There is no difference between the construction of a DC motor and a DC generator. A motor is A machine that converts electrical energy to mechanical energy. The motor works on the principle that whenever a current-carrying conductor is placed in a magnetic field, the conductor tends to move.
To understand the principle of Motor. Consider a conductor which carries no current and is placed in a magnetic field as shown in Fig.
Now pass the current.
the end view of the conductor carrying current into the plane of the paper.
The flux around the conductor is given by the corkscrew rule in clockwise.
it will see that the main field acts in conjunction above the conductor and opposes the main field below the conductor. So, the conductor opposes the main field below the conductor downward. The direction of this force is also given by the Left-hand rule
Now, instead of a conductor, assume that a coil carrying current is placed in the magnetic field shown in Fig.
It will be seen that conductor AB tends to move downward while the conductor CD tends to move upward. So, these two forces tend to form a couple and try to rotate the coil in an anticlockwise direction.
Principle of motor
The DC armature can now be considered to have a number of such coils as shown in Fig.
The armature has 4 coils here. The resultant torque produced tries to rotate the armature in an anticlockwise direction.
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 to each other. Then if the forefinger represents the field direction, the middle finger the current direction, then the thumb will show the direction of rotation of the conductor or armature.
Terms used in D.C. Motor
Torque. It is a term that expresses the turning or twisting movement of force about an axis. It is measured by the product force x Radius at which force acts.
Speed. Total revolution taken by an armature per 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 lines of force. The induced emf. is in opposition to the applied voltage (V) and is called the Back or Counter e.m.f. (E,).
Back e.m.f. is the generated e.m.f. produced in the same way as the e.m.f. of a generator.
Different types of D.C. motors
- Shunt-wound
- Series Wound
- Compound Wound
Shunt Wound: In this, the field winding is connected in parallel with the armature as shown in Fig. The field winding is of high-value resistance with more turns. It runs practically at constant speed at almost all loads. Its starting torque is 1.5 to 2 times greater than the full load torque. The variations in the speed of the motor can be well achieved by a shunt regulator. Uses. These types of motors are useful for driving line shafts to which a number of machines, are belted. These are also useful for driving pumps, lathes, drills, printing presses, etc.
Series Wound: In this type of motor, the field has field 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 the speed for increasing the load, so when there is no load on this motor the speed is very high. Hence, this type of motor is never used without a load. Use This type of motor is useful for trams, cranes, trains, pumps, trolley cars, etc. due to its very high starting torque at starting and constant speed.
Compound Wound Motor
i. Cumulative compound
ii. Differential Compound
Cumulative compound: Cumulative motor is one in which the 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 decreases in the same way as in the series motor. When the load is released suddenly, the shunt field prevents the motor from speeding. up beyond safe value. Hence, such motors are used where series characteristics are required, and also the load is likely to be removed totally such as in coal cutting machines and in machine tools that take sudden deep cuts.
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 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 the shunt field. This will cause the motor to start in the wrong direction at high speed.
Uses. The motors are seldom used since the shunt motor usually satisfies the requirements of constant speed.
