Contactors are essentially switches that can be controlled remotely. They can be designed to operate at voltages similar to the control devices that control their state or they can be designed to operate at voltages similar to the loads that they control.
Contactors have features that distinguish it from a standard relay. In most cases, a contactor is designed specifically with high current usage in mind. Other differentiators to relays are as follows:
The normal configuration of a contactor describes the disposition of the contacts when current is not applied to them. Most contactors are normally open, meaning that they close in the presence of current. There are variations, but this configuration is one of the defining characteristics of contactors and one of the things that separates them from standard relays.
Contactors are typically used for very heavy duty industrial scale applications and their ratings systems is designed to reflect this. They come in different classifications under IEC 60947-4-1. These classifications are expressed as AC numbers, from AC-1 to AC-4. Other classification systems are also used.
The applications of contactors include starting motors, providing control over furnaces, controlling the speed of very large motors and other large-scale applications.