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Contents

Introduction

Three Point Starter

Four Point Starter

Conclusion

Reference

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NECESSITY OF DC MOTOR STARTER

INTRODUCTION

At the time of starting at the DC motor

N = 0 & Eb = 0.

We know that Eb = V IaRa

O = V IaRa

Fa = V / Ra

Suppose

V = 230 V

Ra = 1

Therefore, Ia = 220 / 1 = 220A

Hence at starting the D.C. motor armature will draw very high current &

this high current will damage the brushes, commutator, armature etc.

In order to reduce high value of current, starters are necessary.

3 POINT STARTER

This type of starter is widely used for starting shunt and compound

motors.

OPERATION

To start with the D.C. supply is switched on with handle in the Off position

The handle is now moved clockwise to the 1st stud. As soon as it comes in

contact with the 1st stud the shunt field way is directly connected across

the supply, while the whole starting resistance is inserted I series with

armature ckj

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As the handle gradually moved over to the final stud, the starting

resistance is cut out of the armature CKTS in steps. The handle is now

held magnetically by the no volt release coil which is energized by shunt

field current.

In the supply voltage is suddenly interrupted or of the field excitation is

accidentally cut, the no volt release coil is demagnetized and the handle

goes back to the Off position under the pull of spring. If no volt release coil

where not used.

If motor is overloaded, it will draw excessive current from the supply this

current will increase the ampere turns of the overload release coil and

pull the armature, this short circuiting the no-volt release coil. The no volt

coil is demagnetized and handle is pulled to the Off position by the spring.

Thus the motor is automatically disconnected from the supply.

DRAW BACK

In a 3 Point starter, the no volt release coil is connected in series with

the shunt field CKT so that it carried the shunt field current.

4 POINT STARTEROperation

In a 4-point starter, the no-volt release coil is connected directly the supply

line through a protective resistance R.

Now the no-volt release coil CKT is independent of the shunt field CKT.

Therefore, proper speed control can be exercised without affecting the

operation of no volt release coil

Its working is same as like 3 Point starter.

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Conclusion

We studied that the 4 Point starter construction is same as 3 Point Starter

expect the no volt is connected in series with rheostat.

Reference:

Book by B.L. Thereja

Book by V.K. Mehta