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Safe Starting of Motors: Check for Temperature Increase

Publication: 
Maintenance Technology
Publication date: 
May 2015
Author: 
Jim Bryan, EASA Technical Support Specialist

The most stressful time for an electric motor is during starting, when the shaft speed is zero and the motor current is at its maximum. This condition is termed starting or locked-rotor current.

The starting current is directly proportional to the voltage and inversely proportional to the speed. Although many motor-performance parameters are directly proportional to the current, the main concern here is the extra heat produced during starting. The power lost as heat, measured in kilowatts (kW), is proportional to the square of the current flow through a resistance.

Once a motor has successfully started and reached its load current level, its cooling circuit can dissipate the additional heat produced by the starting current. Restarting the motor before all extra heat has been dissipated, however, will add more heat (kW) to the heat already there. In that case, each subsequent start will add even more heat, raising the motor temperature until some component fails.