End turns bad?

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major
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Re: End turns bad?

Post by major » Mar 18, 2018 5:18 pm

Buk___ wrote:
Mar 18, 2018 3:29 pm
major wrote:
Mar 18, 2018 2:59 pm
Fix = secure?

Image

Electronially commutate, secure the coil and allow the magnets to rotate. Isn't it a 2-pole bldc?
What you would have is an ironless, air-coil motor. ...
major wrote:
Mar 18, 2018 11:39 am
...
And due to the nature of the simple diagram, realize that the armature would contain a ferromagnetic core, there would be back iron, flux would be radial crossing the air gap and the forces would be tangential.
...
Let's pretend there is iron in the coil assembly. Just like pretending there is commutation of some type. It is a simple diagram not a blueprint.

Pretty apparent that Thin Gap thinks their motors are BLDC and that is the first sentence in the link miles posted.

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Re: End turns bad?

Post by Buk___ » Mar 18, 2018 6:57 pm

major wrote:
Mar 18, 2018 5:18 pm
Let's pretend there is iron in the coil assembly. Just like pretending there is commutation of some type. It is a simple diagram not a blueprint.
You win.

I've never encountered the 'Are we there yet!' strategy of scientific debate before, but I'll admit, it's a very effective one.

I withdraw.

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Re: End turns bad?

Post by Alan B » Mar 19, 2018 1:25 am

There is usually more than one approach to analysis of a problem, and they should all produce the same answer (or nearly so, within the accuracy of their approximation). I suspect that either Lorentz calculations or magnetic attraction calculations will, when done correctly, produce the same torque result. They are just two different views of the same system. It would be interesting to take an example and work through the solution both ways, the answer should be the same if both analyses are merely different views. I did see in my motor book they mentioned they were ignoring the rotor fields and just analyzing the stator fields and the forces resulting from those, but in reality either analysis must give the same result if they are correct. A true expert would see both approaches and understand the difference between them, and the assumptions and variances effect on their accuracy.

I further suspect that even though the full turn of the coil does not contribute to the Lorentz calculation that this does NOT mean it has no effect, this is merely a side effect of that particular analytical approach to the solution. Both the permanent magnet field, and the coil field exist in the same space, these analyses are looking at one vs the other. When the analysis is done a different way the ampere-turns of the coils are computed, so there is no apparent loss of contribution with that approach.

I present here a similar situation. How many are familiar with a Polar Planimeter? It is a very clever device that can be used to measure the area of an arbitrary two-dimensional shape. The one we used to have was deceptively simple looking - it had a weighted pivot and two arms with a pivoting elbow. There was a magnifying lens at one end so you could clearly see a small circle on the paper that you would guide along the perimeter of the shape, and on one arm there was a wheel that would ride on the paper and a dial to measure accurately the rotation of the wheel. You would take the magnifier on the end and run the small circle along the perimeter of a piece of property on a scaled drawing, and it would measure the area of that property, if you followed it accurately and started and stopped at the same point and zeroed out the dial at the start.

The wheel would rotate and skid, depending on the direction of the perimeter as you followed it. The dial meant nothing till the end, when it would accurately reflect the area you had encircled.

Some of the time, as you followed the perimeter, the wheel would be skidding sideways, accumulating nothing, other times it moved forward or back, accumulating a vector sum value. This might lead one to believe that portions of the perimeter in certain directions did not contribute to the area of the shape, which is not quite accurate, but is merely an artifact of the measurement technique. Since it achieves the correct answer the technique is verified, however one must be very careful about drawing conclusions about observations made of the measurement process.

http://persweb.wabash.edu/facstaff/foot ... Linear.htm more on the Planimeter

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Re: End turns bad?

