Motor Duty Cycle

The duty cycle

The duty cycle of an electric motor, according to the international standard IEC 60034-1, is divided into 10 main types (S1 to S10).

Choosing the correct duty cycle is crucial, especially in control panel design, PLC programming for timing control, or retrieving status data for SCADA systems. Selecting the wrong motor type can lead to overheating and motor burnout.

Here's a summary of the 10 types:

- Most commonly used types (S1 - S3)
- Types with starting and braking conditions (S4 - S8)
- Types with complex load changes (S9 - S10)

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S1 : Continuous running duty

The duty type S1 can be defined as operation at a constant load maintained for sufficient time to allow the machine to reach thermal equilibrium.

For a motor suitable to this duty type, the rating at which the machine may be operated for an unlimited period is specified.

This class of rating corresponds to the duty type whose appropriate abbreviation is S1.

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S2 : Short time duty

The duty type S2 can be defined as 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 the equilibrium between the machine temperature and that of the coolant temperature.

For a motor suitable to this duty type, the rating at which the machine, starting at ambient temperature, may be operated for a limited period is specified. This class of rating corresponds to the duty type whose appropriate abbreviation is S2

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S3 : Intermittent periodic duty

The duty type S3 is defined as a sequence of identical duty cycles, each including a time of operation at constant load and a time de-energized and at rest. The contribution to the temperaturerise given by the starting phase is negligible

A complete designation provides the abbreviation of the duty type followed by the indication of the cyclic duration factor (S3 30%).

Figure 4 : Intermittent periodic duty: Duty type S3

ΔTc Operation time at constant load

ΔT0 Time de-energized and at rest

Cyclic duration factor = ΔTc/T

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S4 : Intermittent periodic duty with starting

The duty type S4 is defined as 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 a rest.

A complete designation provides the abbreviation of the duty type followed by the indication of the cyclic duration factor, by the moment of inertia of the motor JM and by the moment of inertia of the load JL, both referred to the motor shaft (S4 20% JM = 0.15 kg m2 JL = 0.7 kg m2)

Figure 5 Intermittent periodic duty with starting: Duty type S4

ΔT* Starting/accelerating time

ΔTc Operation time at constant load

ΔT0 Time de-energized and at rest

Cyclic duration factor = (ΔT* + ΔTc)/ T
 

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S5 : Intermittent periodic duty with electric braking

The duty type S5 is defined as 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 a rest.

 A complete designation refers to the duty type and gives the same type of indication of the previous case.

Figure 6 Intermittent periodic duty with electric braking: Duty type S5

ΔT* Starting/accelerating time

ΔTc Operation time at constant load

ΔTf Time of electric braking

ΔT0 Time de-energized and at rest 

Cyclic duration factor = (ΔT* + ΔTc + ΔTf)/ T

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S6 : Continuous operation periodic duty

The duty type S6 is defined as 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.

A complete designation provides the abbreviation of the duty type followed by the indication of the cyclic duration factor (S6 30%).

Figure 7 Continuous-operation periodic duty: Duty type S6

ΔT* Operation time at constant load

ΔT0 Operation time at no load

Cyclic duration factor = ΔTc/ΔT0

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