Contactors and Relays - Starting Motors, Sending Signals...What's the Difference?

Contactors and relays are both electrically controlled switches used to operate loads such as motor starters, solenoids, lights, or simply to change a signal to one voltage to another.

They both consist of a magnetic coil and a set of contacts that are moved back and forth as voltage is applied and removed from the coil.

Contactors will typically be used to start AC 3 phase motors and large loads.  They will have multiple sets of contacts, usually 3 or 4 normally open contacts or poles, and sometimes several auxiliary contacts that can be normally open or normally closed.

Relays are used to switch smaller loads such as fractional horsepower single phase motors, solenoids, lights, and to send signals to other devices.  They may have a single pole (contact), double pole, 3 pole, 4 pole, or even more contacts.  Unlike contactors whose poles are usually either normally open or normally closed (single throw), relays will many times have double throw contacts.  These are contacts that will have a contact that is both normally open and normally closed, connected through a common terminal.


There are a few other types of relays that were not included in this video including solid state relays, latching relays, timing relays, counting relays, potential relays, phase monitoring relays, thermal overload relays, etc.

Learn more about Contactors and Relays.

Learn more about Motor Starter Control Panels.

Next Steps

Go to the Control and Power Wiring Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.