In this article we will discuss about the four point starter. The term Starter is usually used for any device which helps in initiating any action of some other devices under its own power. Starters can be of different types, i.e. anything which initiates like motor starter, which is used for driving or initiating the operation of the motor under its own power. Now we are going to discuss about the very useful and popularly used ‘Motor starter’. When the motor is at rest, we all know that the back e.m.f is zero and now if we provide full supply voltage across the armature of the motor, then it will draw an excessive large amount of current because of very low armature resistance. This excessive current will blow out the fuses and prior to that it may damage the respective winding, commutator and brushes etc.

So to avoid this type of incident and to check the excessive flow of current we provide a Starter which is actually a resistance, connected in series with the armature to limit the starting current to a certain safer range. Now at the same time we get to see that the starter allows a high amount of current to produce a good starting torque. The starting resistance is now gradually decreased and cut out as the motor picks up speed and thus develops the back emf. which is now used for the speed regulation. Now in this context, we are going to deal with a special type of motor starter i.e. 4 Point Starter, which is similar device which helps in initiating the rotation of other device i.e. Compound wound and Shunt wound D.C. motor, under its own power. This type of D.C. motor requires a starter initially for its excitation or starting without which they cannot gain the starting torque.

Contents [hide]

• 1 Constructional Details of 4-point starter
• 2 Working principle of 4-point starter
• 3 What is the main disadvantage of 4-point starter ?

Constructional Details of 4-point starter

A 4 point starter is mainly the no-volt release coil is connected directly across the supply line through a protective resistance R to limit the starting current to a certain safer range. The resistance is subdivided in many variable resistances and this section of resistance contains the contact points called Studs, which are calibrated seperately from OFF, 1, 2, 3, 4, 5 …. ,RUN. The main reason behind the name of 4 point starter is the 4 main points of the starter which are as follows:

•   Line Terminal ‘L’, which is to be connected to the positive of supply.

•   Field Terminal ‘F’, which is to be connected to the field winding.

•   Armature Terminal ‘A’, which is to be connected to the armature winding.

•   A 4th point N which is to be connected to the No Voltage Coil (NVC).

Now we are going to discuss about the constructional details of this 4 point starter, how this each and every parts are interconnected and also the sub-elements or subparts used in the construction of this 4 point starter. With respect to the figure, the only difference between a three point starter and a four-point starter is the manner in which no-volt release coil is connected. However, we can tell that the working of the two starters is the same. The basic difference between three point and four pint stater is the connection of NVC. In 3 point starter, NVC is in series with the field winding while in four point starter NVC is connected independently across the supply through the fourth terminal called ‘N’ in addition to the ‘L’, ‘F’ and ‘A’.
Therefore we can say any change in the field current does not affect the performance of the NVC. Thus it is ensured that NVC always produce a force which is enough to hold the handle in ‘RUN’ position, against force of the spring, under all the operating conditions. Such a current is adjusted through NVC with the help of fixed resistance R connected in series with the NVC using fourth point ‘N’.

Working principle of 4-point starter

As we provide the d.c. supply to the motor, to make it switch on, the handle starts moving against the spring force to make a contact with stud No. 1, which was initially at OFF position. Thus field winding gets supply through the parallel path provided to starting resistance with the help of NVC. This is how we see the entire starting resistance comes in series with the armature and armature current which is high initially, gets checked to certain limit. Similar way the handle starts moving towards the other studs 2, 3, 4 etc., thus reducing the starting resistance gradually from the armature circuit and reaches the RUN position making the starting resistance R equal to zero and the motor keeps on rotating at its normal speed. Thus the working of the 4 point starter is just same as the 3 point starter. But we can say the main difference between a 3-point starter and a 4-point starter is the manner in which no-volt release coil is connected. The three-point starter also provides protection against an open-field circuit which is not at all provided by the four-pointstarter.

What is the main disadvantage of 4-point starter ?

We have seen the only limitation of the four point starter is that it does not provide high speed protection to the motor. Under running condition, field gets opened then the field current reduces to zero. But there is some residual flux remaining and N ∝ 1/Φ the motor tries to run with dangerously high speed. This is called high speeding action of the motor. Thus we see in four point starter NVC is connected directly across the supply and its current is maintained irrespective of the current through the field winding. So it will always maintain the handle in the run position, as long as supply is provided. And thus it does not protect the motor from field failure conditions which result into the high speeding of the motor. The most important difference between a three-point starter and a four-point starter is the manner in which no-volt release coil is connected. The three-point starter also provides protection against an open-field circuit which is not at all provided by the four point starter.