Solving Bearing Currents and Shaft Voltages

High frequency shaft voltages and currents can cause motor and generator failures resulting in hundreds of thousands of dollars in system failure, maintenance and downtime.

The use of variable frequency drives (VFD) with large AC motors induce high electrical voltages on the motor shaft.  Once they exceed the resistance of the bearing lubricant, these voltages discharge to ground (typically the equipment housing), causing fusion craters and fluting in the bearings and motor failure.

What Are Bearing Currents?

Bearing currents occur when an induced voltage on the motor shaft is high enough to overcome the breakdown voltage of the bearing lubricant. This is typically greater than 50 volts. These shaft voltages will do one of the following:

1) Cause the current to flow directly from the shaft, through a bearing, and through the motor or load frame into the ground.

2) Cause the current to circulate from one side of the shaft, through a bearing, through the motor frame, back into the opposite bearing, and back into the shaft .

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 electric 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.

Linqing Nine Star Bearing Science & Technology Co., Ltd 

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