How to determine Kv of unknown motor? - RC Groups
Jan 18, 2002, 08:25 PM
Registered User

# How to determine Kv of unknown motor?

If you have an unknown motor, how can you tell the Kv, or the internal resistance?
Thanks,
Rick
 Jan 18, 2002, 09:14 PM Registered User I THINK you have to spin the motor at a known RPM (like with an electric drill) while measure the output that the motor puts out as if it were a little generator. Divide one number by another for the KV ratio. The resistance you MIGHT be able to measure with an ohm meter. Hope a guru logs on and corrects me if I'm wrong. My memory is not that good
 Jan 18, 2002, 10:21 PM Registered User If I spin the shaft of the motor, it will produce a measurable amt of electricity?
 Jan 18, 2002, 11:11 PM EDF Head Yes the motor will generate electricity if spun. If you have a tach handy you can also make a ply/cardboard disk to act as a prop, apply a known voltage to the motor and read the rpm. Then divide rpm/V to get the KV.
Aug 21, 2008, 10:11 AM
Bertrand MICHELS
Quote:
 Originally Posted by Haldor Yes the motor will generate electricity if spun. If you have a tach handy you can also make a ply/cardboard disk to act as a prop, apply a known voltage to the motor and read the rpm. Then divide rpm/V to get the KV.

does this also apply to brushless motors ? If yes, then how do I know what is whthe applied voltage since the bL motor goes through an ESC that has 3 wires output ! ! !
 Aug 21, 2008, 11:13 AM Registered User Outrunners are easy: wrap a piece of white tape around the bell, and using a black Sharpie color two patches diametrically opposite each other. Point a flashlight at the bell from the side. Applying whatever voltage you wish, read off Io with your Wattmeter, and the rpm with your tach (the tach will see the reflected light as if it were a two-blade prop). Kv will be approximately rpm/voltage. Run a few prop tests too... then you can use Motocalc/Drive Calc/or Peak efficiency to determine Kv and Rm. Inrunners are more difficult because even a tiny pseudo-prop adds a load (but if you only need a crude measure, that will do). They are best measured using an EagleTree!
Aug 21, 2008, 11:49 AM
Bertrand MICHELS
Quote:
 Originally Posted by Dr Kiwi Outrunners are easy: wrap a piece of white tape around the bell, and using a black Sharpie color two patches diametrically opposite each other. Point a flashlight at the bell from the side. Applying whatever voltage you wish, read off Io with your Wattmeter, and the rpm with your tach (the tach will see the reflected light as if it were a two-blade prop). Kv will be approximately rpm/voltage. Run a few prop tests too... then you can use Motocalc/Drive Calc/or Peak efficiency to determine Kv and Rm. Inrunners are more difficult because even a tiny pseudo-prop adds a load (but if you only need a crude measure, that will do). They are best measured using an EagleTree!
I guess this only work with the ESC au FULL trotle since there is no way to mesure voltage between any point between the ESC and the motor (3 wires, not just 2!). SO we must mesure voltage IN the ESC, not IN the motor ? correct ?
 Aug 21, 2008, 12:26 PM Registered User Oops Last edited by jrb1; Aug 21, 2008 at 12:33 PM.
Aug 21, 2008, 12:34 PM
Member
Here's some words from Ken Myers ( http://homepage.mac.com/kmyersefo/M1-outrunners.htm ); the picture shows the calcs in a spreadsheet:

Kv
Kv is a motor constant and is directly related to Kt, the motor torque constant. Kv is most often expressed as RPM/Volt or RPM/v. Kt is often expressed in the units inch ounces per amp. Kv (expressed as RPM/v) * Kt = 1352. The Kv motor constant has nothing to do with the applied voltage. It is part of the motor's physical makeup.
Kv is also known as the generator constant or dynamo constant. When any electric motor is spun physically it generates electricty. It doesn't matter whether it is a brushed or brushless motor.
A typical hobby brushed motor can be spun by a drill press at a constant speed. By measuring the DC voltage across the terminals, with brushes set to neutral timing, and knowing the RPM of the drill press the Kv can be calculated. i.e. 1560 RPM / 1.6v DC measured volts = 975 RPM per volt. (To see how timing affects a motor, read my Timing Test.)
A brushless motor isn't quite as simple to do, but can be done using some math. A brushless motor has three possible lead combinations that need to be measured using AC voltage. i.e. Know the RPM (1560 in this example). Measure the AC voltage on each pair of leads, there are three possible combinations with a brushless motor. Lead combination A - 2.08, Lead combination B - 2.08, Lead combination C - 2.08. Note that most cheap motors do not have all three combinations come out exactly the same, but they do on the better quality motors. Use the average of the three voltage numbers if the measurements are slightly different.
Find the V-peak by multiplying the average AC volts by 1.414 In this example 2.08 * 1.414 = 2.94v
Divide 1000 (a constant) by the RPM (1560 in this case) = 0.64
Ke = V-peak ((2.94) * (in this case 0.64))/1000 = 0.00188
Find the inverse of Ke (1/Ke) (1/0.00188 in this case) = 531
Divide the inverse of Ke by 0.95 = 559 RPM/v or the approximate Kv expressed as RPM/v

Brushless Kv formula using drill press
Kv = (1 / ((Vac * 1.414) * (1000 / drill press rpm)) / 1000) / 0.95
Kt = 1352.4 / Kv

It should be noted that many manufacturers/suppliers, even the good ones, provide inaccurate information on the motor's Kv, so if you can, measure it to be sure you have the motor you want. Nothing really needs to be done to the motor to measure the Kv, so it should be easily returnable if the Kv is not suitable, as the shaft will only have been chucked into a drill press that is set up with a known RPM.
Advancing the timing on a brushed motor (using rotation of the brushes) or brushless motor (via an ESC setting) changes the apparent Kv, increases the RPM and Io (no load amp draw), increases the heat (wasted energy) more than neutral timing, but increases the power out.