Brushless Kv?

[Carl]

How do you measure brushless Kv. Is it the dc voltage that is being fed to the ESC?

Carl

[Mr.RC-CAM]

Yup, Kv is the RPM's per volt. A tach, digital voltmeter, and suitable power supply, is useful to determine the value.

I believe the basic formula is: Kv = RPM / ( V - (Rm * I))
Note: (Rm * I) is the motor's efficiency loss factor.


RC-CAM

[vintage1]

Its not the dc voltage thats being fed to the esc.

Its actually how many volts the motor would generate as a generator, off load, at a given rpm. Or rather the rpm the motor would spin at to generate one volt.

That figure, together with the resisntance of teh windings, and how muchg current the motor absorbs when driving no load at all, allwo you to define the motor's power and efficiency anywhere in its operating region.

Youi can spin the motor and measure the AC voltage up, or you can use the spreadsheet I developed - MUMTATS - to take several measurements of voltage RPM and current, and the spread sheet then calculates the Kv, the Io and the Ro values for the motor.

[modfly]

vintage1..

How to determine Kv from motor specs...for example.....

KM0283010 Motor Specs:

Max RPM: 12,000
Operating Power: 180 W
Operating Voltage: 6-12V
Operating Current:: 15A
Peak Current: 20A (max 15 s)
Shaft Diameter: 3 mm
Diameter: 28 mm
Length: 30 mm
Weight: 60 g

How can I determine KV with this data?

[vintage1]

You can't really :(

More data needed

[modfly]

Ok thanks..need the number of turns and such I guess.

[BMatthews]

More than that you need the number of turns and the number of poles and the number of magnets. Even then I don't think you can calculate the Kv.

It's far easier to test it with a small disc with two marks or two slots to work the tachometer and use a low voltage battery pack. Put it up to full throttle and measure the RPM and the voltage while running. Divide the RPM by the voltage and that's your Kv value.

[modfly]

Thanks BMattews..

[BMatthews]

For some reason I missed Vintage's post about his spreadsheet. That sounds like a great tool he's got there.

It also sounds like it's assuming that the spins to generate 1 volt and the idea that it turns that fast when 1 volt is supplied are the same thing. That right Vintage?