June 2008 Electrical Apparatus

June 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2008  featured technical article,  by Richard L. Nailen, P.E.    

During starting, an induction motor experiences severe short-term stresses in both stator and rotor. Rapid overheating results in differential expansion of core and conductors, while high electromagnetic forces cause mechanical stress in both conductors and insulation. Preventing fatigue failure of those components requires limiting both the severity of each start and the frequency with which starts can be repeated.

Standard motors are designed to safely accelerate loads exhibiting specific values of both inertia and counter-torque.

However, although manufacturers and standards agencies have made some attempts at regulation, starting repetition is not subject to any simple standard, or any established relationship between elapsed time and decreasing temperature as a motor cools following acceleration. That relationship will vary drastically depending upon whether the motor is braked to a stop, coasts to rests, or runs at full speed (loaded or not.) Each situation involves a different rate of heat dissipation from the motor to its surroundings.

In microprocessor-based protective relays, an internal thermal model uses resistor-capacitor networks to simulate a motor’s heat storage and dissipating capacities. However, the model is normally based on the assumption that the motor runs at full load between starts. The cooling rates for coasting and standstill are unknown. Depending upon the application, such an approach may or may not be conservative.

Manufacturers typically instruct the large-motor user to allow coasting to rest before restarting. That can take a few minutes for some drives and hours for others. Also , a limit is commonly placed on the number of starts per day (or per hour for smaller machines.) That invites such question of whether a “day” is 24 hours, 365 times a year, or is a single work shift, five days a week. Six starts spaced evenly throughout 24 hours may be acceptable, whereas six starts in the first hour followed by 23 hours of idleness may be unacceptable.

All these concerns must be weighed carefully by anyone seeking to answer the question, “How often can this motor be started?”

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