Electric motor duty ratings explained

Duty class is an important consideration when selecting and installing an electric motor. Duty class relates to the time a motor operates in one cycle and is stipulated by the IEC (International Electrotechnical Commission) standard 60034-1 on a scale from S1 to S10.

Here’s an explanation of all 10 motor duty ratings.

S1 - Continuous duty

The motor runs continuously for long enough to reach thermal equilibrium (the state reached when all parts of the machine are the same temperature). Most motors tend to be rated S1.

S2 – Short-time duty

Operates for less time than required to reach thermal equilibrium, followed by a rest period for the motor to cool down.

S3 - Intermittent periodic duty

A sequence of identical duty cycles, each at constant load for a period, followed by a rest.

S4 - Intermittent periodic duty with starting

This is similar to S3, but there is a significant period of starting.

S5 - Intermittent periodic duty with electric braking

A series of identical cycles, consisting of starting, operation, braking and rest. 

S6 - Continuous operation periodic duty

A period at load is followed by a period at no load, making up a sequence of identical duty cycles.  There is no rest period.

S7 - Continuous operation periodic duty with electric braking

This is the same as S6, but has significant starting and electric breaking periods with no rest.

S8 - Continuous operation periodic duty with related load/speed changes

A series of identical repeating duty cycles, where within each cycle the motor runs at different predetermined load levels and speed. There is no rest period.

S9 - Duty with non-periodic load and speed variations

The load and speed vary periodically within the permissible operating range. There is a possibility of frequent overloads.

S10 - Duty with discrete constant loads and speeds

Load and speed combinations are maintained long enough to reach thermal equilibrium.

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