Inverter Generator Frequency Question

Discussion in 'General Electronics Chat' started by ShadyOcUK, Aug 29, 2010.

  1. ShadyOcUK

    Thread Starter New Member

    Jul 4, 2008
    I have recently purchased a second hand sdmo booster 2000 inverter generator and have a question about the Frequency. Uk spec 230/240v and 50 HZ

    I have the UK equivalent of the kill-a-watt meter and if set to the frequency function with no load on the genny it shows 21HZ , put a load on and the frequency reading fluctuates between 04-87 HZ.

    If I use the same meter on a mains socket it reads 50Hz as expected, It also shows 50Hz as the output under load from some inverters powered by
    batteries in my caravan (this gives an indication that the meter works).

    I went to look at another identical model generator for sale second hand (from a different person) and this time took the "kill-a-watt" meter with me and backed out of the purchase due to the same readings, i.e 21Hz on load and fluctuating Hz onload.

    When back home , I was wondering if the floating neutral on the generators was confusing the kill-a-watt meter .So I made up a test extension lead with the neutral and earth conected and this time the no load frequency shows as 41Hz and the on load frequency shows as fluctuating between 40-70Hz .

    The tests where done with both a 1000w fan heater (so resistive load) and an old power drill (300w), the frequency fluctuations showed as greater with the power drill.

    It could be that both generators hundreds of miles apart have developed the same fault, the model of generator could be suspect or my method of testing could be suspect!!

    So my questions are ,

    1) should I expect to see a real stable (as advertised) 50hz (or 60hz on a USA model) output of an inverter generator ?

    2) Is a kill-a-watt meter a suitable way of testing the frequency of a generator output.

    3) Would a fluctuating frequency be likely to damage items plugged in?

    Cheers, Shady
  2. DonQ

    Active Member

    May 6, 2009
    You say inverter generator... I am assuming you mean an electronic inverter.

    Inverters generally, except very expensive ones, do not generate sine waves. Many meters expect sine waves for proper readings. I would expect this mismatch is largely what is causing your problems.

    Many frequency meters are easy to confuse when square-waves, ringing, fundamentals, and other such things are included in the signal, like what comes out of most inverters. Changing the resistive load, and especially changing the reactive load on the inverter can really change the details of the output waveform that is confusing the meter.

    Some sort of electronic or mechanical filtering is probably needed before you can measure the inverter frequency with meters not designed for signals with so many harmonics.
  3. ShadyOcUK

    Thread Starter New Member

    Jul 4, 2008
    when I say inverter generator . I mean a petrol powered suitcase type inverter generator that is supposed to put out a clean sine wave 50hz.
    the sdmo boosters use the same engines as the honda eu20i.

    I currently dont have a scope so cant check the actually wave output, but when testing my meter I have used it on both pure sine wave inverters and modified square wave inverters and it reads 50hz output from both.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    To lessen the confusion, a generator is a device that converts mechanical energy into electricity. An inverter converts DC voltage into AC. I believe it causes confusion to combine the terms.

    When the generator is running, does the pitch of the engine stay constant? Or does the pitch (engine RPM) hunt up and down? Most small generators use a mechanical governor to hold engine RPM reasonably constant.

    At 50 Hz, the engine might be expected to run at or around 3000 RPM. A drop to 21 Hz would also drop RPM's to around 1000 - something you would easily hear.
  5. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    A generator will normally have a sine wave output as opposed to an inverter.

    You have a very poorly governed generator with an unstable output frequency.

    Still useful for some purposes but probably not very good for use with anything too sophisticated or expensive.
  6. Heavydoody

    Active Member

    Jul 31, 2009
    I agree. However, in this case, I think the description may be accurate. I found this:
    How does the inverter work?

    Honda 's inverter technology takes the raw power produced by the generator and uses a special microprocessor to condition it through a multi-step process.
    First, the generator's alternator produces high voltage multiphase AC power. The AC power is then converted to DC. Finally the DC power is converted back to AC by the inverter. The inverter also smoothes and cleans the power to make it high quality. A special microprocessor controls the entire process, as well as the speed of the engine.
    Honda uses only high quality inverters in our generators, which produce stable, consistent power.
    The end result? Clean enough power to run even the most sensitive electronic equipment.


    They call it an inverter generator as well...
  7. Heavydoody

    Active Member

    Jul 31, 2009
    BTW, I used one of these Hondas on site powering a Hougan hole punch and multiple Dewalt battery chargers. It was mighty handy and worked well in this application. I never checked it with a meter though, and I can't speak to this brand or your needs...
  8. ShadyOcUK

    Thread Starter New Member

    Jul 4, 2008
    Ok , there appears to be some confusion on what an inverter generator is ,on an old style generator the frequency and voltage are governed by the RPM. With an inverter generator the output from the alternator is converted to DC and then inverted back to AC . This allows a stable voltage and frequency regardless of the engine speed , the engine speed (and noise level) is proportional to the load .

    originaly typed this before the above two replies.
    Last edited: Aug 29, 2010
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
    Sounds like the Honda ends up with an inverter circuit as the final output stage.