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Home / Technical Articles / 10 Different Duty Types (Load Cycle) Of A Three-Phase Asynchronous Motors

Motor Duty / Load Cycle

The term duty defines the load cycle to which the machine is subjected, including, if applicable, starting, electric braking, no-load and rest de-energized periods, and including their durations and sequence in time.

10 Different Duty Types (Load Cycle) Of A Three-Phase Asynchronous Motors
10 Different Duty Types (Load Cycle) Of A Three-Phase Asynchronous Motors (photo credit: ABB)

Duty considered as generic term, for example, can be classified as continuous duty, short-time duty or periodic duty. The percentage ratio between the period of loading and the total duration of the duty cycle is defined cyclic duration factor.

It is the responsibility of the purchaser to declare the duty.

Where the purchaser does not declare the duty, the manufacturer shall assume that duty type S1 (continuous running duty) applies. The duty type shall be designated by the appropriate abbreviation and the purchaser may describe the type of duty based on the classifications according to the indications given below.

When the rating is assigned to the motor (values declared, usually by the manufacturer, for a specified working condition of a machine), the manufacturer must select one of the rating classes. If no designation is given, the rating relevant to the continuous running duty shall be applied.

In compliance with the classification of Std. IEC 60034-1 here are some indications regarding the duty types which are typically considered as reference to indicate the rating of the motor.


Continuous running duty (type S1)

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.

DEFINITION – 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.

Continuous running duty: Duty type S1
Figure 1 – Continuous running duty: Duty type S1

Where: ΔT – Time sufficient to allow the machine to reach thermal equilibrium

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Short-time duty (type S2)

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.

DEFINITION – 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.

A complete designation provides the abbreviation of the duty type followed by an indication of the duration of the duty (S2 40 minutes).

Short-time duty: Duty type S2
Figure 2 – Short-time duty: Duty type S2

  • ΔTc – Operation time at constant load
  • ΔT0 – Time de-energized

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Periodic duty (type S3-S8)

For a motor suitable to this duty type, the rating at which the machine may be operated in a sequence of duty cycles is specified. With this type of duty, the loading cycle does not allow the machine to reach thermal equilibrium.

This set of ratings is linked to a defined duty type from S3 to S8 and the complete designation allows identification of the periodic duty.

If no otherwise specified, the duration of a duty cycle shall be 10 minutes and the cyclic duration factor shall have one of the following values: 15%, 25%, 40%, 60%.

The cyclic duration factor is defined as the ratio between the period of loading, including starting and electric braking, and the duration of the duty cycle, expressed as a percentage.


Duty type S3

(Intermittent periodic duty)

DEFINITION – 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 temperature-rise 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%).

Intermittent periodic duty: Duty type S3
Figure 3 – 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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The duty type S4

(Intermittent periodic duty with starting)

DEFINITION – 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).

Intermittent periodic duty with starting: Duty type S4
Figure 4 – 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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The duty type S5

(Intermittent periodic duty with electric braking)

DEFINITION – 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.

Intermittent periodic duty with electric braking: Duty type S5
Figure 5 – 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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The duty type S6

(Continuous-operation periodic duty)

DEFINITION – 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%).

Continuous-operation periodic duty: Duty type S6
Figure 6 – Continuous-operation periodic duty: Duty type S6

  • ΔTc – Operation time at constant load
  • ΔT0 – Operation time at no load
  • Cyclic duration factor = ΔTc/ΔT0

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The duty type S7

(Continuous-operation periodic duty with electric braking)

DEFINITION – The duty type S7 is defined as a sequence of identical duty cycles, each cycle consisting of a starting time, time of operation at constant load and a time of electric braking. There is no time de-energized and at rest.

A complete designation provides the abbreviation of the duty type followed by the indication of both the moment of inertia of the motor JM and the moment of inertia of the load JL (S7 JM = 0.4 kg m2 JL = 7.5 kg m2).

Continuous-operation periodic duty with electric braking: Duty type S7
Figure 7 – Continuous-operation periodic duty with electric braking: Duty type S7

  • ΔT* – Starting/accelerating time
  • ΔTc – Operation time at constant load
  • ΔTf – Time of electric braking
  • Cyclic duration factor = 1

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The duty type S8

(Continuous-operation periodic duty with related load / speed)

DEFINITION – The duty type S8 is defined as a sequence of identical duty cycles, each 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.

