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Old 04-06-2014, 05:49 AM   #1
garyp1029
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Default ESC Determination

I know the function of the ESC, but I am at a loss as to how the required amperage is determined. For example--my Apprentice has a 15 brushless motor, 3200 mah battery, and a 30 amp ESC. Why, in this case, a 30? Why not a 20? Exactly what factors determine this?
I am considering the Electrifly Super Sportster rxr as my first low-wing sport plane. The SSrxr has a motor of undisclosed specs, uses a 1250 mah battery with an ESC of 18 amps. A web reviewer said he always uses a 1500 battery in his SSrxr. I understand that that will give longer flights (everything else remaining constant), but does increasing the battery amperage REQUIRE a larger capacity ESC? What is the relationship between BATTERY amperage and ESC amperage? I understand (I think) that LARGER motors require more ESC capacity, but often motor specs are rather sketchy at best. Still trying to grasp the fundamentals of all these relationships. Thanks for your patience and comments. Gary
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Old 04-06-2014, 06:09 AM   #2
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Generally the only way to know the amp capacity of the ESC needed is through testing, recommendations, or previous experience. The voltage and prop are going to be main things determining how many amps the motor will pull. Of course the ESC needs to be a bit over the maximum drawn by the motor. The mAh capacity of the battery has almost no effect. Adding a larger capacity battery of the same voltage does not require a higher amp rating on the ESC. With a bigger pack you may see slightly higher amp draw due to less voltage sag. That's one reason you want a little safety margin.
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Old 04-06-2014, 06:15 AM   #3
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hello Gary and welcome to wattflyer, i suggest checking out the most excellent sticky in the beginners section to get the full story on esc's.

Everything You Wanted To Know About Electric Powered Flight ( 1 2 3 4 5 6 )
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Old 04-06-2014, 07:23 AM   #4
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Originally Posted by garyp1029 View Post
I know the function of the ESC, but I am at a loss as to how the required amperage is determined. For example--my Apprentice has a 15 brushless motor, 3200 mah battery, and a 30 amp ESC. Why, in this case, a 30? Why not a 20? Exactly what factors determine this?
I am considering the Electrifly Super Sportster rxr as my first low-wing sport plane. The SSrxr has a motor of undisclosed specs, uses a 1250 mah battery with an ESC of 18 amps. A web reviewer said he always uses a 1500 battery in his SSrxr. I understand that that will give longer flights (everything else remaining constant), but does increasing the battery amperage REQUIRE a larger capacity ESC? What is the relationship between BATTERY amperage and ESC amperage? I understand (I think) that LARGER motors require more ESC capacity, but often motor specs are rather sketchy at best. Still trying to grasp the fundamentals of all these relationships. Thanks for your patience and comments. Gary
When you buy a motor, the motor specs will give a amp limit to the motor, lets say its a 20 amp motor, Use a 30 amp ESC, Why Because a 30 amp ESC will last longer and stay cooler because its not working to hard, the battery cell count and the prop size and pitch will determine the amp draw, you want to keep the amp draw close to the motors max rated AMP specs. You Absolutely Need a Wattmeter to Do this with, A wattmeter is Cheap insurance and will save your electronics and your pocket book, to get a good idea of how much a motor will draw with a given size prop and battery, check out Heads up RC, Jeff list a lot of motors there with testing already done for you, just look for a motor thats the same size and same KV rated as your motor in Jeffs Motor pages, and you will be very very close to the same amp draw with your motors.
Always use a ESC thats 20 to 25% larger than whats needed. and that will keep you safe.

Wattmeter

http://www.hobbyking.com/hobbyking/s...arch=wattmeter

Motor Example on prop and amps

http://www.headsuphobby.com/Firepowe...otor-E-625.htm

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Old 04-06-2014, 08:20 AM   #5
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Originally Posted by garyp1029 View Post
but does increasing the battery amperage REQUIRE a larger capacity ESC? What is the relationship between BATTERY amperage and ESC amperage?
The mAh figure quoted for batteries is not 'amperage', it is charge capacity. It say nothing about how many amps will flow out of the battery when you use it. Think of mAh as being similar to the capacity in gallons of your car's gas tank. Will fitting a larger capacity gas tank make the fuel flow out of the tank faster or make the engine more powerful... Nope!

