Part
12470
Rev. 2 3/25/99
9
1.
Thermal
-
melting alloy or solder pot
2.
Thermal
-
bimetallic strip
3.
Electromagnetic
In thermal type relays, time-current characteristics are obtained by the thermal properties of the
melting alloy or bimetallic strip. In the magnetic type, a damped plunger or moving iron device is
used to produce time delays.
1.
Thermal - melting alloy or solder pot
In this type, tripping is the result of heat generated by the motor load current passing through
a "heater" in the overload relay. This overload relay consists of a brass shaft which is
surrounded by solder. Fixed to one end of the shaft is a small ratchet wheel.
As long as the solder is solid, this assembly is immobile. When the motor control circuit
contacts are closed, a spring is held compressed by the immobility of the ratchet wheel. An
overloaded condition in the motor increases the current through the heater, thus melting the
solder and releasing the energy in the spring. This interrupts the circuit of the holding coil in
the motor starter and shuts down the motor.
The starter may be reset only after the temperature of the heater has cooled sufficiently to
permit the solder to solidify and again make the ratchet and shaft immobile. Reset is usually
accomplished by an external pushbutton on the face of the starter. Many heaters offer a
selection of either manual or automatic reset.
2.
Thermal - bimetallic strip
This type uses a bimetallic strip - two pieces of dissimilar metal bonded together. An
increase in heat will cause movement of this bimetallic unit and eventually open a set of
contacts in the motor control circuit, thus opening the holding coil circuit and shutting down
the motor.
The principle of operation is the same as the melting alloy type. When the bimetallic
element has cooled sufficiently, the motor control circuit may be reset either manually or
automatically.
3.
Electromagnetic
In this type of motor overload relay, a damped plunger or moving iron device is used to
produce the delays required and initiate the trip signal to the interrupting device. The most
