Don Foreman wrote:

On Tue, 27 Dec 2005 14:51:50 GMT, JohnF
wrote:

I have a large expensive contactor/relay that buzzes so hard it has
destroyed the aux contacts mounted on it. Is there some way to stop
the buzzing? A replacement is nearly $3000.00 and money is very scarce
around here right now.

TIA

JohnF

AC relays and solenoids often have a "shading coil" to reduce buzz.
Look for something like a D made of heavy copper that surrounds only
part of the core. Sometimes it's heavy copper wire that goes thru a
hole or slot in the core. If that is missing or open (cracked),
the unit will buzz loudly. It's usually found near the "business" end
of the coil, near the armature that it moves.

How it works: the current in the coil is primarily inductive, so
about 90 out of phase with applied voltage. The current in the
shading coil is determined by transformer action from part of the
flux (since it doesn't encircle the whole core) and its (low)
resistance, so it's more in phase with applied voltage. The result
is "two phase" local magnetic flux which may fluctuate but never
goes thru zero.

If you go with DC excitation as Jeff suggests, first measure the
resistance of the coil with an ohmmeter and look at the rated AC coil
current The right average DC voltage will be the rated current times
the resistance reading. You'll probably find that it's much lower
than the rated AC voltage, so you'll probably need a dropping
transformer before your bridge rectifier.

Some shading coils on older contactors are made with "U" type threaded
rod with nuts. You just have to clean the thing up and it will kill the
noise. I had one a while ago that was on a big motor gen set with the
big mercury vapor tubes, for a 150 hp vertical boring mill . The
noise from the contactor could be heard through the whole plant. The
local maintenance guy couldn't believe that what I told him to do would
fix the noise but after he did it, the thing was completely quiet. I
couldn't do anything for the motor gen noise though. G

John