APPLICATIONS AND SELECTION OF DC MOTORS

As per the characteristics of DC motors, different types of DC motors are applied for different jobs as follows:

1. Separately excited motors: Very accurate speeds can be obtained by these motors. Moreover, these motors are best suited where speed variation is required from very low value to high value. These motors are used in steel rolling mills, paper mills, diesel-electric propulsion of ships, etc.
2. Shunt motors: From the characteristics of a shunt motor, we have seen that it is almost constant speed motor. Therefore, it is used under the following conditions:
   1. Where the speed between no-load to full-load has to be maintained almost constant.
   2. Where it is required to drive the load at various speeds (various speeds are obtained by speed control methods) and any one of the speed is required to be maintained almost constant for a relatively long period.
   As such, the shunt motors are most suitable for industrial drives such as lathes, drills, grinders, shapers, spinning and weaving machines, and line shafts in the group drive.
3. Series motors: The characteristics of a series motor reveal that it is variable speed motor, that is, the speed is low at high torques and vice versa. Moreover, at light loads or at no-load, the motor attains dangerously high speed. Therefore, it is employed under the following conditions:
   1. Where high torque is required at the time of starting to accelerate heavy loads quickly.
   2. Where the load is subjected to heavy fluctuations and speed is required to be adjusted automatically.
   As such the series motors are most suitable for electric traction, cranes, elevators, vacuum cleaners, hair driers, sewing machines, fans and air compressors, etc.Note: The series motors are always directly coupled with loads or coupled through gears. Belt loads are never applied to series motor because the belt may slip over the pulley or it may break. Then, the motor will operate at light loads or at no-load and will attain dangerously high speed that may damage the motor.
4. Compound motors: The important characteristic of this motor is that the speed falls appreciably on heavy loads as in a series motor; however, at light loads, the maximum speed is limited to safe value. Therefore, it is used under the following conditions:
   1. Where high torque is required at the time of starting and where the load may be thrown off suddenly.
   2. Where the load is subjected to heavy fluctuations.
   As such the cumulative compound motors are best suited for punching and shearing machines, rolling mills, lifts and mine hoists, etc.