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Why is my coil making a humming noise?

Due to the alternating currents in an AC system, properties of the contactors within the design tolerances of the magnet frame, can play into the coil giving off a slight hum. In this case, it is considered normal and will not harm the contactor.

Several factors will create an objectionable AC Hum on coils. Pollution of the magnet pole surfaces will create a slight hum coming from the contactor in the closed position. When cleaning, use a soft and dry piece of cloth. Hum can also occur if the pole surfaces are deformed. A louder hum will occur if the shading coil is broken because the magnet will cause the contactor to chatter. If so the magnet frame assembly will need to be replaced. Another reason for AC-hum could be corrosion caused by environmental conditions that exceed the contactor specifications. Contactors need to be protected from condensation in order to keep the pole surfaces of the magnet free from corrosion.

A significant low voltage (more than 15%) on the coil will typically produce a hum and will of course damage the coil if the low voltage condition continues.

Do you have a troubleshooting and maintenance guide for contactors and starters?