Three terminal DC motor starter

A DC shunt motor has a very small armature resistance, so at standstill (E_b = 0) the current V/R_a would be many times the rated current. A starter adds resistance in series with the armature during starting and removes it step by step as the motor gains speed. The 3-point starter has three terminals: L (line), A (armature), F (field).

Construction

 Supply (+)
     |
  L  o---[OLR coil]---o pivot
                       \  handle (soft-iron piece; spring pulls it back to OFF)
                        \
      OFF   (1)  (2)  (3)  (4)  (5) <- studs
             |    |    |    |    |
             +-R1-+-R2-+-R3-+-R4-+----------o A ---(Armature)----+
             |                                                   |
             +--brass arc--[NVC]------------o F ---(Shunt field)-+
                                                                 |
 Supply (-) -----------------------------------------------------+

Main parts:

1. Starting resistance in sections between the studs.
2. Handle (starter arm) moving over the studs; a soft-iron piece is attached to it.
3. No-volt coil (hold-on coil) in series with the shunt field. Acting as an electromagnet, it holds the handle in the ON position.
4. Overload release (OLR) electromagnet with a trip lever, which short-circuits the no-volt coil when the current is excessive.
5. Spring, which returns the handle to the OFF position.

Working

1. With the supply switched on, the handle is moved slowly to stud 1. The full resistance is in the armature circuit, while the shunt field gets full voltage through the no-volt coil. A small starting current flows.
2. As the motor accelerates, back emf builds up, and the handle is moved to the next studs, cutting out resistance step by step.
3. At the last stud, the handle is held by the electromagnet of the no-volt coil, and all the resistance is cut out. The motor runs on full voltage.
4. No-volt protection: if the supply fails, the no-volt coil is de-energised and the spring pulls the handle back to OFF, so the motor does not restart with all the resistance cut out.
5. Overload protection: on overload, the OLR magnet lifts its lever and shorts the no-volt coil, so the handle is released and returns to OFF.

Disadvantage: the no-volt coil is in series with the field, so when the field current is reduced for speed control it may release the handle. The 4-point starter removes this problem.