Relays and contactors are all electromagnetic switching devices, but the former belongs to the switching device operating in the control circuit, while the latter belongs to the switching device operating in the main circuit.

Relay definition: The relay is an automatic electric appliance. It is suitable for long-distance switching and disconnecting AC and DC small-capacity control circuits, and is used for control, protection and signal conversion in electric drive systems.  The input quantity of the control relay is usually the current, the voltage, etc., or the non-electric quantity such as temperature, pressure, speed, etc. The analog output quantity is the electric signal generated by the contact action or the parameter change of the output circuit.  The characteristic of the relay input is that when the input quantity changes to a certain program, the output will undergo a step change.

Contactor definition: Contactors are divided into AC contactors (voltage AC) and DC contactors (voltage DC), which are used in power, distribution and power supply module applications.  The contactor refers broadly to an electrical appliance that uses a coil to flow a current to generate a magnetic field in an industrial power to close the contact to achieve a controlled load.

In fact, the most important difference between a contactor and a relay is that the contactor has an arc extinguishing device, and the relay does not.