Contactors & Auxiliary Switches
Contactors perform a crucial role in motor control, providing a failsafe method of energising and de-energising drives by automatically disconnecting power when there is a power failure. Contactors designed for three-phase operation will have either three or four main contacts rated for making and breaking the rated current, and may also have auxiliary contacts fitted for control purposes. A contactor is controlled by an electromagnetic coil, and its supply voltage may be the same as the phase voltage but more commonly it is lower. Popular coil voltages include 100 and 230 volts AC or DC, with control voltages between 24 and 110 volts DC. Most contactors are rated for operation at phase voltages of 400 volts AC. Contactors can be used for switching DC power, but their breaking current is much lower. Most contactors are designed to accommodate a range of auxiliary switches, which are easily clipped or screwed into place and do not interfere with normal operation. These are used for control purposes, and some also incorporate timers that can be used to control switching of star delta motors, and other control functions. Other accessories that are available include locks to mechanically interlock pairs of contactors. Manufacturers include Siemens, Schneider Electric, Eaton Omron, Crydom and Finder.