Author Topic: What Determines the KV of a Motor? (Read 1378 times)

David Hoover · « **on:** April 10, 2016, 06:51:53 PM »

It would seem to be related to the number of poles and number and type of windings but if that's the case why does a Cobra 2826/12, 14 poles and 12 turn delta windings have a KV of 760 while a Cobra 3520/12, also 14 poles and 12 turn delta windings, have a KV of 820? Clearly there's something else at play here.

Tim Wescott · « **Reply #1 on:** April 10, 2016, 07:12:18 PM »

It's complicated. Number of poles, number of turns, type of winding, strength of the magnets, air gap between rotor and stator, length of the active magnetic area (roughly the length of the magnets and/or stator) & diameter probably play the largest parts.

All else being equal, a longer motor is going to have a lower Kv; I'm not sure about the effect of diameter -- maybe it doesn't have a strong effect, but I don't know. I'd have to do some cyphering, and I'm not a motor designer by trade -- just a motor user. Weaker magnets mean a higher Kv -- but I doubt there's much variation from motor to motor. A larger air gap would also raise the Kv, similar to weaker magnets.

The numbers are not inconsistent with length and diameter both lowering the Kv -- if you toss in a considerable fudge factor.

roy cherry · « **Reply #2 on:** April 11, 2016, 07:19:32 AM »

all the above is true but if you look at it for me it wold be the restance in the wire an e max 28 26 has five turns of 9 strand 29 gage wire so if it had five turns of 27 gage wire the resistance would less allowing more volts to flow freely did you guys do slot car racing try to relate to that

Tim Wescott · « **Reply #3 on:** April 11, 2016, 09:18:15 AM »

Done properly on a normal motor, a Kv rating should only be weakly dependent on resistance.

Kv is the unloaded motor RPM, so the measurement is taken when the current is low. Unless the motor's winding resistance is truly absurd (which you won't find in our motors) then winding resistance shouldn't have much effect.

Mike Anderson · « **Reply #4 on:** April 11, 2016, 05:25:58 PM »

Quote from: roy cherry on April 11, 2016, 07:19:32 AM

all the above is true but if you look at it for me it wold be the restance in the wire an e max 28 26 has five turns of 9 strand 29 gage wire so if it had five turns of 27 gage wire the resistance would less allowing more volts to flow freely did you guys do slot car racing try to relate to that

If the number of turns is the same then the Kv will be the same. But the current capacity will be improved in the one with larger wire so a larger propeller could be used at the same voltage with less heat being developed. Kv is a function of magnetic field strength and the length of the conductor that is moving through that magnetic field. The length of 5 turns will be essentially the same, regardless of wire gauge, therefore the voltage induced will be the same.

When designing the motor, one decides how many turns one needs, and then crams the largest diameter wire possible to fill the slots with copper when wound to the number of turns decided upon. Using multiple turns of smaller diameter allows one to fill the gap with more copper (and less air), increasing efficiency.

Author Topic: What Determines the KV of a Motor? (Read 1378 times)

David Hoover

What Determines the KV of a Motor?

Tim Wescott

Re: What Determines the KV of a Motor?

roy cherry

Re: What Determines the KV of a Motor?

Tim Wescott

Re: What Determines the KV of a Motor?

Mike Anderson

Re: What Determines the KV of a Motor?