First of all, the working principles of relays and contactors are basically the same. Both consist of a coil and contacts. When the coil is energized, the normally open contact closes and the normally closed contact opens. Of course, contactors also have main contacts that also get energized.
Difference
The load capacity of a typical small relay is around 240V and 5A. Please refer to the markings on the relay for specific details. The load capacity of a contactor is much greater, with currents ranging from several tens to several hundreds of amperes.
2. Relays are generally used in the secondary circuit, which is the control circuit, to achieve various control functions. They are relatively small in size. For example, they are used to control solenoid valves and relays; while contactors are mainly used in the main circuit and are larger in size, such as controlling power components with higher power, like electric motors.
3. Relays have numerous normally open and normally closed contacts, including two groups, three groups, four groups, etc. However, if no auxiliary contacts are added to a contactor, it will only have one or two groups of normally open or normally closed contacts. Of course, the addition of auxiliary contacts to a contactor is also a difference from a relay.