



The formula if I understand you correctly is simple.
Get a esc thats rated HIGHER then the motor. Get a battery that can handle the amp demand and then some. EXEMPLE: Without knowing the amp draw frmo the motor and prop combination you can get a esc rated for higher then the max amps of the motor. If you motor is rated for 50A MAX then get a esc rated for higher. Simple really. It wont hurt if you get a too big of esc, all it will do is take more place in the plane and weight a little more. If thats not a big concern then no worries. I got 55A max motor but use a 100A constant ESC that will peak at 150 or higher I think, I bit overkill but hey I mite want to use it in something bigger later and wont have to buy another esc just because I bought one that barely could provide my current setup.. Now if I over prop and draw too many amps then I will smoke the motor. 





If you already know the "draw current" that's pretty much all you need to know. The ESC rating should be at least that much and preferably a bit higher.
You also need to check that the ESC will take the voltage that you are using. Many ESCs are only rated up to 3,4 or 6 cells and using a higher voltage will damage them. Steve 





So what if you don't know the amp of your motor? I just have a little brushed motor with no info on it and I have the battery but I need an ESC, how could I decide which one to buy?






Quote:
Do you have a picture of your motor, and an explanation on what you want to use it for ? 






The actual size of your "little brushed motor" might be useful, also some information about exactly what battery you have. E.g. the average 6mm diameter motor uses much less than 1A, the average 28mm diameter motor probably less than 15A etc.
But basically if you don't know anything about what you have then there's no practical way to decide what you might need to go with it. Steve 





Fortunately, a brushed motor will run without a speed control. Just plug it into the battery and measure the current while running the prop you want to run. Of course you have to have a reasonable estimate of what size prop or you could burn up the motor. I would think that the three watts per gram rule would apply to brushed motors too.












Quote:
Meet the natives forum.rcmpt.com (Portugal) www.miliamperios.com (Spain) Vriendelijke groeten Ron 




The motor formulas will help here:
Torque = Kt * I Kv = RPM / V Kv* Kt = 1 in the MKS system, however most people use the feet, ton, seatofthepants system so the numbers will be different. Notice that all motors with the same Kv rating will draw the same amps when they are driving the same propeller or any other load. The motors with lower power ratings will run hotter and maybe smoke. So all you need to know is the Kv rating and the load to calculate the current draw. 





Quote:
Kinda makes you appreciate our modern systems. 






Yes, it was a lot of fun. I made a twin motor plane out of a Balsa USA Swizzle Stick 05. Two "18 Volt" motors from Radio Shack and a 15 Cell 450 mAh NiCD made from salvaged walkie talkie radio batteries. Over 2 pounds weight and about 200 square inches of flat bottom wing. Turn on the switch and fly it till the batteries ran down and it would land.






Quote:
A smaller motor with the same Kv as a larger motor will likely have a much higher armature resistance even though both motors have the same Kv. The smaller motor will definitely not draw the same current as the larger motor with the same battery and prop because of the larger voltage drop in the armature resistance and its RPM with the same prop load will be much lower. Quote:
I should also point out that Kv is not a rating. It is a constant, a motor parameter that doesn't change regardless of the motor's operating point. 


Last edited by Martyn McKinney; Apr 11, 2014 at 11:34 PM.


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