How VFD operates?
2015-08-31
VFDs regulate the speed of a motor by converting sinusoidal line AC voltage to DC voltage, then back to a pulse width
modulated (PWM) AC voltage of variable frequency. The switching frequency of these pulses ranges from 1 kHz up to
20 kHz and is referred to as the “carrier frequency.” The ratio of change of the ΔV/ΔT creates a parasitic capacitance
between the motor stator and the rotor, which induces a voltage on the rotor shaft. If this voltage, referred to as “common
mode voltage” or “shaft voltage,” builds up to a sufficient level, it can discharge to ground through the bearings. Current
that finds its way to ground through the motor bearings in this manner is called “bearing current.”
modulated (PWM) AC voltage of variable frequency. The switching frequency of these pulses ranges from 1 kHz up to
20 kHz and is referred to as the “carrier frequency.” The ratio of change of the ΔV/ΔT creates a parasitic capacitance
between the motor stator and the rotor, which induces a voltage on the rotor shaft. If this voltage, referred to as “common
mode voltage” or “shaft voltage,” builds up to a sufficient level, it can discharge to ground through the bearings. Current
that finds its way to ground through the motor bearings in this manner is called “bearing current.”