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SECTION 6 - MOTOR GROUNDING
Motors are unique and have characteristics considerably different from other electrical devices such as transformers. The construction of the motor lacks the ability to withstand the mechanical heating effects of short circuit currents. The reactances of the motor are not equal, unlike those of a transformer. The space limitations restrict the amount of insulation that can be installed. Internal faults to the motor ground can result in extremely high current flow that can damage the laminations.
Motors should be grounded using one of the methods described according to the provisions of Section 28 of the CEC. If the overcurrent protection device is a motor short-circuit protector or an instantaneous trip breaker, see Code Section 28-300 for sizing such device. The size of the motor grounding conductor is based on the rating of the protective device. In any design, the equipment grounding conductor never has to be larger than the largest conductor supplying the motor.
Careful attention must be given to motor variable speed devices (VFD). For safe and dependable operation all VFDs must be properly grounded. This normally calls for a grounding conductor to be brought back to a single-point grounding location, usually at the service. In addition, a grounding conductor must be brought back from the motor to the VFDs internal grounding terminal. This direct motor ground to the VFD is required to minimize interference and for the ground-fault protection function of the drive to operate properly.
A good VFD motor cable is meaningless if the metal armour or shield is not properly installed. The shield must be terminated in a gland that is solidly connected to the stator frame.
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