When a relay is used to switch a large amount of electrical
power through its contacts, it is designated by a special name: contactor.
Contactors typically have multiple contacts, and those contacts are usually
(but not always) normally-open, so that power to the load is shut off when
the coil is de-energized. Perhaps the most common industrial use for
contactors is the control of electric motors.
 The top three contacts switch the respective phases of the
incoming 3-phase AC power, typically at least 480 Volts for motors 1
horsepower or greater. The lowest contact is an "auxiliary" contact which
has a current rating much lower than that of the large motor power contacts,
but is actuated by the same armature as the power contacts. The auxiliary
contact is often used in a relay logic circuit, or for some other part of
the motor control scheme, typically switching 120 Volt AC power instead of
the motor voltage. One contactor may have several auxiliary contacts, either
normally-open or normally-closed, if required.
The three "opposed-question-mark" shaped devices in series with each
phase going to the motor are called overload heaters. Each "heater"
element is a low-resistance strip of metal intended to heat up as the motor
draws current. If the temperature of any of these heater elements reaches a
critical point (equivalent to a moderate overloading of the motor), a
normally-closed switch contact (not shown in the diagram) will spring open.
This normally-closed contact is usually connected in series with the relay
coil, so that when it opens the relay will automatically de-energize,
thereby shutting off power to the motor. We will see more of this overload
protection wiring in the next chapter. Overload heaters are intended to
provide overcurrent protection for large electric motors, unlike circuit
breakers and fuses which serve the primary purpose of providing overcurrent
protection for power conductors.
Overload heater function is often misunderstood. They are not fuses; that
is, it is not their function to burn open and directly break the circuit as
a fuse is designed to do. Rather, overload heaters are designed to thermally
mimic the heating characteristic of the particular electric motor to be
protected. All motors have thermal characteristics, including the amount of
heat energy generated by resistive dissipation (I2R), the thermal
transfer characteristics of heat "conducted" to the cooling medium through
the metal frame of the motor, the physical mass and specific heat of the
materials constituting the motor, etc. These characteristics are mimicked by
the overload heater on a miniature scale: when the motor heats up toward its
critical temperature, so will the heater toward its critical
temperature, ideally at the same rate and approach curve. Thus, the overload
contact, in sensing heater temperature with a thermo-mechanical mechanism,
will sense an analogue of the real motor. If the overload contact trips due
to excessive heater temperature, it will be an indication that the real
motor has reached its critical temperature (or, would have done so in
a short while). After tripping, the heaters are supposed to cool down at the
same rate and approach curve as the real motor, so that they indicate an
accurate proportion of the motor's thermal condition, and will not allow
power to be re-applied until the motor is truly ready for start-up again.
Shown here is a contactor for a three-phase electric motor, installed on
a panel as part of an electrical control system at a municipal water
treatment plant:
 Three-phase, 480 volt AC power comes in to the three
normally-open contacts at the top of the contactor via screw terminals
labeled "L1," "L2," and "L3" (The "L2" terminal is hidden behind a
square-shaped "snubber" circuit connected across the contactor's coil
terminals). Power to the motor exits the overload heater assembly at the
bottom of this device via screw terminals labeled "T1," "T2," and "T3."
The overload heater units themselves are black, square-shaped blocks with
the label "W34," indicating a particular thermal response for a certain
horsepower and temperature rating of electric motor. If an electric motor of
differing power and/or temperature ratings were to be substituted for the
one presently in service, the overload heater units would have to be
replaced with units having a thermal response suitable for the new motor.
The motor manufacturer can provide information on the appropriate heater
units to use.
A white pushbutton located between the "T1" and "T2" line heaters serves
as a way to manually re-set the normally-closed switch contact back to its
normal state after having been tripped by excessive heater temperature. Wire
connections to the "overload" switch contact may be seen at the lower-right
of the photograph, near a label reading "NC" (normally-closed). On this
particular overload unit, a small "window" with the label "Tripped"
indicates a tripped condition by means of a colored flag. In this
photograph, there is no "tripped" condition, and the indicator appears
clear.
As a footnote, heater elements may be used as a crude current shunt
resistor for determining whether or not a motor is drawing current when the
contactor is closed. There may be times when you're working on a motor
control circuit, where the contactor is located far away from the motor
itself. How do you know if the motor is consuming power when the contactor
coil is energized and the armature has been pulled in? If the motor's
windings are burnt open, you could be sending voltage to the motor through
the contactor contacts, but still have zero current, and thus no motion from
the motor shaft. If a clamp-on ammeter isn't available to measure line
current, you can take your multimeter and measure millivoltage across each
heater element: if the current is zero, the voltage across the heater will
be zero (unless the heater element itself is open, in which case the voltage
across it will be large); if there is current going to the motor through
that phase of the contactor, you will read a definite millivoltage across
that heater:
 This is an especially useful trick to use for
troubleshooting 3-phase AC motors, to see if one phase winding is burnt open
or disconnected, which will result in a rapidly destructive condition known
as "single-phasing." If one of the lines carrying power to the motor is
open, it will not have any current through it (as indicated by a 0.00 mV
reading across its heater), although the other two lines will (as indicated
by small amounts of voltage dropped across the respective heaters).
REVIEW: A contactor is a large relay, usually used to switch current to
an electric motor or other high-power load. Large electric motors can be protected from overcurrent damage through
the use of overload heaters and overload contacts. If the
series-connected heaters get too hot from excessive current, the
normally-closed overload contact will open, de-energizing the contactor
sending power to the motor.
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