Working of 3-point DC motor starter

A 3-point starter is a device that limits the starting current of a DC shunt (or compound) motor by connecting a resistance in series with the armature at starting, and cutting it out in steps as the motor speeds up. It also gives no-volt and overload protection.

Reason it is needed: at standstill $E_b = 0$, so $I_a = V/R_a$ is very large.

Parts of a 3-point starter: three terminals L (line), A (armature) and F (field); a starting resistance divided into sections with studs; a spring-loaded handle (starting arm) with a brass arc; a no-volt release coil (NVC), an overload release coil (OLR) and a soft-iron piece on the handle.

   L (+)
    |      OLR (overload coil)
    +------[~~~]---+
                   |   handle (spring loaded)
   OFF  1   2   3  4  ON   o----- soft iron
        |   |   |  |   |     piece, held by NVC
        +[R]+[R]+[R]+---+---------> A (armature)
                        |
             +--- brass arc ---[NVC]----[ Field ]--> F
   L (-) ---------------------------- supply -

Working:

1. With the handle at OFF, the circuit is open. The handle is moved slowly to stud 1. The full starting resistance is now in series with the armature, and the shunt field gets full supply voltage through the brass arc and NVC. This gives full flux, so the motor starts with high torque and low current.
2. As the motor speeds up, $E_b$ rises. The handle is moved stud by stud, cutting out resistance sections so the armature current stays within limits.
3. At the last stud (ON), the whole resistance is cut out and the motor is directly across the supply. The soft-iron piece on the handle touches the NVC electromagnet, which holds the handle there against the spring.
4. No-volt protection: if the supply fails, the NVC loses its magnetism, the spring returns the handle to OFF, and the motor does not restart with the resistance out when the supply returns.
5. Overload protection: if the armature current exceeds a set value, the OLR electromagnet lifts its armature and short-circuits the NVC. The NVC releases the handle, and the spring returns it to OFF.

Drawback: the NVC is in series with the field. If the field current is reduced for speed control, the NVC may be too weak to hold the handle. A 4-point starter places the NVC in a separate branch across the supply to avoid this.