Relays are usually cheaper and lower in their performance capabilities for signal switching and load handling than contactors. While contactors and relays are both used in engineering applications to switch signals and power connections, relays are used in applications where speed and power are not as critical.
Solid-state relays are the most commonly used circuit component to control load currents. These relays are constructed from solid-state switches such as triacs, silicon-controlled rectifiers and power transistors. The type of switches used in relay design depends on the amount of load current, potential difference and switching speed that the relay is expected to handle.
Electromechanical relays mechanically open and close electrical contacts to turn a load current on or off. They are capable of handling much larger load currents, but they are much slower in comparison to their solid-state counterparts. These relays are also much cheaper than solid-state ones and can be connected to multiple contacts simultaneously.
Contactors are special-purpose relays designed to rapidly engaging and disengaging high power circuits. High horse-power electric motors are usually controlled with contactors. Because of the criticality of their applications, contactors are usually integrated with overload heaters to prevent catastrophic failure. Contactors can be designed for specific applications, such as HVAC system compressor motors or capacitor switching.