Relays, contactors, and motor starters are widely used as output devices for a variety of control systems. In some cases the control system consists of switches, such as limit switches, proximity switches, and photoelectric switches, which are wired together to provide the control portion of the circuit.
The relay, contactor, and motor starter are all similar to each other in that they each have sets of NO or NC contacts that are controlled by a coil. The relay is typically the smallest of these devices and the current rating for its contacts will be smaller than that of a contactor and motor starter. The basic rule of thumb is that a relay is designated as having contacts that are rated for less than 15 A.
If the device has contacts that are rated for more than 15 A, it’s called a contactor. This leaves room for disagreement, so one manufacture may call a device with a contact rating of 15 A relay, while another may call the same device a contactor. The motor starter is similar to the contactor except it has heaters and overload devices to protect the motor that’s connected to it against overcurrent.