Siemens motor overload relay teardown

This is an optional attachment for contactors that is designed to detect a motor drawing more current than normal, and triggers a latching contact that turns the motor off.

Motors draw a base current that increases with the load applied. There are many things that can cause excess load on a motor, like a physical obstruction, build up of debris in a machine, loss of lubrication or just wear and tear of bearings and guides.

Another common overcurrent situation is loss of a phase, where the motor struggles to turn. That can cause many machines in a factory to trip their motor protection circuits. Some do have phase failure protection that will turn the machine off if it detects a phase failure or accidental reversal of phases.

You do get programmable electronic ones now, but this one is a traditional bimetallic unit with low resistance heaters in series with the load that will cause the internal trip levers to deflect according to the current flowing through them.

When I showed the trip mechanism activating at the end, the cream coloured section that was popping up had both the normally closed and normally open contact plates on it.

These units can save expensive motor replacements. If they start tripping regularly it may be time to check the machine is running properly. Resist the temptation to nudge the trip threshold up above a sensible level.

My suggestion of pressing reset buttons in panels is aimed at technically competent people who may have their own workshop. I don't recommend interfering with panels at a place where you are employed. That's best left to the maintenance department.


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