SOLA Constant Voltage Transformer

Try loading it with a light bulb.

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How are you measuring the voltage?

It is not a sine wave output, which causes inaccurate readings with a typical meter. You need a "True RMS volt meter" to read the actual voltage.

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Most constant-voltage transformers output a semi-square wave (sort of a sinusoid with the tops flattened). It will not measure "properly" with most voltmeters.

Isaac

I don't understand why a ferroresonant transformer / capacitor combination, should output anything other than a sine wave. Many years ago, I worked for a company which made use of these devices to feed radio relay equipment, and as far as I recall, they just had a normal sinewave output. I had a quick look on the 'net at some Sola ones, and there is nothing in the specs to suggest other than a sinewave output. It does, however, state that the output voltage can be have a variance of +10 to -20% of the nominal rating plate value. For the OP's example, that is potentially nearly 12v high worst case, which added to the 118, gets pretty damn close to the 130 odd that he is measuring. I wouldn't have expected his scanner to be that fussed about this sort of level of overvolts anyway. Depends to some extent on the type of power supply it uses. Also, as someone else suggested, loading the CVT will probably bring the voltage down a little, although it wouldn't be a very good CV source, if it dropped too far ...

Arfa

Some of the SOLA units do produce a slightly squared wave. The higher the line voltage, the more the sinewave is flat topped. They also built motorized low distortion regulators that adjusted the line voltage in 1/10 volt steps. I had one at a military TV station in the early '70s. It would add or subtract 20 volts to the line, but the small power grid in that area was so bad that I saw the line voltage drop to

90 volts, and shot up to 210 volts AFTER the regulator one day when some idiot tried to switch the wrong generators to and from the grid. Some of the small towns didn't bother to call the other stations, and weren't very good with the manual controls that were still in use.

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So what's the reasoning behind it then, Michael ? Is it that the core actually saturates to square off the top of the wave ?

Arfa

Yes, and some items are designed to only be powered form these transformers. CATV line amplifiers are typically line powered though the hardline coax by a 60 VAC 15 Amp CVT power supply, which was perfect when the early square wave UPS systems came out. The 15 amps is fed from somewhere near the center of a span, with roughly 50% in each direction. The power supply modules had warnings not to use a sine wave source for bench testing. I've worked with Sylvania/Texscan, Jerrold, Vikoa, and IEE/RCA systems. They all used this system, but the older equipment was 30 VAC at 30 amps.

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Have you checked the SOLA web site? Can you give us a model number?

Bill

Thanks for that Michael. Very interesting.

Arfa

You're very welcome. I like it that some people on these groups actually want to learn more about electronics. I've been studying every aspect of it that I could for over 40 years. The more you know about it, the easier it is to fill in the gaps. :)

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some of the units that i have owned have a large capacitor as part of the assembly. if your unit has one check that the capacitor has not failed or changed value.

Wouldn't it have been easier to just say "yes" since he specifically asked whether it could be the capacitor?

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"Arfa Daily" wrote in news:iwtph.9259$KQ2.5350 @newsfe6-win.ntli.net:

Some Googling turned up more info about constant-voltage transformers.

It is indeed saturation of the core that provides the "constant voltage" feature. (If the input voltage goes up, and the core is already in saturation, the output voltage won't change.)

For this reason, early CVTs had a non-sinusoidal output.

However, modern CVTs can be bought with "low-harmonic" output, meaning it's a sine wave without any harmonics. This is achieved by a third winding on the core, connected to a capacitor to resonate at the power- line frequency. (It acts like a tank circuit, picking out the fundamental frequency and discarding the harmonics.)

Now, if the winding/capacitor resonant frequency differs from the line frequency, the CVT can't be expected to deliver its specs.

1. Buy a 60Hz CVT and connect it to 50Hz, don't expect it to work.

1. Connect a CVT to a gas-engine alternator (poor frequency control) don't expect it to work.

2. Capacitor changes value or open-circuits, don't expect it to work.

Bottom line: Depending on the age and design of the CVT, it may or may not have sine wave output.

P.S. In any case, the core saturates every cycle, so expect the transformer to get hot (hysteresis loss).

I've found a generator to have pretty decent frequency control, the one I borrowed during a recent lengthy outage measured anywhere between

58-60 Hz depending on load. Would 2Hz really be enough to mess up the performance of a CVT?

James Sweet wrote in news:F0Wph.995$q32.463 @trndny01:

Just looking through Sola's site, I saw one of their CVTs rated 57-63 Hz, so I would expect a CVT would probably work OK with the generator you tested. Do they all hold frequency that well?

Heck if I know, that's the only one I've ever used. I'd imagine it depends on how much horsepower overhead the motor has and the condition and adjustment of the governor. The throttle response of the carburetor would be an important factor as well. I'm curious how much the output would be affected as the frequency drifted.

come to think of it, it could be the capacitor.

here are some schematics for those that are interested

James Sweet wrote in news:MnYph.603$Ch1.153 @trndny04:

According to Sola's PDF for its CVS series of constant-voltage transformers,

you can expect the output voltage to rise about 1.5% for each 1% rise in the frequency. So, freq goes up 1Hz, 1/60 = 1.7% x 1.5 = 2.5% x 120V =

3V rise.

This is true for their CVS series transformers---with other models, YMMV.