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.
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 speci¯c 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 therm-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.
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.
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 speci¯c 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 therm-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.
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