In this theory, the duty cycle of the machines is discussed. The duty cycle of the motor is given from S1 and S8 whether the load varies or not. If the duty is not given in the name plate, it is assumed S1 ( continuous rating ) duty.

Types of Duty Cycle

Continuous Rating Duty ( S1 )

It is rating at which the motor may be operated for an unlimited time of duration, considering standard rating of the motor and temperature does not rise to its maximum permissible value in any part of the machine.
It is donated by symbol S1.

Short Time Duty ( S2 )

The motor is operated at some constant load for specific time then it is rested.
The period of time for load is very short therefore the temperature rise of the motor does not reach its maximum limit whereas the rest time for the machine is so long therefore the motor temperature drops to the ambient temperature.
Short time ratings : 10 min, 30 min, 60 min or 90 min

Intermittent Periodic Duty ( S3 )

The motor runs for constant load for some specific time, then load decreases to zero again constant load for specific time.
This cycle continues in the intermittent periodic duty.
The time duration of constant load is too short allow temperature increases its maximum value whereas the time duration of rest / cooling of motor is also too short allows motor to cool down to the ambient temperature.
It consists of sequence of identical cycles.

Duty Cycle ( Cyclic Duration Factor )

The intensity of heating is given by the duty cycle or cyclic duration factor.
It is defined as the ratio of the heating period to the sum of heating period and rest / cooling period

t_p = Heating period

t_c = Cooling period

T_c = Duty cycle = t_p + t_c

Cyclic duration factor ε = t_p / ( t_p + t_c )

Intermittent Periodic Duty with Starting ( S4 )

It consists of sequence of identical cycles in which has starting / operating time, constant load time and rest / cooling time.
The starting and cooling time is kept too short in order to achieve ambient temperature in one duty cycle.
One duty cycle is sum of time for starting time, constant load time and cooling time.
The duty factor is given by the ratio of sum of starting and constant load time to the sum of starting time, constant load time and cooling time.

Cyclic duration factor ε = ( t_D + t_P ) / ( t_D + t_P + t_C )

Where t_D= starting time

t_P = Constant load time and

t_C = Rest time / cooling time

T_c = Duty cycle = t_D + t_P + t_C

Intermittent Period Duty with Braking ( S5 )

This type of motor operated for sequence of periodic starting time, period of operation at constant load, period of braking time and period of rest ( cooling time ).
The starting time and rest time are too short to obtain thermal stability or equilibrium condition during one cycle.

Cyclic duration factor ε = t_D + t_P + t_F/ ( t_D + t_P + t_{F +}t_R )

Where

t_D = Starting time

t_P = Constant load time

t_F = Braking time

t_R = Rest ( cooling ) time

Continuous Operation with Intermittent Periodic Duty ( S6 )

The motor is operated for sequence of cycle which consists of period of constant load time and period of no-load time.
Generally, the constant load time and no-load time is kept very small in order to attain thermal equilibrium condition.

Cyclic duration factor ε = t_P / ( t_P + t_V)

Where

t_P = Constant load time

t_V = No load time

Continuous Operation Periodic Duty with Electric Braking ( S7 )

The motor is operated for sequence of similar cycle in which starting time, time of operation for constant load and braking time is given.
There is no rest or deenergized / cooling time.
The duty factor is always unity.