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Buzzing change over

lyledunn

A fairly loud buzzing noise has recently appeared in a 200A changeover switch. Everything checked for tightness. Assumption is that it is one of the coils but I am unsure if this is an issue of concern. 

Parents
  • 0

    Ah yes that is a really good point - perhaps I am too complacent  in my previous advice - if there is a contactor not quite 'sitting down' so there is an air gap in the magnetic path, it  will draw a lot more current as the inductance of the solenoid, rather than the wire resistance, limits the current  once the armature has moved into the end stops. The coil has a much higher inductance (impedance to AC ) once it has a complete loop of iron core, compared to one with a mm or so missing..
    With an AC supply this is a nice way of giving a huge pull-in current to do things with a satisfying bang, and yet have a much lower holding current one when the contacts have shut. I've never had to do the contact filing thing but I have seen a stray grain of sand in the gap cause a coil to overheat, which I guess is the same problem.
    M.

Reply
  • 0

    Ah yes that is a really good point - perhaps I am too complacent  in my previous advice - if there is a contactor not quite 'sitting down' so there is an air gap in the magnetic path, it  will draw a lot more current as the inductance of the solenoid, rather than the wire resistance, limits the current  once the armature has moved into the end stops. The coil has a much higher inductance (impedance to AC ) once it has a complete loop of iron core, compared to one with a mm or so missing..
    With an AC supply this is a nice way of giving a huge pull-in current to do things with a satisfying bang, and yet have a much lower holding current one when the contacts have shut. I've never had to do the contact filing thing but I have seen a stray grain of sand in the gap cause a coil to overheat, which I guess is the same problem.
    M.

Children
  • 0 in reply to mapj1

    The iron cores used to have what what known as a shading ring fitted to them to cancel out circulating currents - has this shaken loose and fallen off? If so, it can result in buzzy coils as described above.

  • 0 in reply to whjohnson

    Weren't the copper shading rings not designed to create a "remanent" circulating current at each armature face, to maintain the magnetic attraction during zero cycles?

    Jaymack 

  • 0 in reply to Jaymack

    Not sure, all we were taught was that they were there to 'short out' circulating or rather 'eddy currents' as they were known as back then,  in the iron core, otherwise it got hot and buzzed.

    We didn't go that deeply into it at the time - C&G 200 series 1970's.

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