3- Point Starter For DC Shunt Motor
Working of 3 Point starter:
The starter resistance R, is divided in different steps and are connected to brass-studs. When the arm 'A' is moved by the handle to stud number 1. then R. is connected to armature circuit.
The field and the armature is connected to the supply and the motor starts with the safe current. The arm is then moved to the next studs successively and the starter resistance (Rs) is gradually cut-out and finally the arm is moved on the last stud.
The soft iron piece (S.I.P.) of the arm is attracted by the N.V.C. electro-magnet. The N.V.C. electro-magnet gets the supply voltage and energized.
The attractive force of N.V.C. is more than the spring tension of the arm. Now if the supply fails, then the N.V.C. gets de magnetized and the handle is thrown back to off position due to the action of the spring.
Now if the supply resumes then the operator has to move the arm and motor starts with R in the circuit.
If N.V.C. and spring were not thereto then there would be a chance that motor may start directly when the supply resumes. Thus, spring and N.V.C. protects the motor from direct starting.
R = Resistance,
OLC = Overload coil,
FR = Field regulator
S = Spring,
NVC = No volt coil,
There is a second electro-magnet O.L.C. This is connected in the motor circuit and carries the motor current.T
The full current of the motor is say = 10 Amp. Then this will also pass through the coil of this electromagnet (O.L.C.) Due to this current, the OL.C.
will try to attract the strip beneath it. But the strip is kept at the certain distance so that due to this force it is not attracted. But, in case of overloading (i.e. motor takes current more than, normal full load current say 20 Amp).
The overload current also passes through the O.L.C. and this creates more force of attraction. This force is now sufficient to attract the strip upwards.
When the strip is attracted, then the N.V.C. is short circuited and N.V.C. is de magnetized.
Due to the action of the spring, the arm is thrown back to off position and hence motor stops. It is protected from overloading, O.L.C. comes in play under overload condition only and not in normal condition.
Thus, starter is the important device, which limits the starting current to the safe value. In addition, it protects the motor from direct starting and protects from overloading.
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| 3-Point D.C. shunt motor starter |
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