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How to calculate Amps from a brushless motor?


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#1 the darrener

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Posted 10 November 2010 - 09:05 PM

:newbie: Hello all.

Newbie here & having read & read the forum topics I am at a loss over brushless motors & esc's.

I have seen combo for sale with for eg: 9t motor & 35A esc, then same 9t motor & 60A esc. What I can't find out is how to calculate the Amps a brushless motor takes when I have no idea on the watts used by the motor.

To use Ohm's law you need the power divided by the voltage to workout the Amps but descriptions of motors only give the turns & the Kv's but not the wattage so how do I know which motor goes with which Esc & what volt battery I could use?

Edited by the darrener, 10 November 2010 - 09:06 PM.


#2 Si Coe

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Posted 10 November 2010 - 09:33 PM

Its a bit complicated since you can't calculate current draw without knowing a massive bunch of variables.

The current the motor draws will depend on:

What the cells can provide (can't exceed this!)
What the gearing is
Mechanical drag from the drivetrain (relevant if comparing the same motor in a 2wd vs 4wd for example)
What tyres you are using
What surface you are running on
Whether you are moving at a constant speed or accelerating.


Running full spikes on grass cause more drag than running slicks on tarmac, so you get a higher current. Hard acceleration causes bursts of massive current, constant (even high) speed generates lower currents, but for much longer. Higher gearing draws more current.

In the case of the Ezrun 9T: The 35A esc is adequate for most bashing purposes, but people racing the system really need the 60A esc. The constant braking and accelerating found in racing overheats the 35A very quickly, normally in under 5 mins (annoying, cos races last 5 mins!). For bashing however, where more time is spent at constant speed, or stationary, the esc has a chance to cool down and I can get 30 min run times with the 35A esc.




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