Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How to calculate a PMA's efficiency

  1. May 10, 2013 #1

    I want to start by stating I am a bookkeeper and do not have much experience or depth of knowledge in electronics...so get you chuckles now.
    What I would like to do is augment my electric supply with a Windtronics type generator. I am opting for this style as I am not able to put a 30 foot tower with a regular turbine in my urban yard but I could mount a Windtronics type to my roof.
    I have found that Windtronics are not the most reliable and their after purchase customer service is questionable, so I would like to build my own.

    So what I do know...
    according to my electric bills I average 43000 watts per day
    What I thought I knew....
    I always believed the US was on a 110 volt system but as I do my research I have been led to believe we are actually on a 120 volt system
    So I use either 391 or 358 amperes per day.

    Let's assume...
    I would like to go totally off grid and I have purchased the required 12 volt deep cycle batteries to provide me with 400 amperes per day.
    My design is a windmill with neodymium magnets mounted on the outside rim spinning inside a hub with the coils mounted inside the hub.
    The weather service states that the average wind speed is 6 mph but mostly on calm days it is only 2 mph.
    I can have as many windmills as I want.

    So how do I generate the maximum amperes at 12 volts?
    Is it the size of the magnets?
    Is it the number of magnets?
    Is it the size of the coils ie: diameter?
    Is it the number of coils?
    Is it the gauge of the wire?
    Is it the number of wraps in the coil?

    Is there a simple to use Linux based CAD type program which will allow me to explore the different variables?

    Any advice would be most appreciated.

  2. jcsd
  3. May 10, 2013 #2
    i would like to know this too. build your own turbine? this dosnt seem hard at all.. lol
  4. May 11, 2013 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    As a book keeper, you must have come across a load of people who have failed to make as much money as they thought they would. It will sometimes have been just bad luck but mostly it will have been because they didn't know enough about the business they were investing in.
    I have a feeling that you may be at risk of heading down the latter route. The cheapest way to acquire an alternator will certainly be to buy one. You need to know the capacity that's needed, 'that's all'. You want an installation that will pay for itself and justify your investment. That's a pretty professional engineering job you're talking about. For a start, you need to learn the difference between Watts and WattHours and between Amps and AmpereHours. They correspond to £ and $, which you certainly do know about.
    There's no doubt that you could have a lot of fun and satisfaction in producing an arrangement that will charge a battery and give you a modest electricity supply to light some bulbs but the cost per Unit of Energy would be pretty high.
    The first thing you need to know is the actual wind speed statistics in your particular location (not the local weather station's version). You can get home weather stations very cheaply, which will allow you to see the state of the wind over a year or two, if you log and analyse the results on your PC. That will give you an idea of what energy you can expect over a long time and the peak energy, which will tell you how big your turbine(s) need to be.
    If you don't approach it this way, you could end up with under-sized turbines which cannot give you all the available energy or over-sized turbines which will not pay you back what you spent on them.
    Another approach might be to find a local 'green energy' organisation and get some advice about the practicalities of DIY power in your part of the world.
    The main thing that home power generation has going for it at the moment is selling energy back to the supplier, of course. The deal can be soooo attractive for some home owners. But you can only get the Feed In Tarif if your installation is pukkah and totally up to their requirements.
  5. May 11, 2013 #4
    What is the charging and discharging formula? Is it

    If the battery is say 22AH, if I charge it at 5A it would take 4.4 hours to charge. thats without a current draw.
  6. May 11, 2013 #5
    The charging amps must be more than the current draw in order to keep the batteries charged? This seems complicated as the output won't be constant.
  7. May 11, 2013 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Of course it's complicated but simple, in principle. It's like financial income and expenditure. You have a regular salary and spend at an irregular rate. With batteries and wind turbines it's more complicated, income and expenditure are both irregular, you may not have an overdraft so you need to work out your very worst spending month and make sure that you will have enough Ahr available even if the worst charging month happens to be at the same time.
    This is what I meant by needing to have big enough turbine / generators to use what wind is available when it's there. etc. etc. But you always have a fallback in that you can use the mains when you are running out.
    There is, however, the small problem with voltages. For best efficiency, you would need most of your loads to be Low voltage DC and only AC for equipment like washing machines , so that you don't need to be using an inverter to get mains volts all the time.
    This is really not simple. Do you really have the knowledge and experience to make it work? I must say, I would think twice about it and I have spent my life in Engineering and Physics. But perhaps I'm just a realistic chicken heart.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook