Induction Motors Duty Cycles

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Introduction to Motor Duty Cycles

Electric motors do not always operate under the same load or running conditions. In real-world applications, a motor may run continuously, intermittently, or with frequent starts, stops, and braking. These patterns of operation are known as duty cycles, and they directly influence the motor’s heating, cooling, and overall lifespan. International standards (IEC/IS) classify duty cycles into eight standard types (S1 to S8), helping engineers select the right motor for each application.



Types of Motor Duty (S1 to S8)

Electric motors are designed to operate under different duty cycles, depending on the application. The duty type defines how long the motor runs, rests, and how heat builds up and dissipates in the windings. Below are the standard duty classifications:

S1 – Continuous Duty

  • Motor runs at constant load long enough to reach thermal equilibrium.

  • Example: Pumps, compressors, fans.

S2 – Short-Time Duty

  • Motor runs at constant load, but not long enough to reach thermal equilibrium.

  • After operation, the motor rests long enough to return to ambient temperature.

  • Example: Crane hoists, valve actuators.

S3 – Intermittent Periodic Duty

  • Motor operates in a sequence of identical run and rest cycles with constant load.

  • Thermal equilibrium is never reached.

  • Starting current is negligible for heating.

  • Example: Press machines, conveyors.

S4 – Intermittent Duty with Starting

  • Motor cycles involve starting, running at constant load, and rest.

  • Starting current significantly influences heating.

  • Example: Elevators, electric traction systems.

S5 – Intermittent Duty with Starting & Braking

  • Each cycle involves starting, running at constant load, and electric braking.

  • No rest periods.

  • Example: Rolling mills, heavy-duty cranes.

S6 – Continuous Duty with Periodic Loading

  • Motor runs in identical cycles of constant load followed by no-load running.

  • No rest periods.

  • Example: Compressor with air reservoir, reciprocating pumps.

S7 – Continuous Duty with Starting & Braking

  • Motor cycles include starting, constant load running, and electric braking.

  • No rest periods.

  • Example: Automatic machines, printing presses.

S8 – Continuous Duty with Periodic Speed Changes

  • Identical duty cycles with different load and speed conditions.

  • Performed by motors with speed control (e.g., pole-changing motors, VFD motors).

  • Example: Metal-cutting machines, CNC machines.

✅ Tip for Students & Engineers:
When selecting a motor, always check its duty cycle (S1–S8) rating to ensure reliable performance under actual operating conditions.

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