Classes of Motor Duty

CLASSES OF MOTOR DUTY

various load time variations encountered into eight classes as

i.        continuous duty

ii.       short time duty

iii.      intermittent periodic duty

iv.      intermittent periodic duty with starting

v.       intermittent periodic duty with starting & braking

vi.      continuous duty with intermittent periodic loading

vii.     continuous duty with starting & braking

viii.    Continuous duty with periodic speed changes.

Where,

TL – Load torque in N-M,

Ө- Temperature in Deg.centigrade,

t- Time in seconds.

1. Continuous duty:

          This type drive is operated continuously for a duration which is long enough to reach its steady state value of temperature.

          This duty is characterized by constant motor torque and constant motor loss operation. Depicted in fig.1 (a) & (b).

          This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors.

Examples:

Paper mill drives ,

Compressors

Conveyors,

Centrifugal pumps and

Fans ,

2. Short time duty:

                In this type drive operation, Time of operation is less than heating time constant and motor is allowed to cool off to room temperature before it is operated again.

                 Here the motor can be overloaded until the motor temperature reaches its permissible limit. Depicted in fig.2 (a) & (b).

                This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, DC series motors, universal motors.

Examples:

Crane drives ,

Drives for house hold appliances

Turning bridges

Sluice gate drives

Valve drives and

Machine tool drives.

3. Intermittent periodic duty:

                In this type drive operation, It consists of a different periods of duty cycles

                I.e. a period of rest and a period of running, a period of starting, a period of braking.

                Both a running period is not enough to reach its steady state temperature and a rest period is not enough to cool off the machine to ambient temperature.

                In this type drive operation, heating due to starting and braking is negligible.

                Depicted in fig.3 (a) & (b).

                This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, universal motors.

Examples:

Pressing

Cutting

Drilling machine drives.

4. Intermittent periodic duty with starting:

                This is intermittent periodic duty where heating

                Due to starting can‟t be ignored.

                It consists of a starting period; a running period, a braking period & a rest period are being too short to reach their steady state value.

                In this type of drive operation, heating due to braking is negligible.

                 Depicted in fig.4 (a) & (b).

                This type of duty can be accomplished by three phase induction motors and DC series motors, DC compound motors, universal motors.

Examples:

Metal cutting,

Drilling tool drives,

Drives for forklift trucks,

Mine hoist etc.

5. Intermittent periodic duty with starting & braking:

Ø              This is an intermittent periodic duty where heating during starting & braking can‟t be ignored.

Ø               It consists of a starting period, a running period; a braking period & a rest period are being too short to reach their steady state temperature value.

Ø              Depicted in fig.5 (a) & (b).

Ø              This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, DC series motors, DC compound motors, universal motors.

Examples:

Billet mill drive

Manipulator drive

Ingot buggy drive

Screw down mechanism of blooming mill

Several machine tool drives

Drives for electric suburban trains and

Mine hoist

6. Continuous duty with intermittent periodic loading:

Ø             This type of drive operation consists a period of running at constant load and a period of running at no load with normal voltage to the excitation winding in separately excited machines.

Ø              Again the load and no load periods are not enough to reach their respective temperature limits.

Ø             This duty is distinguished from intermittent periodic duty by running at no load instead of rest period.

Ø             This type of duty can be accomplished by single phase/ three phase induction motors and DC compound motors, universal motors.

Examples:

Pressing

Cutting

Shearing and

Drilling machine drives.

7. Continuous duty with starting & braking:

Ø             It consists a period of starting, a period of running & a period of electrical braking.

Ø             Here period of rest is negligible.

Ø             This type of duty can be accomplished by single phase/ three phase induction motors.

Examples:

The main drive of a blooming mill.

8. Continuous duty with periodic speed changes:

Ø        It consists a period of running in a load with a particular speed and a period of running at different load with different speed which are not enough to reach their respective steady state temperatures.

Ø        Further here is no period of rest.

Ø        This type of duty can be accomplished by single phase/ three phase induction motors and DC series motor in traction.

Examples:

All variable speed drives.