Power and Duty Cycle

Power Output

The unit of power output is kW (1HP = 0.75kW; 1 kW = 1.34 HP). The power output values specified are based on maximum power at continuous load at what the thermal stability of the motor winding is attained. The continuous load is referred to as S1 duty cycle. Electric motors can be used at various duty cycles, short-term or intermittent (S2, S3, S4, etc.). A higher power output may be achieved whilst used at short-term or intermittent duty. The main factor for determination of the maximum power output at a short-term or intermittent duty cycle, is the temperature rise, which, when added to the (standard) ambient temperature of 40°C, must not exceed the limit temperature of the insulation material used in the electric motor winding. In order to achieve the optimum efficiency a correct combination of the size of the motor should be used for the given application.

 

Duty Types

The duty cycles (S1 to S10) have been defined in IEC 60034-1 standard indicating the run cycles of electric motors and in case of the intermittent duty also the frequency of the run periods. The power output limit up to which an electric motor can be utilized is determined by the maximum permissible temperature of the stator winding. The electrical and mechanical parameters of electric motors are based on duty cycle as mentioned below.

 

S1: Continuous duty

Operation with a constant load during such a period of time that a thermal equilibrium is attained. The power that is specified on the rating plate may be taken up continuously. The rating plate specifies: S1.

 

S2: Short-time duty

Operation at constant load for a given time, less than that required to reach thermal equilibrium, followed by a time de-energized and at rest of sufficient duration to re-establish machine temperatures within 2 K of the coolant temperature. The run time of the motor is not longer than the given time, after which the motor cools down to the ambient temperature. The rating plate states e.g.: S2 60 min (alternative: 5, 10, 20 or 30 min).

 

S3: intermittent periodic duty

A sequence of identical duty cycles, each including a time of operation at constant load and a time de-energized and at rest. In this duty, the cycle is such that the starting current does not significantly affect the temperature rise. The on/off period is usually set on 10 minutes. The rating plate states e.g.: S3 25% (alternative: 30, 40 or 60%).

 

S4: intermittent periodic duty with starting.

A sequence of identical duty cycles, each cycle including a significant starting time, a time of operation at constant load and a time de-energized and at rest. Like duty type S3, however, with the influence of starting. In this example the duration factor is 25% (7,5 sec of one cycle 0,5 min + 120 c/h + max. moment of inertia). The rating plate states: S4 25% 120 + max. moment of inertia.

 

S5: Intermittent periodic duty with electric braking

A sequence of identical duty cycles, each cycle consisting of a starting time, a time of operation at constant load, a time of electric braking and a time de-energized and at rest. Like duty type S4, however, with the influence of electrical braking. The rating plate states e.g.: S5 – 25% + max. moment of inertia.

 

S6: continuous duty with intermittent load

A sequence of identical duty cycles, each cycle consisting of a time of operation at constant load and a time of operation at no-load. There is no time de-energized and at rest. The motor is running continuously, however, during 40% of the cycle (usually10 min.) the load equals the power mentioned on the rating plate. In this example the rating plate states: S6 – 40%.

 

S7: Continuous-operation periodic duty with electric braking

A sequence of identical duty cycles, each cycle consisting of a starting time, a time of operation at constant load and a time of electric braking. There is no time de-energized and at rest. Like duty type S5, however, without resting period. The rating plate states e.g.: S7 (+ cycle data).

 

S8: Continuous duty with periodic changes in load and rotation

A sequence of identical duty cycles, each cycle consisting of a time of operation at constant load corresponding to a predetermined speed of rotation, followed by one or more times of operation at other constant loads corresponding to different speeds of rotation (carried out, ‘for example, by means of a change in the number of poles in the case of induction motors). There is no time de-energized and at rest. The rating plate states e.g.: S8 (+ cycle data).

 

S9: Duty with non-periodic load and speed variations

A duty in which generally load and speed vary non-periodically within the permissible operating range. This duty includes frequently applied overloads that may greatly exceed the reference load. For this duty, a constant load appropriately selected and based on duty type S1 is taken as the reference value ('Pref”) for the overload concept. The rating plate states e.g.: S9 ( + cycle data).

 

S10: Duty with discrete constant loads and speeds

A duty consisting of no more than four discrete values of load (or equivalent loading) and if applicable, speed, each load/speed combination being maintained for sufficient time to allow the machine to reach thermal equilibrium. The minimum load within a duty cycle may have the value zero (no-load or de-energized and at rest). For this duty type a constant toad appropriately selected and based on duty type S1 shall be taken as the reference value (‘Pref’) for the discrete loads. The rating plate states e.g.: S10 (+ cycle data).



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