Shaft-Voltage occurs in motors powered by variable-frequency drives. Variable Frequency Drives (VFD) induce shaft voltages onto the shaft of the driven motor because of the extremely high speed switching of the insulated gate bipolar transistors (IGBT) which produce the pulse width modulation used to control AC motors in heating, ventilation, air conditioning and refrigeration systems.[1] The presence of high frequency ground currents can cause sparks, arcing and electrical shocks and can damage bearings.[2]
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Techniques used to minimise this problem include: insulation, alternate dishcharge paths, Faraday shield[3], insulated bearings, ceramic bearings, grounding brush [4] and shaft grounded ring.
An electrostatic shielded induction motor (ESIM) is one approach to the shaft-voltage problem, as the insulation reduces voltage levels below the dielectric breakdown. This effectively stops bearing degradation and offers one solution to accelerated bearing wear caused by fluting, induced by pulsewidth modulated (PWM) inverters.[5]
Grounding the shaft by installing a grounding device provides an alternate low-impedance path from the motor shaft to the motor case. This channels the current away from the bearings. It significantly reduces shaft voltage, and therefore bearing current, by not allowing voltage to build up on the rotor.
A shaft grounding ring (SGR) is similar to a grounding brush, except that this brush makes use of conductive micro fibers, creating a low impedance path from the motor.
Insulated bearings eliminate the path to ground through the bearing for current to flow. However, installing insulated bearings does not eliminate the shaft voltage, which will still find the lowest impedance path to ground. This can potentially cause a problem if the path happens to be through the driven load or through some other component.
High frequency grounding can be significantly improved by installing shielded cable with an extremely low impedance path between the VFD and the motor. One popular cable type is continuous corrugated aluminium sheath cable.