I picked this up this past week and thought it would be great for the bench to keep the line voltage regulated at 118 VAC.

This is a Sola Constant Voltage Transformer Model 20-13-210 and has the following ratings:
Primary: 90-135 VAC
Secondary: 118 VAC
Frequency: 60 Hz
Amps: 8.48

I wired it up to test and found it hums/buzzes like mad. My house voltage this morning was at 128.5 and the output of this unit was also 128.5 unloaded. I put a light 200 watt load on it and no change on the output voltage.

I popped it apart and there seem to be a series of large AC caps. Any thoughts on if replacing the caps would restore the functionality and give me the 118 v I'm expecting? Also, is the buzzing the nature of the beast? I had hoped to mount next to my electrical panel and hardwire it to my bench as "regulated power", but would not be able to live with the noise.

Also - the caps are not labeled. Thoughts on values?

Looking for general thoughts, comments, opinions.

(1) Is this worth fixing (next to no cost so far)?
(2) Is the transformer buzz just normal noise on a unit this size?
(3) Would it draw too much current ($) to leave run for bench power?
(4) Guesses on the capacitor values?
(5) Anyone have any experience with these?

Mark

Sola is still in business and making regulating transformers. Very expensive if bought new.

http://www.solahd.com/products/transformers/Index.htm

Those caps should still be good but notice there is some physical damage. You may hear some mechanical hum from the transformer but in operation it should clean up some electrical noise.

Thanks Norm. I did send them an email.

The mechanical noise would be too much to have to listen to, but maybe I'll look again and see if the transformer bolts are tight.

The one cap does have a dent so it could be damaged internally.

Wouldn't the "buck" function of the transformer work regardless of the caps?

Mark

You need the caps for regulation. Not really a buck function but more like a tuned circuit.

Okay - I had assumed based on the spec plate that it had a buck function.

Primary: 90-135 VAC
Secondary: 118 VAC
Frequency: 60 Hz
Amps: 8.48

Most of those Solas make a racket. That's why I don't use mine.

Mark,

Those old Sola CVTs are amazing devices. Most models can serve as isolation transformers, and if working correctly will correct for changes in the incoming AC line. They are usually quite inefficient and can be pretty noisy, as you have noticed.

They work by having a resonant secondary loosely coupled to the primary, with the secondary part of the core operating in partial saturation.

The resonating capacitance value is critical, too much or too little capacitance will fail to resonate at the line frequency and the output will not be properly regulated. Often the capacitors are connected to a separate secondary “tank” winding that is closely coupled to the output winding, but is wound for a higher voltage. Be careful with the covers off, there may be far more than 120 V in there! This higher voltage is determined by the economics of capacitor construction. Up to a point, the required capacitors are less expensive as the AC voltage is increased.

A 60 Hz rated unit will not work at 50 Hz, and vice versa. They are not of much use on a small gasoline generator set, where the frequency wanders around.

The output of most models is a semi-square wave, not a sine wave. They are designed to regulate the RMS value of the output voltage. An attached light bulb should maintain constant brightness as the line goes up and down.

The funny waveform requires an RMS responding meter for proper measurement. A common, average-responding voltmeter will read high on the squared-off waveform. Offhand, the 128.5 V value your meter registered seems a bit too much above the nameplate secondary value, but the transformer may be working correctly.

If yours is actually working, or you can get it working, you can connect a variac after the CVT for working on radios.

The idle current is pretty high on these things. You do not want to leave one running all the time.

With the squared-off waveform, the DC voltages in a radio will run a bit lower than they would on a sinewave line, assuming the heater voltage is set at the nominal value. This might cause some confusion when checking DC voltages.

Ted

Excellent info Ted - thanks. I did find a generic schematic from Sola that helps me (visually) understand.



I did find the capacitor values and they are all 5 MFD @ 660 WVAC, wired in parallel. Maybe replacing these would bring it back into proper resonating frequency?

