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Necessity of Starter

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Rural Technology


Necessity of Starter -

If an AC motor is stated an full voltage it will draw from two to six times its normal running current. Because the motor is constructed to withstand the shock of starting no harm will be caused by this excessive flow of current. However vary large motors it is generally desirable to take some measure to reduce the starting current otherwise damage may be done to the machinery driven by the motor & line disturbances may be created that affect the operation of other motors on the same line.

Direct On Line Starter (D. O. L) -

It is a simple and ordinary starter suitable for squirrel cage induction motors having a capacity up to 5 hp. Push buttons are used it in place of a handle. The green push button starts the motor while the red push button switches off the motor. A latch fitted near the red push button can be adjusted over it so that the motor may not be started. This provision is used only when the motor is defective. Mainly it consists of contact system thermal relay unit & a solenoid coil. The contact system consists of silver tipped contacts which provide a quick ON-OFF system. The bi-metallic elements of the thermal relay protect the motor from overload. The solenoid coil is an electromagnet which works as an operating level for the contact system.

Fig. 8 D. O. L. Starter Fig shows the pictorial view of the D. O. L. starter & its main components. The solenoid coil or no-volt coil is connected across the two phases (it can be connected between a phase & neutral if it is designed to work at 230V) The overload release or thermal relay unit is connected in series with the outgoing ends of each phase from the starter. On pressing the ‘START’ button (green push button), the solenoid coil becomes magnetized & it attracts the iron plunger which closes all the contact points. In this way the supply terminals L1, L2 & L3 are connected to the motor terminals M1, M2 & M3 & the motor operates by completing its circuit through the thermal relay coils. To stop the motor, ‘STOP’ button (red push button) is pressed. It breaks the solenoid coil circuit & finishes its magnetism. As a result, the plunger is released & the contact points become open. The thermal relay protects the motor by breaking the solenoid coil circuit through thermal relay switch which is placed in series with the operating coil.

Fig. D. O. L. Starter for ¼ hp Motor As the motor becomes overload draws more current & the bi-metallic strips of the thermal relay unit brings the thermal relay switch to the ‘OFF’ position. The size of a starter depends on the hp capacity of the motor In Text Question – (b) Fill in the blanks
1. The induction motor, rotor is made of _________.
2. Stationary part of motor is called ___________.
3. The rotating part of motor is called __________.
4. For refrigerator _________ motor is used.
5. For food mixer __________ motor is used.

True or False

1. Split phase motor runs on running & starting winding.
2. Capacitor is connected in series with running winding.
3. Armature is a part of universal motor.
4. Shaded pole motor has squirrel cage rotor.
5. A thermal relay is used in D. O. L. starter for safety of motor.

Terminal Questions -

1. What is AC Motor?
2. What is an AC current?
3. What are the types of rotor used in AC induction motor?
4. State the advantages of AC current?
5. On what principle AC Motor operates?
6. Explain the operation of alternator with figure.
7. Desire the operation of capacitor?
8. What is a stator?
9. Explain the uses of universal induction motor?
10. What is the necessity of starter?

Answers of In text Questions -

Fill in the blanks (a)
1. Electromagnet Induction
2. Silent type rotor & Cylindrical type rotor
3. 250V
4. Rotor
5. AC Current

Fill in the blanks (b)
1. High Grade Steel Laminated
2. Stator
3. Rotor
4. Capacitor Motor
5. Universal Motor True or False 1. False 2. False 3. True 4. True 5. True

What you have learnt -

In this lesson we learn A. C. Generators & A. C. Motors. Principles of a motor, operations of A. C. Motor, types of A. C. Motors, necessity of starter, 3-phase motor, D. O. L. starter.

Suggested Activities -

Practical -1


Connect, Start, Run & Reverse a capacitor motor.

Required Material-

Tools -

Insulated Plier Capacitor start motor any hp Supply connection Electrician Screwdriver

Material -

Test leads with series test board

Procedure -

  1. Fix & arrange a suitable supply for the capacitor motor.
  2. Open the terminal block of motor.
  3. Connect the motor as per the circuit diagram.
  4. Switch ON the supply & see motor running.
  5. Switch OFF the supply.
  6. Reverse the wire of running winding
  7. Switch ON the supply & look out rotation of the motor.

Practical - 2


Disassemble & Assemble Split ( any motor) phase motor. Required Materials -


Electrician plier. Electricians Screw driver. Spanner set.

Procedure -

1. Open the terminal block. 2. Disconnect the wire from terminals. 3. Remove nuts. 4. Remove end plates. 5. Placed the end plate again properly. 6. Tighten the nuts. 7. Connect the winding leads to the terminals as shown in figure. 8. Connect the motor to supply. 9. Switch on the supply.

Observation -

When it is connected to supply & after switch on, the motor runs.

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Rural Technology