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I think i have flawed results and I need somebody's opinion. Answered

I have been working on a small generator and i need a dynamo to generate the electricity so I did a little experiment to find out what motor i had was most efficient at generating power so I know what I should use a in the upcoming project. On each motor which was spun at the same speed by my Dremel all the results were ordinary except for one motor which surprised me with its super high reading. Upon seeing this I thought it might be my multimeter going dead and giving me false readings i changed out my multimeter and got the same exact readings for the motor. Is this because it really can produce 120 watts or am i doing something wrong? My formula for finding watts was V*A=W the motor was producing around 8.5 amps at 14 volts. I'm pretty sure the formula is correct as well. Help would be appreciated greatly thanks.
     Harry

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steveastroukBest Answer (author)2013-12-21

I bet you
a.) Measured the voltage with a voltmeter, across the open terminals
b.) Measured the current into an ammeter directly across the terminals.

Right ?

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steveastrouk (author)harry882013-12-21

You see, when you measure voltage at zero current, you have no power.
When you measure current at zero voltage, you have no power.

What you CAN say is that the generator has an output resistance of opencircuit volts/ short circuit current.

If we take your numbers, you have 14/8.5 Ohms of resistance - for the sake of argument, we'll believe the current number, though that seems VERY high, unless this really was a drill motor. So you have 1.6 ohms of output resistance. The MAXIMUM power from the generator will be delivered into 1.6Ohms, and that will be = (open circuit volts/2)^2/1.6, about 30W.

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harry88 (author)steveastrouk2013-12-21

OK thank you I do believe this motor was built to run on low amounts of solar power so I think the current could be correct. what kind of motor would you recommend I use?

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steveastrouk (author)harry882013-12-21

What physical size is your 120W motor ?

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harry88 (author)steveastrouk2013-12-21

maybe 1.5 inches long. that's why the results surprised me that the tiny motors were getting the higher wattage readings I'll have to retry my experiment i'm looking to trickle charge 12 volt batteries with this a half amp or less would be perfect output.

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steveastrouk (author)harry882013-12-21

12Watts is MUCH more likely, but remember that will only be at the same speed the motor was DESIGNED for. A drill motor is much more likely to work than a toy.

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rickharris (author)2013-12-21

Just as a passing thought you will be better off using a stepper motor or a brushless motor - Some will be rated at 120+watts although you may not get that out of them as a generator.

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harry88 (author)rickharris2013-12-21

thank you I'll look into that I read that a motor generates approximately 80% of its watt ratings so that's still pretty good.

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rickharris (author)harry882013-12-22

Remeber you will get 3 phase AC from a brushless motor and rather less than the motor absorbs as a motor.

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harry88 (author)rickharris2013-12-22

Yes I plan to use a 3 phase rectifier. Like I said before I read specificly "stepper and brushless motors can generate approximately 80% of their power rating.

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harry88 (author)rickharris2013-12-21

do you think something like this would work?

http://www.nitroplanes.com/86ma15-2220-1880kv.html?gclid=CN_VsMPuwbsCFcU5Qgod9R0AdA

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framistan (author)2013-12-21

Most likely you are reading the meter wrong. Carefully look at the display, not just the numbers it shows. Next to the numbers in the display, the display will also say "A" for Amps... or it might say mA meaning milli-Amps. For example 8.5 mA is the same as 0.0085 Amps. So you would use 0.0085 as your amps reading in your equation, not 8.5.

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steveastrouk (author)2013-12-21

There's no scale. What are the motors from ? Electric drills ?

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rickharris (author)steveastrouk2013-12-21

The "120 watt" motor is a standard "toy" type motor 3 volts and around 10,000rpm off load. by the looks of it.

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rickharris (author)2013-12-21

1. Your motor isn't giving you 120 watts - Nor 8.5 amps. It's far too small and should give approx the voltage it is rated at to run - Probably around 3 to 6 volts at a few milli amps.

2. How are you measuring the amperage?

3. How are you measuring the voltage?

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