Problem:

A customer was using a CA7-43 contactor. Since the equipment was installed, the contactor was replaced 6 times in 6 months of operation, about once every 4 weeks. Upon disassembling the contactor the contacts were found to be worn or burnt. The motor duty cycle on the application could be up to 200 times per day. Sometimes the contactor was not used at all for up to 5 hours. Sensors and a program from a PLC Controller were used to energize and de-energize the contactor. What caused the power contacts to have such a shortened lifespan and premature wear?

Cause:

Sometimes a "telegraphing" pilot device can cause the contactor contacts to wear out prematurely. A "telegraphing" pilot device is telling the contactor to turn on and off very rapidly. We have seen this happen with a pressure switch or a temperature switch when the differential is set too tightly.

You may need to look at the control voltage with a scope in order to catch this condition. In the control circuit scheme between the sensors and the PLC controller, something may be telling the contactor to turn on and off rapidly, which could be a false trigger to turn on the contactor coil. Think of it as a bouncing contact signal if you were using a mechanical device like a relay output.

Solution:

With the PLC Controller being a programmable device, the programming could be modified to provide a slight time delay based on an input going into the PLC controller which could take care of a bouncing or telegraphing input signal.

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