Most motors used with Variable Speed Drives have some degree of shaft current induced in normal operation. That is the spinning rotor in cutting lines of flux, induces some voltage on the shaft. Stray capacitance puts voltage onto the shaft each time the VSD switches. If a loop through ground exists, a shaft current will flow. This happens with dc motors, and ac induction motors. Although I have not verified this, I suspect Permanent Magnet motors are not immune. About 1% of drive applications have damaging shaft currents.
The shaft current typically passes through one or more of the bearings (usually a motor bearing) to ground. Each pulse of the VSD (4000 Hz or higher for ac drives) pits a few atoms of the bearing race. Because the timing of the pulses and the speed of the motor is usually constant, the bearings repeatedly pit in the same spots. The damage manifests as a unique pattern (fluting) on the bearing race and can be readily identified after the damage has occurred.
Symmetrical design of the rotor is the strongest tool in reducing induced shaft current. In the 1980s, we were working with one 1000-HP dc motor which had severe shaft current problems. Tests with a cable and clamp-on ammeter drew an arc and we read shaft current over 100 Amps at low frequency. Needless to say the bearings failed within a week. In this case, a design flaw put rotor stabilizing bars on only one of two armature windings, resulting in an asymmetry.
The higher frequency shaft currents due to VSDs have a number of solutions. Special cables are available for VSDs. These have three grounding cables symmetrically arranged with the current carrying cables and surrounded by a properly grounded shield. The motor frame should be properly grounded (high frequency). I have seen one instance where motor cables heated all metal through which they passed to the point thin metal burnt away. Shaft currents were also a problem here.
Other steps that may be required in problem situations are:
- Insulate the motor coupling
- Shaft grounding brush
- Insulated motor bearings with grounding brush on the driven end of the motor
- One insulated motor bearing with grounding brush (location specified by engineering)
A failed grounding brush or one installed on the wrong end of the motor can move the problem from the motor to the machine bearings.