A contactor is an electrical switching device used to regulate power circuits that run heavy-duty loads such as electric motors, heating and air-conditioning units, and industrial lighting. It is a lot like a relay, but able to handle much higher current flow. The contactor itself may well directly be directly connected to the power source, or it could be controlled by a separate economizer circuit that carries a great deal less power than the one controlled by the contactor. Contactors can certainly run on either alternating current (AC--the type received from the power grid of utility companies) or direct current (DC--from batteries, solar cells or windmill generators).
AC contactors are essentially the most common, because most household and industrial power supplies are AC. Each and every contactor has three components: 1) the contacts; 2) an electromagnet; 3) an enclosure. The enclosure is only a frame to hold the contacts and the magnet; it is made from an insulating--non-conductive--material such as Bakelite, nylon or some other type of plastic. The electromagnet is really a coil of wire surrounding a movable soft iron core. It is the driving force which usually closes the contacts (switches) so as to complete the electrical circuit and deliver power to the load device. The contacts actually carry the current. The resting condition of the contactor, once the power is off, is generally with the switches open; no power is delivered to the load; electrical engineers call this a 'normally open' circuit. Each contact has a fixed pole and a movable pole; the movable pole is attached to a spring which is activated by the electromagnet. When power is applied, the magnetic field generated by way of the wire coil moves the iron core and, with the use of the spring, pulls the movable pole of the contact toward the fixed pole until they are touching; this wraps up (closes) the circuit and delivers power to the load device.
A contactor has at least three switches or contacts, but can have many more depending on the complexity of the device or system it controls. There will also be different kinds of contacts on the same contactor: main power contacts or auxiliary contacts. Depending on the quantity of contacts, there may also be several electromagnets. This arrangement can be used for a system in which automatic acceleration is required. An economizer circuit lowers the amount of power necessary to keep the contacts closed and minimizes heat buildup. It is always utilized for DC contactors, but only for very big AC contactors.
The defining characteristic of alternating current is often that it reverses direction in a cyclic fashion; as a result the magnetic contactor can be pulled in alternating directions until some mechanism smoothed or averaged the current flow. AC contactors have a mechanism that does that. A shading coil is placed around a small portion of the iron core; it serves to slightly delay the flow of the electromagnetic current.
There are lots of companies that make as well as sell AC contactors. AC contactor suppliers is often brick-and-mortar home center stores or even wholesale electrical supply companies. In addition, there are plenty of online sources.