Double your efficiency with a VFD!

That’s what one who visits Utterpower’s pages suggests.

Here’s an email snippet..

“What I am trying to say is that i think that PMG only requires 1hp electric motor for every 1kw output and that should be clearly seen at the input of the VFD in the form of about approximately 1150w input for every 1kw output at the gen head, thanks troy”

More background if you need it, or simply scroll down:

hi george, i talked with you earlier about connecting your pmg to your 10 hp motor and vfd to do a comparison analysis. you said you had some test gens that were in your shop. i believe if you connect the motor to the gen with a lovejoy coupling and wire it 240vac output and connect purely resistive lights to it at 3kw load it will read 240vac at 12.5 amps to the load from the gen output. with a two pole electric induction motor and a vfd that is converting single phase to three phase there should be about a 10% increase in input at the vfd to the out put of the gen head. you should see about 14-15 amps at 240vac single phase to the vfd and 12.5 amps on the output to the load. now in the field they say 2hp per 1kw which would mean 6hp for 3kw. however 6hp on that 10hp motor and vfd will be about 25-27 amps at 240vac on the single phase vfd input coming from the panel box to the vfd. I want to prove point to the gearheads in my area that your drive, motor(prime mover), and gen head at 3 kw output will only consume around 3.4kw from the panel box to produce 3kw at the gen head. if it takes 2hp then thats around 2.2kw from the panel box and so that gen would only be at most 50% efficient and i think it is higher than that. with the vfd losses of 1%, motor of around 5%, and gen around 5%, the input single phase should only be 10-15% higher than the output at the gen head. bottom line i want to couple that gen head to an electric motor and vfd and strap a 3kw load to it and test the input kw to the vfd. i bet if done correctly 3kw out of the gen only requires 3.4kw input, not 6-6.4kw.  i can buy one from you test it and see or pay you to test it for me, i probably won’t use the gen after that i just to test that theory and answer for myself. my phone is 803-xxx-xxxx when you figure what you want to do please give me a call.

thanks troy

My response:

OK, so let me give this a shot, we are going to post this at Utterpower because there are other people who believe in similar stuff.

horsepower is horsepower period, it matters not what makes it, it is simply RPM times torque, matters not if a mule makes it or an electric motor.

We know that one inch of copper wire has a loss, so when we put electrical energy into one end, we expect less out the other end period.

So in your case… you believe we can add a variable frequency drive, followed by an electric motor (with known losses), a drive coupler and then the PMG and effectively double the efficiency over the rule of thumb, 2HP=1KW?

In stead of offering to pay me to set this up on the test bench and prove your theory, why not spend an hour thinking or learning?

If what you say is real, then a lot of really smart people are ignoring a real simple way of addign efficiency.. Imagine, engineers have been struggling to produce fractional gains in efficiency for years, and you’re going to approximately double it? 

Now I do agree that about 30% of Americans think the same way you do.  There’s little difference between ‘life’s energy’ and other forms of energy. We seem to have a bumper crop of people who would vote for a minimum wage of let’s say $95 and hour.  This would create a lot of income and then we could tax the hell out of them and have enough money for every government program. Patty Murray, Barbara Boxer, Nancy Pelosi are in this camp.  

Just a few minutes of  ‘critical thinking’ make it all clear, if anything was clear to start with..  wait a minute! even critical thought is not helping right now….

George B.

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11 Responses to Double your efficiency with a VFD!

  1. David says:

    From the somewhat convoluted explanation in the original e-mail it sounds like the person proposes building a motor-generator set with the motor driven by a VFD to prove that the HP to KW ratio is not 2:1. He is absolutely correct on that point. And building a good MG set could prove it, though simply looking at the definitions of a watt and a horse power should be sufficient. I do not believe his intent was to develop a more efficient, or above unity design.

    The overall problem seems to be Troy’s confusion converting between HP and KW, and the purpose of the 2-1 thumb-rule.

    Both KW and HP are measurements of power: rate of energy transfer. However, they are NOT the same thing. HP in the United States is an imperial or American measurement based on foot-pounds (torque), and RPM (rotational speed), or pounds of force and speed. Watts are a metric unit based on joules per second.

