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Alternator Inductor Coil Output Equation (?)

by Elektrostatik
Tags: alternator, coil, equation, inductor, output
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Elektrostatik
#19
Mar4-12, 03:28 PM
P: 20
Magnetics is not well taught.
That's actually more than true, given that most of the population are quite familiar with basic electric units :
Volt, Ampere, Watt, Ohm.. even those with no interest in electronics, but relatively few people have ever
heard of SI magnetic units : Weber, Tesla, henry.. even within the magnet 'industry' the old cgs units :
Gauss, Oersted and Maxwell are still in standard use, but most likely due to the size of the cgs units being more managable.
Amazingly the photomagnetic effect has been unknown until only a year ago,
and would likely have remained unknown - if all electromagnetic science and engineering research
were based strictly on standard equations.

So - build yours with "hula hoops" of whatever you planned, but leave yourself ability to substitute iron and see whether voltage increases.
The hula hoop analogy is intended only to build a mental picture of the rings, not to be taken literally of course.
The actual inductor support mounts are flat, 20 mm thick polycarbonate rings, the same diameter as the flywheel.

The inductor coils were originally designed with soft iron cores, due to the high permeability of iron.
Undoubtedly iron cores would produce a much higher output than an air core, my only concerns are eddy currents
and frying the insulation with excessive current.

The air gap between the magnets and inductor cores will be kept to a minimum,
mechanically limited by clearance requirements that account for deviation and distortion
of the flywheel during operation at maximum angular velocity on high precision bearings.

I've calculated a tensile stress of 18 MPa on the flywheel at 20 rev/s,
which allows a 4-fold tollerance below the 75 MPa tensile limit of Polycarbonate.

The only real variables in the design now are the inductor core material and wire gauge.

With the information you've given me, I've been able to do most of the calculations needed to make predictions,
mind you, all the values that count will come from real-world testing.
SirAskalot
#20
Mar4-12, 04:58 PM
P: 141
In my mind your coils are, by definition, armature coils. Also your coils are stationary hence they compromise the stator! Different name, same sh**! Your flywheel with PMs ("field winding") are rotating hence the name rotor.

But what the heck, thats just the industry standard, lets use some other weird name for it!

Unless you can come up with a drawing who says different, I do believe I have the correct design and operation of your machine. It is widely used in some applications, but with 'some' modifications i order for customers to want to buy it.

Take advice from professionals like Jim. It can save you from much frustration.
jim hardy
#21
Mar4-12, 07:50 PM
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Well, HERE's a neat demo of field surrounding a magnet !

http://www.kjmagnetics.com/magneticfield.asp
Elektrostatik
#22
Mar5-12, 04:14 AM
P: 20
In my mind your coils are, by definition, armature coils. Also your coils are stationary hence they compromise the stator! Different name, same sh**! Your flywheel with PMs ("field winding") are rotating hence the name rotor.
There is no armature and no stator in the design.

Here is an image of an armature :

http://www.gearseds.com/curriculum/i...nd_brushes.jpg

the magnetic field induced in the armature coils creates an electromagnet
which repells the permanent magenets in the stator, causing the armature to rotate.

the rotating armature is the part of the motor that does mechanical work.


In my design the permanent magnets are mounted on a rotating flywheel,
there are no coils on the flywheel, the flywheel is neither a stator nor an armature.

no part of the design does mechanical work, the flywheel is free spinning on it's axis
and only continues to spin by angular momentum. the coils have no effect on this rotation.

for clarity - we can forget that there is more than 1 coil
the device operates whether there is 1 coil or 1000000 coils

this single coil is stationary and sits like a magnetic sensor 1 mm from the side of the flywheel,
so as the flywheel rotates, the magnets in the flywheel pass over the stationary sensor coil and induce a current.

this coil cannot be called a an armature, it has no motion and no contact with anything that has motion.
it is absolulety not a stator, it is only a single coil of insulated wire epoxied to a flat sheet of polycarbonate,
it's only function is that of a transducer.

Here is an image of a permanent magnet stator

http://lh6.ggpht.com/_X6JnoL0U4BY/S2...C57_thumb3.jpg
jim hardy
#23
Mar5-12, 06:59 AM
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Actually, "Armature" means the coils wherein the magnetic field and current interact.


4. That part of a dynamo or electric generator or of an electric motor in which a current is induced by a relatively moving magnetic field. The armature usually consists of a series of coils or groups of insulated conductors surrounding a core of iron.
http://dictionary.webster.us/armature



You are correct that an open circuited coil is a flux 'transducer', voltage induced is per Faraday's law.

i say go ahead and build it, and see how well it works.
Open circuit voltage will tell a lot about your magnets and their field strength. You could connect your coils to an electronic integrator and o'scope to observe flux.
If you DO connect your coils to an external circuit,, plot torque versus current at some constant speed..
Carl Pugh
#24
Mar5-12, 11:04 AM
P: 384
Actually I may have what you describe in my car.
It's a magnet rotating around that induces a voltage in a coil when the spark plug is to fire. At least that how they did it 20 or 30 years ago when they got rid of points.
Elektrostatik
#25
Mar5-12, 03:25 PM
P: 20
Well, HERE's a neat demo of field surrounding a magnet !

http://www.kjmagnetics.com/magneticfield.asp
nice demo*

In the north of Sweden you can see the earth's magnetic field in living colour (green)
otherwise known as the 'northern lights'.

