In brief
A contactor is an electromagnetically operated switch that uses a small control voltage to switch large load currents on and off.
Contents
What is Contactor?
A contactor consists of a magnet coil, a movable armature and contacts. When control voltage is applied to the coil, the armature pulls in and closes the main contacts; without voltage, springs open them again. The contactor thus galvanically separates the control circuit from the load circuit. It is designed for a very large number of operating cycles and is used for operational switching, not for short-circuit protection. It differs from a relay mainly in its higher switching capacity.
Important parameters are the control voltage and type of current of the coil, the rated operational current or power in a utilisation category, and the number of main and auxiliary contacts. The utilisation category to IEC 60947-4-1 describes the load: AC-1 for virtually resistive loads, AC-3 for starting and switching off squirrel-cage motors. Contactor relays switch control currents only. Mirror contacts and mechanically linked contacts are used in safety circuits.
Contactors are installed in almost every control cabinet, for example for motors, heaters and lighting. Wear parts are the main contacts, which erode through arcing, and coils, which fail due to incorrect voltage or overheating.
In practice: maintenance and procurement
For a replacement, state the coil voltage with type of current, the rated current in the appropriate utilisation category, the number of poles and the auxiliary contact configuration. For larger contactors, contact kits and coils are often available separately, which speeds up repair.
Synonyms and abbreviations: power contactor · motor contactor · contactor relay · magnetic contactor · AC contactor
Frequently asked questions
What is the difference between a contactor and a relay?
Both work on the same principle: a coil actuates contacts. Contactors are designed for high load currents, often have arc-quenching devices and are used for motors and other loads. Relays switch smaller currents, mostly control and signal circuits. Contactor relays lie in between and are used for control, not for switching motor power.
What does AC-3 mean on a contactor?
AC-3 is a utilisation category to IEC 60947-4-1 for starting and switching off squirrel-cage motors during operation. The associated rated current or motor power in kW shows which motor the contactor can switch reliably. AC-1 with higher permissible currents applies to resistive loads, while stricter categories apply to inching and plugging.
What information is needed to replace a contactor?
Required are the coil voltage and type of current, that is, AC or DC, the rated current or motor power in AC-3, the number of main poles and the number and type of auxiliary contacts as normally open or normally closed. Frame size, mounting, type of connection such as screw or spring-loaded terminal, and fitted accessories are added.
Why does a contactor hum?
AC contactors hum slightly when the pole faces of the armature and magnet core are dirty or worn, or when the shading ring in the core is damaged. Control voltage that is too low can also prevent the armature from pulling in cleanly. Loud humming is often accompanied by heating of the coil and should be checked.