Classes of motor duty

The duty class of a motor describes how its load varies with time: continuous running, short running, periodic on/off, starting, braking and speed changes. Because temperature rise depends on the duty, the motor rating is specified for a duty class (IEC 60034-1; the eight classic classes are S1 to S8, and newer editions add S9 and S10).

Classes with waveforms

In the sketches, P = load, $P_v$ = losses, $\theta$ = temperature.

S1 Continuous duty: operation at constant load long enough to reach thermal equilibrium.

 P  |_________________
 th |   ___-----------  (reaches steady temp)
    +-----------------> t

Applications: pumps, fans, compressors, conveyors.

S2 Short-time duty: constant load for a short time $t_L$ (10, 30, 60, 90 min), then rest until the motor cools to ambient.

 P  |_____
    |     |__________________ rest
 th |  /\
    | /   \______ (never reaches final temp)
    +-----------------> t

Applications: sluice gates, swing bridges, valve actuators, kitchen mixers.

S3 Intermittent periodic duty: identical cycles of constant load and rest; starting losses negligible; temperature fluctuates around an average.

 P  |__    __    __
    |  |__|  |__|  |__
 th |  /\/\/\/\/\/\  (saw-tooth)
    +-----------------> t
     on off

Specified by cyclic duration factor $\text{CDF} = \dfrac{t_{on}}{t_{on}+t_{off}} \times 100\,\%$ (15, 25, 40, 60 %). Applications: cranes, hoists, presses.

S4 Intermittent periodic with starting: as S3, but each cycle has a significant starting period. Applications: lifts, machine tools.

S5 Intermittent periodic with starting and electric braking: start, load, electrical braking, rest. Applications: rolling-mill auxiliaries, centrifuges.

 S5:  start load brake rest
 P  |  /----\
    | /      \_______
    +-----------------> t

S6 Continuous duty with intermittent periodic loading: load and no-load periods, no rest.

 P  |__    __    __
    |  |~~|  |~~|  |~~ (no-load, motor running)
    +-----------------> t

Applications: pressing, drilling, punching machines.

S7 Continuous duty with starting and braking: start, load, electric braking repeatedly, no rest period. Applications: reversing rolling mills.

S8 Continuous duty with periodic speed changes: load at one speed then at another speed, no rest. Applications: multi-speed machine tools, pole-changing motor drives.

Summary

For variable duty, the motor is sized by the rms (equivalent) torque or power over a cycle and checked for maximum torque.