I have a good friend who was hired as the QA engineer for Sola back in the 1960s. Their approach for reducing acoustic noise was to whack away at the transformer laminations with a BFH while powered on the test bench - a technique that could indeed reduce the hum with appropriately placed impacts. Unfortunately, the array of meters to monitor performance leaped off the bench with the same impacts...a response that could hardly be conducive to accuracy or consistent results.

Being a person of high integrity, he didn't last long in that job...

Anyway, a plywood box stuffed with fiberglass batting does a decent job of reducing the hum. Heat may be an issue...it has greater efficiency at design loads than it does lightly loaded.

I wonder if really goobering up the transformer laminations with varnish or paint would stick them together and quiet it down some? First I would make sure all the bolts going thru the laminations are good and tight.
Curt

I probably won't bash the laminations unless it falls off the bench on to my foot... However, I did read on the Sola site that this "Ferroresonance" type conditioner is by default noisy "due to the alternating flux in the core producing a phenomenon known as magnetostriction" and that ~45db SPL would be normal. So, I need to dig out the SPL meter and see where we are.

I'm not sure this is going to be worth the effort, but I will try replacing the caps and might even look at varnishing the laminations. I did get at the bolts and snugged them up - they were a little loose, but it made no difference with the noise. I guess I'll also wait and see what Sola replies with.

Ted - one more thing; I was using my Fluke 77 (gen 1) so it may well be the case that I am reading high.

I use the same model in the ham shack and with various fans running I dont notice any noise.

Another is used on the upstairs PC desk, where two PC are used, only when Im on generator power. I can hear the hum but with a radio on its gone.

SOLA's do not like to be run unloaded, they get hot and cranky as well as raise your electric bill if run 24/7.

Carl

There are two kinds of SOLA, CVS which makes a sine-wave output, and CVN, which you probably have, makes almost a square wave. The square wave is much easier on the following rectifiers, as it does not have a sharp peak. The square wave is easier to filter to pure DC, as well. Don't try to hit the laminations, they are not loose. The original capacitors are oil-filled and probably still good, although the old ones had PCB oil in them. At 1000 watts, this one is probably way too big to power a few radios properly. It wants to power a transmitter or minicomputer or a large server. I have a 500 watt unit powering PCs for the last 25 years and never lost a supply or hard drive.
Don

Can't add much except to say that I rescued one from a local datacenter with the idea of using it on the bench but gave up because it was too noisy. It's also my understanding that magnetically regulated items like this require a load coming close to their rating to function effectively.

--------------------------------------------------------------------------------

Found on curb today, what would it have been used for??
Name plate reads,
Sola Constant Voltage Transformer
Volts input 95-130 X190-260
Output 120Volts
VA250 60 freq.
Has line cord and 3 prong female receptacle on the front, Has a flange on back for wall mounting.
5"x5"x12" size.
Forwarded from April Finds & Loses.

I did hear back from Sola. Their answers are prett much what everyone here has offered:

Question: I understand about the buzz/noise associated with this type of transformer, but this is quite loud - well over 65 DB SPL. This is measured sitting on a test bench from 1 foot.
Answer: Noise is associated if unit is not running under load, recommended is at least 40-50% Do not overload as well.

Question: Do the capacitors need replacement as part of maintenance?
Answer: Yes

Question: Do I need to observe Hot/neutral on the input and output connections?
Answer: Yes

Question: Should the output be at 118 VAC as listed on the label? I am not using a true RMA meter to read.
Answer: Please use a true RMS meter

Did anybody mention, these things run VERY HOT?

Also, these devices are a load themselves. Do not leave them plugged in unless you are actually using them.

If I end up using this, I may mount it in the garage. The other side of the wall in the basement is my house panel and next to that my shop space. So, maybe the noise and heat would not be too big a factor.

One more thing I don't understand is how it handles hot/neutral. There are two terminals for in and two for out and no indication of hot/neutral - which I understand looking at the schematic above (it's a transformer). But - if I pick one input terminal and wire to the hot side, I read 128 vac from the case to safety ground! Flipping the hot/neutral on the input terminals and I get no reading.

I also am wondering what to do with my safety ground. As you all can see from my picture above I wired the ground straight through while bench testing. Should safety ground be tied to the transformer case?