Electric Motors: Dynamic/Online Testing

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In a previous post I discussed electric motor testing, specifically static or offline testing. This week I would like to tackle the subject of dynamic or online testing of electric motors.

As the name suggests, dynamic tests are performed while the electric motor is running i.e. when it is “online”. While the motor is running, you can take current and voltage readings that can be analyzed to indicate the following faults:

• Short circuiting between conductor or coil windings

• Contaminated winding

• Uneven air gaps, possibly caused by an eccentric rotor

• Broken rotor bars

• Insulation to ground faults

• An uneven flux field

Vibration data may be collected for online tests to check for broken rotor bars, an uneven air gap, or an eccentric rotor.

Current readings can be taken using a current clamp on each phase of the motor, usually at the electrical panel. Amperage readings between each phase should be relatively similar unless there is an unusually high or low load. The current spectrum can also indicate issues such as an uneven air gap, a bent or bowed rotor, and damaged rotor bars.

A flux coil can measure the flux field of the motor. Analyzing this data can review if there are an issues with the motor windings.

The advantages of dynamic or online testing are that the motor does not have to be shutdown and it can help identify potential issues, if they arise; however, offline testing does provide more comprehensive methods to detect and identify faults, and for troubleshooting operational problems.