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Old Nov 16, 2010, 03:13 PM
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Calculate ESC amps

Hi,

Is there any formula to calculate how many amps a esc must have according to motor, battery voltage and draw current?
If this has already been discussed in other thread i am sorry but i couldnt find it..

Thanks
JOse - Portugal
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Old Nov 16, 2010, 04:03 PM
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Canada, ON, Rockland
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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.
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Old Nov 17, 2010, 03:52 AM
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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
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Old Nov 17, 2010, 06:12 AM
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Thats simple... Thanks
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Old Apr 08, 2014, 03:08 PM
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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?
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Old Apr 08, 2014, 03:56 PM
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Originally Posted by Prune Tang View Post
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?
The 'amps' will depend on what the load is, (usually the size of prop etc), and the battery voltage, (more volts, more amps).

Do you have a picture of your motor, and an explanation on what you want to use it for ?
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Old Apr 08, 2014, 04:36 PM
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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
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Old Apr 08, 2014, 11:53 PM
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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.
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Old Apr 09, 2014, 03:53 AM
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I would think that the three watts per gram rule would apply to brushed motors too.
Definitely NOT! That would make a Speed 400 (73g) good for around 220W whereas in the real world 80-100W is about all it can manage.

Steve
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Old Apr 09, 2014, 11:56 AM
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... Is there any formula to calculate how many amps a esc must have according to motor, battery voltage and draw current? ...

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Old Apr 09, 2014, 06:49 PM
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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, seat-of-the-pants 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.
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Old Apr 09, 2014, 08:08 PM
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Originally Posted by slipstick View Post
Definitely NOT! That would make a Speed 400 (73g) good for around 220W whereas in the real world 80-100W is about all it can manage.

Steve
Yup, I really got that one wrong. My old Astro Cobalt 05 was only good for 200 watts or so, and it weighed 5.5 oz, or about 150 grams which would be 450 watts, so maybe we could say 1 - 1.5 W/g?

Kinda makes you appreciate our modern systems.
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Old Apr 10, 2014, 03:48 AM
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Yes we've got it easy these days with lightweight powerful brushless motors and no more of those seriously heavy NiCds to carry around. But we had fun just trying to get anything to fly didn't we ?

Steve
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Old Apr 10, 2014, 07:57 PM
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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.
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Old Apr 11, 2014, 11:17 PM
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Originally Posted by mjsas View Post
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.
This is not a completely accurate statement.

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:
Originally Posted by mjsas View Post
So all you need to know is the Kv rating and the load to calculate the current draw.
Both the armature resistance and No-Load current are also a key factors in accurately determining the current draw. With high efficiency motors, the No-Load motor current Io may not be a significant factor, but can also influence the current draw dependent on the motor's operating point.

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