How do you get a motor, such as the one on your well pump, to switch on automatically? You equip it with a contactor, which converts incoming current from a pressure-, temperature- or light-sensitive sensor into a magnetic field that closes the main electrical contacts and allows power to flow.
Of all types of contactors used in industry, magnetic ones are the most common, and they bear little resemblance to the manual switches in use in the early 1900s. The general types of magnetic contactor fall into two broad categories, those approved by the National Electrical Manufacturers Association (NEMA) and those approved by its European counterpart, the International Electrotechnical Commission (IEC). They all work in basically the same way, though, and have basically the same parts.
How Does a Magnetic Contactor Work?
A magnetic contactor has two incoming circuits which include the main circuit to power the load and an auxiliary circuit to operate the contactor itself. The auxiliary circuit connects to an induction coil, and when current flows through the circuit, the coil generates a magnetic field. The field attracts a second magnet, which may be a permanent magnet or an electromagnet.
A pair of fixed contacts is attached to the contactor housing and a pair of moveable ones is attached to the electromagnet, and the force exerted by a spring or gravity keeps them apart. When the coil is energized, the contacts close, and power flows to the load.
All Types of Magnetic Contactor Have These Parts
Magnetic contactors can be small enough to fit in your hand or as large as a meter in length. No matter the size, the purpose is always the same: to close a normally open switch and allow power to flow. To this end, every contactor has to have the following components:
- Input and output terminals: The size and number of these terminals depends on the voltage of the incoming power and where the power source is single-phase or three-phase.
- A magnet and a coil: The magnet is often a horseshoe magnet that fits through a core around which the coil is wound. The core is made from a non-ferrous material to ensure it doesn't retain a magnetic field when the power is off. Other designs have a rectangular or cylindrical magnet inside a coil-wound solenoid.
- A spring: The function of the spring is to keep the contacts open and the power to the load off. It may push the movable contacts away from the yoke or pull from the other side. In some models designed for vertical installation, gravity may take the place of the spring.
- An enclosure: The enclosure keeps all components electrically isolated and protects users from accidental exposure. The housing is made from plastic, Bakelite or Nylon 6.
Arc Suppression in Magnetic Contactors
Many contactors are designed to work with high voltage power, and these usually have some type of inbuilt arc-suppression mechanism. Electrical arcing occurs as the contacts are opening and closing, and even though it's momentary, the high heat quickly degrades the contact points.
Not all kinds of magnetic contactor need arc suppression. Contactors that operate with AC current at less than 600V usually rely on the surrounding air to quench arcing. These devices have arc suppression hoods that protect the rest of the components. Larger contactors, and those that operate on DC current, often need active suppression which can take many forms, including the use of a resistor and capacitor in the circuit.
To counteract the effects of arcing, the contacts often have a protective coating or are made with a non-corrosive material, such as silver tin oxide or silver cadmium oxide.