Circuits
27 Control Circuit’s Source of Supply
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.
A Separate Source
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.
Direct from the Line
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.
A Control Transformer
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.
In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.
In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
In motor control terminology, a three-wire circuit utilizes a magnetic motor starter with a holding contact, along with momentary contact pushbuttons. A three-wire circuit provides low-voltage-protection.
A momentary contact device that has a built in spring to return the button to its normal position once release. Available with either normally-open, normally-closed or both sets of contacts.
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The conducting part of a switch that makes or breaks a circuit.
A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.
Used when additional auxiliary contacts are needed in a control circuit, a control relay is a magnetic contactor which is not designed for the energization of motors, and does not have built in overload protection.
A small lamp connected in the control circuit to indicate the status of a motor or other situation.
