We know phase resistance of an electric motor can change drastically with temperature. As temperature rises the resistance rises as a result of copper having a positive temperature coefficient. However, on a three phase motor even though the phase resistance values change with temperature the balance of resistance between the three phases should not change. For your medium and high voltage motors (> 4kV) the balance of resistance is often very low (<1%). In fact, it is quite common to see values of imbalance in the one-hundredths of a percent (ex. .03%). The standard MCEGold balance of resistance setpoints for a medium voltage motor is 1% (caution) and 2% (alarm) so a baseline balance of resistance of .03% would have to increase ~3000% to reach the 1% alarm setpoint. Therefore, it is important for you to consider establishing a change from baseline alarm setpoint for your balance of resistance in these situations to alert you to large changes in the circuit condition.
Rotor Problems?
The Rotor Influence Check (RIC) is performed by rotating the rotor in specific increments (determined by the number of poles) over a single pole group, and recording the change in inductance measurements for each phase of the motor. Have you ever seen a Rotor Influence Check (RIC) that showed no sinusoidal activity and changes in rotor position resulted in only minor variations in inductance? If so, you may be looking at test results from a low influence rotor. Click here to learn more about low influence rotors.
This tip of the week from Decatur Industrial brought to you by our friends at PdMA.
Reliability – How do you want to spend your time?
In the electric motor maintenance business, motor reliability is paramount. When a motor fails unexpectedly, it means downtime, less of productivity, expensive maintenance fees, and a major pain in the neck! Wouldn’t it be great if you could predict motor failure before it happened? Watch this short video from PDMA and talk to Decatur Industrial about a preventative and proactive maintenance program!
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