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Wes Lemarr
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Joined: 07/09/2008
Location: United States
Posts: 35

Posted: 05:45pm 07 Dec 2008

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I am trying to work out a way to measure exactly how many Kwh of power my new generator is delivering at any given RPM. I am running it on a test bench driving it with a variable speed motor. If I take the three AC outputs directly from the generator & hooked them directly to a "grid connect inverter" the type that is used to feed AC back into the grid & then hooked the grid connect inverter up to an ordinary house electric meter, I think that I should be able to get an exact Kwh reading that the generator is producing at the RPM that I am running (testing) the generator at? Please let me know what you think? Thanks for your comments.

Wes LeMarr

KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 06:24pm 07 Dec 2008

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Hi Wes
If your intention is to measure the performance of the generator I would suggest using that and as little other equipment as possible. I believe the conventional way of doing this is to use a dummy load and directly measure the current and voltage directly from which you can immediately calculate the watts.

Kwh, as you know, are 1000s of watts per hour and if your Kwh hour meter is anything like those installed by the power company I expect it will not directly read Kwh but instead you will have to run your rig for a rather long time before you can make a useful reading.

There has been discussion on dummy loads on this site, I am sure a search will find it.Edited by KiwiJohn 2008-12-09

GWatPE

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Joined: 01/09/2006
Location: Australia
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Posted: 08:55pm 07 Dec 2008

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Hi Wes,

The grid connect inverter is already behaving as the dummy load. If you are to record the instantaneous voltage and current to this inverter, then this can provide the power reading. There is no instant kWhr reading, as this measurement is the integrated power that flowed during that integration period.

Gordon.

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KiwiJohn
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Joined: 01/12/2005
Location: New Zealand
Posts: 691

Posted: 08:57pm 07 Dec 2008

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Gordon, where is the load? I understood Wes to be saying he was connecting his grid connect inverter to a meter, not to the mains?

Wes Lemarr
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Joined: 07/09/2008
Location: United States
Posts: 35

Posted: 02:57am 08 Dec 2008

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Kiwi John,

Yes you are correct what I am trying to determine is what a realistic Kwh rating for my new generator would be. Your suggestion about "use a dummy load and directly measure the current and voltage" & then just calculating the watts, is a much simpler & better approach. What should I use as a "Dummy Load" that won't burn out? At 500 RPM on the test stand I am getting voltages as high as 80 volts DC with current in the range of 39 amps. But I am not sure that I am measuring everything correctly.

Thanks
Wes LeMarr

KiwiJohn
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Joined: 01/12/2005
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Posted: 03:09am 08 Dec 2008

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Wes, if you do a search on this site you will find a lot of material relating to dummy loads. The figures you mention are over 3 kilowatts so you need something that can handle that amount of power.

If you can find something like a clothes dryer element of that range, I mean the ones that have a long wire element at the back, you could use it. If it is a 240V machine just divide the element into three and join them in parallel. A good thing about the clothes dryer and other wire type dummy loads is that you can see when they are getting too hot and back off a little.

However have a look at what others have done on this board too.

John

Wes Lemarr
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Joined: 07/09/2008
Location: United States
Posts: 35

Posted: 03:59am 08 Dec 2008

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John,

I have a simple single element 220 volt "Hot Plate" I think that should work?

Thanks
Wes LeMarr

Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5118

Posted: 04:06am 08 Dec 2008

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Hi Wes

Are you planning to use this kwh figure to rate your windmills? A 3kw alternator on the test bench wont make anything like 3kw as a windmill.

If your looking for a watt rating as a windmill, I believe you need to use a battery as a load and record watts against wind speed, not RPM. Maybe someone here knows of a accurate way to rate a windmill?

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
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KiwiJohn
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Location: New Zealand
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Posted: 04:36am 08 Dec 2008

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Hi Wes

The 200volt hot plate wont pass much current at 80volts. One of the formula for Watts is Volts squared over Resistance, the resistance will be the same but volts only one third.

Otherwise, I dont disagree with Glenn.

GWatPE

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Joined: 01/09/2006
Location: Australia
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Posted: 08:26am 08 Dec 2008

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Hi kiwijohn,

I tried to read between the lines. My mistake. I would not have mentioned a grid connect inverter unless I had one to use. I do use the grid as my main diversion load, as you would be aware.

Gordon.

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