In our previous issue, we noted that unbalanced current flowing through the bearings can destroy them. But where does this current come from and what can you do about it?
Three sources and their solutions are:
- Internal circulating current. Solution: Insulate the motor bearing opposite the drive end; even better, insulate the one on the drive end too.
- Common mode current using the motor shaft as a return path. Solution: Add a bonding strap between the motor and the load frame; insulate motor bearings at drive end and opposite; insulate coupling between motor and load shaft.
- Capacitively coupled (to common mode voltage) shaft voltage discharging through bearing in motor or load. Solution: Add shaft grounding brush across one motor bearing; install Faraday shield.
To measure these currents, you need to use an instrument with oscilloscope functionality. At one time, that meant an oscilloscope, but today there are DMM-style devices designed for exactly this kind of work.
Size equipment grounding (bonding) conductors per Article 250, Part V of the NEC.