Calculating how many amps a system will draw is quite complicated because you have to consider battery voltage (number of cells), motor size and kv, plus very importantly prop diameter and pitch. As Chellie indicated the normal way to go about sizing an ESC would be to get one that had a rating at least as high as the motor's maximum amp rating, then check that you are within that limit with a wattmeter. Adjust prop size if required to get in the range you want. You can however perfectly safely use an ESC that has a lower rating than the motor, providing you check that you are within the ESC's range with the wattmeter.
Also note that good quality ESC's have a 'burst' limit that is around 20-30% higher than the 'label' rating. So you don't need to worry if your amps slightly exceed the ESC 'label' during ground testing with a fresh battery. You wont see those amps in flight anyway.

Motor manufacturers sometimes publish recommended set up information which is useful as a starting point (though not always 100% reliable) and there is software out there that calculates a figure that is a good guide, such as Drive Calculator: http://www.drivecalc.de/ , or eCalc: http://www.ecalc.ch/motorcalc.php?ecalc&lang=en


One of the most important things about an ESC isn't even it's rating, it's the quality of the BEC (Battery Eliminator Circuit) that's built into it. Unless you are flying very small models (or large ones that use an external power source for the radio gear) always look for an ESC that has a 'Switching Mode' BEC, and try to get one with the highest BEC amp limit you can find.
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Old 04-07-2014, 03:10 AM   #6
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Just for reference for you, I fit 30Amp ESCs with SBECS on my Quads, however, at hover the motors only draw 5 Amps each and slightly more when climbing so in many cases you can get away with a smaller ESC if you are not into wide open throttle every time you fly. As JetPlane Flier says, it is best to chose a Switching BEC and make sure they are rated to the battery eg. 3S or 4S or more depending on your battery pack. Of course with a Quad each motor has its' own ESC. (In most cases)
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Old 04-07-2014, 05:33 AM   #7
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Originally Posted by garyp1029 View Post
I know the function of the ESC, but I am at a loss as to how the required amperage is determined. For example--my Apprentice has a 15 brushless motor, 3200 mah battery, and a 30 amp ESC. Why, in this case, a 30? Why not a 20? Exactly what factors determine this?
I am considering the Electrifly Super Sportster rxr as my first low-wing sport plane. The SSrxr has a motor of undisclosed specs, uses a 1250 mah battery with an ESC of 18 amps. A web reviewer said he always uses a 1500 battery in his SSrxr. I understand that that will give longer flights (everything else remaining constant), but does increasing the battery amperage REQUIRE a larger capacity ESC? What is the relationship between BATTERY amperage and ESC amperage? I understand (I think) that LARGER motors require more ESC capacity, but often motor specs are rather sketchy at best. Still trying to grasp the fundamentals of all these relationships. Thanks for your patience and comments. Gary
Yeah, there is a lot of new stuff involved with these electric models. First thing you find out is that the current pulled by an existing motor and battery pack is pretty much determined by the propeller size and pitch on the motor. What appears to be a minor change, like going from a 11 inch to a 12 inch prop, or from a 3S (3 Series) to a 4S LiPo battery pack can easily double the watts pulled by your motor.

Put to big of a prop on a glow engine, and it will lug down and overheat. Put way to small of a prop on a glow engine, and if you can get it to run, it will over-rev and maybe damage something.

With electric power, put to big of a prop on it, and it will put out a LOT of power. Until the magic smoke comes out of the motor/ESC/battery pack. Put way to small of a prop on an electric motor, no damage, just that the model won't go anywhere.

If you don't all ready have one, one of the first things you should consider buying is one of those wattmeters to actually measure the current being pulled by your motor, along with the battery voltage, and watts pulled by that motor. With a wattmeter, if you use it while setting up your prop size, you'll find out in plenty of time that you're overloading the motor, the ESC or the battery before burning anything up. And, along with that, if your prop is to small, it will show up as two few watts and/or amps being pulled by your motor, at least as compared to what your motor is rated for.

You'll also run across that "C" term with this electric stuff. Here is a description of what "C" is, and how to use it.

"C" and what it is
http://www.wattflyer.com/forums/showthread.php?t=65869

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Old 04-07-2014, 07:15 AM   #8
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Amps = wattage/volts.
wattage = volts x amps

You need to know them all to calculate actual draw and esc sizing.

personally, every motor I've tested from heads up has been bang on. if weight isn't an issue, a large esc won't hurt anything. i had a 60 amp opto esc and a 70 amp esc on my slow stick bipe twin pulling a grand total of 49 amps.
Exact science. These things never work out quite the way you expect them to.

many motor manufactures do not build motors to exact kv ratings. another reason i dig jeffs motors. actual prop testing results. but things still vary quite a bit. due to my 4500ft elevation, i have to prop up a bit to get max rated specs on most of my motors

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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