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Leccy basic question

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fly boy329/07/2017 21:50:37
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Hi all, for example only, will my motor try to pull more amps from my lipo to obtain it's specified watts if I change from a 3s to a 2s, or due to less volts available, will the motor have less watts available to work with. Cheers from leccy newbie.

Dwain Dibley.29/07/2017 22:04:33
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Fly Boy, here's a calculator that shows amps and watts for a given voltage. Very basically,Less volts means more amps.

Other factors like the prop will make a difference to the Amps pulled and watts produced.

D.D.

Edited By Dwain Dibley. on 29/07/2017 22:05:37

Geoff Sleath29/07/2017 22:11:04
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It's more complicated than that. The power rating of a motor is a measure of what it can withstand without letting out the proverbial smoke rather than what it will consume regardless.

These comments are all to a first approximation as things tend to interact in various ways but they're good enough.

One motor parameter is the rpm/volt (kv) rating and is the speed a motor will try to run depending on the input voltage. So changing from a 3S to a 2s will simply make the motor run slower. The power consumption depends on the load ie in our case, the size of the prop. Changing the prop is the way to change the power of the system but a change should always be checked by measuring the current draw to make sure you don't damage either the motor or the esc.

Therefore changing the battery voltage from 11.1 to 7.4 will just reduce the power.  To maintain the power you'd need to increase the prop size but with a lower voltage you'd need to draw more current and that could damage the motor if it's not rated at the higher the current. 

Power is measured as VxI (volts x amps) or I^2 xR (current (in amps) squared x resistance in ohms) or V^2/R (voltage squared / ohms)

Geoff

Edited By Geoff Sleath on 29/07/2017 22:16:50

Dwain Dibley.29/07/2017 22:18:31
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Very nicely put Geoff, as I said, it's all relative, but that calculator does explain the interaction between Volts and Amps for the Novice, if all else (watts) remains constant.

D.D.

Geoff Sleath29/07/2017 22:21:12
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Thanks, Dwain. Just to say I was typing before your comment so my saying 'It's more complicated than that' referred to the original question rather than your post which is fine.

Geoff

Steve Hargreaves - Moderator29/07/2017 22:21:18
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It will pull less curent fb3.....for a given circuit a lower voltage always results in a lower current as voltage & current are linked via the circuit resistance. Also the motor will run slower because of the lower voltage meaning the prop will also turn slower. This will also result in lower current.

The only way to get the current to it's former level would be to increase the prop size. This would make the motor work harder & thus draw more current.

Remember though that even if the current was raised to it's previous level you would still have less power because of the reduced voltage (Power = volts x current)....

HTHs...

Dwain Dibley.29/07/2017 22:28:01
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That's Cool Geoff...It crossed my mind we were all typing at the same time. LOL

So as we have all said then, to get the same watts to fly said plane, a bigger prop with less Volts to draw more Amps.

The question to Flyboy is why would you want to swap to two cells ????

D.D.

fly boy329/07/2017 22:35:44
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Thanks all, very well put I must say. I was aware of old schooldays ohms law, but wondered as a motor was involved it might try to pull extra amps that would fry some thing else. Got a lot to learn lol. Cheers and thanks. Hi Dwain, I used battery analogy as I was unsure of the correct volts of 2 and 3cell at the time.  Cheers

Edited By fly boy3 on 29/07/2017 22:40:36

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