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A thermal overload relay is an electro-mechanical relay that is operated by heat developed in the relay. |
C When the level of current in a circuit reaches a preset value, the increased temperature opens |
a set of contacts. |
C The increased temperature opens the contacts through a bimetallic strip or by melting an |
alloy that activates a mechanism that opens the contacts. |
C Two types include melting alloy and the bi-metallic strip. |
Melting-Alloy Thermal Overload Relays: |
These are probably the most popular type of overload protection. |
C The motor current passes through a small |
heater winding and under overload conditions, the heat causes a special solder to melt allowing a ratchet wheel to spin thus opening the control circuit contacts. |
C Must be reset by hand operation |
C Heater coil and solder pot in one unit — |
non-tamperable |
Bimetallic Thermal Overload Relays: |
This design uses a bimetal strip associated with a current-carrying heater coil. |
C When an overload occurs, |
the heat causes the bimetal to deflect and actuate a tripping mechanism which opens a set of contacts in the control circuit interrupting power to the coil and opening the power contacts. |
C Most relays are adjustable |
over a range from 85% to 115% of their value. |
C They are available with ambient compensation. An ambient compensated devices’ trip point |
is not affected by ambient temperature and performs consistently at the same value of current. |