Introduction to Circuit and Motor Protection – MOTOR PROTECTION (3)

Overload Components and Operation
The assembly of a mechanical overload relay is formed by the heater coil, also called an overload heater, and the contact mechanism (see Figure 30).

Although there are different types of heaters made by different manufacturers, all of them work by the same principle of measuring the heat generated by flowing current (see Figure 31). There are two design types that are commonly used: the eutectic melting alloy and the bimetal overload relays.


The eutectic melting alloy overload uses a heater element surrounding a solder-like mechanism that holds the pin of a rachet wheel element (see Figure 32). Eutectic means low-melting temperature solder.

When excessive current passes through a heater, the solder will melt and will transform into a liquid state very quickly without going through a plastic stage (see Figure 33). This will cause the
rachet wheel to release the pawl it is holding. Once the pawl is released with pressure of the loaded spring, the normally closed contacts will open (see Figure 34). When the solder-like melting alloy has cooled down to a solid state, the overload relay contacts can be reset to the normal state. If you try to reset the overload relay contacts while the alloy is still liquid, the contacts will open because the rachet will not be able to hold the pawl mechanism.

to be continued…………

to be continued………..

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