DC Motors

D.C. Motor:

What is a D.C. Motor ? Explain the principal of motor.

There is no difference between the construction of D.C. motor and D.C. Generator.

Motor is that machine which converts electrical energy to mechanical energy. The motor on the principle that whenever a current carrying conductor is placed in a magnetic field, conductor tends to move.

To Understand principal of Motor:

Consider a conductor which carries no current and is end view of the conductor carrying current into the plane of paper. The flux around the conductor as given by the cork screw rule in clockwise.

The resultant of field due to main field and that due to current in the conductor is shown. It will be seen that the main field acts in conjunction above the conductor pushers the conductor downward. The direction of the force is also given by the left hand rule.

Principle Of Motor

Now, instead of a conductor, assume that a coil carrying current is placed in the magnetic field the sides AB and DC which carry current in the direction A to B and D to C produce flux of their.

represents the resultant flux around the conductor. it will be seen that conductor AB tends move downward when the conductor CD tends to move upward. so, these two forces tend to form a couple and try rotate the coil anticlockwise direction.

Principle Of Motor

Fleming's Left Hand Rule:

This  rule is applied to find out the direction of rotation of an armature in the motor. According to this rule, spread the forefinger, middle finger and the thumb of your hand in such a way that each should be at 90^o to each other. then if the forefinger represent the field direction, middle finger, the current direction, then the thumb will show the direction or rotation of conductor or armature.

Fleming's Left Hand Rule

Terms Used in D.C. Motor:

Torque:-it is a term which expresses the turning or twisting movement of force about an axis. it is measured by the product force × radius at which force acts.

Speed:- total revolution taken by an armature par minute is called speed. back e.m.f. or counter e.m.f. when the armature of a motor rotates, an e.m.f. is induced in the conductors as they cut the line of force. the induced e.m.f. is in opposition to the applied voltage (V) and is called the back or Counter e.m.f. (E_b)
Back e.m.f. is the generated e.m.f. produced in the same way as the e.m.f. of a generator.


What are the different types of D.C. motors ? Explain with diagrams.

There are three types of D.C. motors

1. Shunt Wound 
2. Series Wound
3. Compound Wound

1. Shunt Wound: in this the field winding is connected in parallel with the armature the field winding is of high value resistance with more turns. it practically at constant speed at almost all load. its starting torque is 1.5 to 2 time greater then the full load torque. the variations of speed of the motor can be well achieved by a shunt regulator. 

Shunt Wound

2. Series Wound: in this type of motor the field has few turns of heavy conductor which is connected in series with the armature, so that the load current flows from the field and armature. this motor has the characteristic of decreasing of the speed for increasing the load, so then there is no load on this motor, the speed is very high. hence, this type of motor is never used without load. uses this of motor is useful for trams, cranes, trains, pumps, trolley cars, etc., due to its very high starting and constant speed.

Series Wound

3. Compound Wound Motor: in this, two windings series and shunt are used. According to their field connection, these are of two types

(1) Cumulative Compound (2) Differential Compound

1. Cumulative Compound: Cumulative motor is one in which series field is so connected that it assists the field due to the shunt winding. the armature torque is increased under heavy load at starting. as the load increases, the speed decease in the same way as in series motor. when the load is released suddenly, the shunt field prevents motor from speeding up beyond safe value. hence, such motors are used where series characteristics are required also the load is likely to be removed totally such as in as in coal cutting machine and in machine tools which take sudden deep cuts. 
2. Differential Compound: Motor is one in which series winding is so connected that the field due to it opposes that due to shunt winding, it tends to keep the speed the constant. when starting up the motor, the series winding needs to be short-circuited otherwise the series field winding would rise to its full value before shunt this will cause the motor to start in the wrong direction with high speed.uses the motor are seldom used since the shunt motor usually satisfies the requirements of constant speed.

Cumulative Compound  / Differential Compound