Download - 3 Point or 4 Point Starter
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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