D.C motor

D.C motor is state that the machine whose convert electrical energy in to mechanical energy that's called D.C motor .

There is no difference between the construction of D.C. motor and DC. generator. Motor is that machine which converts electrical energy to mechanical energy. The motor works on the principle that whenever a current carrying conductor is placed in a magnetic field, conductor tends to move.

To understand Principle of Motor. Consider a conductor which carries o current and is placed in a magnetic field 

 (a). Now pass the current. 

 (b) shows the 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.

(c) 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 and opposes the main field below the conductor. So, the concentrated flux above the conductor pushes 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 . The sides AB and DC which carry currents in the direction A to B and D to C produce flux of their own .

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

The d.c. armature can now be considered to have a number of such coils . The armature has 4 coils here. The resultant torque produced tries to rotate the armature in anticlockwise direction.

FLEMING'S LEFT HAND RULE : 

in this rule you will find direction of rotation armature in the d.c motor .in this rule soared the forefinger , middle finger and thumb of right hand .each finger should be at 90 degree to each other. Firefighter indicate the field detraction , middle finger indicate direction of current and thumb is indicate direction of rotation of an armeture 

TYPES OF D.C MOTOR: 

There are three types of D.C motor 

1. Shunt wound 

2. Series wound

3. Compound wound

1.Shunt Wound : 

Shunt wound motor

In this motor armeture are connected in parallel with field winding . The field winding is of high value resistance and more turns. It runs practically at constant speed at almost all load. Its starting torque is 1.5 to 2 times greater than the full load torque. The variations of speed of the motor can be well achieved by a shunt regulator.

Shunt wound motor 

Uses. These types of motors are useful for driving line shaft to which a number of machines are belted. These are also useful for driving pumps, lathe,drill, printing press, 

2. Series Wound :

Series wound motor

 In this  motor  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 when there is no load on this motor, the speed is very high. Hence, this type of motor is never used without load.

3. Compound Wound Motor :

 It has two windings Series and Shunt

are used. According to their field connection, these are of two types: 

(i) Cumulative compound

 (ii) Differential compound.

(i) Cumulative compound :

 Cumulative motor  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 decreases in the same way as in 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 which take sudden deep cuts.

(ii) Differential compound:

 Motor is one in which series winding is so connected that the field due to it opposes that due to shunt winding .

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 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 e.m.f. 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 .

STATOR : At starting, the speed of motor is zero so that the back e.m.f.in the armature is zero and since the armature resistance is low; so, if the armature is connected directly to the main supply, the starting current will be very high and will be given by I= V/R. So, to save this excess value, a rheostat is necessarily inserted in series with the armature.

THREE POINT STARTER : 

Y

Three point stator

This stator used in very small motor  directly to the mains to limit the current with slightly bigger size, starter is used .

To start the motor, handle is moved from off position to the first contact. The total resistance is in the armature circuit and the field is connected directly to the supply line through no volt coil . As motor takes speed, the armature develops back e.m.f. and the current falls. The handle is moved over steps, the back e.m.f. rises and current fall in each step. Finally, at the last step, the armature is connected directly to the supply. In some motors field is connected directly to the supply, but through no volt coil .while in some the total starter resistance is included in the field circuit. Now, its effect is to reduce the excitation and giving some more speed. At the last contact the handle is kept in position by the electro-magnet of no volt coil against the action of the coil spring which pulls the handle towards the off position.

Four-point starter : 

The internal connections of four-point starter .

If the speed of a shunt motor, connected through 3-point starter is increased, the field strength of motor field and of no-volt coil is decrease (Because both are in series). Hence the no-volt coil becomes so weak that it cannot hold the handle in position and the motor stops. So, we are to use 4-point starter.

In four-point starter the no-volt coil is connected in series with a resistance direct on line but in the three-point starter, the no-volt coil is connected in Series with shunt winding.

So, the four-point starter is quite independent of the current of the shunt winding. Other parts of the starter are same as of three-point starter. This starter is used where speed variations in motor are required.

Series motor starter with no-load release : 

This type of starter is used with such series motors, the load of which is to be totally removed at a time. At this time the starter cuts off the motor from supply.

Series motor starter with no-volt release : 

Such starters are used on such motors which are directly coupled with load .such stators cot off the motor when the supply is off

SOME IMPORTANT QUESTIONS ANSWER : 

1. How can e.m.f. be produced in a coil of wire ? On what factors does its value and direction depend?

Ans= By rotating the coil in magnetic field. Its value depends upon velocity and number of conductors. Its direction depends upon direction of rotation and direction of field.

2. On what principle a D.C. generator works?

Ans= On the principle of Faraday's Law of Electro-magnetic Induction.

3. What are the main parts of a generator ?

Ans=Armature, field, end covers, Bearings, Brush Holder, Brushes.

 4. Is there any difference between D.C. motor and generator by construction?

Ans= no

5. Why the armature is laminated?

Ans= to reduced Eddy current loss

6. Can we use a wooden armature ? If not, why?

Ans= No, because wood cannot withstand high rotation and the reluctance will be more.

7. What is the work of commutator?

Ans= To change A.C., produced in armature, to D.C. and conducts current to or from the external circuit.

8. What type of windings are done on the D.C. armature? 

Ans= Lap or wave (Simplex or multiplex).

9. What are the types of D.C. generator with respect to their field excitation?

Ans= Two types: separately excited and self-excited.

10. What are the types of generators?

Ans= Series, shunt and compound.

11. What are the further types of compound generators ? 

Ans= Short shunt cumulative compound, short shunt differential compound, long shunt cumulative compound, long shunt differential compound

12. Where are the series generators used?

Ans= For lighting arc and used as a booster