Permeability in a DC Motor

Limits on DC motor rotor speed comes from bearings burning up, brush float (on a brushed motor) or, once you've solved that problem, the rotor failing (or at least deforming) from centripetal acceleration. On some motors there's probably a secondary limit on the armature overheating because you're exceeding the design limit on eddy current or (possibly, I don't design motors) B-field strength in the iron.

As a test, take your cheap Mabuchi motor, oil the bearings with light oil (20W is probably perfect, but "sewing machine oil" would work), then attach it to a variable power supply that'll go up to twice the motor's rated voltage. Ramp the voltage up to 2x (or 3x or 4x) the motor's rated voltage and observe the speed -- you'll see that it goes right up. Keep this up until the motor breaks. Then disassemble, and figure out which bit broke.

A friend of mine had a job testing brushless motors to destruction. The project goal was to get the most powerful motor in the smallest space. The limit was keeping the magnets on the rotor. They regularly exceeded

30,000 RPM.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

Hi Tim W.,

I understand that the maximum RPM is limited by a destructive event, but my point is not about that limit. I am proposing a new law of motor generator s. To test it, use a low current so the motor is not destroyed. Calculate t he average velocity of electrons in the armature current:

ve = I / NQA

ve = electron speed, I = current, N =electron density in copper 8*10^

28/meter^3, Q is electron charge, A is area of wire

Calulate speed of a proton (vp) in an Iron magnet, relative to the orthogon al copper coil:

vp = 2 pi R omega

R is rotor radius, omega is angular velocity, radians per second

Law proposed

vp/ve < 797,000

where 797,000 meters = 1 / permeability of free space

Law proposed: The speed of a motor is less than 797,000 times the speed of an electron in the driving current.

That is in an ideal case where the RPMs are low enough that the motor is no t damaged. The permeability of free space is Henrys per meter. Magnetic fie ld H is amps per meter. Magnetic flux density B is Webers per square meter. using these standard terms, the Law will show that the magnetic field is a velocity:

H = meters per second = B/permeability

H = second^-1 / Henry*meter^-1

Weber = Coulomb per second = Ampere

therefore ... law under construction...

Oh, I'm sorry -- I thought you were seriously interested in something real.

My bad.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

On Wednesday, June 1, 2016 at 9:43:24 AM UTC-10, Tim Wescott wrote:>

Oh, I'm sorry -- I thought you were seriously interested in something real .

Real motors is what I am discussing. The motors that are not damaged can be tested to prove my new Law of motor-generators. For example, for a motor w ith a maximum allowed RPM of 9200, run it at less than 9200 RPM so it is no t damaged. Force a small current so it runs at 8000 RPM.

Calculate the velocity of the motor divided by the velocity of the electron current. The ratio of those two speeds is always below the inverse permeab ility. This is handled with mathematics that employ primitive units of meas ure: meters and seconds

B = second^-1 = magnetic flux density

H = meters per second = Magnetic field intensity

mu zero = 1 Henry per 797,000 meters

B = (mu zero) H

B/ mu zero = velocity

where B is one line of flux for one electron and one proton in a pair.

In that case, an outer-rotor brushless motor should last longer, since centripetal forces simply press the magnets harder into the rotor..?

Michael

I don't know if they tried that -- it was long before outrunners were commonly used. They did have rotors with titanium bands shrunk on over the magnets -- which would fail, leaving magnet-sized inverse dimples in the ring.

It didn't have squat to do with the ratio between electron velocity in the wires and the rotor velocity; I know that.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!

For real DC permanent magnet motors the only thing that limits rpm is the load on the motor. If there was no load, and the motor materials were of infinite strength, the motor would rotate at just below the speed of light. No matter how fast the electrons were moving in the motor windings. Eric

And how is that useful?

Hi Tim, I have been reading up on motors and the websites do not discuss these electrical engineering standard variables:

H Magnetic Field Intensity

B Magnetic Flux Density

mu permeability

Magnetic motor websites have equations about voltage, current, torque, and rpm, but few even hint that a Magnetic Field Intensity is involved in a motor equation. mu is not in their equations. I am trying to put the H, B, and mu in the motor equations.

1/mu zero = 797,000 meters

That inverse permeability is profound and mysterious.

Stronger than a band of titanium! Maybe they could have marketing put a positive spin on that... :p (hey, was that a pun?)

On one hand, I hear electrons in atomic orbitals move at a significant fraction of the speed of light; on the other, I hear that due to electrons frequently bumping into each other, I can run faster than the electrons move through a wire. Which is it? :p

Have you tried Freelancer.com?

Cheers,

Michael

You're thinking of a shunt-wound motor, not a permanent-magnet motor. A friction-free DC motor (or one with superconducting coils) turns at a constant speed proportional to voltage.

And I suspect that in a shunt-wound motor the rotor inductance would limit things, even in the absence of any loss mechanism.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

Individual electrons in a wire move pretty quick. But the average speed of any one electron in a wire is dog slow. However, that average is superimposed on a whole lot of shakin'.

Electrons in orbitals move pretty quick, too, but you don't have to make relativistic corrections until you get to the really heavy atoms, like gold (which is a different color than the initial computations indicated, because some of the inner orbitals are smaller than predicted without using relativistic corrections -- once you correct for those electrons' mass being bigger, then things work).

That's all I know about this -- I'm just barfing out things I've read; I couldn't do the math to save my life.

--
Tim Wescott 
Control systems, embedded software and circuit design 
I'm looking for work!  See my website if you're interested 
http://www.wescottdesign.com

Average velocity of free electron = v

v = I / NQA

I is current, N is density of electrons in Copper, 8*10^28/meter^3, Q is electron charge, A is area of wire

for copper wire 1mm square, 0.01 amp

v = 0.01 / (8*10^28 1.6*10^-19 10^-6)

v = 10^(-2-29+19+6)

v = 0.000001 meter per second

Non relativistic speed, but magnetic enough to turn a motor at 796 cm per second

Whoops! I better go back and read that book again. Eric

...

r a

Yup, gold, and lead, which is almost as heavy as gold, and a lot cheaper, t oo :)

I'm pretty sure the "1s" orbital is the one that gets super tiny in lead an d gold, and the electrons in there are going so fast you need relativistic corrections, but I'm trying to find the article I found years ago and I'm n ot finding it :/

... but I found this :)

I'm reminded of a TV show I saw as a kid - they were making fun of Soviet n ews broadcasts, where a reporter was reading the news with a pistol aimed a t his head, just to the side (but within view of the camera). My Fluid Mec hanics class sure felt like that. "Learn this or die."

Michael