Post by JanComputerman » Mar 19, 2018 1:51 am

Missed a few days here and wow!
My brother has been researching eBike motors for about 5 years on forums he seems to find that geared hub motors wear and fail yearly and given that his background is turbine engines navy flight engineer and has done extensive work in the ice motorcycle field I respect his opinion. We are both in our 60's btw. My background is EE microcomputers design and programming and power electronics and battery management.
Acceleration on my upgraded BladeZ with that 12 magnet bldc motor which has a phase windings of 5 parallel 15 gauge magnet wire (not awg?) about 9 feet long for a resistance of 13mohms ... Let that soak in for a few seconds ... Can handle 225amps max ... Kicks butt off the line at 100-150 phase amps and with a gates belt reduction still gets a 40mph top speed at 48volts using 4 sla batteries. This motor is 3 inches thick including covers with 7/8 inch shaft bearing on both sides, core is 1.25 inches thick.
Real numbers here no opinions!
Efficiency ... Ok there are losses at low power levels with bearings and core Eddy currents ... Not much you can do about them and the faster you spin the motor the more losses you have so ideally you design for the lowest RPM that gives you the required amp turns to generate enough torque to accelerate/climb a hill and enough battery voltage to reach your desired top speed. One thing going for you is the PWM of FET driven controllers that can take the 50 plus volts and give you high current low voltage starting torque into a low resistance winding. THAT is where the magic happens and if you can get the phase resistance low that DD hub motor with 30-50 amps going through it should give plenty of torque ... I.E. 50 volts 1amp at the battery to 1 volt 50amps at the phase windings. Once you start moving the back emf starts to kick in. This essentially is your electric "gearing" and you get 50amps of torque from the motor with 50watts from the battery! At full speed it's another matter and that is where HP (horsepower) comes into play but acceleration is all about the torque.
So ... Low resistance windings, lots of copper stuffed in the slots, at the right turns ratio, and long active conductor length (width), in a big diameter low RPM motor, makes it more efficient. Plus low rds FETs that make it all possible! And screw the weight ...it's not flying it's rolling on the ground and what you use in energy going up a hill you get back going down the other side.

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Re: End turns bad?

Post by JanComputerman » Mar 19, 2018 2:02 am

Ok now for the end turns...I mean the wire between the coils around a stator tooth ... The entire length of the end turn is ... The circumference of the stator core ... This is most likely less that the motor lead outside the motor going to the controller. DON'T WORRY ABOUT IT! You can't reduce it much, it has to be there, get over it!

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Re: End turns bad?

Post by Punx0r » Mar 19, 2018 4:04 am

Buk___ wrote:
Mar 18, 2018 3:16 pm
And to be honest, I'm kind of tired of trying to find new ways to express, what is to me, staring us all in the face. I get that you disagree, repeating it adds nothing. And my repeating my arguments adds nothing.
How frustrating for you. Your opinions remain in direct conflict with established theory and practical experiment.

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Re: End turns bad?

Post by Miles » Mar 19, 2018 5:20 am

JanComputerman wrote:
Mar 19, 2018 2:02 am
Ok now for the end turns...I mean the wire between the coils around a stator tooth ... The entire length of the end turn is ... The circumference of the stator core ...
No, that's not what we're discussing in this thread.

It's the section of each turn of the coil returning it to the other side of the tooth. So, there are twice as many endturns in the coil as there are complete turns of the coil. Mostly.....

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Re: End turns bad?

Post by Miles » Mar 19, 2018 5:57 am

Alan B wrote:
Mar 19, 2018 1:25 am
I further suspect that even though the full turn of the coil does not contribute to the Lorentz calculation that this does NOT mean it has no effect, this is merely a side effect of that particular analytical approach to the solution. Both the permanent magnet field, and the coil field exist in the same space, these analyses are looking at one vs the other. When the analysis is done a different way the ampere-turns of the coils are computed, so there is no apparent loss of contribution with that approach.
For motors with an iron stator core, the motor constant scales linearly with the width of the core. This is the "active" motor length. It's difficult to explain this if the endturns make any significant contribution.

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Re: End turns bad?

Post by madin88 » Mar 19, 2018 8:48 am

Miles wrote:
Mar 19, 2018 5:57 am
For motors with an iron stator core, the motor constant scales linearly with the width of the core. This is the "active" motor length. It's difficult to explain this if the endturns make any significant contribution.
Thats a good point.
I already have thought about the relationship between the width of the core and motor constants.
As it scales linearly, the biggest part of the initial question should have been answered now.

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Re: End turns bad?

