An automatic starter would include a triple pole contactor to control the stator circuit, together with rotor-resistance grids short-circuited by the necessary number of accelerating contactors, the last of which must be continuously rated to carry the full-load rotor current. Also required are the necessary number of overload relays and timers controlling the duration of the starting period. The number of timers and accelerating contactors correspond to the number of steps of rotor resistance that are provided.
A wiring diagram -of an automatic slip ring motor starter with two steps of rotor resistance is shown. Control terminals are provided for pushbutton control from one or two positions, or alternatively, for automatic control (for use with thermostat, float-switch or similar switching) with or without a try-out switch.
When the ‘start’ button is pressed (or the automatic switch closes), the control circuit is made through the coil of the stator contactor M. The stator contactor closes, connecting the stator to the line. At the same time the first timing relay is TR1 is energized. At this stage, the rotor is complete through the whole resistance since the accelerating contactors 2R and 3R are open. After an adjustable delay, the contacts of TR1 close, thus energizing the accelerating contactor 2R which short-circuits a portion of the rotor resistance and energizing the second timing relay TR2. When in turn the contacts of TR2 close the second and, in this case, final contactor 3R is energized and closes, short-circuiting the whole of the rotor resistance. The overload relays are in circuit during starting and running. For automatic (2-wire) remote control, hand-resetting overloads are essential.