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Old Apr 30, 2012, 04:01 PM
R0yalty is offline
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Brushless Motors and Amps


What is it that decides how many Amps an Esc should handle, battery or motor?

As I understand it, basically the battery isn't really a factor when determining which Esc to get.
It is the motor that decides how many Amps it needs, and if the battery can supply that, the Esc should be able to handle it.
Am I right?

If so.. Amps are specified on Esc, and for batteries they are easy to calc by just multiplying the mAh and C rating, But I have never seen an Amp rating for a motor.

So if i would to buy new motor, esc and batteries, how would I know how to pair it up?
If I would know the Amps required for the motor, I could just pick an Esc that could handle it, and a battery that would be able to supply it.
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Old Apr 30, 2012, 04:23 PM
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No, battery (mainly series cell count), motor and prop all play a part in how many amps your ESC will see.

The battery cell count has a huge influence. For instance, if you connected up a three cell pack in place of a two cell then your amps would more than double. Battery 'c' rating and mAh capacity can have some effect but it's very small in comparison to the effect that the number of cells has.

The size of prop has a major effect too, as obviously does the motor itself.
Old Apr 30, 2012, 04:28 PM
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Thanks. But could you be a bit more specific. Iow, what are the relations of the components. How do I calculate what ESC I need for X batteries, Y Motor and Z prop.

One thing that confused me are the components that I received with my RTF Dynam Spit. I got a 30A and a 3s 2200 mAh 25C battery.
As the battery can deliver 55Amps (2,2 x 25) and the ESC can only take 30..
The components worked fine together of course, but i would like to know how to calc these stuff so I can pick together my own stuff.
Old Apr 30, 2012, 04:30 PM
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Originally Posted by R0yalty View Post
But I have never seen an Amp rating for a motor.
Where are you looking at motors? Most reputable sites will list motor specs, to include max and continuous amps.
Old Apr 30, 2012, 04:31 PM
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Where are you looking at motors? Most reputable sites will list motor specs, to include max and continuous amps.
I've looked at various places, but all I can see is the kv rating.. I might of course have overlooked it.
Old Apr 30, 2012, 04:34 PM
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Originally Posted by R0yalty View Post
Thanks. But could you be a bit more specific. Iow, what are the relations of the components. How do I calculate what ESC I need for X batteries, Y Motor and Z prop.

One thing that confused me are the components that I received with my RTF Dynam Spit. I got a 30A and a 3s 2200 mAh 25C battery.
As the battery can deliver 55Amps (2,2 x 25) and the ESC can only take 30..
The components worked fine together of course, but i would like to know how to calc these stuff so I can pick together my own stuff.
It all works because the motor and prop combo determine the amp draw and the combo you have will only pull somewhere less than 30. The battery is capable of delivering 55 amps, but it doesn't "force" the motor to take that many amps, it only delivers what the motor is demanding.
Old Apr 30, 2012, 04:36 PM
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It all works because the motor and prop combo determine the amp draw and the combo you have will only pull somewhere less than 30. The battery is capable of delivering 55 amps, but it doesn't "force" the motor to take that many amps, it only delivers what the motor is demanding.
So I did understand it right. The battery isn't really a factor when determining ESC capability.
So I should start off determining the Amps the motor will need, and then choose batteries and ESC based on that..
Old Apr 30, 2012, 04:37 PM
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I've looked at various places, but all I can see is the kv rating.. I might of course have overlooked it.
Take a look at this page and scroll down to the specs section.

http://www.headsuprc.com/servlet/the...ushless/Detail

"Current = maximum of 20 amps or 220 watts for 30 seconds"
Old Apr 30, 2012, 04:41 PM
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So I did understand it right. The battery isn't really a factor when determining ESC capability.
So I should start off determining the Amps the motor will need, and then choose batteries and ESC based on that..
There you go.

Just remember that the motor amperage draw will depend on the size of the prop and the battery voltage. (Higher voltage will result in a higher RPM and an increase in amperage.) The mAh and C rating only determine if the battery can supply what the motor is demanding, and will factor into the total run time you will get from the battery.
Old Apr 30, 2012, 04:47 PM
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I also think that once you have determined what amps your motor needs you should scale up the esc 10 - 20 % more than the motor requires.
Old Apr 30, 2012, 05:07 PM
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Originally Posted by R0yalty View Post
So I did understand it right. The battery isn't really a factor when determining ESC capability.
So I should start off determining the Amps the motor will need, and then choose batteries and ESC based on that..
No, that's only approximatly true if you stick with a certain battery voltage (i.e. cell count).. Change the cell count and you effect the amps massively.
Old Apr 30, 2012, 05:11 PM
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To be honest there isn't an easy way to determine motor amps without manufacturers test data. There are just so many variables: battery voltage, motor kv, motor winding resistance, prop pitch, prop diameter, number of blades, blade area.. Even the ESC itself can have a significant effect on how many amps the system sees because different ESC's have different internal resistance.

Best bet is to look at the manufacturers data or get advice from someone running a similar set up.

The uncertainty is why the wattmeter is the most important tool you can buy. You must test a new system with the meter to make sure everything is operating within spec. Adjustment one way or another can be done by changing props.

Steve
Old Apr 30, 2012, 05:18 PM
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There you go.

Just remember that the motor amperage draw will depend on the size of the prop and the battery voltage. (Higher voltage will result in a higher RPM and an increase in amperage.) The mAh and C rating only determine if the battery can supply what the motor is demanding, and will factor into the total run time you will get from the battery.
That's true as a rough rule of thumb but in reality battery 'c' rating and mAh capacity can have a significant effect on the amps you pull. that's because high c and/or large mAh batteries hold their voltage higher under load, and more volts makes for more amps.

the same goes for the health of a battery. the voltage of old tired batteries sags badly under load significantly reducing watts and amps.. that's why your plane feels sluggish when the batteries are getting past their best.

This can easily account for a 10% change in amps, possibly more.
Old Apr 30, 2012, 05:27 PM
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The uncertainty is why the wattmeter is the most important tool you can buy. You must test a new system with the meter to make sure everything is operating within spec. Adjustment one way or another can be done by changing props.

Steve
But to measure with a wattmeter I would have to hook everything up. And if something is out of range, wouldn't it be to late?
Old Apr 30, 2012, 05:35 PM
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Is this sufficient for a Watt Meter?


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