What are the Reasons of AC Humming, Chattering in Magnetic Contactor ?

Chatter, humming or buzzing coming from a contactor shows problem in contactor. There are three common issues that may cause unexpected noises from an installed contactor.

  • The control voltage supplying the coil is not in the operating voltage range of the coil.
  • The control source is not capable to provide necessary current to pick up the coil
  • There is debris on the pole faces of the yoke and armature


Coils of contactor are designed to pick-up and operate between 85% and 110% of their rated voltage. The supply voltage to coil must be within the operating range of the coil for relaible operation of the contactor. The voltage drops up to 60 % of the rated voltage when contactor is switched on. If the supply voltage is lower than the rated voltage, the contactor draws a larger current for its pick up and cause voltage drop. This further leads to chattering of the contactor.

The supply voltage to coil must not be more than 110 % of the rated voltage. It may cause over heating of the coil & may cause coil burning. AC Hum & Contact bounce reduces the life of the contactor, therefore it must be operated at rated voltage.


The reliable operation of the contactor depends on the pick up current supplied by the supply source. The coil draw a large inrush current during pick-up and pick-up current reduces when the armature and yoke start to come together. To avoid the AC Hum & Contact bounce in magnetic contactor, the supply source VA rating should be more than the coil VA rating.


Debris like plastic or metal shavings may cause coil to draw more current that its rated current. The deposits of debris may take place with use of contactor. Therefore, there is need of periodic maintenance of the contactor pole surfaces with soft cloth.

The contactor pole surfaces has tendency to deposit debris in the humid and corrosive environment.

This is all about AC Hum & Contact bounce in Magnetic Contactor.