Electrodynamometers & Meter Coil Rewinding

Discussion in 'General Electronics Chat' started by ac4aq, Nov 4, 2010.

  1. ac4aq

    Thread Starter New Member

    Nov 4, 2010
    Hello, all. This is the first time I've posted, I hope that this is a new subject which is of interest.

    I have a Weston model 432 electrodynamometer-type wattmeter, which I have been unable to use as the needle is stuck. I suspect that the moving-coil has been overheated.

    Most likely this came about because I connected the E-input to an AC source of the wrong frequency. It has a dual input, terminals marked 75 and 150 plus a common, also the dial reads "DC and 25- Cycles". I gathered from this that the instrument could be used on a line voltage of 150 maximum, either DC or AC from 25 Hz up to some unknown frequency (why there is a blank after 25- is unknown). I made up a cord, with the E-input connected across the line (117 VAC, 60 Hz) and the I-input in series with the neutral and attempted to measure the power consumption of some small loads. The readings I got were low and I noticed the meter needle became stuck. After disconnect, the needle soon became free again. Foolishly I tried again and now the needle is stuck for good.

    Unfortunately I cannot find any data on this instrument on the web and the maker is long gone. Also I find no commercial repairer of meter movements, only some discussion for home-repair of meters in antique radios. The makes sense, as analog meters are used less and less all the time and most meter movements last long unless abused, also few meters have ever been worth repair. While this meter is almost certainly done for, I also have a Biddle insulation tester (a "megger") with an open moving coil. It feel it might be worth an attempt to rewind it.

    I ask anyone to speculate on what went haywire with the Weston, if you please. Did the blank after the 25- mean this instrument was for DC only? Also anyone with experience in rewinding a moving coil bobbin, please tell. It seems to me the difficulty here is the parts are tiny and delicate, one must have the touch of a watchmaker. Also the problem of determining the wire guage and number of turns in the original coil. And the wire is as fine as a silken thread, winding it without stretching or breaking isn't going to be easy.

    I know that I can buy a digital wattmeter, accurate for most power factors or waveforms, for a low price. I'm interested in restoring these two instruments, along with several others.

    Thanks for any thoughts on this,

    Last edited: Nov 4, 2010
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    If you think it is of Antique Value, then don't mess with it yourself unless you understand and have built meter movements before. They are VERY VERY delicate, but built from some very stiff and hard materials as well. Eye loop, small paint brush, shellac, fish paper and maybe some cotton gauze as well, small screwdrivers, and perhaps even some copper brazing might be called for. That old and you might discover some hardened steel magnets. If it has old style magnets, they will be worthless as magnets after so much time has passed. Steel demagnetizes over long periods of time without special care. So the meter may not even be close to accurate anymore. If the magnets are changed the scale would change due to the different level of magnetic flux this would introduce into the meter.

    Find an instrument repair shop and ask if they know of someone who repairs meter movements. It is a dying art these days. Very few individuals are left doing that anymore

    The twenty five was for 25Hz, a very common operating freq in industrial settings at the start of the 20th century.