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.