AC Hum & Contact bounce in Magnetic Contactor?

AC hum & contact bounce in a magnetic contactor cause premature failure of the contactor. In this post we will discuss in detail what is AC hum & contact bounce.

AC Hum in magnetic Contactor

AC humming and chattering sound occur in the contactor under specific conditions. The magnetic pole surfaces must be uniform for proper closing of the contactor. Slight dust or foreign material deposits on the pole surfaces may cause pole surfaces unevenness, and during closing of the contactor it creates humming sound. Thus, the deformation of the pole surfaces cause humming sound.

In order to reduce the contactor hum problem, we should protect the contactor from the polluted environment.

The main reasons of AC hum in the contactor are as follows.

  1. Dirt or other materials deposit on the pole surfaces
  2. Mechanical deformation of the pole surfaces
  3. Breaking of shading coil of the contactor- It cause louder hum.

Contact Bounce in Magnetic Contactor

The contactor coil has minimum and maximum voltage range for its operation. As per IEC60947-4-1, the operating voltage limit of the contactor is between 85 and 110% of the rated coil voltage. The voltage lower than the minimum operating voltage leads contact bounce in magnetic contactor at closing.

Also, the higher voltage above limits cause the contact bounce problem that further leads to increased contact wear. The main reason is the electromagnetic force increase with higher voltage that causes contact wear.

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.