SOLUTION

An intermittent duty rating refers to a sequence of identical duty cycles.

The intermittent duty cycle consists of an on-load and off-loads period with the motor coming to rest during the latter. The on-load period during one cycle is too short for the motor to reach a steady thermal condition, and the off-load period is likewise too short for the motor to cool to the temperature of the cooling medium.

Starting and braking are not taken into account on the assumption that the times taken up by these events are too short in comparison with the on-load period, and therefore do not appreciably affect the heating of the motor.

The load torque during one cycle may be greater than the rated torque of the motor. The load torque during one cycle may be greater than the rated torque of the motor.

When starting the motor power for this form of duty, it is also necessary to state the cyclic duration factor:

cyclic duration factor = (on time/cycle time) x 100%

The standards specify that the duration of one cycle must b shorter than 10 min. Cases, where the duty cycle is longer than 10 min, must be brought to the attention of the motor manufacturer.