PDA

View Full Version : How to emperically measure Kv?


dditch66
08-05-2009, 09:16 PM
Is there a way to take a motor and imperically measure the Kv of the motor through an experiment?
I have a motor, esc a way to measure RPM (eagletree) and a variable DC power supply at my disposal.

I have one motor that I think has a higher KV than what the MFG says. I'm mostly curious to see if it can be measured without too much more than what is at my disposal.

simibill
08-05-2009, 10:19 PM
Is there a way to take a motor and imperically measure the Kv of the motor through an experiment?
I have a motor, esc a way to measure RPM (eagletree) and a variable DC power supply at my disposal.

I have one motor that I think has a higher KV than what the MFG says. I'm mostly curious to see if it can be measured without too much more than what is at my disposal.

Check out http://homepage.mac.com/kmyersefo/

Look about half way down the page for info and spreadsheet to calc KV using a drill press to spin the motor being tested.

dditch66
08-06-2009, 12:09 AM
Is it true that a motor will never exceed it's Kv rating?

In other words, no matter how you load it, a 1000Kv motor will never run over 12000RPMS if you have a pack that is at exactly 12V?

Larry3215
08-06-2009, 06:23 AM
Is it true that a motor will never exceed it's Kv rating?

In other words, no matter how you load it, a 1000Kv motor will never run over 12000RPMS if you have a pack that is at exactly 12V?


Yes.

Even with no prop the motor will never exceed its kV x voltage. Brushless motors do not "runaway".

With the prop on its a good idea to be sure your getting around 75% - 80% of kV under load at a minimum. That will keep the motor running relatively efficiently.

slipstick
08-06-2009, 10:36 AM
Your Eagletree should be able to measure the rpm with no load and that will give you a reasonable approximation of Kv. E.g. 10,000rpm, 10V = 1000 Kv. If you want it a bit more accurate take a second reading with a prop on and stick it into MotoCalc's motor test screen.

Steve

dditch66
08-06-2009, 12:51 PM
Your Eagletree should be able to measure the rpm with no load and that will give you a reasonable approximation of Kv. E.g. 10,000rpm, 10V = 1000 Kv. If you want it a bit more accurate take a second reading with a prop on and stick it into MotoCalc's motor test screen.

Steve

10V will be the pack voltage and 100% throttle?

Cool,
It's on 450 sized helicopters so I can vary the pitch and throttle curve.

Larry3215
08-06-2009, 04:16 PM
Here is another way to determine kV with a drill press. Its explained a little more clearly than the spread sheet simibill linked to above but its basically the same thing.

http://www.bavaria-direct.co.za/models/motor_info.htm

Look towards the bottom of the page. He gives 4 different methods that have varying degrees of accuracy.

For the most part, just using the simple rpm voltage thing is close enough for guestimating power draw with the motor calcs.

dbcisco
08-07-2009, 07:19 AM
It is difficult to measure the voltage because it is not a continuos DC voltage. An oscilloscope, tachometer and a calculator are best for an accurate KV determination. You also need to know what the manufacturer is using to come up with the specs. Is it RMS voltage at the motor or DC voltage applied to a specific ESC. My guess would be the former methed as it will yield a higher rating.

slipstick
08-07-2009, 09:48 AM
Except that for determining Kv it's sufficient to measure at 100% throttle and simply assume that the battery voltage (which is easy to measure) is equal to the motor voltage.

It's true that this is not as accurate as other more complex methods that most people can't use, not having an oscilloscope readily to hand....but then motor Kvs aren't often stated with any real accuracy either ;).

Steve

cbatters
08-07-2009, 02:09 PM
If you can measure RPM with Eagletree, why not put on a low pitch prop and simply measure RPM?

You can also measure RPM with a free oscilloscope program running on a PC and a microphone (I posted a link in another thread)

dbcisco
08-07-2009, 02:27 PM
Or just put on the prop ESC RX and battery you are going to use with it,then check the speed at various throttle settings on your TX with a tach. At least you will get real world data.

Larry3215
08-07-2009, 05:52 PM
If you can measure RPM with Eagletree, why not put on a low pitch prop and simply measure RPM?

You can also measure RPM with a free oscilloscope program running on a PC and a microphone (I posted a link in another thread)

Can you post that link here? Id like to play with that scope software. :)

cbatters
08-08-2009, 10:53 AM
Can you post that link here? Id like to play with that scope software. :)

http://www.wattflyer.com/forums/showthread.php?t=13403&highlight=scope

stephen.robin
09-14-2009, 10:01 AM
Hi
The volt is the derive of electromotive force, commonly called voltage. It is also the unit for the related but slightly different quantity electro potential also called "electrostatic potential difference". :blah::red:

Dr Kiwi
09-15-2009, 12:57 PM
Another way to determine the "raw" Kv... that is, simply, RPM/V... is to get one of these:

http://www.rcgroups.com/forums/showthread.php?t=1091136

Using several no-load current and RPM measurements, and several prop tests, then entering those data into DriveCalc should get you a slightly more accurate figure for Kv, than the crude/raw RPM/V.

Moxus
09-23-2009, 07:53 AM
Yes.

Even with no prop the motor will never exceed its kV x voltage. Brushless motors do not "runaway".

With the prop on its a good idea to be sure your getting around 75% - 80% of kV under load at a minimum. That will keep the motor running relatively efficiently.

or to be 100% exact, it is true that electromotors (brushed or brushless) wont runaway as long as they have permanent magnets.
brushless or brushed motors can runaway if they use electromagnets for both rotor and stator.
this is true because the kv rating is directly dependant of the magnetic field strenght. and electro magnets have variable magnetic strength, depending on the current going through them.

this also means that brushed motors also has a kv rating, although this is not ususally mentioned within out hobby.

ron_van_sommeren
09-23-2009, 12:08 PM
Yes, brushed and brushless motors are the same, in their behaviour, they use the same constants for modelling the motor and for power calculations:
http://www.rcgroups.com/forums/showthread.php?t=185271&highlight=bergmeyer

Brushed motors have mechanical commutation (http://en.wikipedia.org/wiki/Commutator_%28electric%29) (brushes&copper segments), brushless motors have electronical commutation (ESC)
http://en.wikipedia.org/wiki/Commutator_%28electric%29

Vriendelijke groeten ;) Ron

nick_cool
10-09-2009, 12:11 AM
Here it is a thread about a Kv meter:

http://www.rcgroups.com/forums/showthread.php?t=1091136

It is easy to use, and inexpensive