Motor Duty Class and its Classification

Nowadays, almost task is performed by the electric motor and control of the such motor is achieved by the electric drive. But the operating time for all motors is not the same. Some of the motors run continuously, some of the motors run for short time, and some of the motors run in intermittent period.

The motor duty class is divided into the following categories-

1.            Continuous duty(S1)

2.            Short time duty(S2)

3.            Intermittent periodic duty(S3)

4.            Intermittent periodic duty with starting(S4)

5.            Intermittent periodic duty with starting and braking(S5)

6.            Continuous duty with intermittent periodic loading(S6)

7.            Continuous duty with starting and braking(S7)

8.            Continuous duty with periodic speed changes(S8)

Continuous duty(S1):

The duty type S1 can be defined as an operation at a constant load maintained for sufficient time to allow the machine to reach thermal equilibrium.

Where: ΔT – Time sufficient to allow the machine to reach thermal equilibrium.

Short time duty(S2):

The duty type S2 can be defined as an operation at a constant load for a given time, less than that required to reach thermal equilibrium, followed by a time de-energized and at rest of sufficient duration to re-establish the equilibrium between the machine temperature and that of the coolant temperature.

Intermittent periodic duty(S3):

The duty type S3 is defined as a sequence of identical duty cycles, each including a time of operation at constant load and a time de-energized and at rest. The contribution to the temperature rise given by the starting phase is negligible.

Intermittent periodic duty with starting(S4):

The duty type S4 is defined as a sequence of identical duty cycles, each cycle including a significant starting time, a time of operation at constant load and a time de-energized and at a rest.

Intermittent periodic duty with starting and braking(S5):

The duty type S5 is defined as a sequence of identical duty cycles, each cycle consisting of a starting time, a time of operation at constant load, a time of electric braking and a time de-energized and at a rest.

Continuous Duty with Intermittent Periodic Loading(S6):

In this type of motor duty, everything is the same as the periodic duty but here a no-load running period occurs instead of the rest period. Pressing, and cutting are examples of this system.

Continuous Duty with Starting and Braking(S7):

It is also a period of starting, running and braking and there is no resting period. The main drive of a blooming mill is an example.

 
Continuous Duty with Periodic Speed Changes(S8):

In this type of motor duty, there are different running periods at different loads and speeds. But there is no rest period and all the periods are too short to attain steady-state temperatures.