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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.
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