A three-point starter is used for starting a dc shunt or compound motor. The coil of the hold-on electromagnet is connected in series with the shunt-field coil. Figure shows the schematic diagram of a 3-point starter with protective devices connected to a D.C. shunt motor.
Operation :
When supply is switched on, the handle should be moved clock-wise to the first stud. As soon as it comes in contact with first stud the field winding is directly connected across supply and is not disturbed at all, where as starting resistance Rs is inserted in series with armature. Due to high starting resistance Rs, the current drawn from the source will be decreased.
As the handle is gradually moved to the final stud, the starting resistance is cut out of the armature circuit in steps. When the handle comes in contact with the final stud, the entire starting resistance is cut out of the circuit. Thus motor will be protected from high rush in current at the time of starting.
When supply is switched on, the handle should be moved clock-wise to the first stud. As soon as it comes in contact with first stud the field winding is directly connected across supply and is not disturbed at all, where as starting resistance Rs is inserted in series with armature. Due to high starting resistance Rs, the current drawn from the source will be decreased.
As the handle is gradually moved to the final stud, the starting resistance is cut out of the armature circuit in steps. When the handle comes in contact with the final stud, the entire starting resistance is cut out of the circuit. Thus motor will be protected from high rush in current at the time of starting.
Working of No volt Coil ( NVC ) :
In the case of disconnection in the field circuit due to internal failure or field-rheostat failure, the control arm will return to its OFF position by spring tension. This is necessary because the shunt motor will over speed in the same manner as the series motor if it loses its field excitation. The starter will be switched off in case of low supply voltage or complete failure of supply voltage. This protection is therefore called no volt coil ( NVC ).
Working of Overload Coil ( OLC ) :
Overload protection for the motor can be incorporated by connecting another electromagnetic coil in series with the armature. It protects the motor when it is overloaded has it draws a heavy current, this currents will also flows through the electromagnetic coil. The electromagnet then pulls an iron piece upwards which short-circuits the coils of the hold-on electromagnet ( NVR coil, see figure ). The hold-on electromagnet gets de-energised and therefore the starter arm returns to the OFF position, thus protecting the motor against overload. The complete circuit connection for a three-point starter with NVC and overload coil ( OLC ) is shown.
It is to be noted that points L, A and Z are the three terminals of a three-point starter. Use of a brass arc enables connection of the field circuit directly with the supply instead of via the starter resistance.
Drawback of Three-point Starter :
A three-point starter may not be suitable where a large field current adjustment by using a field regulator is needed. This may cause weakening in field current to an extent that hold-on electromagnet may not be able to keep the starter arm in the ON position and the motor is disconnect from the supply. This problem can overcome by using a four-point starter.
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