The Workings and Uses of Contactors
A contactor is a type of relay which is used to conduct larger currents. The contactor is used to open and close the connection to devices which require regular switching such as lighting, motors and heating. The contactor can be normally open, but closes when activated or vice versa. Contactors can be either used in AC or DC applications, but AC is more commonly used.
The basic design of a contactor
The contactor consists of three basic elements:
- The spring which allows it to return to a set position.
- The contacts which make or break the circuit.
- The coil when energised, is used to move the contacts into the open/closed (O/C) position.
The contactor usually has a three or four pole switch in three phase circuits or a double pole switch in single phase circuits, which in its natural position is open. The switch consists of stationary and moveable contacts usually coated with a silver alloy. The two contacts are held apart with the use of the spring. The moveable contacts are connected to an armature. A separated circuit which operates at a much lower voltage (usually 220Vac/380Vac/440Vac or 12Vdc/24Vdc) is used to power a coil wound around a magnetic core forming an electromagnet. When the electromagnet is energized it attracts the armature which then brings the two contacts together closing the circuit. The switch can either be held closed by keeping the electromagnet energized or by mechanical methods.
[A schematic representation of a simple contactor. Image created by Author.]
The contactor contacts are closed as fast as possible to avoid arcing between the two closing contacts. The arc between the two contacts needs to be extinguished as fast as possible as this could damage the contacts in the long term. The contacts are coated in silver as this prolongs the use of a contact before failure. In today’s contactors the contacts are brought together at such a speed that the contacts bounce off each other. This bouncing can cause secondary arcs which can also cause damage to the contacts. It is thus desirable for contactors to have as little bounce as possible, as contactor bouncing decreases the life expectancy of the contactor.
It is for this reason that contactors should only be purchased from a reputable authorised supplier to ensure that a premium quality product is used for your installation.
The IEC rates contactors based on the design philosophy followed to determine the life-expectancy of the contactor. The following categories are used by the IEC to rate contactors.
It is important to follow the guidelines set out by the manufacturer when installing a contactor. Most contactors will fall into one of the listed ratings.
As can be seen in the table contactors are mostly used to operate motors. These contactors are generally of the three or four pole type due to most motors utilise three phase power. The two pole contactors are mostly used to switch lighting and heating in a building due to these systems utilizing one phase power.
Article by: Jannes Smit, 3rd year Electrical Engineering student at the University of the Witwatersrand. firstname.lastname@example.org