Three-Phase Motor Tips: How To Evaluate Winding Temperatures

Before pulling what might be a hot-running unit, take the time to confirm your suspicion.

By Mike Howell, Electrical Apparatus Service Association (EASA)

Suspect a three-phase motor is running hot? If you’re right, the unit is either producing more heat than it’s designed for or dissipating less. With excess heat, the main concerns are typically the health of the bearing-lubrication and the winding-insulation system.

Before incurring the expense of pulling the motor, evaluate its winding temperature. Here’s how.

Once the winding temperature of a suspect motor has been determined, compare it to the unit’s nameplate (allowable) temperature. Often, only the insulation class is listed. For electric motors, the class is usually B, F, or H. Motors that get close to or exceed their class temperature limits could be having problems.

Measure the stator current of all three phases.

Compare these readings to the nameplate ratings and, if possible, to the current readings of any sister motors in the same application. The heat produced by the stator winding is proportional to the winding resistance and square of the current. Extra current equals extra heat.

Don’t conclude the winding is too hot by simply touching the motor’s frame.

Winding temperatures can’t be properly evaluated based on outside frame temperatures.

If the motor is equipped with temperature detectors that are capable of providing readings, use them.

A controller isn’t always necessary. Thermistors may require a controller, but thermocouples and resistance temperature detectors (RTDs) can be easily read with common, handheld meters and reference tables.

A thermocouple or RTD can be affixed to the stator core back iron, using proper electrical-safety precautions.

In such cases, the winding temperature is usually around 5 to 10 C above the back-iron temperature. If temperature detectors are not installed or operable, winding temperature can be extrapolated using a time series of winding-resistance measurements taken after shutdown. (Ask your service center for assistance with this if needed.) While this approach requires shutting down and opening the motor terminations, depending on the unit, it may be worth it.

Once you know the winding temperature, compare it to the motor’s nameplate temperature, i.e., allowable temperature.

Frequently, only the insulation class will be listed. For electric motors, the class is usually B, F, or H (see table). If the motor is getting close to or exceeding its class temperature limit, you may indeed have a problem.

Be proactive

If your evaluation of the winding temperature points to a problem, take steps to solve it. Don’t wait for a failure to occur. While overheating in the stator winding can lead to stator winding failures, it can also damage the stator core and/or mechanical components due to heat transfer from the stator winding to the bearing lubrication. MT

Mike Howell is an electrical support specialist with the Electrical Apparatus Service Association (EASA). Based in St. Louis, EASA is an international trade association of more than 1,900 electromechanical sales and service firms in 62 countries. For more information, visit www.easa.com.