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Starters

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Rural Technology

DC MOTOR

DC motors & DC generators are essentially the same. In a motor electrical power forces the armature to turn and the moving armature. A DC motor converts electrical energy into mechanical energy.

fig. 4 D C Motor Do you know on what principle motor works? when a current flows through the coils a coil itself acts as a magnet & the coil is moved by the force between the magnetic field. This is the principle of operation of DC motors.

Classification of DC motors -

There are three main types of motors which are as follows :
1. Series Motor
2. Shunt Motor
3. Compound Motor

1) Series motor -

The motor in which the field winding is connected in series with the armature, is called a series motor. The field winding consists of a few turn Sothic enameled copper wire as shown in figure - fig. 5 Series Motor A series motor should not be run without a load is necessary for controlling its speed or otherwise it will run at a very high speed. Therefore it is used for such jobs in which a load is always present at the meter.

2) Shunt motor -

The motor in which the field winding is connected across the armature is called a shunt motor. The field winding consists of a large number of turns of fine enameled copper wire to provide a high resistance as shown in figure - Fig 6 Shunt Motor The speed of a shunt motor remains almost constant & is not effected by the load variations. The motor cant be started with a heavy load. It is suitable for stable & light load.

3) Compound motor -

The motor in which one part of field winding is connected in series & the other part is connected across the armature is called a compound motor.

STARTERS

Necessity of Starter -

In the starting of a motor the back e.m.f. is zero. It rises gradually with a rise in speed. In the absence of the back e.m.f. the motor will draw an excessive current which can burn the motor windings. Therefore it becomes necessary to control the excess flow of current. For this purpose a variable resistor is connected in series with the field. As the motor speeds up the external resistance is gradually cut out of the circuit. At full speed the whole of the variable resistor is cut out of the circuit and the induced back e.m.f. controls the armature & field currents. The current controlling device is termed as a hand starter.

3 Point starter -

A three point starter is used with shunt motors. It consists of variable resistance no-volt & over load release coil as shown in figure - fig. 7- point starter These two coils are known as protective devices.

A] No- volt coil -

It consists of an electromagnet which is magnetized by the field winding current. The handle of the starter has a piece of soft iron fitted in it. The spring of the handle keeps it in OFF opposition. When the handle is brought to ON position then the electromagnet attracts the handle & holds it firmly against the tension of spring. In the event of supply failure the electromagnet is demagnetized & the handle returns back to OFF position under the action of spring. In this way the whole arrangement protects the motor against any damage which may be caused due to supply failure & the re-establishment of supply.

B] Overload coil -

It consists of an electromagnet which is connected to the line & the handle. Its coil consists of a few turns of thick copper wire. There is a metallic leaf near the electromagnet. It can short circuit current the two N.V.C. terminals close to it. If the load & thus the supply current exceeds release coil attracts the metallic leaf which short circuits the no-volt coil & thus disconnects the motor from the supply. In neither case can the motor be re-started without the starting resistance being in the armature circuit & therefore any damage to the motor windings is prevented.


Function of starter -

As the starter handle is moved in a clock wise direction it comes into contact with first stud. At this stage the full starting resistance is connected in series with the circuit, which reduces the armature current. The resistance in the armature circuit is gradually cut out as the motor speeds up, & the back e.m.f. replaces the starting resistance. When the starter handle is at ‘ON’ the starting resistance is completely cutout of the circuit & by this time the motor achieves its full speed. Now, the starting resistance is no more required. At ‘ON Position’, the starter handle is attracted by the no volt coil magnet against the spring, & is firmly held by it. If the supply has failed or the motor is over loaded then the no-volt coil is demagnetized & the starter handle is released to reach at ‘off’ position, under the action of spring. The motor can be re-started, If the supply has re-stored or the overloading has finished.

D] Care-

The starter handle should be moved gently in the clockwise direction from ‘off’ to ‘ON’ positions. The armature winding can burn if the starter handle is rotated quickly. 2] In Text Question- a) Fill in the blanks- 1. D.C motor converts electrical energy into _______ . 2. D.C motor classified in ______, ________ , _______ motor. 3. A Series motor should not be run without ______. 4. Shunt motors speed remains _______. 5. When we start the motor back e.m.f. is ________. b) True or False. 1. The motor in which field winding is connected across the armature is called series motor. 2. The motor in which the field winging is connected in series with the armature, is called Compound motor. 3. If there is absence of back e.m.f. the motor winding can burn. 4. Overload coil consist of an electromagnet which is connected to the line & handle. 5. Starter handle moved in Clock-wise direction.

Terminal Questions -

1. Why the D.C. Motor & D.C. generator initially same? 2. State the principle of operation of generator? 3. Write a short note on D.C. Motor. 4. Classify the D.C. Motor or state the types of D.C. motor. 5. What is a starter & now it works? 4. 6. What care should be done while handling a hand starter?

ANSWERS-

1] In text Questions - a) Fill in the blanks-
1. Magnetic field
2. Induced Voltage
3. Induction
4. Slip rings
5. Self excited generators b) True or False.
1. False 2. True 3. False 2] In text Questions - a) Fill in the blanks-
1. Mechanical energy
2. Series motor, shunt motor, compound motor
3. Load
4. Constant b) True or False -
1. False 2. False 3. True 4. True 5. True

What you have learnt -

In this lesson we seen principle of operation of generator self-excited generators what is a D.C. motor’s principle, what is a starter how it works, what care should be given while handling the starter.

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Rural Technology