DC motor

A DC motor is an electric motor that runs on direct current power. In any electric motor, operation is dependent upon simple electromagnetism. A current carrying conductor generates a magnetic field, when this is then placed in an external magnetic field, it will encounter a force proportional to the current in the conductor and to the strength of the external magnetic field. It is a device which converts electrical energy to mechanical energy. It works on the fact that a current carrying conductor placed in a magnetic field experiences a force which causes it to rotate with respect to its original position.



Principle of DC Motor


When a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. In other words, when a magnetic field and an electric field interact, a mechanical force is produced. The DC motor or direct current motor works on that principal. This is known as motoring action


The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger, and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of current, then the thumb represents the direction in which force is experienced by the shaft of the DC motor.


Types of DC Motors


Direct motors are named according to the connection o the field winding with the armature. There are 3 types:

  1. Shunt wound DC motor
  2. Series wound DC motor
  3. Compound wound DC motor


 DC shunt motor


A DC shunt motor (also known as a shunt wound DC motor) is a type of self-excited DC motor where the field windings are shunted to or are connected in parallel to the armature winding of the motor. Since they are connected in parallel, the armature and field windings are exposed to the same supply voltage. Though there are separate branches for the flow of armature current and field current – as shown in the figure of below.

Series Wound DC Motor or DC Series Motor


A series wound DC motor like in the case of shunt wound DC motor or compound wound DC motor falls under the category of self-excited DC motors, and it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.


The electrical layout of a typical series wound DC motor is shown in the diagram\



Compound Wound DC Motor or DC Compound Motor


A compound wound DC motor (also known as a DC compound motor) is a type of self-excited motor, and is made up of both series the field coils S1 S2 and shunt field coils F1 F2 connected to the armature winding as shown in the figure below.

Both the field coils provide for the required amount of magnetic flux that links with the armature coil and brings about the torque necessary to facilitate rotation at the desired speed. As we can understand, a compound wound DC motor is basically formed by the amalgamation of a shunt wound DC motor and series wound DC motor to achieve the better off properties of both these types. Like a shunt wound DC motor is bestowed with an extremely efficient speed regulation characteristic, whereas the DC series motor has high starting torque.

So the compound wound DC motor reaches a compromise in terms of both these features and has a good combination of proper speed regulation and high starting torque.



Applications of DC Motors

Series Motors

The series DC motors are used where high starting torque is required, and variations in speed are possible. For example – the series motors are used in Traction system, Cranes, air compressors.

Shunt Motors

The shunt motors are used where constant speed is required and starting conditions are not severe. The various applications of DC shunt motor are in Lathe Machines, Centrifugal Pumps, Fans, Blowers, Conveyors, Lifts, Weaving Machine, Spinning machines, etc.

Compound Motors

The compound motors are used where higher starting torque and fairly constant speed is required. The examples of usage of compound motors are in Presses, Shears, Conveyors, Elevators, Rolling Mills, Heavy Planners, etc.