Need of DC motor starter and three-point starter

A starter is a device that puts a resistance in series with the armature of a DC motor while starting, and cuts it out step by step as the motor speeds up. It also provides protection against no-voltage and overload.

Necessity of a starter

In a DC motor, $I_a = \dfrac{V - E_b}{R_a}$. At the instant of starting, speed is zero, so back emf $E_b = 0$:

$$I_{a,start} = \frac{V}{R_a}$$

$R_a$ is very small, so this current is huge. For example, a 220 V motor with $R_a = 0.5\ \Omega$ would draw $220/0.5 = 440$ A, perhaps 15-20 times full-load current. This would:

• burn the armature winding and damage insulation,
• cause heavy sparking at the commutator and brushes,
• give a sudden large torque that can damage the shaft and load,
• cause a big voltage dip on the supply.

So an external resistance $R$ is added at start: $I_{start} = V/(R_a + R)$, and it is cut out as $E_b$ builds up. Small motors (below about 1 hp) can be started directly because their $R_a$ is higher and inertia low.

Three-point starter

It has three terminals: L (line), F (field) and A (armature). It is used for shunt and compound motors.

 L (+) o----[OLR coil]----o Handle pivot
                           \
           studs: 1  2  3  4  5  ... ON
           o--R--o--R--o--R--o--R--o
           |     (starting resistance)
           |                       |
       to A (armature)        [NVC]--- to F (field)
                                 (holding magnet)

 Supply -ve ------ motor armature and field return

Parts

• Starting resistance divided into sections between studs.
• Handle with a soft-iron piece, held in OFF position by a spring.
• No-volt coil (NVC) / hold-on coil: in series with the shunt field.
• Overload release (OLR) coil: in series with the line, with an armature that shorts the NVC when it is lifted.

Working

1. The handle is moved from OFF to stud 1. The field gets full supply voltage through the NVC, so full flux is produced. All starting resistance is in series with the armature, so starting current is limited.
2. As the motor speeds up, $E_b$ grows and current falls; the handle is moved stud by stud, cutting out resistance.
3. At the last (ON) stud all resistance is cut out of the armature circuit. The NVC, energised by field current, holds the handle against the spring.

Protections

• No-voltage protection: if supply fails or the field circuit opens, the NVC loses its magnetism and the spring returns the handle to OFF. So the motor cannot restart at full voltage without the starter when supply returns. It also prevents dangerous over-speeding if the field opens.
• Overload protection: if current exceeds a set value, the OLR lifts its armature and short-circuits the NVC. The handle is released to OFF and the motor is disconnected.

Drawback: for speed control by field resistance, a large field resistance weakens the NVC current, and the handle may release unwantedly. This is overcome by the four-point starter, where the NVC is connected directly across the supply.