Types of Duty of Induction Motor

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Type of Duty - Induction Motor
 

Cyclic Duty Factor ( CDF ): The cyclic duration factor is the time during which motor heating takes place. It is the ratio between the total time which contributes to electrical heating in one cycle and the complete cycle time.

This is the most common and available in market and industry.


Type of Duty - S1

Operation at constant load long enough for a thermal steady state condition to be reached.

P = Load

Pv = Electrical losses

θ = Temperature

θmax = Max. temp attained

t = time





Type of Duty - S2

Operation at a constant load for a given time which will not allow the motor to reach thermal equilibrium followed by rest and de energized. 

You may find the usage in siren of any Industry . It will run for a while and then again after a specific period.

P = Load

Pv = Electrical losses

θ = Temperature

θmax = Max. temp attained

t = time

∆tp = Operation time at constant load



Type of Duty - S3

Combination of duty cycles which consist of Operation at constant load, rest and de energized period. The cycle is such that the temperature rise is not affected by starting current.

If you run a S1 duty motor with VFD in place of S4 duty / crane duty, then the same will act as S3 duty as the starting will not affect the heating of the motor, hence no of start / stop is not a critical factor to consider.

tr = CDF
tb = On load time within a cycle
tst = Rest time within a cycle

tr =  tb / tb + tst



Type of Duty - S4

All are same as S3 with an additional significant starting period which allows a significant heating.

Crane duty motor with specific no. of start/stop with grid supply. 

tr = CDF
ta = Starting time
tb = On load time within a cycle
tst = Rest time within a cycle

tr = ta + tb  / ta + tb + tst




Type of Duty: S5


It is the duty which is similar to S4 but with an addition of Electrical Breaking. Which means there is Starting period, Running with load, Breaking, then rest and lastly de energized..

tr = CDF
ta = Starting time
tb = On load time within a cycle
tst = Rest time within a cycle
tbr = Breaking duration


tr = ta + tb + tBr / ta + tb + tBr + tst














Type of Duty - S6

It is a duty cycle consist of period of operation on load and at no load. There is no rest, de energized period.

tr = tb / tb + tL

tb = period at on load
tL = period at no load














Type of Duty - S7


There is no rest or de energized period , not even No load period. Only Starting period, Running period on load, Electrical Breaking period involved. In this case, the CDF = 1.

















Duties other than S1 - Additional information to select an Induction Motor

S2: Output and duration of operation ( 10 mins if nothing is specified )

S3, S6: Output, Cycle time ( If available ) and CDF

S4: CDF, No of Starts / Stop, GD^2 and T-S curve of the load

S5, S7: Same as S4 with type of breaking. For S7, CDF is not required as it is 1


It is important to select an induction motor with the appropriate duty rating for a given application to ensure optimal performance, reliability, and longevity. The duty rating should match the load and operating conditions to avoid motor overheating, excessive wear, or premature failure.

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