Humming Contactors

I don't help ANONS

As Spades indicates, something is causing an air gap in the closed magnetic circuit.

Disassemble the contactors, most likely rust corrosion on pole faces. Sand both faces clean (on a flat surface) and the buzzing will stop. Also check that the shaded pole winding is intact.

You can't put paint on the faces as this will create an air gap. I'm not not aware of a conformal coating that will inhibit corrosion and keep the air gap minimized. Someone here may know of this.

I'm assuming these are located in an area where humidity can cause this corrosion. It does not take much.

First disconnect and lock-out/tag-out the supply source(s).

Good Luck, Old Salt

Many good manufacturers spray a fine mist of anticorrosive oil which percolates by capillary action between laminations and helps reduce magnetostictive noise. It also oozes onto the surface and forms a very fine (5 microns or so) layer which does not interfere with the normal operation of the magnet. If I recall, the name of the chemical is Tectyl 472.

The best way to reduce (even eliminate) noise is to use a DC coil. Many elevator/escalator operating contactors use DC coils with a built-in rectifier since the incoming coil supply is AC.

You do not write if only some contactors hum more than others or all hum objectionably. Many contactor coils are AC powered. This is simple and reliable but inherently produces 100Hz or 120 Hz mechanical vibration and noise.

Basically, the smaller the air gap in the iron path, the greater the inductance and the smaller the current and noise (current and noise will increase with coil voltage). But there is a minimum gap to avoid lock-on due to remanent magnetism while the coil is de-energised.

"Spades"has given reasons why some may be more noisy than others.

You might be afflicted by a particularly noisy contactor design or panelling which resonates at the vibrating frequency. A microphone and amplifier with level meter might track down a particularly noisy item. Resilient rubber mountings (for each contactor) might work or stiffening a panel to move "off resonance - but these are difficult beyond the design/construction stage.

Modern panels close to personnel would use DC contactors where noise could be an irritation. This is not a feature of lowest cost design. But you might [on questioning the contactor maker] find that the coils on your contactors can be "field-changed" for DC coils with built-in diode bridges - the shading turn by the gap, needed for AC, will slow DC pick-up and drop-out and need to be removed.

If your contactors are fed from a common supply, such as a control transformer, it could be that the normal voltage "out" is at the top end or above the contactor coil specified band - you might be able to use a higher voltage transfo primary tap (also cuts transfo noise). Control transfos may be oversized for a small number of contactors and be cheap [with poor regulation and high voltage on small loads]. N.B. It is possible your primary supply voltage is high, which would affect motor efficiency, flux and noise as well as contactors.

If you suffer low supply frequency, this increases magnetising current and noise but nothing can be done about that!

Basically the contactor laminated E- core surface properly worn out or dirt accumulated on the surface of the laminated E -core. This cause the closing gap point between the upper laminated E -core and the lower laminated E -core are not properly closed and during energize the contactor the gap cause the humming sound.

The only best thing to do is to replace the contactor.