electric flight

I know this may be a very simple question to answer to those that know the answer..

but working the size of motor required ,(Watts..) is the amps that from the battery or what the motor can take.?
Im a little confused, no funny comments from you younger ones,

I know ..Watts = Amps xVolts if the battery say gives 10v and is1000mAh x10c this means 100 Watts but say the operating amps of motor is only 7amps,would the motor burn out?

Andy if you reply...please please make it simple...thanks

OKey dokey John, I'll keep it simple

It's both

If you know how much power you are looking to "have" in a model, (eg you have worked out the required power to weight ratio) then you should work backwards from the motor. You have to size the motor (in terms of how much power it can handle, before it goes pop) appropriately. You then must size the battery pack, so that it can supply the motor with the required amount of power.

For example, in my next Electric wing, I have decided I want a power to weight ratio of around 150Watts per lb. I know my model will weigh approximately 1.5lb, so I must look for a motor that can handle power levels around 225 watts (plus a bit for headroom if you want). I'm going to choose a Mega 16/15/4 brushless motor based on this..

Now, the motor requires volts and amps fed into to generate all that power, so I must now choose a battery pack. If I choose a 3s lipo pack (11.1v nominal) then this will require 20.27A pulling out of it to generate 225 watts in the motor (225/11.1 = 20.27). You can then size the prop to pull this current from the motor. A bigger prop = more current.

As you can see, you need to carefully select the motor/battery/prop combo to make sure it produces the power you want, with the prop you want to use on the batteries that you want too. If starting from scratch, it can be a very iterative process if your suppliers don't have good datasheets/specs availibility. That's why it is usually best to use something that has already been done before

Also, don't forget that the loads attached to the battery determine the current pulled from it. IN your example, the motor will only pull as much current as it asks for. The battery may be capable of putting out hundreds of amps, but it will only give what is asked. So, your motor should not burn out.

HTH
Andy

thanks Andy...I think Ive understood all that.You should go into Science teaching...long holidays in the summer

johnoxleydean wrote:
but working the size of motor required ,(Watts..) is the amps that from the battery or what the motor can take.?

Hi John
I use motocalc which runs in a pc on any windows program.
It is very easy to use and U can input any data you want. It gives worked examples of brushed, brushless, lipo's nicads and more.
You can ask it to give an in flight performance forecast of your design. It then makes recommendations of what to change. So you can soon get the hang of electric model mysteries.
Available on 30 days trial off internet but there is a charge after that if U want to continue.
I bought mine on a floppy disc from Gordon Tarling at Chester Rodee a long time ago. Since then it has been upgraded to a much bigger prog foc for me and even when the pc has crashed, they have provided me with a replacement free.
download from
http://www.motocalc.com

Or /and
try these 2 which are both free on the internet

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One free one I is P-calc. Its web based but not downloadable.
http://brantuas.com/ezcalc/dma1.asp

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Here's one that is downloadable:

http://www.drivecalc.de/

and athough it is de it works in English
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I've used the P-calc one quite a bit when sizing the motor/battery/prop combo for my big electric glider. It works well for giving you ball-park figures, although it does tend to get a bit far from the truth when you start pushing a motor hard.

For example, the motor I had is rated for 45A continuous. When I start pushing it past that, the difference between what P-Calc tells me, and real life, starts to get quite large (greater than 10% or so).

They are useful tools though, especially for seeing how the figures change when you change a prop size

Cheers
Andy

thanks PDQ and again Andy. This forum is really useful,thanks Mark for setting it up