Technical brushless motor ?

Is a brushless motor max amp draw constant or does it increase as voltage increases?It's my understanding that a motor will draw more amps at higher voltage all other things being equal.So do I need more powerful batteries to run my motor at 22v then I do at 14v? Here is what was posted on a different site.

When talking about energy and work, remember nothing is free, there is always give and take. Think of voltage as the "pressure" that pushes the current out of the battery pack. If that energy is forced out more quickly the battery will empty faster.

It's actually quite simple. Voltage(V) = Current(I) x Resistance(R), or I = V/R. For the purpose and accuracy of this discussion and with all other things being equal, the internal resistance of any given DC motor is fixed by its wire size and wire length.

For example, assume a DC motor has 0.5 ohms resistance and we use nominal pack voltages:
For a 6-cell pack, 7.2 volts; (I) = 7.2 volts/0.5 ohms or 14.4 amps
For a 7-cell pack, 8.4 volts; (I) = 8.4 volts/0.5 ohms or 16.8 amps

Because we run battery packs with fixed capacities, i.e. 4.6 amp-hr, potential runtimes can be calculated:
For a 6-cell pack; 4.6 amp-hr/14.4 amp = 0.32 hr or approx. 19 minutes
For a 7-cell pack; 4.6 amp-hr/16.8 amp = 0.27 hr or approx. 16 minutes

Is this correct or is there something I'm missing?

[kostaktinos_mt]

it`s less than half the truth... 1/3rd actually...
load and impedance are the other 2/3...

eg: if a motor pulls 20amps with a particular load at 10volts, then it may pull 10amps at 20volts with a much smaller load...

so no, voltage to current isn`t that simple, nor there is anything stationary/static when it comes to current draw... the load progresses/varies when driving an r/c, and so does current draw... same can be said with voltage [batteries aren`t power supplies with imaginary/perfect/static behaviour] and motors as well...

as a general expression, a motor will pull more current when accelerating from a dead stop [large load] and the current will dive as speed advances [depending on rpm/v, the dynamic impedance increases with speed advance].


it can be analyzed with math to reach 'legitimate' conclusions, but we might not need to go there...

Thanks for that input,very helpful.So at a higher voltage I could get away with a smaller rated battery?As long as the load stays the same,correct?

[kostaktinos_mt]

'load' is defined as: vehicle weight,rotating mass [mostly weight of wheels&tires], aerodynamics,etc.
thus gearing, is the means of manipulating/harnessing the load related to the motor.
a lower kv [sometimes physically larger too] motor with higher voltage is more efficient and consumes less current [thus allowing you to work with 'smaller' batteries].

however, keeping the same load [and motor and gearing] and doubling the voltage, will decrease the efficiency and will require better batteries.

assuming same genre motors and same vehicle, and without being too technical, you can say that:
a 4000kv motor with a 3s battery and geared at 20/60 [example that is], is less efficient and needs a 'better' battery than a 2000kv motor with a 6s battery with the same gearing [20/60]. also notice, that the top speed will theoretically be the same, while runtime and working temperatures will be different [due to the overall efficiency difference].

Well it's sounding to me like the higher voltage is the way to go.I guess 6s will be the way to go with my medusa 36-60-2000v2 thats should be here before to long.Thanks for your help.

[kostaktinos_mt]

you are welcome...


6s with the 36-60-2000v2 will be too powerfull... not sure if you want that much power...
the 36-60-1500v2 is a much better fit for 6s...
what kind of vehicle are we talking about?

too powerfull

Is there such a thing?I thought thats what the throttle was for.I'm running an Emaxx 3905.But I'm really planning on revo.I don't really plan on 6s all the time.I'm afraid any lower kv won't perform to good on 4s.I'm a little bit spoiled on power.Until I got my maxx on wednesday I usually drove my Stampede with novak hv4.5 and 3s lipo.I was totally unable to hit full throttle. :) I don't really want that much power to weight in a big truck .Right now I have my 4.5 in my maxx,but it just isn't quite enough.Though it did hit 41mph and stays pretty cool.Under 150.

[kostaktinos_mt]

"I'm afraid any lower kv won't perform to good on 4s."
true... 6s on 2000kv is too powerfull [imo], 5s is perfect, 4s is quite conservative [performance-wise].

Well I think I'll be shooting for that perfect 5s.6s when I got the radar out and 4s to let someone else drive it.