What is causing the humming noise a magnetic contactor


AC contactors are normally not completely noiseless, there is always a degree humming due to magnetostriction. Magnetostriction is the change in dimension of magnetic materials during magnetization. Although when this humming becomes very audible such that it can easily be described as a chatter it raises some questions in the mind of the electrical technician. What causes a contactor to chatter, what is the effect of chattering on the performance of contactor and what can I do to fix it.

Common causes of humming in a magnetic contactor.

Coil Voltage
According to IEC 60947-4-1 the operational limit of the contactor is between 85 and 110% of the rated coil voltage. You should first measure the voltage at your coil terminals A1 and A2 then compare with the rated coil voltage to ensure that incoming voltage to the coil match coil datasheet rating. Undervoltage can cause your contactor to chatter. A 50/60Hz supply voltage would create a magnetic field with a magnetic pull that changes 50 or 60 times in a second. This would which induce excessive mechanical vibrations if the supply voltage is low because the field produced by the coil wouldn't have sufficient force to keep the contactor armature fully sealed against the spring pressure.

Debris
Any condition that does not allow the armature to close and seal properly will cause noise. it could be carbon deposit, metal shaving, tiny cockroach, rust, dust etc Thus is common with old contactors that are used in a highly corrosive environment like a urea granulation plant.

Mechanical Restriction
Experience taught me this one, While installing a NEMA size 2 Contactor it fell to the ground and began to hum after installation. I opened it up to discover a misalignment between the Armature and the Magnet Assembly. This slight misalignment didn't allow the armature to travel fully and sit properly thereby causing a jam.

Excessive carbon deposit
 Carbon deposit on the contacts of the contactor can cause high resistance connection to develop in one of the phases. The higher resistance will cause a voltage lag which in turn could lead to contactor chattering.


Other factors like loose lamination, DC coil being used on AC can cause contactor chattering and the general recommendation would be to disassemble the contactor and clean it with contact cleaner. Clean and gently sandpaper the contactor but do not sandpaper the armature just clean. There was a time where I had to cut the kick-out spring a little to reduce the spring pressure as I suspected that the coil was old and no longer creating a strong magnetic field.

Finally, you can just replace the contactor because in most cases, when contactors begin to chatter, they are worn out due to age or misapplication. The most common ways people misuse a contactor is
1) Continuously Feeding the coil it with overvoltage and

2) Using it in an area with High Ambient temperature.

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