Electrical motors are the cornerstone of many industrial processes today, converting electric energy into the mechanical energy necessary to operate many critical equipments, from pumps to engines and more. With the increasing focus on energy efficiency across various industries, more and more organisations use a variable frequency drive (VSD) to change the frequency and voltage of their motors instead of running them at a constant speed, which causes plenty of wastage. However, VSDs are not without their shortcomings, mainly increasing the likelihood of bearing failure from common-mode voltage.
Read on to learn how to resolve this issue and prolong the lifespan of electric motor bearings.
What causes bearing failure?
VSDs are crucial in reducing a motor’s energy consumption at the cost of premature failure should it not be grounded properly. While many other factors can lead to a motor breakdown, bearing failure is generally the culprit wherever VSDs are used, with the damage caused by common-mode voltage.
In a three-phase AC system, this voltage is defined as the voltage difference between the neutral point of a three-phase load and the power source ground. The neutral point of the load equates to the motor’s stator neutral if the load is an AC motor. Moreover, its fluctuations bring about a voltage on the electric motor’s shaft via electrostatics that discharges along the bearings or windings.
This continual arching causes the bearing’s surface areas to become more brittle, with tiny pieces more likely to break off within the component. Furthermore, the damaged materials could get stuck between the balls and races of the bearings, causing grinding that leaves tiny pittings. Motors that continue to operate under such conditions will only lead to worse damage until signs of trouble—like strange noises—arise to indicate substantial damage.
How to extend the lifespan of your electric motor bearings
Attaching a shaft grounding mechanism at a motor shaft’s end can prevent bearing failure, especially in applications highly susceptible to common-mode voltage. Shaft grounding entails connecting a motor’s rotating rotor to earth ground through its frame. This solution is more cost-efficient when done before motor installation or, better yet, already comes preinstalled on the unit to avoid costs from maintenance and downtime. Shaft grounding devices come in various types, the most common being:
1. Shaft-grounding rings
A shaft-grounding ring is a device with many electricity-conducting fibre strands arranged inside a ring around the shaft, with brushes riding on the surface of the motor shaft and direct electric current to ground, while the ring’s exterior is generally mounted to the motor’s end plate.
2. Carbon brushes
These brushes enable electrical contact between the rotating and stationary components of the motor’s electric circuit and direct the current in the motor rotor to a ground, preventing the electrical charges from accumulating on it and ending up discharging via the bearings.
3. Ceramic bearings
Bearings with non-conductive materials like ceramic help to prevent unwanted current discharges within the motor. Their ceramic-coated balls ensure shaft current does not flow to the motor bearings, reducing the likelihood of wear from electric currents.
4. Ground-bearing isolators
This grounding device combines two technologies: an isolated conductive filament ring and a non-contact isolation shield that keeps out contaminants and a metal rotor that directs current from the shaft away from the motor bearings.
There are many ways to observe your motor’s energy efficiency and lifespan. Apart from electric motor rewinds, using a VSD is another way to improve a motor’s efficiency. When installing a VSD, there are a few things to take into account to mitigate common-mode voltage: ensure the motor system and the motor is properly grounded; have the right carrier frequency balance for reduced voltage imbalance and noise levels; and choose the right shaft grounding for the application should one be necessary. It is highly recommended to engage in professional services that offer mechanical and electrical engineering in Singapore when it comes to any type of motor failure.