There are mainly two types of dc motors. These are shunt DC motor and series DC motor. There is another type of dc motor which has features of both shunt and series motor. Since it is hybrid of both shunt and series motor, we call is as compounded dc motor. Depending upon the kinds of dc motor, we employ them in various uses. Now we explain the applications of dc instrument one by one as follows
We know that a shunt motor has a definite speed at its no-load condition. Hence, it does not run with excessively high speed when we suddenly withdraw the mechanical load from its shaft without disconnecting its supply and field circuit.
Applications of DC Shunt Motors
Before look into the applications of dc shunt motors, we need to discuss some of its unique features to get a clear idea about its uses or applications. We know that a shunt motor has a definite speed at its no-load condition. Hence, it does not run with excessively high speed when we suddenly withdraw the mechanical load from its shaft without disconnecting its supply and field circuit. Also, a shunt field dc motor does not reduce its speed significantly when it carries the full mechanical load. Practically, the speed falls slightly from no-load to full load condition. Hence we can refer this motor as a constant speed motor.
Although by varying the shunt field current, with the help of a rheostat, we can quickly change the speed of the motor at a particular loading condition.
It has a general shaped efficiency curve. A shunt dc motor has low starting torque. But it does not mean that the motor cannot start with heavy mechanical load imposed on its shaft. It may begin its rotation with heavy mechanical load, but in that case, the motor draws excessive starting current from the supply mains. On the other hand, other types of dc motor such as series motors and compounded motors take much less starting current compared to that of a shunt dc motor.
Example of Use of DC Shunt Motor
Considering the above-explained characteristics of dc shunt motors, we use them
- when we require a constant speed operation with a variety of loading conditions. Even when we want the speed of the driving motor should be approximately the same from its no-load to a full-load condition.
- when we need to maintain an almost constant speed for a wide range of loading condition and then to change the speed as per requirement. Such as for driving a lathe machine. Besides, the constant speed feature of shunt dc motor, it provides easy and economical speed control methods. That means speed control of dc shunt motor is simple, easy and economical.
Applications of DC Series Motors
Before discussing the applications of series motors, let us recall the typical features of a dc series motor.
The speed of dc series motor prominently drops when we increase the mechanical load of the motor. So, a series motor cannot sustain a steady speed with fluctuating mechanical load at its shaft. Hence we usually do not use a dc series motor for constant speed operation.
But at a certain load, even at the full load, this motor takes less starting current than a dc shunt motor. In other words, for a particular armature current, a dc series motor develops more starting torque than a dc shunt motor. For that reason, we use dc series motors in street cars, cranes, hoists and electrical railway tractions. Because, all these applications, require massive on load starting torque.
In addition to the much starting torque, a dc series motor increases its running torque, with increasing mechanical load. The speed is inversely proportional to the torque in a dc series motor. Hence, with increasing mechanical pressure, the speed of the motor decreases and thereby torque of the motor increases.
But a shunt motor takes larger currents from the supply mains to maintain the constant speed at an increased torque.
Example of Use of DC Series Motor
Suppose we fit a shunt motor with the wheel shaft of an electric car. When the vehicle ascends an inclined road, the shunt motor tries to maintain its ground level speed. As a result, the engine takes higher electric power to supply extra torque to keep the constant velocity of the vehicle during ascending.
When we use a series motor for the same purpose, the engine takes much less power during ascending of the vehicle. While the car goes up, the speed of the dc series motor decreases. The decreased speed causes additional torque for climbing up without taking much power from the battery.
Summary of Applications of DC Motors
The dc shunt motor has a steady constant speed, adjustable speed, and medium starting torque. Because of these features, we use dc shunt motors for constant speed line shifting, lathes, centrifugal pumps, machine tools, blowers, fans, and reciprocating pumps.
The dc series motor has variable speed, adjustable variable speed, and high starting torque. For these mentioned reasons, we use dc series motors for electric traction, rapid transit system, trolley cars, cranes, hoist and conveyors, etc.
Like dc series motor, a dc cumulatively compounded dc motor has also variable speed, adjustable variable speed, and high starting torque. We use cumulatively compounded dc motors for intermittent high torque loads, shears, punches, elevators, conveyors, heavy planers and rolling mills, etc.