Kv and motors etc

[Tim Mackey]

Hamish McNab asks....

 

Kv = RPM/volt    -  How?

 

Trying to get my head round how voltage can alter the speed of a brushless motor.  Back to my earlier training a 3ph motors speed was determined by the frequency of the supply and the number of poles in the winding, voltage provided current to give torque.

 

Whilst motors can be wound with different poles to give speed variations the speed changes in models are provided by the changing the frequency by the ESC.

 

Am I correct in assuming that the ESC detects the voltage and therefore ups the frequency to give higher rpm?

 

If this assumption is correct why do motors have so many Kv ratings or is this just a figure if exceeded they will disintegrate?

 

 

 

[Tim Mackey]

Speaking in very general and simplistic terms....

If a motor is wound differently then it will spin at different RPM

 for  the same input voltage

( other factors apply also such as magnet strength etc but lets leave that aside.)

A motor with a low wind and thick gauge wire can carry heavy currents and will create a stronger magnetic effect, spinning the armature faster but with little "torque".

 

A high wind motor will spin slower, and consume less current but have greater torque.

 

The ESC does control the speed by varying the "pulses" of power being fed to the windings. Crudely put, full power is applied for very brief periods on and off and the motor reacts accordingly, then, as throttle is increased these pulses are held "on" for longer and longer, until at WOT its virtually on constantly, so the motor is seeing the full battery potential, albeit phased across the windings ( which creates the motion ).

 

Therefore, a low Kv motor ( with higher winding count ) will be spinning slower on the same battery, than the higher Kv motor. Exploiting the various winds / Kv factors of the different motors is what gives them such flexibility, especially when used on different voltage batteries.

Hope that helps?

 

Edited By Timbo - Administrator on 02/01/2010 10:03:29

[Hamish]

Thanks Timbo.

 

I can understand now that as the magnetic forces are stronger due to current the poles will move quicker to align.

 

I am afraid my HNC is 40+yr old when we had c/s and not these fancy Hz 

[Myron Beaumont]

Timbo

How can you get more Torque from less current through more windings ? I appreciate that more windings equals more torque . One only has to compare a car starter motor with a generator  Surely the "less current" factor is because of the greater  physical  resistance of the windings -the limiting factor when all said and done thus avoiding melt down of said windings .

Now I'm confusing myself !

[Vecchio Austriaco]

Myron, 

 

magnetic force is determined by Current (Amps) x number of windings.

so you get the same magnetic force from 10 windings at 10 Amps as from 100 windings at 1 Amp.

VA 

[Myron Beaumont]

Thanks Vecchio

I knew that  Didn't I ?

Bonne annee

[Hamish]

Vicchio

 

As the magnetic force is the same this must be achieved quicker for the low windings resulting in the higher revs.

 

Something to do with Henry's

[Myron Beaumont]

Does that mean that Henry the 8*th was slower than Henry the 1st 'cos he has a history of being wound up more? & he got rid or the revs (reverands ) didn't he ?

[hamish Finlay]

Hi anyone,

 

I need help here. some motors spec the number of Turns as well as the rev/V. What is the relevance of Turns ?. Are they the same as winds. How does the nember of poles affect things. If three wires go in to motor, are there three phases, and should No.of Turns and No. of poles be functions of three?. I' m off to lie down.

 

Hamish Finlay 

[Bruce Richards]

Number of turns is the same as winds and is related to KV (low number of turn = High KV) I do not think there is any exact conversion you can do between turns and KV.

 

Number of poles is not really relevant to this.

 

3 wires is 3 phase and I do not think turns need to be a whole multiple of 3.

 

Not sure what you are trying to do but hope this helps.

 

[Phil Claridge]

well i'm all wound up and i've just had a funny turn, and now my heads spinning, so what amps am i running

[Tim Mackey]

38

[Chris Bott - Moderator]

Wouldn't it be 42? (The answer to life the universe and everything...).

[hamish Finlay]

Thanks Moderator. That helps. I am just trying to understand as much as I can. The ads for Scorpion motors provide a bit more information, ie turns and no of strands.

Since I posted my question I have dismantled and unwound an old motor and have a better idea of things. It seems to me that no of poles must be divisible by three but turns can be any number. ( usually below 15.) I have A level Physics from 50 years ago, but I fear much of my grey matter has rotted since then.

Cheers

 

Hamish

[Tim Mackey]

I think you are on the right lines there Hamish - I understand your desire for the extra knowledge and info, but remember that the main things you really need to know are the motors Kv and power handling capacity.

[Mike Downs]

I don't know if I am in the right bit of the forum for this question! but here goes....

I am converting my old multiplex twin star to lipo and brushless......

 

I was recommended as follows,

2X1000kV brushless....ppb 2834-1000 12 turn 180W

2X22 A ESC's

2X8x6 props

 

On setting everything up one motor seems to be a bit tighter to turn by hand and is approx 20 degrees C hotter after a short run..on a couple of runs it came loose from the backing/mounting plate ( I have tweaked these up) and are now a bit better.

 

When turning the 'good ' motor by hans I can feel 12 jumps per 360 degrees.

When turning the other one I cannot feel these jumps.

I checked the temp by remote temperature gun.

 

Any ideas?

Cheers