Poorboy's watt meter? - RCCanada - Canada Radio Controlled Hobby Forum
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Old 03-21-2004, 09:45 AM   #1
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Poorboy's watt meter?

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Have owned a Radio Shack digital multimeter for some years now and never had much use for the 10 amp range, but now I do, I think. Was curious one day as to what my speed400 motor draw was so connected it in series with the battery. This was my first lesson in electrics as I was sort of shocked to see over 10 amps! Have since learned this is pretty nomiminal.
Sure, I'd like to go out and buy a wattmeter but is this method of checking motor draw fairly accurate?
A word of warning though, don't try this for too long as the shunt in the meter could burn out if current goes over the rated value. There are also test lead losses to consider.
Maybe I could use a little advice here but it was obvious the battery pack I was using didn't have that capacity.
Taking it one more step and measuring the running voltage we have watts, or are there some other factors to consider? W=VA
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Old 03-21-2004, 08:25 PM   #2
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You are right voltage into Amp draw equals watts. As a general forumla, take the number of cells in the battery packs e.g 8 cells, and multiply by the current draw, lets say 10 amps, you have 80 watts. The rounding of voltage of each cell from 1.2 to 1 volt gives you the margin of losses.

As far your test, depending on what prop and how many cells were you running on the sp400, current draw could vary alot. You can push the Sp400 upto a 6x4 prop and 8 cells, that draws roughly around 14 Amps. At this level, sp400 does not last long, but again its only 10 bucks to replace.

If you can't affrord a watt meter yet, download motocalc software, punch in your motor, prop and battery pack and it will tell you how much is the current draw.

The website is http://www.motocalc.com/
and its free trial for 30 days.

Hope that helps.
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