High-power circuits are repeatedly established and interrupted by special-purpose relays called contactors. Contactors control most integral horsepower motors (1 hp and above) and overload heaters in each controlled phase are frequently incorporated by the contactors.
Contactors are packaged as per NEMA or IEC standards and IEC devices reduce heat at their contacts through electromagnetic arc quenching techniques whereas NEMA devices heavily rely on mass to dissipate heat from arcing. A typical approach is that the blowout coils which generate a magnetic field designed to push the arc away from the contact area are used with a second set of arcing contacts which is designed to reduce the amount of arcing across the main contacts. Differences in rating practices can lead to IEC devices being smaller by 30% to 70% than their NEMA counterparts.
Overload heaters are often incorporated by contactors in each phase and an assembly or a low-resistance strip of metal is the heater element that heats up as the motor draws current. A normally closed switch will open if the heater temperature reaches a point that corresponds to a moderate overloading of the motor. The opening of the normally closed contact automatically de-energizes the relay as it usually connects in series with the relay coil and powers down the motor.
Special-purpose components which are not found in conventional relays are incorporated by contactors. Quenching the arc that forms when high currents are interrupted is one category. Magnetic blowout coils are often incorporated by the contactors that push the arc away from the contacts to quickly cool the arc.
Overcurrent protection for large electric motors is provided by the overload heaters which is different from fuses and circuit breakers which provide overcurrent protection for power conductors.
Compressor motors powering HVAC systems use the definite-purpose contactors which are designed to last only till their intended application whereas NEMA-rated devices may outlast the equipment on which they are installed and tend to be rated conservatively.
Definite-purpose contactors have their applications in other types of cooling, heating and refrigeration applications and generally, there are two categories:
- Compact one and two-pole contactors (handling between 20 and 40 A): used for residential central air-conditioning.
- Standard-frame devices (handling between 15 and 360 A): used for larger industrial/commercial rooftop air-conditioning units.