Thermal overload protection of motor

Why frequency must varied to avoid motor thermal over load in starting

In order to avoid overloads, motors must not be operated with just any starting frequency. With frequent start of motors the rated current of the motor doesn’t play the decisive part anymore but much rather the higher starting current (typical 6 x the rated motor current) as well as the starting frequency, the start-up time and the power-on time.

A periodical operation of the motor; current-/time-curve below:

A periodical operation of the motor is examplified in the current-/time-curve below:
Istup starting current of motor; Irated rated current of motor; tstup start-up time; tpon total power-on time; ttot cycle time

Motor start by frequency converter – frequency varied

The frequency converter with overload relay simulates the thermal behaviour of a motor in a model. The tripping threshold of the overload relay is situated at (1.125 ± 0.075) x rated current.

The temperature of the motor can have the following characteristics.The mean temperature of the motor stays beneath the permissible maximum temperature, however, on start-up the motor temperature can exceed
this temperature barrier. In this case the overload relay trips.
starting of motor to avoid thermal overload

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