- Motor Duty Class Definition: Motor duty class is defined as the categorization of motors based on their operating times and cooling periods.
- Continuous Duty: These motors run long enough to reach steady state temperature, used in paper mills, compressors, and conveyors.
- Short Time Duty: These motors have short operating times with longer cooling periods, ideal for cranes, household appliances, and valves.
- Intermittent Periodic Duty: These motors operate with alternating periods of running and rest, seen in presses and drilling machines.
- Classes of Motor Duty: Different classes include continuous duty, short time duty, and intermittent periodic duty, each suited for specific applications.
The load, speed, starting, braking and rest pattern determines how electric motors heat and cool. electrical drives must therefore be rated for the actual operating cycle, not only the peak power. A motor duty class describes that cycle in a standard form. IEC 60034-1 defines duty types S1 to S10. The eight types illustrated on this page are:
- S1 continuous running duty.
- S2 short-time duty.
- S3 intermittent periodic duty.
- S4 intermittent periodic duty with starting.
- S5 intermittent periodic duty with electric braking.
- S6 continuous-operation periodic duty.
- S7 continuous-operation periodic duty with electric braking.
- S8 continuous operation with periodic changes in load and speed.
Continuous Duty
S1 applies a constant load for long enough that the electric motor reaches thermal equilibrium. Paper-mill equipment, continuously operated conveyors and process compressors can use S1 when their real load is sufficiently constant.
Short Time Duty
S2 applies a constant load for a stated time that ends before thermal equilibrium. A sufficient de-energised rest then returns the motor close to the cooling-medium temperature. The rated duration must be stated, such as S2 30 min. Valve actuators and other infrequently operated mechanisms can fit this pattern; an application name alone does not determine the duty.
Intermittent Periodic Duty
S3 consists of identical cycles with a constant-load operating period followed by a de-energised rest. Neither period is long enough for thermal equilibrium, and starting losses do not materially affect the temperature rise. The designation includes the cyclic duration factor, such as S3 25%. Presses can fit S3 when their measured cycle matches this definition.
Intermittent Period Duty with Starting
S4 consists of identical cycles of starting, constant-load operation and de-energised rest. Starting losses affect its temperature rise. S5 adds an electric-braking period, so its cycle contains starting, constant load, electric braking and rest. The operating periods are too short to reach thermal equilibrium. Hoists, manipulators and mill auxiliaries may require S4 or S5 after the actual cycle and inertia are checked.
Continuous Duty with Intermittent Periodic Loading
S6 consists of identical cycles with a constant-load period followed by a no-load running period. The motor remains energised and there is no de-energised rest. Its designation includes the cyclic duration factor. Some pressing and cutting machines fit S6 when they continue to run between loaded operations.
Continuous Duty with Starting and Braking
S7 consists of starting, constant-load operation and electric braking with no rest or de-energised period. Starting and braking losses both affect the rating. Reversing mill drives are a common example when their cycle follows this pattern.
Continuous Duty with Periodic Speed Changes
S8 consists of repeated operation at several constant loads with corresponding speeds. There is no de-energised rest, and no one period is long enough to reach thermal equilibrium. The designation states the load, speed and cyclic duration factor for each condition. IEC 60034-1 also defines S9 for non-periodic load and speed variations with possible frequent overloads, and S10 for specified discrete constant load and speed conditions.
