I have a 2-part question about electromagnetism and motors

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  • Thread starter James Birt
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797
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Copper resistance at cold and hot not taken into account and friction losses are practical variants apart from room temp.. which can alter the calculations I think.
Please correct me but I assumed the amount of voltage induced in a conductor is not affected by the conductor's resistance. (Only when current flows does resistance have an effect.) And the speed at which a conductor is moving is not affected by how much friction the rotor needed to overcome to attain that speed.
 
jim hardy
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What formula do you use to find average magnetic flux?? I can get the other numbers, and thanks for the feedback I'm stuck on this.
the basic flux formula for a closed, uniform, magnetic circuit is
Φ = μμ0NIA/length
Φ = flux
μ and μ0 are permeability
N = number of turns
I = current
A = area of path
length is of path
a search on that formula will turn up tutorials

for you motor,
The easiest approach is to turn it around, assume flux you want and solve the formula for MMF

your magnetic path is not uniform but it is closed.
So break it up into segments . Look at that sketch in post 24.
Air gap is two segments, rotor is one, each pole piece is a segment, and the round yoke is yet another.
Each segment has its own area, permeability, and length.
So for each segment of your closed loop magnetic circuit,
calculate the MMF required to magnetize that segment to desired flux then add them up.

It's easy in principle. If you're as neat and orderly with your arithmetic as you doubtless are in your milling machine setup you'll have no difficulty.

Let us know what % of your MMF is consumed in each segment of your proposed design? Then for fun, halve relative permeability of the iron and see how much that changes.. Then halve the air gap and see how much it changes. In my day we did that by slide rule, you'll no doubt use a spreadsheet.

Will your iron tolerate B of 1 Tesla ?

Have fun - motors are fascinating. DC motors are my favorite.

It's all F=QVcrossB

old jim
 
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I was under the impression that the electrical resistance would be in the electrical calculations, the armature resistance would be in the motor or RPM calculations and for the magnetism I would only have to concern myself with reluctance. Is that not correct? As per the thread David is referring to.

And good ole' Jim. I'm using old fashion pen and paper... with a scientific calculator of course. I do understand my first step here in calculation is the easy part even though I'm having trouble and what should work in theory doesn't always work in application but that will most likely be because of some variable I didn't account for... it's hard to say till I actually build and field test, but once I'm comfortable with the calculation I will be. And thanks to you and David I'm approaching a comfort zone.
 
797
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The material for your armature core is not selected solely on the basis of permeability. You would also like retentivity to be as low as possible because the core carries alternating magnetic flux.
 
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So, low magnetic retention to be considered also?
 
797
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Yes. It takes more energy to demagnetize the core than you spent magnetizing it, so every time you demagnetize and reverse the polarity of flux in a ferromagnetic material, some energy will be lost to magnetic hysteresis.
 
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I didn't even think of that, back to the drawing board.. Thank you again, always something insightful!
 
jim hardy
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this guy's videos might help you get started

you'll need headphones, his accent i find difficult but i do like the first presentation.

 
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I listened, but that presentation is not in English and trying to read subtitles and watch the examples is confusing.
 
jim hardy
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that presentation is not in English

Oops sorry about that
i thought it was my old ears on this terrible laptop speaker !

will look for a proper one...

for starters try


and
 
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Haha, old school, I like it.
 
797
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What the video calls the neutral plane (I call it the geometric neutral plane) contains the motor's axis of rotation and is perpendicular to lines of field flux when no armature current is flowing.

Armature reaction (caused by the magnetic field that arises from armature current) tends to rotate field lines against the direction of motor rotation. The video terms this rotated neutral plane the adjusted neutral plane. I call it the electrical neutral plane.

When an armature conductor passes through the electrical neutral plane, it's moving parallel to magnetic lines of force, and hence no back EMF is induced as a result of the conductor's motion. However, some additional back EMF is self-induced as armature current decreases (caused by armature inductance). These two phenomena work together to rotate the electrical neutral plane in the same direction.

When voltage in an armature conductor is zero (that is, when it passes through the electrical neutral plane), this is the ideal point at which to commutate the current.
 
797
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Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)
Correction: Average magnetic flux should be average magnetic flux density. (The SI unit for flux density is tesla.)
 

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