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SOLA Constant Voltage Transformer

Discussion in 'Electronic Repair' started by dre, Jan 11, 2007.

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  1. dre

    dre Guest


    New poster here, just wondering if anyuone can help me with this beast.
    I'm sort of hoping to use it as an isolation transformer for my
    scanner. Overkill, I know, but it cost me all of 15 bucks. The
    problem is the voltage it produces on the secondary is 132 volts with
    line voltage in. The plate states secondary voltage is 118, so what's
    causing the overage? Do I not have it loaded enough (rating, IIRC is
    2.5 A) or could it be the capacitor failing? Any advice is


  2. CJT

    CJT Guest

    Try loading it with a light bulb.
  3. How are you measuring the voltage?


  4. 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

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  5. isw

    isw Guest

    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.

  6. Arfa Daily

    Arfa Daily Guest

    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 ...


  7. 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.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  8. Arfa Daily

    Arfa Daily Guest

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


  9. 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.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  10. Bill Jeffrey

    Bill Jeffrey Guest

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

  11. Arfa Daily

    Arfa Daily Guest

    Thanks for that Michael. Very interesting.


  12. 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. :)

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  13. TimPerry

    TimPerry Guest

    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.
  14. CJT

    CJT Guest

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

    Jim Land Guest

    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.

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

    3. 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).
  16. James Sweet

    James Sweet Guest

    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?
  17. Jim Land

    Jim Land Guest

    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?
  18. James Sweet

    James Sweet Guest

    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.
  19. TimPerry

    TimPerry Guest

  20. Jim Land

    Jim Land Guest

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

    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.
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