Circuits
25 Circuits Overview
When discussing motor controls, there are two circuits that need to be well understood.
The first is the power circuit. This circuit is relatively straightforward. It is a 3-phase circuit which powers the motor and can operate as high as 600A at 600V or even beyond.
The second is the control circuit. This is the circuit which controls the power circuit. This circuit can be as simple as a two-wire pilot device, or it can be very complex with many interlocking features and multiple starter and timer coils. The control circuit will often operate at a lower voltage relative to the power circuit.
We will be spending the majority of our time discussing common control-circuit designs, so it’s best to start with a quick overview of the power circuit.
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.
An auxilary device that provides indication or control of a process to an operator. Pilot devices include automatic switches such as float and pressure switches, as well as indicating lights.
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).