# Cheapest tool to measure Total Harmonic Distortion

#### Peter2016

Mar 19, 2016
24
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.

Thanks.

Jun 20, 2015
2,342

Mar 19, 2016
24

#### dorke

Jun 20, 2015
2,342
Is your furnace 3 phase or single phase?

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:

#### Peter2016

Mar 19, 2016
24
Thanks for providing a schematic. I will give it a try.

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

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.

#### dorke

Jun 20, 2015
2,342
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.

#### Peter2016

Mar 19, 2016
24
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

#### dorke

Jun 20, 2015
2,342
I think the black one would plug directly to the outlet and it is lower voltage.
So the black one.

#### Peter2016

Mar 19, 2016
24
Working like a charm. Thank you very much!

Last edited:

#### dorke

Jun 20, 2015
2,342
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:

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).

#### Peter2016

Mar 19, 2016
24
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.

Thanks a lot!!!!!

#### dorke

Jun 20, 2015
2,342
Please post the signals of both generators(oscope and frequency).
it would be interesting to compare them.

#### Peter2016

Mar 19, 2016
24
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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