Actually, "Armature" means the coils wherein the magnetic field and current interact.
Aha, okay.. I've always interpreted the term 'armature' as the rotating electromagnet and drive axis of a motor.

Actually I may have what you describe in my car.
Hm.. what model car do you have?

Most cars since the mid 80's have microprocessor controlled electronic ignition, which have no moving parts.


Unless you can come up with a drawing who says different, I do believe I have the correct design and operation of your machine.
Here is a diagramme of the machine :

1. EDGE VIEW
2. SIDE VIEW
a. axis - high precision bearing
b. polycarbonate flywheel
c. neodymium N55 permanent magnet 30 mm x 30 mm
d. electromagnetic transducer coil
e. bridge rectifier
Attached Thumbnails
generator.jpg  
Elektrostatik
#26
Mar5-12, 03:31 PM
P: 20
This is a simplified example of the design - showing only 1 transducer coil

in the actual working model - there are 120 transducer coils operating independently
60 on either side of the flywheel, Group A and Group B - 90 out of phase with Group A.

The target output for each transducer coil is 18 - 30 V with a peak current of 10 A


You are correct that an open circuited coil is a flux 'transducer', voltage induced is per Faraday's law.

i say go ahead and build it, and see how well it works.
Open circuit voltage will tell a lot about your magnets and their field strength. You could connect your coils to an electronic integrator and o'scope to observe flux.
If you DO connect your coils to an external circuit,, plot torque versus current at some constant speed..

Thanks Jim, I will follow your advice, and let you know the results.
Elektrostatik
#27
Mar6-12, 01:25 PM
P: 20
Jim, do you think a 10 mm spacing between magnets will produce an unbroken sine wave?

I will not be able to test the output with an oscilloscope until both the flywheel and 1 transducer coil are completed,
and once I build the flywheel I've already invested quite a bit of time and money in materials,
so I need to get it right the first time.
jim hardy
#28
Mar6-12, 06:12 PM
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do you think a 10 mm spacing between magnets will produce an unbroken sine wave?
Knowing how much trouble manufacturers take to produce a sine wave flux i will be surprised if you get a precise one with just magnets.

Here's a calculator that figures flux a point in space near a magnet of given shape
http://www.kjmagnetics.com/fieldcalculator.asp

i used it to figure a couple points 15mm above a 30mm X 30 mm N42 cylinder; it didnt offer me N55.

Y component on center 1597 gauss
halfway between center and edge 1417
at edge 879
5mm beyond edge 550
10mm past edge 290
40mm past edge it's 6.3, essentially zero
so the flux is highest in center like a cosine function
and if we take 40cm as distance from peak to zero
879/1597 is arccos of 56.6 but 15/40 of 90 is 33.6
similarly 550/1597 is arccos of 69.8 but 20/40 of 90 is 45
so flux from a single magnet only resembles a sinusoid at that distance, it's too peaky.. as one would expect.

What will the adjacent magnet do to field?
I assume your magnets will alternate N-S ?
My intuition says that'll pull the flux down away from coil and sideways over into adjacent magnet. So a short fat coil with its turns near the magnet would be better than a tall skinny one. It'll also move zero crossing of flux Y-component nearer to magnet, probably reducing that 'peakiness'.
Use that button on calculator labelled "See magnetic field", to lower right . It makes clear that you need coil close to magnet. And something to encourage flux to stay inside the coil would be a nice touch.

Forty years ago i might have tackled that flux calculation myself but not anymore. In air it's a field problem, in iron it'd be just algebra.

Keep studying this. It's interesting.

But to answer your question, i think you will get a function that rises and falls and is curvy amd symmetric and is continuous like a sine wave but will be a poor mathematical fit to one.

For making DC a flat-topped wave is better anyhow, because the valleys between peaks are narrower easing job of filter capacitor.

Objective is to learn, eh?
Have fun.

old jim
Elektrostatik
#29
Mar7-12, 05:40 AM
P: 20
Y component on center 1597 gauss
halfway between center and edge 1417
at edge 879
5mm beyond edge 550
10mm past edge 290
40mm past edge it's 6.3, essentially zero
so the flux is highest in center like a cosine function
and if we take 40cm as distance from peak to zero
879/1597 is arccos of 56.6 but 15/40 of 90 is 33.6
similarly 550/1597 is arccos of 69.8 but 20/40 of 90 is 45
so flux from a single magnet only resembles a sinusoid at that distance, it's too peaky.. as one would expect.

What will the adjacent magnet do to field?
I assume your magnets will alternate N-S ?

Aha, so for N42 the centre to centre spacing between magnets would need to be 110 mm for a zero to peak sinusoid,
and for N55 an even wider spacing.

For making DC a flat-topped wave is better anyhow, because the valleys between peaks are narrower easing job of filter capacitor.
of course, a square wave would be ideal, since a square wave would rectify to flat DC
without the need for a second wave 90 out of phase.


