Explains the ten IEC 60034 motor duty classes (S1 through S10) that define how a motor's rated power relates to its actual thermal load. Covers continuous duty (S1), short-time duty (S2), intermittent periodic duty (S3, S4, S5), continuous-operation periodic duty (S6), continuous cyclic duty with electric braking (S7), continuous with related load and speed changes (S8), non-periodic load and speed variations (S9), and discrete constant loads (S10). Includes the two derating parameters — cyclic duration factor and load-related characteristic — and worked examples for pump, crane hoist and lift-truck applications.
A three-phase induction motor's nameplate rated power is defined at a specific thermal duty. Running the motor at the same shaft power but a different duty profile changes the thermal loading — a motor rated for continuous duty (S1) at 15 kW can deliver considerably more than 15 kW momentarily under short-time (S2) or intermittent (S3) duty, and correspondingly delivers less than 15 kW continuously if operated under a heavier duty class than rated. IEC 60034-1 defines ten duty classes covering the common load-profile patterns; selecting a motor to the correct class prevents both under-utilisation and thermal failure.
| Class | Name | Description | Typical use |
|---|---|---|---|
| S1 | Continuous running duty | Motor runs at constant load long enough to reach thermal steady state. | Fan, centrifugal pump, conveyor |
| S2 | Short-time duty | Motor runs at constant load for a defined time (typ. 10, 30, 60, 90 min) then de-energises and cools completely. | Sluice gate actuator, lock valve, dam gate |
| S3 | Intermittent periodic duty | Sequence of identical duty cycles; each cycle = period of constant load + period at rest. Starts do not significantly influence temperature. Cyclic duration factor (CDF) 15%, 25%, 40%, 60%. | Crane hoist, punch press, valve actuator |
| S4 | Intermittent periodic duty with starting | Like S3 but with a starting phase that appreciably influences temperature. | Frequent-start conveyor, small crane trolley |
| S5 | Intermittent periodic duty with electric braking | Like S4 with an added electric-braking phase that also influences temperature. | Machine-tool spindle, lift-truck traction |
| S6 | Continuous-operation periodic duty | Cycles of load + no-load, but motor stays energised (no cooling by shutdown). CDF 15%, 25%, 40%, 60%. | Lathe main drive, mixer, mill drive |
| S7 | Continuous with electric braking | S6 with electric braking rather than no-load rest phase. | Reversing rolling mill, hoist |
| S8 | Continuous with related load and speed changes | Continuous operation with periodic load + speed changes; typically pole-changing motors. | Two-speed pump-fan, elevator |
| S9 | Non-periodic load and speed variations | Continuous operation with load and speed varying non-periodically within the permissible operating range. | VFD-driven mixer, ship propulsion |
| S10 | Duty with discrete constant loads | Sequence of loads, each held long enough for the motor to reach thermal steady state; typical thermal life expressed by TL rating. | Multi-stage boiler feed, batching plant |
For S3, S6 and related classes, the load period as a fraction of the full cycle is the cyclic duration factor. A motor nameplated at CDF 40% delivers rated shaft power during 40% of the cycle time; the remaining 60% is rest (S3) or no-load (S6). Attempting to run at CDF 60% requires derating to about 85% of rated power on the same frame; running at CDF 100% (i.e. continuous) requires further derating to about 70% of nameplate.
A duty-class nameplate lists the class code, the load period (or CDF), the number of starts per hour where applicable, and the thermal class. Example: "S3 40% 240 c/h" reads as intermittent periodic duty at 40% cyclic duration factor, 240 cycles per hour. If a motor lists only "S1" with no CDF, it is a continuous-duty motor and the rated shaft power is deliverable indefinitely at rated ambient and cooling.