A magnetic starter is an electrical switching device commonly used as a initiating mechanism for electric motors and other high current equipment. Also known as a contactor, the magnetic starter relies on an electromagnetic field to close a set of contacts which then transfer power to the motor. This electromagnetic field is supplied by a two part laminated steel core and a wire coil connected to the control circuit of the starter. When the start button is pressed and the coil energizes, it creates the magnetic field which pulls the contact mechanism closed and starts the motor. Magnetic starters may have from two to four sets of main contact points and will often feature built-in sets of auxiliary contacts and thermal overload cut-outs.
Most electric motor and heavy equipment installations use a magnetic starter to initiate operation. Often called contactors or relays, magnetic starters allow for remote starting of equipment and, depending on the specific design, also offer overload protection and auxiliary switching. The main contact points in a magnetic starter act like switches to make or break the main supply circuit to the motor. In the case of smaller single phase motors, only two contact points will be required — one each for the live and neutral lines. Three phase motors will naturally require three contacts, one for each phase.
These starters consist of two laminated steel cores and two sets of contact points which serve as a switch to control the motor's electrical supply. One steel core and one set of contacts are attached to the body of the magnetic starter and do not move. The second core and contacts can move and are connected together under spring tension to keep them separated from the static units. A wire coil is placed around the static core which, when energized, creates a electromagnetic field that pulls the moving core up against the static one. The moving contacts travel with the moving core and press firmly against the static contact points to complete the motor supply circuit.
The circuit which supplies current to the coil is known as the control circuit and passes through the stop and start buttons, thus allowing remote control of the motor. There are usually at least one or two sets of auxiliary contact points separate from the main contact sets included in magnetic starters. These are used as interlocks, retainers, and to switch remote lamps which indicate the running status of the motor. The starters may also include thermal overload devices which cut the power supply to the coil and stop the motor should it overload.