SOLUTION

Short Time duty is preferred if the load requires constant power for a short period of time and rest for a sufficient longer duration.

A short time rating of an electric motor can be defined as the extrapolated overload rating of the motor which it can supply for the specified short time without getting overheated.

In this type of duty, the load occurs on the motor during a small interval and remains idle for a long time to re-establish the equality of temperature with the cooling medium. The variation of the load against time for short-time duty is shown in Fig.

When a motor is used for this purpose, the duration of the load on the motor is less than the heating time constant of the motor or the time required for obtaining thermal equilibrium. The period of rest is sufficient enough to cool the motor to the ambient temperature.

The rating of the motor is called short-time rating. The motors may have 10 minutes, 30 minutes or 60-minute rating based on this criterion. The motor is thermally well utilized.  Nowadays the machines are being designed and manufactured for a short duration having sufficient overload torque capability.

Usually, such type of short-time duty occurs in bridges, lock gates, and some, other household appliances such as mixies.