March 2009 Electrical Apparatus

March 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus March 2009 featured technical article,  by Richard L. Nailen, P.E.     

A motor is said to be overloaded when its thermal capacity is exceeded. That occurs most often when mechanical power output at the shaft exceeds the rated value. However, lower output can also cause thermal overload if voltage is low or unbalanced. Overloads are tolerable under two conditions: when they’re only intermittent, or when ambient temperature is below the standard limit. Determining the acceptability of an overload requires knowing winding temperature at a known operating condition. For outputs between 2/3 and 1-1/3 times rated load, motor current is considered directly proportional to power output. Since copper loss in the winding varies as current squared, a common assumption is that temperature rise varies as output squared.

Although conservative, that’s inaccurate, because core loss (also a contributor to winding heating) does not vary as current squared. A more accurate assumption is that winding temperature rise varies as the 1.5 power of output.

That suffices for a continuous overload. When load varies cyclically, two methods of temperature estimation are available. In the root-mean-square method, individual products of output power squared and the time duration period for each output are summed up, the total divided by the total cycle time, and the square root of the result taken as the effective output. Maximum torque must of course be sufficient to sustain peak horsepower.

The heat balance method is better suited to cycles involving speed-changing losses, such as frequent starting. The winding heat-dissipating or cooling capability will be the losses producing rated temperature rise. That value is compared to the heat-producing losses throughout the cycle, including starting or plug-stopping losses. If the cooling capability exceeds the heating developed, the application is acceptable.

In any overload situation, overcurrent devices must be capable of allowing the desired operation without either nuisance tripping or loss of protection.

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