Electric motors are important pieces of equipment for industrial processes. They turn electrical energy into mechanical energy, thus enabling crucial operations that require movement. Industrial motors operate pumps, compressors, and fan a group of machines that need major efficiency improvements. Unfortunately, the applications usually run at constant speed even when not required, thus causing energy wastage and unnecessary CO2 emissions.
For this reason, variable frequency drives (VSD) are essential to vary the voltage and frequency fed to the motor, thus regulating its rotational speed. VSDs reduce energy consumption, thus saving on operations cost over the motor's life. But despite the crucial role of drives in electric motors, they cause bearing failure resulting from common-mode voltage.
This article explains how to overcome the challenge and extend the life of your motor bearings.
How Bearing Failure Occurs
VSDs are useful for reducing power consumption but lead to premature motor failure if not grounded properly. Of course, other things can cause motor failure but bearing failure is the most common issue where drives are used—the damage results from common-mode voltage.
You can define common-mode voltage in a three-phase system as the imbalance between the three phases resulting from the pulse width modulated drive power or the voltage difference between the neutral point of the three-phase load and the power source ground. The fluctuation in common-mode voltage induces a voltage on the motor shaft through electrostatics. This voltage may then discharge via the windings or through the bearings. Consequently, movement of electrical current in the bearings causes arching in a process dubbed electrical discharge machining (EDM).
The continued arching causes brittleness on the surface areas of the bearing, and tiny pieces can start breaking off inside the bearing. Additionally, the damaged materials may find their way between the bearing's races and balls, leading to a grinding effect that can form tiny pitting or washboard-like ridges. Some motors can continue working as the damage worsens until the first sign, usually an audible noise indicating substantial damage.
Extending the Life of Motor Bearings
Adding a shaft grounding device to an end of the motor shaft, particularly in applications susceptible to prevalent common-mode voltage, can prevent the problem. A shaft ground means connecting the rotating rotor of the motor to earth ground via the frame of the motor. It is cost-efficient to add a shaft grounding device to the motor before installation or buy a preinstalled rather than incurring maintenance costs linked with replacing beating, not to mention the cost of downtime in your facility. The common types of shaft grounding devices include:
1. Carbon Brushes
Carbon brushes offer the electrical connection between the stationary and rotating parts of the motor's electrical circuit and draw the current from the rotor to the ground, so the charge does not accumulate on the rotor and discharge through the bearings.
2. Shaft-Grounding Rings
Shaft-grounding rings have numerous strands of electric conducting fibers organized inside a ring around the shaft. The brushes ride on the motor shaft's surface and direct the current via brushes to the ground while the outside of the ring typically mounts to the endplate of the motor.
3. Grounding Bearing Isolators
The grounding devices combine two technologies: a non-contact isolation shield that prevents entry of contaminants and an isolated conductive filament ring plus a metallic rotor that drives shaft current away from the bearings.
4. Ceramic Bearings
Using bearings whose material is non-conducting can help prevent the discharge of current. Ceramic coated balls in ceramic bearings prevent shaft current from flowing via the bearing to the motor. Current-induced wear is rare since no electrical current flows via the motor bearings.
When installing speed drives, there are three considerations necessary to help reduce common-mode voltage. The first one is to ensure proper grounding of the motor and motor system. Besides, establish the proper carrier frequency balance, which will reduce noise levels and voltage imbalance. Finally, where a shaft grounding device is crucial, choose one that suits the application.
At Mader Electric, we understand that no one-size-fits-all solution exists when a bearing current is present in your system. Therefore, our experts work with you to determine the most appropriate solution for the particular application. Contact us for assistance in extending the life of your motor bearings.