Now a days, in
almost every applications, electric motors are used, and to control them electrical drives are employed. But the
operating time for all motors are not the same. Some of the motors runs all the
time, and some of the motor's run time is shorter than the rest period.
Depending on this, concept of motor duty class is introduced and on the basis
of this duty cycles of the motor can be divided in eight categories such as
1. Continuous duty
2. Short time duty
3. Intermittent periodic duty
4. Intermittent periodic duty
with starting
5. Intermittent periodic duty
with starting and braking
6. Continuous duty with
intermittent periodic loading
7. Continuous duty with
starting and braking
8. Continuous duty with
periodic speed changes
Continuous Duty
This duty denotes that, the motor is running long enough AND the electric motor temperature reaches the
steady state value. These motors are used in paper mill drives, compressors,
conveyors etc.
Short Time Duty
In these motors, the time of operation is very low and the heating time is much
lower than the cooling time. So, the motor cooks off to ambient temperature
before operating again. These motors are used in crane drives, drives for house
hold appliances, valve drives etc.
Intermittent Periodic Duty
Here the motor operates for some time and then there is rest period. In both
cases, the time is insufficient to raise the temperature to steady state value
or cool it off to ambient temperature. This is seen at press and drilling
machine drives.
Intermittent Period Duty with Starting
In this type of duty, there is a period of starting, which cannot be ignored
and there is a heat loss at that time. After that there is running period and
rest period which are not adequate to attain the steady state temperatures.
This motor duty class is widely used in metal cutting and drilling tool drives,
mine hoist etc.
Intermittent
Periodic Duty with Starting and Braking
In this type of drives, heat loss during starting and braking cannot be
ignored. So, the corresponding periods are starting period, operating period,
braking period and resting period, but all the periods are too short to attain
the respective steady state temperatures, these techniques are used in billet
mill drive, manipulator drive, mine hoist etc.
Continuous Duty with Intermittent Periodic Loading
In this type of motor duty, everything is same as the periodic duty but here a
no load running period occurs instead of the rest period. Pressing, cutting are
the examples of this system.
Continuous Duty with Starting and Braking
It is
also a period of starting, running and braking and there is no resting period.
The main drive of a blooming mill is an example.
Continuous Duty with Periodic Speed Changes
In this type of motor duty, there are different running periods at different
loads and speeds. But there is no rest period and all the periods are too short
to attain the steady state temperatures.