Post by Buk___ » Mar 19, 2018 5:00 pm

Alan B wrote:
Mar 19, 2018 1:25 am
There is usually more than one approach to analysis of a problem, and they should all produce the same answer (or nearly so, within the accuracy of their approximation). I suspect that either Lorentz calculations or magnetic attraction calculations will, when done correctly, produce the same torque result.
Thanks for trying Alan.

I suspect that people get away using Lorentz torque calculations on reluctance torque motors, because in essence, both derive from the same inputs. The field from the magnets interacting with field created by the current in the coils. But, as with that MIT Lazar scooter example, they perform the Lorentz torque calcs, build their motor and when it doesn't perform quite the way they planned, they fudge it by adding more turns, or removing a few, or use a different gauge wire, to get the characteristics they really wanted.

They eventually get what they want, and never go back to work out why they didn't get what their math predicted first time.

That MIT example is classic, and the formula they use quite amusing:
The field strength for Neodymium magnets is pretty close
to 1T, which makes the math easy. Since there are 30 windings per tooth, 2 teeth per phase, and
2 phases active at any given time, 120 windings contribute to the overall electromagnetic
interaction. Ideally, if the steel can carry it, the total current in all the loops combined (1200A)
interacts with a 1T field on each side of a tooth. The total force produced by this configuration is
therefore given by:

F = (10A)(120)(2)(0.0254m)(1T) = 61N.

This is a pretty high estimate of the force the wheel could produce at 10A
They are assuming that two 1/4" thick N42 neodymium magnets 3.438" apart will create a uniform 1T field between themselves; where the reality is that the closest turn to the magnets on the inner layer gets little more than 0.1T, and those furthest away gets less than 1.5e-3T.

Their only saving grace is that the concentrating affect of the iron cores in the solenoids, transfers the field created by the coils (end-turns and all) directly to the airgap to interact with the fields from the magnets.

It ain't Lorentz force, but it does derive from the same to basic inputs, and by eliminating (most) of the 1/R^3 fall of in the flux density, the calculation comes passably close to reality. Of course, if they included the forces from the end-turns, it'd get closer; but hey, they can always fudge it:
We probably won’t pick the exact right number of turns on the first try, but we can measure the performance of the first motor and use it to tweak the windings. Table 1 shows some possible scenarios and what we might be able to do to accommodate them. Motors are very forgiving, so it’s unlikely that we will make a motor that just completely doesn’t work, but this will help us figure out how to make it better in just one iteration.

Table 1: Possible scenarios after motor testing and the course of action that could be taken. --- Scenario Possible Course of Action --- Why?

Motor works perfectly on the first try. -- Celebrate --- Because.

Motor speed is good at 36V, but the torque is too low --- Heats up too much if we try to push more current. --- Use thicker gauge magnet wire or double-stranded magnet wire, but keep the same number of turns. The number of turns determines the back EMF, which also sets the top speed at a given voltage. So if the speed is good, we should keep the same number of turns and just make the wires themselves larger to carry more current.

Motor speed is too fast at 36V and torque is too low. --- Use the same gauge wire, but wind more turns. --- This will simultaneously decrease the top speed at 36V and increase the torque, since the total number of amp-turns contributing to the magnetic interaction will increase. This is sort of like increasing the gear ratio.

Motor torque is fine, but the speed is too fast. --- Use a lower voltage. --- Why would something ever be TOO fast?


Fag packet calculations and fingers in the wind. Why not.

Thanks again.

Buk.

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Re: End turns bad?

Post by JanComputerman » Mar 20, 2018 10:46 pm

Ok so the end TURN if not contributing to the Pole flux at all or at least significantly can be kept short by having more slots/teeth closer together in the core thus increasing the number of stator slots thus increasing the number aka total active length of the windings increasing the motor torque and decreasing the size of the required Magnets which usually reduces the cost. (More smaller magnets are usually cheaper than fewer bigger magnets keeping the circumference the same.)

From the just prior post... now you are getting it ... Wind the core for the amp turns to get the desired torque BUT also once you get the number of turns you want/need wind the slot with appropriate wire size and parallel strands to fill the core slots and reduce the I²R losses.

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Re: End turns bad?

