Posted on January 28, 2013 by David Manney

The main cause of the failure of the motor bearings is the electrical damage. Electrical discharge machining is a technical phrase used for this damage. Do you know from where this damage comes and how can you prevent it from happening in the bearings of your machine? The excess electric charge trapped on an object is known as static electricity and it remains on the object until it moves to an object having opposite or weaker electric charge by means of electric discharge. A static electric charge is created when one out of two objects has high resistance to electrical current.

Electrostatic discharge can damage the bearing of a machine or can change its rotor dynamics, which may damage to the shaft of a machine. The rework of a shaft is very expensive; therefore, proper inspection and maintenance of voltage mitigation system can help you to avoid this problem. Usually all VFD-driven motors are at risk to bearing damage but you should not ignore the shaft grounding to protect bearings of the motor. An effective method of shaft grounding is essential if you want to save money.

Although the manufacturers of AC induction motors provide inverter-ready models but these motors have insulation only for the windings and bearings are often ignored. NEMA (national electrical manufacturers association) has recommended that motor bearings should have protection against damaging electrical discharges. It is a well-known fact that VFDs (variable frequency drives) can create motor shaft voltages that can destroy bearings which may also cause premature failure of the motor. The utilization of VFDs to direct the operations of AC motors is increasing dramatically these days. In addition to the high-efficiency and low operational costs, they also save a considerable amount of energy.

Minimizing the harm caused to AC motors from the current in the shaft is a big challenge for the engineers and the designers. As soon as the operation starts, VFD stimulates critical voltages on the shaft of motor until the discharge path to the frame is found. Generally, the bearings of motors form the pathway of slightest resistance. As soon as the voltage overcomes the resistance offered by the oil layer, shaft current starts to discharge in the ball bearings of the motor. This phenomenon persists until the time the bearings turn out to be rough with extreme noise, which finally leads to failure. How can you eliminate unwanted currents? In case of static discharge, check the grounding i.e. straps may be corroded or have become loose. If you are suspecting VFDs as a source of electric discharge then have a look at the quality and the length of the cable.