4 Point Starter is almost identical in performance features to the 3 Point Starter. The 4 Point Starter limits the starting current in the absence of back EMF(Eb). Also, Four Point Starter functions as a protective device.
Working Principle of 4 Point Starter
A 4 point starter limits the starting current & protects the armature of a DC motor. The DC motor takes a high starting current because the back EMF is absent at the start.
The 4 point starter is very much similar to 3 point starter. The construction and functional similarity of both starters are almost the same.
Difference between 4 point & 3 Point Starter
There is some difference in the 3 points & 4 point starter, although the operational characteristics of both the starters are the same. A 4 point starter does not have the holding coil in its shunt field circuit. The holding coil has a connection across the line with a current limiting resistor.
Now let us discuss in detail the operation of the Four-point starter.
Construction and Operation of Four Point Starter
- ‘L’ line point connected to positive supply
- ‘A’ Armature terminal point
- Field winding connected to NVC(‘ F’ point)
- Resistance(R) connected to NVC ( ‘N” point)
Where – NVC is the no-volt coil
The starter has the contact points – studs and is shown separately as OFF, 1, 2, 3, 4, 5, RUN, over which we can regulate the starting current of the motor by operating the handle manually.
Initially, the starter handle is at the off position. In this condition, the armature of the motor does not get supply. Now, when we put the handle at stud ‘1’, the armature of the motor connects in the series with R1, R2, R3 & R4. Thus total resistance of the armature at stud position 1 is;
R(Armature circuit Resistance) = (R1 + R2 +R3 + R4 ) + Ra
Thus, the total armature resistance is maximum when we start the motor. Why is maximum resistance in series with armature resistance needed at starting of the motor? Obviously, there is no back EMF at starting & series resistance only can limit the starting current.
Now, as the motor accelerates, the series resistance decreases, and at stud position ‘5’ motor runs at its rated speed.
R(Armature circuit Resistance) = Ra
How the handle remains stuck at the position’5′?
The no-volt coil gets supply and it gets energized. During energizing period, the NVC attracts the soft magnetic piece attached to the handle. It will keep the handle at the last position till the supply is available and the motor current is within its rated current.
The motor armature must be cut off from the supply source when power is cut off. If not cut from the supply source, the motor will start by taking an enormously high current when supply restores.
How the 4 point starter protects the motor under overload?
When the motor takes more current than its rated current, the overcurrent relay operates. The contact of the overload relay shorts the supply across the no-volt coil. And, the no-volt coil gets demagnetized. No volt coil can not attract the magnetic piece mounted on the handle no more. The handle comes to at it OFF position. This way, the starter protects the motor armature from burning.