I found this,
https://www.solidstatecontrolsinc.com/-/media/ameteksolidstatecontrols/documentation/white-papers/ferroresonant-transformers-white-paper.pdf
which says in section 1.8 on page 8,
1.8 FREQUENCY SENSITIVITY
Referring to Figure 1.3, we see that the slope of the straight line
varies inversely with the square of the input frequency. As the frequency
increases, the slope of the straight line decreases and point C moves up
to a higher value of flux density. The output voltage of the regulator
is, therefore, sensitive to frequency variations, the relation being a
1.4% change in output voltage for a 1% change in input frequency
Chris
On Wednesday (07/16/2025 at 09:06PM +0000), Brendan McNeill via cctalk wrote:
> Hi Jon
>
> I restored a PDP-8 straight 8 that had a similar PSU. I am here in New
> Zealand with 230v 50Hz. I used a step down transformer 230v -> 110v but of
> course retained the 50hz. It worked just fine. I did replace the
> capacitor in question however. They contain PCB’s so use gloves and dispose
> with care.
>
> Kind regards
> Brendan
>
>
> --------------//----------------
> [email protected]<mailto:[email protected]>
> +64 21 881 883
>
>
>
> From: Jon Elson via cctalk <[email protected]>
> Date: Thursday, 17 July 2025 at 02:50
> To: Tom Hunter via cctalk <[email protected]>
> Cc: Jon Elson <[email protected]>
> Subject: [cctalk] Re: Ferroresonant transformer mystery
>
> On 7/16/25 09:13, Tom Hunter via cctalk wrote:
> > Recently I got a nice and complete PDP-8/s from the US. The power supply
> > uses a ferroresonant transformer which in addition to the standard primary
> > and secondary windings has a separate 2.3H winding connected in series to a
> > 2uF 660VAC capacitor forming a resonant "tank" circuit. The transformer's
> > secondary side and the resonant circuit are operated in saturation. There
> > is a magnetic shunt to prevent the primary side going into saturation as
> > well. It accepts a wide input voltage range, but is very sensitive to the
> > input frequency of 60Hz. This is quite a nice if not elegant design for the
> > period in question, but maybe not the most efficient.
> >
> > As I live in Australia I get 240VAC and 50Hz as opposed to the US 115VAC
> > and 60Hz.
> >
> > I can easily convert our 240VAC to 115VAC with a step-down transformer, but
> > cannot easily supply 115VAC at 60Hz. So I was considering using a step-down
> > transformer to get the 115VAC, but modify the resonant "tank circuit" for
> > 50Hz.
> >
> > Unfortunately there is some magic I don't understand. The resonant
> > frequency of a LC circuit with L=2.3H and C=2uF is about 75Hz not the
> > expected 60Hz.
> >
> > Otherwise I could just solve the standard LC resonant circuit formula for C
> > and plug in 50Hz and 2.3H to get the required C.
> >
> > Obviously ferroresonant transformers are more complex than this former
> > software engineer can grasp. Could any experienced EE with relevant
> > transformer knowledge please chime in and help me understand how to
> > redimension the tank circuit to use 50Hz instead of the original 60Hz input.
> >
> > Obviously I could replace the entire power supply with two modern switch
> > mode supplies to create the two rails, but it would be really nice to keep
> > the original supply and just reversibly adapt it for 50Hz.
>
> That L is probably different when the transformer is excited
> at full mains voltage and near saturation. Of course, this
> seems like it will reduce L and therefore drive the
> resonance higher!
>
> What I might do is make up a cap bank that is 1.2 X larger
> than the 2 uF and power it up. Then, measure the output
> voltage, and if it is within range just use it like that.
> If you want to get fancy, put it on a Variac and sweep the
> input voltage. You will note a reverse slope, as input
> voltage rises through the range, output voltage will decline.
>
> Jon
--
Chris Elmquist