    There are several different definitions of horsepower, but what we deal with in the United States is based on imperial, not metric, units and in no system does 1 HP = 1KW.

    Just like 1 kilogram does not equal 1 pound and 1 degree Celsius does not equal 1 degree Fahrenheit, 1 Kilowatt does not equal one 1 Horse Power.

    Since they are both measurements of power, you can convert from one to another. The conversion of HP to KW is:
    1 HP = 0.746 kw or about 4 HP = 3 KW

    The thumb-rule is in no way a measure of efficiency (and is not intended to be). It simply provides a means of sizing an engine for a generator without having to know complicated conversion factors or do any engineering.

    Using the 2 HP to 1 KW helps ensure you:
    -take into account all system losses: friction, resistive, magnetizing losses, etc.
    -operate the engine close to its most efficient loading (full rated load is rarely the most efficient).
    -have additional power available to help start motors (they require much more power to start than to run)

    • George B. says:

      David, there may be more that one interpretation of Troy’s theory.
      As I received it, Troy’s thought was that the VFD offers a significant gain in efficiency WHEN the VFD and a three phase motor are used to power the PMG versus other types of single phase motors. The comparison point is using a clamp on amp meter and voltage probe feeding the VFD and of course comparing this to the input of the single phase motor.
      The reason I post this is there are a lot of people who are thinking the VFDs are magic, and maybe they are? That’s the topic, and maybe that wasn’t clear ??

      I did correct some grammar and soften my presentation of this email, as I am even more confused after reading it again, perhaps Troy wiLL visit and explain further?

      George B.

  2. John Laswell says:

    Sorry, but there is no Santa Claus & there are not perpetual motion devices. There is a Conservation of Energy Law. My engineering reference reads: one hp = 0.746 KWs. That is for 100% efficiency. Which is like saying 2+3 = 5. There are no other answers. In the real world there are copper, core, friction & winding, & stray-load losses. A small 1 hp motor might be 75 % efficient. A small generator (PMG) might be 80 % which gives a final efficiency of 60 % of input power. Auto alternators are about 50 – 62 % efficient.
    There are no magic black boxes that can increase 1 hp more than 3/4 KW!!

  3. Bill knighton says:

    In the letter Troy says that the pmg has losses of only 5%. Is that correct?
    I was not able to find specifications for the pmg efficiency but it seems like I read someone estimated it at 75%. That is a vague recall on my part and it may be way off. Are there some measured values out there? Or has someone compared fuel used with an st-5 setup and a pmg?

    • George B. says:

      5 percent overall loss? You wouldn’t catch me claiming that.
      When we look at the PMG design, we note that we do not use any electrical energy to power the field. With that note, we might look at a standard synchronous generator of similar size and accurately measure the field power used there and expect the possibility of being more efficient by that amount or less.
      Yes, there are other factors, and in the PMG, we use a generous fan to help us keep the voltage droop flatter from no load to full load. In other words, we are more concerned about power quality than highest efficiency.

  4. Greg says:

    Dude, you are harshing his mellow!

  5. Eugene F. Augustin says:

    I’m anxious to talk with you about a Lister engine but don’t know what ‘verify’ means on the contact page. Can you help me with that, please.

    • George B. says:

      I’m using a reply function for this contact.. I will check to see if it ‘approves the post’ as well.
      Lister engines are now banned, you buy them used, or you find the parts and build one. Of course this means you need to address certification.

    • George B. says:


      Things change, and many of us have been sitting back to see if the Agriculture community is going to put up wiht the EPA’s decision to Ban our beloved Stationary engines. Up to this point, it appears that this ban is going to stick. What that menas is you’ll need to find a used Lister engine and I don’t know exactly where you should go about looking, there are parts out there, and some will certainly use those parts to build their own engines.

  6. George B. says:

    Eugene asked what that verify thing was on the contact page…..
    It’s a tool to stop autobots from delivering spam.. just copy what you see to the right into the verify box..

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