I was planning a coil with a square cross section, which has a 3:1 aspect when the core is also square,
and this gives a low and fat inductor profile.

I also considered using a Tesla spiral coil http://www.theoldscientist.co.uk/Bif...esla_Coil.html which has the most extreme voltage drop
between turns, but maybe this is a bit over the top.


Yes, the poles are alternated N - S

Thanks very much for this information, it's extremely helpful!

Integral equations give me a headache
jim hardy
#30
Mar7-12, 09:36 AM
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Integral equations give me a headache
i'm okay with those. It's dels and curls that make me hide under the bed.

Try some graphical attacks on your magnets side by side geometry.
We solved fluid flow problems that way in early 60's before programmable calculators.

And dont worry about producing a sinewave flux yet.
Your challenge will be in making intense flux go up through your coils instead of straight across into adjacent magnet.

Output can be filtered if you need a sine wave. The coarse power inverters to which i am accustomed have only third and fifth harmonic filters. Okay for most electronic loads.

It is rewarding to see your thoughts progressing. That's how we learn, small steps.
When you start in with the hand tools , design in escape routes for those ' what if's '.

Good luck!
Elektrostatik
#31
Mar7-12, 01:02 PM
P: 20
i'm okay with those. It's dels and curls that make me hide under the bed.
Dels and curls are what make Maxwell's equations the next best thing to a nervous breakdown.

Yepp, small steps in the right direction are way better than giant leaps towards catastrophy
I'm glad I decided to post these problems here, your input has been extremely helpful.

Hopefully after some days of milling, winding, soldering and blasphemy,
I should have something resembling a working generator.

Thanks Jim!
Elektrostatik
#32
Mar8-12, 08:07 AM
P: 20
I've updated the design to account for adjacent magnetic field geometry at 110 mm centre to centre spacing.

To keep the flywheel size within 1 m I've reduced the magnet count to 20
so at 20 rev/s the frequency is now down to the aviation standard of 400 Hz

This not only improves the sinusoidal output and transducer efficiency but also reduces the cost of N55s to 33%
Attached Thumbnails
generator2.jpg  
jim hardy
#33
Mar8-12, 09:44 AM
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Can you get some iron filings and place two magnets under a sheet of paper? That'd let you see what flux does.

Something is counterintuitive here. Dynamos squeeze flux together but you are spreading your magnets apart.

not saying it's wrong, just i'm perplexed by the logical conflict in my alleged brain.
Chewing on it.

Check this thread in General Physics

"" Increase magnetic field density near surface of magnet "
http://www.physicsforums.com/showthread.php?t=584655



Perhaps one of the fellows from that thread will have a suggestion.

old jim
Elektrostatik
#34
Mar8-12, 02:24 PM
P: 20
Can you get some iron filings and place two magnets under a sheet of paper? That'd let you see what flux does.
I will pick up 2 sample N55s and put them under my green magnetic field viewer film

I based the design changes on your N42 zero to peak gauss curve - 110 mm centre to centre.
I just assumed it would be more efficient to allow a full zero to peak sinusoid by spacing 40 mm from the edge.

Y component on center 1597 gauss
halfway between center and edge 1417
at edge 879
5mm beyond edge 550
10mm past edge 290
40mm past edge it's 6.3, essentially zero
so the flux is highest in center like a cosine function
and if we take 40cm as distance from peak to zero
879/1597 is arccos of 56.6 but 15/40 of 90 is 33.6
similarly 550/1597 is arccos of 69.8 but 20/40 of 90 is 45
so flux from a single magnet only resembles a sinusoid at that distance, it's too peaky.. as one would expect.
But then.. what seems intuitive to me doesn't necessarily make it right.

Traditional dynamos are DC generators that operate on the reverse DC motor principle using a commutator,
so would these same density rules apply to an open alternating principle?
jim hardy
#35
Mar8-12, 06:17 PM
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I just assumed it would be more efficient to allow a full zero to peak sinusoid by spacing 40 mm from the edge.
i was trying to demonstrate that the sine function results from design of the flux field
i dont see connection between its shape and "eficiency". Sine wave is aesthetic though.

best 'efficiency' will be achieved by maximixing the fraction of field flux from your magnets that links (goes through) your windings.

That's why your iron cores in middle of your coils were a good feature IMO.

You do understand that the magnetic path for each magnet is from N to S pole , and you want as much of that path as possible to be through the coil.
Iron core can encourage the majority of flux to take that route. An iron return path will help too.

EDIT :: and yes, a synchronous machine can motor or generate.
Elektrostatik
#36
Mar9-12, 03:30 AM
P: 20
I have a habit of relating magnetic field lines to acoustic standing waves
and since inverse acoustic waves (and inverse electronic audio signals) cancel each other out,
I thought the same should be true of inverse magnetic fields.

I was expecting that the Group B iron inductor cores which are 90 out of phase
with Group A on the opposite side of the flywheel would privide a return path for the flux
in the magnets. Would that not be the case?

I think to be safe I will first mill 2 test flywheels in wood, 1 with wide spacing
and 1 with tight spacing to check what kind of power output readings I get with my Ammeter.


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