Compared to the power circuit, the control circuit is where things get interesting.
Control circuits can be simple two-wire on/off devices that energize a motor starter, or they can be more involved three-wire circuits with multiple pushbutton locations and motors that start in sequence.
One of the key advantages of control circuits is that they can operate at a lower voltage than the motor that they control. This allows an operator to push a 120V pushbutton, which then energizes a 120V starter coil whose contacts then close on a 600V motor. If a dangerous fault occurs, the high-voltage switching happens in a location away from the operator.
Since the value of voltage between conductors in a 3-phase system could be as high as 600V and the control circuit is usually a single-phase 120V AC circuit, some form of control-circuit voltage must be available. Power for the control circuit can be supplied in one of three different ways.
This system can provide a lower voltage for the control circuit, but requires additional wires and breakers to be installed at the main-load centre to supply the control circuit.
This system does not allow the control circuit to operate at a lower voltage than the power circuit. The advantage is that no additional circuits need be drawn from a load centre. Power is taken directly from the power circuit after the three-phase disconnect and overcurrent protection. This ensures that if either of these opens, the control circuit will also be disconnected from its source of supply.
The connection is similar to when power is taken directly from the line, but instead of connecting directly to the control circuit, the line voltage is first stepped down, usually to 120V AC. The secondary of the control transformer then acts as the source of supply for the control circuit and must be of sufficient size to supply power to the motor starter and any other associated contactor equipment such as control relays, timers or pilot lights.