Permanent magnet motors require shaft grounding rings to reduce shaft voltage levels

Unlike asynchronous AC induction motors, where the speed of rotation is slower than the speed of the magnetic field, permanent magnet AC (PMAC) motors are synchronous motors that rotate at the same speed as the magnetic field generated by the stator windings. In this way, they eliminate rotor conductor losses, thereby significantly improving efficiency and power factor, and improving performance and speed control. In fact, the loss of PMAC motors is usually 15-20% less than that of NEMA high-quality induction motors of the same size. They provide higher flux density than similar induction motors. In short, they can provide more power (torque) within a given physical range, or provide the same torque in a smaller package. Compared with AC induction motors, they have a wider speed range.

PMAC motors are also known as permanent magnet synchronous motors (PMSM) and brushless AC motors. These motors use rare earth permanent magnets, which are very magnetic and generate high magnetic flux in the motor. They are so powerful that they can actually interfere with pacemakers, hearing aids, as well as electronic devices (mobile phones, tablets, watches, etc.) and even credit cards. However, once the rotor is enclosed within the motor housing, the radiated magnetic energy is no greater than that of the induction motor.

Almost all true permanent magnet motors are operated by variable frequency drives (VFDs). Therefore, just like AC induction motors, they also have the risk of damaging the electric bearings by the shaft voltage induced by the VFD. And because they are usually more expensive than AC induction motors, they are more expensive to replace.

Laboratory tests on two PMAC motors (45 kW motor and 300 kW motor) operated by pulse width modulation (PWM)VFD showed that permanent magnet motors are not immune to electric bearing damage caused by the motor.

Bearing current, shaftless grounding ring VFD

Both motors were tested at a speed of 5000 RPM. The shaft voltage read from a 45 kW motor without bearing protection shows a peak-to-peak voltage of 21.8V. This waveform illustrates the common mode voltage from a PWM driver capacitively coupled to the motor shaft. After installing the shaft grounding ring on the motor, a second reading was taken. The smaller waveform shows that the shaft grounding ring significantly reduces the shaft voltage, guiding it from the bearing and safely grounding it. In fact, the shaft ground ring reduces the harmful shaft voltage by more than 70% (peak-to-peak value from 21.8V to 6.6V) and eliminates EDM discharged through the motor bearings.

When the same test was repeated on a 300 kW motor, the results were even more impressive. The shaft grounding ring reduces the shaft voltage level from 23.4V peak-to-peak to 4.8V peak-to-peak by 80%. The shaft grounding ring also eliminates EDM discharge through the motor bearings.

Install shaft grounding ring, shaft voltage on same motor

Field tests also show that the shaft voltage and bearing current present on the new PMAC motor are sufficient to damage the motor bearings. Readings taken from the shaft of an unprotected 5 HP PMAC motor operated by a PWM drive (designed for PM motors) showed a peak-to-peak common mode shaft voltage spike of 14.8V (typical bearing discharge) and an actual 12.1 volt dv / dt bearing discharge. After installing the shaft ground ring on the motor, the peak-to-peak voltage drops to 3.28 volts, a 78% reduction!

Therefore, although PMAC motors have many advantages over AC induction motors (higher magnetic flux density, greater torque, higher energy efficiency, and wider speed range), they are all like AC induction motors prone to damage to EDM bearings due to VFD-induced shaft voltage discharge. However, the good news is that the shaft ground ring is equally effective in preventing VFD-induced EDM bearing damage to these high-performance motors, just as it is on AC induction motors that prevent such damage.