Small slip-ring starters usually consist of a contactor for the stator circuit and a face plate-type starting resistance for the rotor circuit. The basic essentials are shown, the three wires from the stator going to slip-ring terminals R, S and T on the motor. An actual wiring diagram is also shown. The starter must be fitted with interlocks to ensure that the resistance is all-in when starting.
With a contactor controlling the stator supply, interlocking is simply effected, as shown, through electrical contacts on the arm of the rotor starter, no current reaching the contactor coil ‘I’ unless the arm is in the starting position. The start button must be kept depressed until all resistance has been taken out; this ensures that the motor is not accidentally left running with some of the rotor resistance still in circuit. When the operating arm of the face- plate is in the ‘run’ position the start button is short-circuited.
If the motor is fitted with a device designed to lift the brushes and short-circuit the slip rings when the motor is up to speed, an interlock must be arranged in the control circuit to ensure that the brush-gear is in the starting position before the stator contactor can close.
For larger motors, a stator oil switch is usual and may be used in conjunction with a liquid resistance or an oil-immersed grid resistance in the rotor circuit.