) have been in existence in a number of forms since the early 1970s. In the early 1980s the “pulse width modulated” (PMW) VFD versions began to appear. This form of motor control produced less motor rotor and stator harmonic heating and delivered smoother torque control at slow motor speeds.
Thirty years on PWM VFDs are still the most popular form of motor speed control on the market for low voltage (LV) and medium voltage (MV) applications. This article discusses some of the problems which still need to be addressed, especially in critical applications.
Ian C Evans & Rudy Limpaecher, Resonant Link Technology Limited