Post by Miles » Mar 21, 2018 2:39 am

JanComputerman wrote:
Mar 20, 2018 10:46 pm
Ok so the end TURN if not contributing to the Pole flux at all or at least significantly can be kept short by having more slots/teeth closer together in the core thus increasing the number of stator slots thus increasing the number aka total active length of the windings increasing the motor torque and decreasing the size of the required Magnets which usually reduces the cost. (More smaller magnets are usually cheaper than fewer bigger magnets keeping the circumference the same.)
This is well trodden ground... It's the total amount of active copper and iron that counts. Increasing the number of magnet poles/slots does allow you to scale back the material needed for stator and rotor yokes but also decreases the effective magnet area. I don't think the proportion of end turn is reduced.
From the just prior post... now you are getting it ... Wind the core for the amp turns to get the desired torque BUT also once you get the number of turns you want/need wind the slot with appropriate wire size and parallel strands to fill the core slots and reduce the I²R losses.
Wind the core to get the desired torque per amp!. :)

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Re: End turns bad?

Post by JanComputerman » Mar 21, 2018 6:58 am

Very big slots would result in thicker stacks of windings reducing the cooling than more slots that have less thicker layers of copper. I guess that the extreme limit of very low counts of poles would not occur because the magnets would have to be big and very curved. Is there an ideal air gap distance between rotor and stator?

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Re: End turns bad?

Post by Miles » Mar 21, 2018 7:30 am

Everything needs to be balanced. There has to be enough iron in the teeth to avoid saturation etc.

There's no ideal distance. It varies a bit depending on the scale of the motor. For our uses around 0.6mm to 1.4mm. Obviously, the shorter the distance the greater the flux for a given magnet.

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Re: End turns bad?

Post by Voltron » Mar 21, 2018 11:27 am

Been trying to follow along with this one, and one thing keeps jumping out at me in that I don't see how the windings around the teeth are like solenoids.

"Their only saving grace is that the concentrating affect of the iron cores in the solenoids, transfers the field created by the coils (end-turns and all) directly to the airgap to interact with the fields from the magnets."

This is strictly in my laymans thought experiment world, but aren't most solenoids round and hollow, with a plunger moving thru the middle when the field is activated? But the plunger is moving across the field lines at the same angle to all the windings, and there really are no end turns anymore, and the entire field is helping move the plunger, as the plunger is completely contained in the field.
But with windings around core teeth, the angle the the magnets are cutting thru the field at starts to matter, hence wanting a core to help align the field? And the end turns don't contribute to torque in a productive direction due running parallel to the magnet field except contributing in the sense of completing the electric circuit around the tooth?

Re. why would something be too fast...

"Motor torque is fine, but the speed is too fast. --- Use a lower voltage. --- Why would something ever be TOO fast?"

I'm a throttle jockey with open spaces, but for many, if you're always running way under the no load speed be it traffic or survival instinct or whatnot, than you're taking some big efficiency hits.

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Re: End turns bad?

Post by Alan B » Mar 21, 2018 11:57 am

Solenoids don't need to be hollow or round, Relays are one plentiful example of that.

There is always more than one way to analyze a circuit, the same is true for magnetics. Many of the descriptions about what is happening have things that are not correct in them, assumptions that are made that are not quite right. They are tools to help people understand, full accurate details detract from easy understanding.

Motor efficiency doesn't drop as much as people have come to think. The simulator has really colored the issue in people's minds, but the graph is misinterpreted - it doesn't take into account that power is being used for acceleration so the true efficiency is much higher than the sim shows off the equilibrium point. Motor efficiency drops 6 percent or so at half speed, not all that much of an impact.

For a quick example, bring up the ebikes.ca sim, turn human power to zero, efficiency at full throttle is 81.5%.
Now set the throttle to 50%, efficiency is 75.1%. A 6% drop for half the speed.
Don't try to use the efficiency graph, it is not efficiency except at the one point where equilibrium exists. Everywhere else it includes acceleration or deceleration so the equation is not pure efficiency.

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Re: End turns bad?

Post by Voltron » Mar 21, 2018 12:23 pm

I guess I was thinking as solenoid as a distinct group from other electromechanical devices, like a relay where the coil isn't hollow.... as in the definition below.

"Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug (termed the armature). The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet."

The point I was heading for is whether its solenoid or relay, can the end turns could be productive in both of those, but not be productive for producing torque in the direction needed for a motor, where the armature isn't moving thru the center of a homogeneous field?

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Re: End turns bad?

Post by major » Mar 21, 2018 12:45 pm

major wrote:
Mar 08, 2018 1:31 am
...
BTW, I used the term "core" when addressing Ampere's Law because of the motor context. It is actually the magnetic path. So in your solenoid examples the magnetic path completely surrounds the coil, therefore there are no end turns. In the motor FEMM you see only a cross section. Only leakage flux will encompass end turns which, in a decent design, will be very little.
downloadfile.jpeg
downloadfile.jpeg (16.88 KiB) Viewed 355 times
Notice the magnetic path (thru air) completely surrounds the coil, so there are no end turns. However when you have a ferromagnetic path (core, back iron, and such completing the magnet circuit) then there are portions of the coil turns outside the path which are noncontributing end turns. This is described in Ampere's Law as I explained in my first post, second reply in this thread.

major

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Re: End turns bad?

Post by spinningmagnets » Mar 21, 2018 5:14 pm

Thanks for posting that. Over the years, there has already been quite a bit of discussion around here over how to improve motors. I am no expert. However I have found it useful to visualize the optimum form of the facet at hand, and then compare the compromises that it forces onto us.

Thinner laminations have fewer eddy current waste-heat losses, but...they cost more.

Stronger magnets make a more power dense motor, but...also increase cogging.

Larger diameter motors have more torque-per-watt, but...only allow for short spokes.

So...it seems instead of simply choosing the feature we like, we must also consider the compromises we hate the least. If we use a perfectly cylindrical electromagnet, what is the optimum magnet shape on the rotor? How would weight and cost be affected in order to get the same wheel power per input watts?

Even though laminations typically run lengthwise, I'm sure we could attach a ferrous permeable "ears" onto the stator tooth to project over the sides too?

In that situation, should the rotor magnets extend farther to the sides to make the best effect of the available magnetic flux?

Even if the electromagnet (ferrous core solenoid?) Was closer to a rounded cube (as opposed to cylindrical) would that cause an unexpected compromise that is very undesirable compared to the "wasted potential" of being saddled with motors that have end turns?

I don't know...

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Re: End turns bad?

Post by billvon » Mar 21, 2018 5:25 pm

Voltron wrote:
Mar 21, 2018 11:27 am
"Their only saving grace is that the concentrating affect of the iron cores in the solenoids, transfers the field created by the coils (end-turns and all) directly to the airgap to interact with the fields from the magnets."

This is strictly in my laymans thought experiment world, but aren't most solenoids round and hollow, with a plunger moving thru the middle when the field is activated?
The term 'solenoid' refers to a type of electromagnet where wire is wound in tight loops, so it ends up looking like a helix with one or more layers. It is sometimes used as slang for a type of electromagnetic actuator - but that's just because the device is basically a solenoid with a movable iron core. For reasons of efficiency, most solenoids are wound around a high permeability magnetic core.
--bill von

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Re: End turns bad?

Post by Buk___ » Mar 21, 2018 6:31 pm

major wrote:
Mar 21, 2018 12:45 pm
Notice the magnetic path (thru air) completely surrounds the coil, so there are no end turns. However when you have a ferromagnetic path (core, back iron, and such completing the magnet circuit) then there are portions of the coil turns outside the path which are noncontributing end turns.
Those portions of the coil outside "the path", are still generating a field (and flux 'lines') around them. Those lines, by definition, must be endless and pass through the centre of the coil.

If the coils are wound tight to the core, those lines *must* pass through the core, when within coil. Therefore they are contributing to the density of lines (flux density:B) within the core; and will traverse the airgap, and create linkages with the magnets.

The flux 'lines' -- that would be evenly distributed around the coil (or the coil and entwined core) if the tooth were isolated from the rest of the elements -- will seek the path of least resistance (reluctance) to complete their circuits around the coil, when that coil is a part of the magnetic circuit.

