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how to measure

Old 11-05-2003, 09:51 AM
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fnp
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Default how to measure

Hi
how do you measure current with refernce to a brushless motor as it has three terminals and not two. Also how do you calculate thrust . Thanks Naji
Old 11-05-2003, 11:50 PM
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Dr Kiwi
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Default RE: how to measure

To measure the amp draw under load you need to have the motor/gearbox and prop set up on a test stand or in the aircraft, but with a Whattmeter or a clamp-on ammeter between the battery and ESC. (That's why it doesn't matter that a B/L motor has three wires from the ESC - it still only has two between the battery and the ESC). You obviously need your transmitter on, and your receiver connected too. As you open the throttle you will be increasing your motor/prop speed and thus the amount of current drawn will increase too. An amp reading at wide-open throttle will give you an indication of the maximum current your set-up is drawing under static conditions. You'll notice that your voltage will drop somewhat under load, so that the 'power' (W = VI) your motor produces is not simply the nominal voltage (V) of your battery (say, 6-cells = 7.2v) times the current (I). Depending on your battery, at only 1A draw your voltage might be close to the nominal 7.2v, but at 10A draw the voltage may have dropped to only 6.2v.

Hope this helps, I'll post a photo of my test stand/thrust measuring device in a following post.

Cheers, Phil
Old 11-06-2003, 12:00 AM
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Dr Kiwi
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Default RE: how to measure

Here's the link to a photo of a test stand set-up for measuring thrust, amps, volts, rpm and mA used [go to post #21]

http://www.rcgroups.com/forums/showt...d&pagenumber=2

Cheers, Phil
Old 11-06-2003, 07:49 PM
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Uncas
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Default RE: how to measure

To calculate 3 phase power (watts) don't you have to divide by square root of 3?
Old 11-06-2003, 10:24 PM
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Dr Kiwi
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Default RE: how to measure

Way too deep for me - I just believe the watts reading on my Whattmeter. I last took a physics course in high school in 1964, but shouldn't power[W] = volts[V] x amps[I] still work?

I'm sure someone will set me straight on this.

Cheers, Phil
Old 11-06-2003, 11:51 PM
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Default RE: how to measure

Sorry, it should work, you are measuring from the battery to the esc, not a phase current of the motor.

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