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07-31-2007 04:25 PM  13 years ago
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concho

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montreal canada

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KV TO WATT CONVERSION
HI RR FOLKS
I TRY TO FIGURE OUT IF THERE A WAY CO CONVERTE KV I TO WATTS
I KNOW THAT IF YOU MULTIPLY THE KV BY THE VOLTAGE YOU GONNA KNOW THE EXAT RPM EX(3400KV X 11.1VOLT =37.740RPM)

IF SOMEBODY HAVE THE AWNSER (KV INTO WATTS)
THEN I BE ABLE TO KNOW EXACTLY THE AMONTS OF WATTS THE MOTOR PRODUCE
IT GONNA MAKE LIFE MUCH MORE EASYEAR TO BUY THE RIGHT MOTOR ESC BATTERIES
THANKS
CONCHO
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07-31-2007 07:59 PM  13 years ago
HugeOne

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Quebec, Canada

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Kv can't be converted into watts, some people believe Kv is like HP for electric motor, but that's not the case. "Kv is the voltage the motor will produce (back EMF) at a certain speed"

-Hugo
Raptor e620 w/V-bar, Tango 45-06, Phoenix 85HV, TrueRC 12S1P 4000mAh
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07-31-2007 08:10 PM  13 years ago
MJWS

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Airdrie, AB - Canada

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Yep. Watts measures work done. It takes a load which for our purposes is variable. So there is no way to convert directly.

The good news is there aren't many systems that aren't very well documented and known. The community is huge, rcgroups has an even larger electric user base. There is a good chance someone can answer your question.

A more typical definition for kv is 'the rpm produced per volt applied'.

Drop the caps and just ask the question. Someone will be able to help you. There are some rules of thumb that work well for electrics.

Mike
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08-04-2007 03:41 AM  13 years ago
tryan02

rrProfessor

Canton, Missouri

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actually kv can be converted into whats by ohms law V * I = P.
but motor manufacturers misuse the KV rating it translates to rpm/volt (4100kv motor * 11.1 lipo) = 45,000 rpm with no load on the motor no prop or even a pinion. And as far as EMF electro magnetic field if it was returning 4100kv the motor would not be able to spin forward with only 11.1v forward and sure wouldn't be running at around 80% efficiency as most brushless do.

download one of the pinion calculators it will help you with your motor decisions. And then look at motor burst current to pick your esc.
No Really I am the club President!!!
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08-04-2007 04:05 AM  13 years ago
MJWS

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Airdrie, AB - Canada

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actually kv can be converted into whats by ohms law V * I = P.
Tell me again how kv can be directly converted to watts? You are missing (I) in your equation. Even if it is only the resistance and loss created by the motor. There is no constant you can use to solve for watts given only kv.

Mike
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08-04-2007 04:36 AM  13 years ago
Dennisu

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Langley, B.C. Canada

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concho

The letters "KV" on a motor rating have nothing to do with "KiloVolts". It is simply a way of saying what RPM the motor will turn, unloaded, for 1 volt of applied EMF from the battery.
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08-04-2007 12:58 PM  13 years ago
tryan02

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Canton, Missouri

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you didn't understand I was stating that kv times current equals power. And you are right first of all the voltage would be the battery voltage and the current free load current of the motor would be given on motor spec sheet. Although you are correct in saying we dont know his individual load current due toprop or whatever load. I ordered a 3100RPM you'll find thats how helihobby rates their motors. 11.1v * 2.5A=27.75W. In short we need to quit the kv rating and call it rpm.No Really I am the club President!!!
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08-04-2007 04:16 PM  13 years ago
MJWS

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Airdrie, AB - Canada

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Unloaded watts are not directly related to kv. Unloaded current (Io) among roughly constant (kv) motors can show you a rough picture of efficiency.

Kv * Current does NOT equal power. Kv is not related to input voltage (in this case you have randomly picked 3s). You can NOT make this substitution.

Kv is RPM per volt (minus losses). Another way it is expressed is:
RPM of motor: kRPM = (V - RmI) / Kb

Here is a great link. A little technical but not bad:
http://www.aveox.com/technical/dc.html

Mike
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08-05-2007 01:01 AM  13 years ago
tryan02

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Canton, Missouri

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OK no more p-ing contest. I Have been in the broadcast electronics business for 11 years and quite well respected even worldwide. I know rf voltage to static voltage I don't need a lesson I understand what the original poster wanted is not possible. I understand the KV rating of motors is invalid they have no KV rating its an rpm/volt. Sorry I added any misinformation to spark your fire.No Really I am the club President!!!
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08-05-2007 02:02 AM  13 years ago
MJWS

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Airdrie, AB - Canada

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Heh. No fire. Sounded like you were going a little fast or from a related field. A couple links and a little discussion will help someone else understand.

Mike
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08-05-2007 02:49 AM  13 years ago
tryan02

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Canton, Missouri

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Cool although when purchasing a brushless motor get all specs like these:
TWF Brushless Motor outrunner
Color: purple
Motor weight: 40g
Motor length: 28mm
Motor diameter: 25.9mm
Shaft diameter: 2.3mm
KV: 3100RPM/V
Free load current: 2.5A
Max efficiency: 80%
Max current: 13A
Speed contorller requiement: 25A

notice speed controller size is almost double max motor current their is a reason for that
No Really I am the club President!!!
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