Pros/Cons of Ferroresonant Transformers

[mklarich]

I posted a question the other day about rectifying 90-125AC down to 48VDC. I ran into the problem I knew I would, voltage regulation over the voltage range.

After a little research I found ferroresonant transformers, they claim to hold output voltage within 3%.

I know that they heat up when they are near their minimum load and maximum load and actually cool off when they are at nominal load. The graph would be an upside down bell curve essentially.

I know they are slightly more expensive then a standard linear transformer.

I don't know any other pros or cons about ferroresonant transformers. My question to everyone what is their personal experience and knowledge about these transformers??? I figure from how good they look more people would be using them, but I haven't heard much about them before.

Thanks in advance,
Matt Klarich

 

[Skogsgurra]

They are good. I use them to protect my equipment (rural grid) from voltage deviations and transients.

They produce a humming sound that drove my collegue crazy (well, not crazy - but irritated). They also produce some heat.

A good thing is that they hold the voltage during deep dips. If lightly loaded, they can keep voltage up for up to 50 milliseconds, which can be good in some cases.

I had one problem with one of these. A gas analyzer had been connected to a stabilizer and when the mains was switched off (primary, switch before stabilizer) the switch-mode power supply in the analyser died.

The problem was that the PSU (an old one without primary voltage supervision) continued to try delivering full power when the input voltage went down. It was only when the voltage had reached less than ten percent of nominal that the PSU switched off entirely. In the meantime it had been drawing increasingly more current and just before switching off altogether, we had ten times normal current - which killed the switch transistor.

So, make sure that your switch-mode power supplies can handle the decaying voltage that a magnetic stabilizer delivers when the primary is switched off.

Otherwise, they are as good as you say. The slight voltage distortion that they introduce when input voltage is high is not a problem

 

[jraef]

I used to use them all the time, and still recommend them to others having voltage disturbance problems. There are issues with using them however, most being very application specific as illustrated above by skogsgurra. If you do a Keyword search (use the drop-down box at the top of this column) on the word "Sola" (the inventor and now largest nfgr of ferros), and then again on "CVT" (for Constant Voltage Transformer, the generic name for them), you will get hits on quite a number of good discussions in this forum.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Warpspeed]

CVTs are good, but not without problems of size, cost, heat, noise, and efficiency. But they sure work.

One feature of interest might be that they are current limited. You can dead short the output without damage. Conventional ac transformers will carry high short term overloads (within reason) a CVT will not. Beyond a certain load current the output voltage of a CVT will just suddenly collapse.

When used with a rectifier driving one load that might be a very handy feature. When used with a large dc distribution system it might not be. If it is heavily loaded, and a sudden extra inrush current is demanded somewhere, it might cause very severe voltage sags. I suppose it depends on the application, CVT ratings, and your expectations.

A normal phase controlled rectifier with tightly regulated dc output voltage might be better.

 

[Vinnyd]

Does't ferroresonance have to do with iron core transformers? I understand on a power system it is undesireable to make them alive from the low voltage side with the primary side breakers open. This can cause damage to the transformer?

Vinnyd

 

[mc5w]

According to the IEEE Orange book a CVT will convert frequency excursions into spikes when running off of a generator. If you do need to run off of a generator you might consider a dual conversion (genuine) uninterruptible power supply that converts the incoming AC into DC, runs that past the battery bank, then inverts back to AC.

The cheaper UPSs use an inverter and solid state transfer switch. The load normally runs off of line power and the inverter cuts in and carries load curing a power failure. These take advantage of the matter that computer power supplies can ride out a 1/2 to 1 cycle loss of power.