The same mechanical task can impose very different thermal loads on a motor. A rating such as 10 kW is therefore incomplete without operating time, rest periods and starting frequency.
S1 — continuous duty
The motor operates at constant load long enough to reach thermal equilibrium. Typical examples are continuously running pumps, fans and production machinery.
S2 — short-time duty
The operating period is shorter than the time needed to reach thermal equilibrium. It is followed by a de-energised rest long enough for the motor to cool back to the reference temperature. The duration is stated, for example S2 30 min.
S3 — intermittent periodic duty
Identical cycles of operation at constant load and de-energised rest repeat. Starting does not significantly affect heating by definition. The cyclic duration factor is stated as a percentage.
| Duty | Load profile | Additional value |
|---|---|---|
| S1 | Continuous until thermal equilibrium | No time value |
| S2 | One short operating period followed by complete cooling | Operating time in minutes |
| S3 | Repeated load/rest cycles | Cyclic duration factor in percent |
What about S4 to S10?
IEC 60034-1 defines ten duty types. The others cover profiles including starting and braking or uninterrupted periodic duty with changing load. S1 to S3 cover many industrial and special-purpose drives; more complex profiles require a separate technical assessment.
Why duty matters
A low cyclic duration factor may provide thermal headroom, but does not replace checks of torque, current, commutation, starts and mechanical strength. Our motor-sizing service considers the complete load profile.
Frequently asked questions
What does S1 mean on a motor nameplate?
S1 means continuous duty: the motor runs at constant load long enough to reach thermal equilibrium.
What does S2 30 min mean?
The motor may run at the stated load for 30 minutes, followed by a de-energised rest long enough to cool back to the reference temperature.
How is S3 duty stated?
S3 is stated as a cyclic duration factor. For example, S3 25% in a ten-minute cycle means 2.5 minutes on and 7.5 minutes off.