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Author Topic: Estimating motor/battery combo  (Read 439 times)

Offline phil myers

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Estimating motor/battery combo
« on: November 13, 2010, 02:28:53 PM »
Have been given a dc motor to play with but don't know much about it. I should point out that I'm still using the dirty ic engines, I'm messing about with basic DC motors and lead acid batteries for the moment just for fun . How do you determine the ideal battery size for it, it works ok with a 6v 6ah LA but I don't know if a 4 or 7v  3ah battery for example, may be better. Rather than buying loads of different batteries, is there a way to figure this out say, with a multimeter and some maths? Not planning to use this in a stunt plane!! Just playing and having fun learning.
Thanks Phil
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Offline Tim Wescott

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Re: Estimating motor/battery combo
« Reply #1 on: November 13, 2010, 02:56:24 PM »
Is it brushed?  If you can just connect it to the battery without a speed controller it's brushed.

The ideal battery size depends on the application and the motor -- generally motor manufacturer's rate a motor by the maximum voltage supply that they think is going to work, but it's not uncommon for some application to use less (or more rarely more) voltage than the manufacturer rates the motor for.

Probably the best thing to do with a brushed motor -- particularly if it's the cheapo 'can' type motor -- is to snip the wires off close to the motor body and use it around the shop as a handy weight.  If it's an Astro Flight motor then it's probably still good for model aviation, but more for things like RC scale or gliders, and less for control line.

If it's a brushless motor made for model aviation use then chances are high that the manufacturer's name and identifying info will be on it -- if so, then see if you can find reference to it on the web, or post the numbers here.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline phil myers

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Re: Estimating motor/battery combo
« Reply #2 on: November 13, 2010, 05:54:43 PM »
Tim, Thanks for the reply.. Its a brushed cheapo can type! (nothing written on the motor) I'm not worried at this stage, Its just for faffing (messing about) with. Is there a way of determining the ideal battery size or is it just trial and error?
Phil
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Offline Tim Wescott

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Re: Estimating motor/battery combo
« Reply #3 on: November 13, 2010, 05:59:33 PM »
Measure the RPM with a known battery voltage, and calculate the motor Kv (Kv = RPM / voltage).  The best battery voltage is the one that would make it spin somewhere between 16000 to 20000 RPM without a prop, and the best prop is one that'll bring that down to about 10000 - 12000.

It'll be heavy.  And inefficient.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

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