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Overload time constant

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Hi everyone


When the overload constant of a motor is expressed in minutes,(let us say 10 minutes) does it mean that the motor reaches it's 100% thermal capacity when the motor is subjected to some amount of overload (example 110%)for a period of 10 minutes.I shall be very thankful if someone can make it clear to me.


Thank u in advance



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hello merajdab


Thermal time constants of motors vary with speed due to the variation in cooling. It is common to rate a motor with the maximum locked rotor time, or the maximum load inertia. Both of these characteristics relate to the thermal time constant of the rotor and can be used to select the appropriate overload curve.

Some overload systems such as thermal overloads have a number of overload trip curves which you select to match the motor. More comprehensive overloads require you to specifiy the locked rotor current of the motor and the maximum locked rotor time.

Unfortunately, there are variations in requirements depending on the manufacturer. It is best to consult the manufacturers literature about the particular protection relay rather than consider general terms.

Best regards,

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