Motor shaft voltage testing has become an important predictive and preventive maintenance test over the last several years.
With continued efforts to conserve energy, be more efficient, and more reliable, the use of variable frequency drives or VFDs has become common. VFDs are now used for everything from residential appliances to heavy industrial applications.
Due to the electronics required to create the output need to operate a motor, an unrealized consequence has been discovered. It is an electrical current that circulates in the motor as it runs. Though the current is not hazardous, the associated high frequency voltage can eventually cause damage to the motor bearing.
It is estimated that today 30% of motor failures are caused by bearing damage due to these voltages. The benefit of motor shaft testing is to discover problems before they become costly and time consuming to repair.
As the motor operates on a VFD, the circulating current voltage rises.
Typically, the voltage ranges from 10 to 40 volts peak. At approximately 6 volts, the bearing grease can no longer prevent the voltage from jumping or discharging from the motor shaft to the grounded case of the motor.
When this happens, the small electrical arc it creates leaves what is called a fusion crater on the bearing race. The heat from the arc is hot enough to damage the bearing race and contaminate the bearing grease.
An oscilloscope can be used to measure the voltage and from the wave, form an estimate of the damage to the bearing race can be determined. An oscilloscope is a beneficial tool in motor shaft testing.
These discharges are so frequent that before long the bearing race has numerous pits called frosting.
Frosting may result in what is called fluting of the bearing race which is wash-board like ridges in the race. Fluting causes excessive noise and vibration.
Depending on what the motor is driving, the noise and vibration can be magnified and transmitted through the facility.
The result is a motor failure that could be very expensive in repairs and lost production.
OMI Infrared is an Ohio company founded in 2013 and based in the northwest corner of the state. The main focus of OMI Infrared is infrared inspection for preventive and predictive maintenance for industrial, commercial, utilities and residential use. The use of an infrared inspection not only helps increase reliability but will also assist in reducing energy and maintenance costs.