[AMRadio] Ferroresonant transformer revisited

Jim Candela jcandela at prodigy.net
Sun May 7 00:01:33 EDT 2006


    The ferroresonant transformer is not really
resonant at all since the winding that feeds the
capacitor is wrapped around a portion of the core
that is a saturable reactor. Resonance and saturable
reactors don't really go together, but in this case we
do have circulating energy in a ferroresonant
transformer which consumes about 20% of the VA rating
of the device. So for a 1 KVA ferroresonant
transformer the power draw when unloaded may be about
200 watts, and this is pretty much a constant.
Therefore with a 1 KVA load, the power input would be
1.2 kva. Every line cycle runs the core into
saturation, and this creates eddie core losses, heat,
buzz, and a quasi square wave output. The output is
not a sine wave unless yours has a harmonic filter.
Therefore a true RMS meter is needed to accurately
measure the AC voltage.

These are really neat devices using very old

--- Rick Brashear <rickbras at airmail.net> wrote:

> A while back we had a lengthy discussion about
> ferroresonant 
> transformers.  I received a lot of excellent advice
> and help.  I have 
> finally been able to apply what I learned to a large
> ferroresonant 
> transformer that produces about 24 volts AC and when
> all is said and 
> done the rectified and filtered voltage is about 33
> vdc, but drops to an 
> average of about 27 vdc when loaded with a 5 a amp
> load.  The 
> transformer itself is capable of 50 - 75 amps.  It's
> a big one.  It 
> seems during the discussions someone made the
> statement that these 
> transformers had a considerable "buzz" and I can
> verify that, it hums 
> like crazy.  It was also said that they run rather
> warm or even hot.  
> That's what I'm curious about.  How hot?   I left it
> on for about an 
> hour with no load at all and I can barely put my
> hand on the top of the 
> core.  Is this normal?  Should I use a fan with it? 
> It uses a 15 mfd @ 
> 660 vac capacitor to attain resonance and it is new,
> so I don't suspect 
> a problem there.  It draws approximately .9 amp from
> the ac line when 
> idling, so I don't feel that is excessive.  Do they
> just run that hot?
> Thanks for wading through all of this.
> Rick/K5IZ
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