Electrical Apparatus, March 2014

Electrical Apparatus, March 2014

This is a summary of the Electrical Apparatus March 2014 featured technical article,  by Richard L. Nailen, P.E.  

In modern solid-state relays for protecting induction motors from overheating, a capacitor-resistor network creates an electrical analog to the motor heating and cooling behavior. Such a “thermal model” mimics the heat storage and dissipation behavior of the stator and rotor. The model is designed to produce relay operating characteristics (similar to those detailed in IEC 60255-8 or IEEE C37.112) compatible with the motor thermal damage curve.

During the past 40 years, many technical papers have explained the model theory. The literature includes many assumptions and misconceptions, however, possibly resulting from the authors’ unfamiliarity with motor design.

For example, large motors are said to be thermally limited during starting by rotor temperature alone. This is not always true. It’s assumed that motor cooling time, cooling curves, or cooling time constants are readily available from manufacturers. They’re not, because coastdown time and cooling rate are governed by motor load inertia and counter-torque that are not present at the factory.

Some papers suggest methods of deriving motor equivalent circuit constants from nameplate data or field measurements. This is seldom practical. Most authors treat the squirrel cage rotor as a single resistance that exhibits a single temperature rise, overlooking the large difference (varying with accelerating speed) between rotor bar and end ring thermal behavior.

According to one paper, if the safe locked-rotor time has not been supplied, it can be estimated by adding three seconds to the “average start time,” for which no justification is given. Another author requires on-line voltage measurements at the motor terminals—a practice both difficult and dangerous.

Although many authors refer to IEEE 620, a guide to the format of motor thermal damage curves, they seldom comply with it. Motor designers also construct the curve in various ways, for several reasons—particularly in the acceleration region where motor cooling is changing rapidly.

Because of all of this, motor users must work closely with both motor and relay designers to be confident of adequate protection based on the reality of motor thermal behavior.

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