Need of starter and four-point starter

Need for a starter

In a DC motor $I_a = \dfrac{V - E_b}{R_a}$. At start the speed is zero, so $E_b = 0$ and $I_a = V/R_a$. Since $R_a$ is very small (e.g. 0.5 Ω), a 220 V motor would draw about 440 A, many times rated current. This would burn the winding, cause heavy sparking at the commutator, give a damaging torque jerk and a dip in supply voltage. A starter puts a resistance in series with the armature at start and cuts it out step by step as $E_b$ builds up. It also gives no-voltage and overload protection.

Four-point starter

It has four terminals: L (line), A (armature), F (field) and N (no-volt coil return to supply). The key change from a three-point starter is that the no-volt coil (NVC) is connected directly across the supply through a protective resistance, not in series with the field.

 L o--[OLR]--o pivot/handle
              \
   studs 1 2 3 4 5 ... ON
   o-R-o-R-o-R-o-R-o
   |                      F o---- shunt field
   A o---- armature         (brass arc from stud 1)
                          
   [NVC]--[R protective]--o N ---- supply (-)

Working

1. The handle is moved from OFF to stud 1. Current divides into three parallel paths: (a) armature through the full starting resistance, (b) shunt field through the brass arc, getting full voltage, and (c) the NVC with its protective resistance, across the supply.
2. As speed rises, $E_b$ increases; the handle is moved stud by stud, cutting out the starting resistance.
3. At the ON position the armature is connected straight to the supply and the NVC holds the handle against its spring.

Protections

• No-voltage release: if supply fails, the NVC is de-energised and the spring returns the handle to OFF.
• Overload release (OLR): on excess current the OLR attracts its armature and shorts the NVC, releasing the handle.

Why four-point

In a three-point starter the NVC carries field current. When a large field regulator resistance is used to raise speed, field current falls, NVC becomes weak and the handle may release while the motor is running. In a four-point starter the NVC current does not depend on field current, so field control over a wide range is possible without false tripping.

Drawback: if the field circuit opens, the NVC still holds the handle, so there is no protection against over-speeding due to field failure.