The duty cycle of a motor refers to the ratio of time that the motor operates (or is energized) compared to the total time of a complete operating cycle. Duty cycles are commonly expressed as a percentage and are used to characterize how frequently and for how long a motor operates within a given time period. Different applications require motors to operate under different duty cycles, depending on factors such as load requirements, environmental conditions, and motor specifications.
IEC (the International Electrotechnical Commission) uses eight duty cycle designations to describe an electrical motors operating conditions:
| S1 | Continuous duty | The motor works at a constant load for enough time to reach temperature equilibrium. |
| S2 | Short-time duty | The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature. |
| S3 | Intermittent periodic duty | Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise. |
| S4 | Intermittent periodic duty with starting | Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise. |
| S5 | Intermittent periodic duty with electric braking | Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods. |
| S6 | Continuous operation with intermittent load | Sequential, identical cycles of running with constant load and running with no load. No rest periods. |
| S7 | Continuous operation with electric braking | Sequential identical cycles of starting, running at constant load and electric braking. No rest periods. |
| S8 | Continuous operation with periodic changes in load and speed | Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods. |
