SELECTION OF MOTORS FOR DIFFERENT DUTY CYCLES

   SELECTION OF MOTORS FOR DIFFERENT DUTY CYCLES :

1. Continuous duty cycle :

                         It denotes the motor operation at a constant load torque to reach steady state temperature. The load time and. temperature time graph are shown in fig 1.1

EXAMPLES :

              Paper mill drives compressors, conveyers, centrifugal pumps and fans

                                             fig. 1.1

2. Short time duty :

                  It denotes the operation of motor at constant load for short period followed by rest to cool down to the original starting temperature. Short time duty timings are generally 10, 30, 60 and 90 minutes. The load time and the temperature time graph are shown in fig 1.2 

EXAMPLES :

              Crane drivers, drives for household appliances, sluice gate drives, valve drives and machine tool drives 

                                            fig 1.2

4. Intermittent period duty with starting :

                          This is intermittent periodic duty where heal losses during starting can not be neglected Thus it consists of a period of starting, a period of operation at a constant load and rest period. The operating and rest periods are too short to attain the steady state ternperature in one duty cycle. Its characteriestics are shown in fig 1.3. In this duty heating of machine during braking is considered to be negligible. 

EXAMPLE :

        metal cutting, drilling tool drives, mine hoist drives for lift trucks.

                                              fig 1.3

5. Intermittent periodic duty with starting and braking :                                           This is the periodic duty where heat losses during starting and braking can not be ignored. Thus it consists of a period of starting, a period of operation with a constant load, a braking period, and a rest period. Thermal equilibrium is not reached in one duty cycle. Braking is done electrically and is quick. Its characteristics is shown in fig 1.5.

EXAMPLES :

           Several machine tool drives, drives for electric suburban trains and mine hoist

                                             fig 1.5

6. Continuous duty with intermittent periodic loading :

                        The operation of motor has a sequence of indentical duty cycle, each consisting of a period of operation and a period of operation on no load. Thermal equilibrium is not rached in one duty cycle. Its characteristics are shown in fig.1.6. This duty is distinguished from the intermittent periodic duty by a period of running at constant load is followed by a period of running at no load instead of rest. 

EXAMPLES :

              Pressing, cutting, shearing and drilling machine drives 

                                          fig 1.6

7. Continuous duty with starting and braking :

                           The operation of motor consists of period of starting, a period of operation at constant load a period of electric braking and there is no rest period. The characteristics are shown in fig 1.7.

EXAMPLES :

                    Blooming mill

                                             fig 1.7

8. Continuous duty with periodic speed changes  :

                    Operation of the motor has a sequence of indentical duty cycle, each cycle is having a period of running at one load and speed and followed by another period of running at different speed and load. There is no rest period. Its characteristics are shown in fig 1.8. 

                                         fig 1.8

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