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Cheapest tool to measure Total Harmonic Distortion

Discussion in 'General Electronics Discussion' started by Peter2016, Mar 19, 2016.

  1. Peter2016

    Peter2016

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    Mar 19, 2016
    Hi all,

    this is my first post. I have an expensive natural gas furnace using an ECM motor (convert AC to DC for blower motor) and it is very sensitive to voltage, Hz and THD. Basically THD needs to be less than 5% for optimal operation and anything above 8% will end up damaging the control board.

    I bought a generator as back-up power for blackouts but I would like to be able to measure the THD of my gen. I would have loved to use a Fluke 43B but $4K is a bit too much for the occasion ;) And rental isn't really practical. I would prefer buying a tool, if financially possible

    I'm not versed into electronic equipment. So my question is; is there another more affordable tool one can use to determine the THD of the electricity produced by a generator?

    Thank you all for your help.
     
  2. dorke

    dorke

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    Jun 20, 2015
    Is free of charge cheap enough for ya ?;)
    This software scope can measure THD as well.
    You will need to reduce the level of your signal to less than 0.7V peak to peak...by a transformer and voltage divider.
     
    Last edited: Mar 19, 2016
    hevans1944 likes this.
  3. Peter2016

    Peter2016

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    Mar 19, 2016
    Thanks a lot.

    But I have one question for you. It says it requires a PC with a sound card. But if the PC has a built-in sound card on the motherboard itself instead of a sound card inserted into a slot on the motherboard, is that the same thing?

    Thanks.
     
  4. dorke

    dorke

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    Jun 20, 2015
    Yes, it is the same.
    Remember,
    you have to reduce the voltage you are feeding it in order not to damage it.
     
  5. Peter2016

    Peter2016

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    Mar 19, 2016
    I am not too versed in electricity, so I am concerned I could destroy the PC...

    So if I were to buy a clamp meter to measure THD, which one would you recommend?
     
    Last edited: Mar 19, 2016
  6. dorke

    dorke

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    Jun 20, 2015
    As you already know,these professional tools are very expansive.
    If you are not going to use them on a daily baisis I wouldn't recommend one for you.
     
  7. Peter2016

    Peter2016

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    Mar 19, 2016
    Based on my brief research, it seems I could find something in the $200-$300US, or are those garbage? I only need it for this 'project', but I would like to also measure the THD out of my 3 APC UPS, 2 voltage regulators and I will end up with two different brand/model of generators. One on gasoline and the other on propane. So I really would like to know as it is far cheaper than changing the board in the furnace/freezer/fridge.

    Thanks.
     
  8. dorke

    dorke

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    Jun 20, 2015
    Please post a link to those testers.
     
  9. Peter2016

    Peter2016

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    Mar 19, 2016
  10. dorke

    dorke

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    Jun 20, 2015
    Is your furnace 3 phase or single phase?

    Can you please post the specs. of your generator.
    If it is a diesel/fuel generator it will probably have a clean sinusoidal signal while running (very low THD).
    The problems will probably be when you switch it on and off.
    Doing that willingly you should disconnect the furnace/generator load switch before you do.
    But think about the case it runs out of fuel and dies out while the furnace is still connected to it...

    You have nothing to fear about connecting to a PC line-in.
    It is quit simple anyone can do it.

    Here is how in your case:
    If you have a small,low power 120 to 12V AC Transformer (any low voltage will do 3VAC-18VAC or so ) .
    It needs to be AC(not DC!) and not an "electronic one",
    simply a pure transformer.
    There are wall-warts like that available or "bare transformers",in case you need to buy one(for almost nothing).

    Connect like this:
    transf.jpg
     
  11. Peter2016

    Peter2016

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    Mar 19, 2016
    Thanks for providing a schematic. I will give it a try.

    Is your furnace 3 phase or single phase? Single phase.

    Can you please post the specs. of your generator.

    The gasoline is an inverter with less than 5%THD, so I'm safe for the furnace. Moreover the furnace will first go thru a Line-R 1250 voltage regulator. And that V.R. will be connected to the inverter. So I'm very safe for the furnace.

    If it is a diesel/fuel generator it will probably have a clean sinusoidal signal while running (very low THD).

    The other portable gen is propane and has supposedly a THD of 13%, which is damaging to all sensitive electronics, battery chargers, etc. But that is not an issue as the furnace will be on the inverter. I want to use the propane gen with chest freezers and fridge (no fancy display panel). The only question is will the electric motors overheat due to the excessive THD? Hz and voltage are actually very good with the propane gen.

    The problems will probably be when you switch it on and off.
    Doing that willingly you should disconnect the furnace/generator load switch before you do.
    But think about the case it runs out of fuel and dies out while the furnace is still connected to it...

    Not an issue as one must never let a gen run out of fuel while it is producing electricity to a load. One must always first unplug any load from the gen and then let it run at idle for 2-3 minutes in order to cool off and only after that turn it off. Letting any generator 'die' while there is a load on it will eventually kill the electronic components of the gen.
     
  12. dorke

    dorke

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    Jun 20, 2015
    The only question is will the electric motors overheat due to the excessive THD?

    I don't know.
    The best would be to contact the manufacture and ask them.
     
  13. Peter2016

    Peter2016

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    Mar 19, 2016
    Will do. Which one would you prefer to use between those two?

    The Freedom phone actually provides 10.6V, while the other actually provides 16.5V

    100_0029.JPG 100_0027.JPG
     
  14. dorke

    dorke

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    Jun 20, 2015
    I think the black one would plug directly to the outlet and it is lower voltage.
    So the black one.
     
  15. Peter2016

    Peter2016

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    Mar 19, 2016
    Working like a charm. Thank you very much!
     
    Last edited: Mar 21, 2016
  16. dorke

    dorke

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    Jun 20, 2015
    The scope is showing a square wave,no wonder it has 45% THD.

    That can be because of the PC Line-IN volume control is set too high, so the audio card is saturated.
    Or that the input signal is still too high.

    Just to be clear,
    Don't use the mic-input,use the Line-In.
    You should select only the Line-In to be active(in the PC mater-volume/selector) and set the volume relatively low.

    so a sine wave should be displayed in the scope.
    like so:
    lin-in 1.jpg
    line-in 2.jpg

    If that doesn't help, we need to further reduce the input signal level.
    we can do that by reducing R2 from 100 ohm say to 50 (placing another 100 ohm in parallel with the existing one).
     
  17. Peter2016

    Peter2016

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    Mar 19, 2016
    Looks like we were both writing at the same time. I replied before seeing your latest reply. I went into the control for the microphone and reduced the boost to zero as well as the mic set to zero.

    I will now read your reply. Btw, THD is now 4.9Hz :)

    Thanks a lot!!!!!
     
  18. dorke

    dorke

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    Jun 20, 2015
    Please post the signals of both generators(oscope and frequency).
    it would be interesting to compare them.
     
  19. Peter2016

    Peter2016

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    Mar 19, 2016
    Will do, but it might take me a few hours of days?

    You see, I got the PC to work fine by playing with the mic and boost setup. Then I plugged it in the Asus netbook (I can't drag the desktop outside lol!) and at first it was working fine without me doing any changes. But then I got some crazy values again. I'm using Windows 7 and maybe my menu is slightly different than yours. So I need to play more with the setup of the netbook to find and adjust the needed values. The netbook has Windows 7 basic(???) While the desktop has Premium(???).

    Anyway I will post them after I can get the Netbook setting right. It might just take me a little bit of time (hours or days).
     
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