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 moment of inertia of the motor JM and by the moment of inertia of the load JL, together with the load, speed and cyclic duration factor, for each speed condition (S8 JM = 0.7 kg m2 JL = 8kgm2 25kW 800rpm 25% 40kW 1250rpm 20% 25 kW 1000 rpm 55%).

Continuous-operation periodic duty with related load/speed: Duty type S8
Figure 8 – Continuous-operation periodic duty with related load/speed: Duty type S8

  • ΔT* – Starting/accelerating time
  • ΔTc1; ΔTc2; ΔTc3 – Operation time at constant load
  • ΔTf1; ΔTf2 – Time of electric braking
  • Cyclic duration factor = (ΔT*+ΔTc1)/T; (ΔTf1+ΔTc2)/T; (ΔTf2+ΔTc3)/T

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Non-periodic duty (type S9)

Duty with non-periodic load and speed variations

For a motor suitable to this duty type, the rating at which the machine may be operated non-periodically is specified. This class of rating corresponds to the duty type whose appropriate abbreviation is S9.

DEFINITION – The duty type S9 is defined as a duty in which generally load and speed vary non-periodically within the permissible operating range. This duty includes frequently appplied overloads which may greatly exceed the reference load.

Duty with non-periodic load and speed variations: Duty type S9
Figure 9 – Duty with non-periodic load and speed variations: Duty type S9

  • ΔT* – Starting / accelerating time
  • ΔTs – Time under overload
  • ΔTc – Operation time at constant load
  • ΔTf – Time of electric braking
  • ΔT0 – Time de-energized and at rest

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Duty with discrete constant loads and speeds (type S10)

For a motor suitable to this duty type, the rating at which the machine may be operated with a specific number of discrete loads for a sufficient time to allow the machine to reach thermal equilibrium is specified.
The maximum permissible load within one cycle shall take into consideration all parts of the machine (the insulation system, bearings or other parts with respect to thermal expansion).

The maximum load shall not exceed 1.15 times the value of the load based on duty type S1. Other limits as regards the maximum load may be given in terms of limits of temperature of the winding. The minimum load may have the value zero, when the machine operates at no-load or is de-energized and at rest.

This class of rating corresponds to the duty type whose appropriate abbreviation is S10.

DEFINITION – The duty type S10 is defined as the operation characterized by a specific number of discrete values of load maintained for a sufficient time to allow the machine to reach thermal equilibrium. The minimum load during a duty cycle may have value zero and be relevant to a no- load or rest condition.

A complete designation provides the abbreviation of the duty type followed by the indication of the per unit quantities p/Δt for the partial load and its duration, and by the indication of the per unit quantity TL which represents the thermal life expectancy of the insulation system related to the thermal life expectancy in case of duty type S1 with rated output, and by the quantity r which indicates the load for a time de-energized and at rest (S10 p/Δt = 1.1/0.4; 1/0.3; 0.9/0.2; r/0.1 TL = 0.6).

Duty with discrete constant loads and speeds: Duty type S10
Figure 10 – Duty with discrete constant loads and speeds: Duty type S10

Where:

  • ΔΘ1; ΔΘ2; ΔΘ2 – Difference between the temperature rise of the winding at each of the various loads within one cycle and the temperature rise based on duty cycle S1 with reference load
  • ΔΘref – Temperature at reference load based on duty type S1 t1; t2; t3; t4: time of a constant load within a cycle P1; P2; P3; P4: time of one load cycle
    (Pref: reference load based on duty type S1)

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Duty for equivalent loading

For a motor suitable to this duty type, the rating, for test purposes, at which the machine may be operated at constant load until thermal equilibrium is reached and which results in the same stator winding temperature rise as the average temperature rise during one load cycle of the specified duty type.

This class of ratings, if applied, corresponds to the duty type designated “equ”.

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Reference // Three-phase asynchronous motors Generalities and ABB proposals for the coordination of protective devices

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About Author

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

4 Comments


  1. Finux Leong
    Jun 22, 2018

    Hi, my appilcation matches S3, but if I using VFD, do I need to look for S3 75% as well??


  2. Muhammad Saad
    Jan 17, 2018

    Great work & an example/motivation for me to contribute my expertise so that anyone may benefit from this. I have learnt alot through this group.
    Hope & pray this will continue


  3. Jose Maria Parreira Junior
    Dec 11, 2017

    Edvard Csanyi.

    Excelente your work.great profissional.
    My admiration.

    Kind

    Joe Maria

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