Relays, Contactors and Motor Starters

Relay, contactor or starter: how heavy is the load?

This aisle splits by how much power the part must switch. A relay uses a small control signal to switch a modest load. A contactor is a heavy-duty relay with larger contacts for motors, heaters and lighting banks, often in 3-pole form to switch three-phase supply. A motor starter pairs a contactor with overload protection so a stalled or overloaded motor is cut off. Timers, such as spring-wound and defrost types, switch a load for a set time or on a repeating cycle. Choose by what the load draws, not by the size of the box.

Coil voltage, contact rating and pole count

The coil voltage must match your control supply, whether 12 V for low-voltage and vehicle circuits, 24 V for control panels or 120 V and above for line-voltage switching. Check AC against DC, because a coil built for one will not work properly on the other. Contact rating states the current the contacts can switch, and motor and lamp loads draw a surge on start, so read the rating for the load type, such as AC-3 for motors. Pole count tells you how many circuits switch together: single-pole for one line, three-pole for three-phase.

Questions about Relays, Contactors and Motor Starters

What is the difference between a relay and a contactor?

Both are electrically operated switches. A relay is built for lower currents and often switches signals or small loads, while a contactor has heavier contacts and better arc handling for motors, heaters and lighting. Contactors normally have no overload protection unless paired with an overload relay.

Does the coil voltage have to match exactly?

Yes, within the tolerance on the label. A coil on too low a voltage may chatter or fail to close, and one on too high a voltage overheats and burns out. Match the voltage and the AC or DC type to your control supply.

Do I need an overload relay with a contactor?

A contactor only switches; it does not protect the motor. For motor loads add a thermal or electronic overload relay set to the motor's rated current, or choose a motor starter that already includes one.