Can anyone explain brushless Kv ratings?

[cgpro856]

I'm a bit confused by Kv ratings on a brushless motor. It seems to me that what is causing a brushless motor to spin would have to be a pulsing or rotating field generated in the windings by the ESC. And since you're not just putting raw voltage to the comm like in a brushed motor it seems that the same motor with the same battery could be make to spin at very different speeds by two different ESC's. Is this correct? and if so how are Kv ratings made?

[jamesgorman]

You seem to understand everything correctly. The kV rating comes from the back emf of the motor windings. As the motor is spun up it will generate a voltage on its windings known as the back emf. (true for both brushed and brushless) You can think of the back emf as in series with the windings. When the motor is stopped, there is no back emf so the only thing limiting the current is the winding resistance. When the motor moves faster the back emf reduces the effective voltage applied to the motor windings, to the point where at max speed, the input voltage matches the back emf.

The kV rating comes from the amount of back emf that is generated at speed. Higher kV, lower back emf at the same rpm, high top speed at the same voltage.

The more turns there are in the motors windings the more back emf. (Voltage adds in series)

[GLwagon]

In a nut shell...
KV rating is developed from the resistance to flow of electrons & the resulting RPM of the motor.

If motor "A" & motor "B" were given 1 volt each (amperage not relevant)
If "a" = 4100 kv, & "b" = 5700kv
"A" would = near 4100rpm
"B" would = 5700rpm

For that to happen "B" would have lower internal resistance & be a "hotter" wound motor & draw more amperage & have less back EMF.

Batteries have voltage limits, & amperage isn't as much of a concern on a smaller scale.

A 4000-4200kv motor is usually some where around a 10.5 turns of of copper per pole.
A 5700kv motor is usually some where around a 7.5 turns of of copper per pole.

When the Windings are done differently with smaller gage (awg.) of wire, or larger awg. it can still be a 10.5 or 7.5 but the way it reacts & its efficiency may change greatly & the "KV" rating will show that outcome.

It is very possible to have two different KV motors & generically have the same number of turns.

Other factors come into play such as Delta or Wye windings & are only a concerns for 1/8th scale & larger motors & tend to not be listed for smaller motors.