When a dc motor is at rest , its speed N is zero and
therefore back emf (Eb =P
Ï•ZN/60A)
is zero. Now, if the motor is directly switched to mains then as per motor
equation , V=Eb+IaRa , armature current , Ia
= V/ Ra
, will be very high (as high as 20 times!) and it can burn the motor
armature . Therefore , resistances must be
inserted in series with armature to limit this current. 3-point starter is one such device which
serves this purpose.
A three point starter can be used for dc shunt and dc
compound motors only where the field is in parallel with the armature .
Three Point Starter Construction and Working
It consists of starting resistance divided into
several sections and connected in series with the armature. The tapping points
of starting resistance are brought out to a number of studs.
The motor field winding is connected in series with
the no-volt release coil and this series connection is then connected to power
supply terminal and first stud terminal.
Fixed spring attached end of the starting resistance
is connected to supply through overload release coil. The other end moves
against the pull of the spring and makes connections with studs during starting
operation of the motor cutting more and more resistance as it passes over each
stud in clockwise direction .
Operation of a 3-point starter
- To start , keep starting handle in OFF position and switch on the power supply.
- The handle is now moved clockwise to the first stud. The shunt field winding is now directly connected across the supply while the whole starting resistance is inserted in series with the armature circuit.
- As the motor gains speed , the handle is gradually moved over to the final stud , the starting resistance is cut off in steps. The handle is now held magnetically by the no-volt release coil which is energised by the field current.
- Now if supply is interuppted or field excitation is accidentally cut , the no-volt coil is demagnetised and the handle moves back to OFF position under the pull of the spring. If the spring was not used , then the handle would have left on the ON position and on restoration of supply the motor would have directly connected to the supply , thus damaging the motor.
- If the motor is over loaded, it will draw excessive current from the supply . This large current is sufficient to magnetically pull the magnetic rod which will short the no-volt release coil . The no-volt coil is demagnetised and starting handle is pulled to OFF position under the pull of the spring . Thus , the motor is automatically disconnected on overloading.
3-Point
Starter drawbacks :-
- When dc motor speed control is carried out by field weakening method , resistance is inserted in field circuit. Due to this , field current becomes so small that mmf produced by no-volt release coil is not sufficient to hold the handle magnetically against the pull of the spring. Thus , handle goes back to off position even during this normal operation and motor is disconnected from the supply. This is undesirable and therefore 4-point starter is used to overcome this.
- Use of 3 point starter is limited to dc shunt and dc compound motors only.
