Q10Electrical Machine Design
Question
Q.10. Mention various duty cycles of motor.
Answer
Standard motor duty cycles (per IS:4722/IEC 60034-1) include S1 (continuous), S2 (short-time), S3-S5 (intermittent periodic, with variants including starting/braking), S6 (continuous with intermittent loading), S7 (continuous with starting/braking), and S8 (continuous with periodic speed changes).
The standard duty cycles of a motor, as classified by IS:4722 (aligned with IEC 60034-1), are: S1 — Continuous duty (constant load, run until thermal equilibrium); S2 — Short-time duty (constant load for a limited time, then rest to full cooling); S3 — Intermittent periodic duty (repeated load/rest cycles, not reaching thermal equilibrium, characterized by a cyclic duration factor); S4 — Intermittent periodic duty with starting; S5 — Intermittent periodic duty with starting and electric braking; S6 — Continuous operation with intermittent periodic loading (no rest period, but load varies periodically); S7 — Continuous operation with starting and electric braking; and S8 — Continuous operation with periodic changes in load and speed. Each duty type requires a different approach to thermal design, since a motor rated for a demanding intermittent duty (like S3 with a low cyclic duration factor) can deliver a higher peak output for the same frame size than one rated for continuous S1 duty.