Jim Bryan
EASA Technical Support Specialist (retired)

The most stressful time for electric motors is during starting. The speed-current curve in Figure 1 illustrates why. At starting, the motor current is the highest it will ever be. This is referred to as starting or locked rotor current. These different terms describe that when the shaft speed is zero, the current is maximum. Note also the impact of applied voltage to the current characteristics. This will be discussed later.

Many performance parameters of the motor are directly proportional to the current. The parameter of most concern in this article is the heat produced which is proportional to the square of the current as represented by P = I²R. Where P is the power lost in heat (kilowatt-hours [kW•h]) due to the square of the current flow (I²) through a resistance (R). Once the motor has been successfully started, the load current level is reached and the cooling circuit of the motor is able to dissipate the additional heat produced by the starting current. Restarting the motor before this additional heat has been dissipated means more heat in the form of kW•h will be added on top of that which is there. Each subsequent start before the additional heat has been dissipated will add more heat — raising the temperature until some component in the motor reaches its failure point.

The limiting factor as determined by the design is the temperature increase resulting in component failure in a squirrel cage induction motor of one of three components: the winding, the rotor bars or the rotor shorting end rings. Depending on the design, the thermal “weak link” could be any of these.

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