If the density within all elements of the magnetic path -- the tooth core, magnets, back-iron, adjacent teeth -- is low, they will bend their paths to pass around the magnet circuit, because that will have a lower reluctance than the direct route through air;

But if the flux density within the core is high enough, the path of lowest reluctance may be to exit the magnetic circuit at some point (eg. through the sides of the magnet or the back-iron) and pass through the air before rejoining the magnet circuit at the other end of the coil.

But in all cases, one part of their circuit *must* (by definition) pass through the middle of the coil, and if that middle is filled with core, then through that core; and that means they are (must be) contributing to the flux density within the core, thus at the tooth face, thus to the reluctance torque.

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Re: End turns bad?

Post by flathill » Mar 21, 2018 7:08 pm

water seeks its own level

forget rotary motors and just think about a linear motor (unrolled rotary motor)

imagine two moving magnets on the top of the assembly held securely 3 inches apart,
each magnet a sqaure block 1x1x1 inch cubed,
placed on rails such that they can only move left/right,
one magnet pointed up and one pointed down

imagine a square coil wrapped around a square piece of steel,
the steel is straight, 3 inches long, 1x1 inch sqaure, centered below/between the two magnets

the square coil is only wrapped around the center third (one inch),
10 TURNS,
THE CENTRAL AXIS OF THE COIL IS ALIGNED LEFT/RIGHT


the square coil is wound such that on two sides (half the coil length) the current is up/down,
on the other two sides (other half) the current is in/out


the moving magnets and the stator are already perfectly aligned in the in/out direction (aligned by the rail system),
we want to move the magnets left or right by activating the coil and sending current thru it

which sides of the sqaure coil can "steer" the flow of flux to align with the magnets?

the coils wrapped around the steel pole piece interact with the flux flowing thru the pole piece,
steering is done by "dragging" on one side and by "propelling" on the other side,
we only need to steer in the X/Y plane, so only the current moving in/out "steer"

now lets take the same motor but this time make the steel pole piece u-shaped,
the u-shape still has a 1x1 in sqaure profile but now there are two coil on each end of the U,
on the left side of the u-shape the coils has is 0.5 inch long, wound clockwise,
on the left side of the u-shape the coils has is 0.5 inch long, wound counter-clockwise,
THE LEFT COILS IS 5 TURNS AND THE RIGHT COIL IS 5 TURNS,
THE CENTRAL AXIS OF BOTH COILS IS ALIGNED UP/DOWN

note we still have the same situation with regards to "steering" irrespective the alignment of the coil axis

each square coil is wound such that on two sides (half the coil length) the current is left/right,
on the other two sides (other half) the current is in/out


in both case only 50% of the total coil length is active (steering) for a squAre winding,
Ket=ratio of motor coil length, including the end turns, to active coil length (twice the stator stack length in/out of the page in our case)

here we have 4 inches per turn times 10 turns so the total coil length is 40 inches,
20 inch are going up/down in the first straight steel bar example,
20 inches are going left/right in the second u-shaped steel example,
20 inches are going in/out in both examples
in both cases only the in/out coil is "active" (steering)
ratio=active:total=20:40 in both cases so Ket=50% in both examples

now imagine a tubular linear motor or a tubular rotary motor,
the current in the coil is NEVER inline with the allowed direction of travel on a proper tubular motor,
hence there are no end turns in a tubular motor/actuator, so Ket=100%

the flux "steering" happens as naturally as water finding its own level,
nothing is actually actively steering the water to reach equilibrium,
nothing is actually actively steering the flux,
balance/imbalance springs from the geometric symmetry/asymmetry
the motor moves in search of balance


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Re: End turns bad?

Post by major » Mar 21, 2018 9:05 pm

Below there is flux map of a magnetic circuit with an air gap on the right (green line) and a 4 turn coil on the left providing excitation (mmf). Due to the cross sectional view, the coil shows as 8 black dots which represent conductors perpendicular to the viewing screen. There are 4 conductors passing through the high permeability core and the 4 conductors to the far left are outside the core. You can see there are only 3 or 4 lines of force (flux) encompassing those conductors outside the core. And none of those lines cross the air gap where the work will be done. This represents leakage flux and is not useful in motors.

The 4 conductors passing through the core are responsible for the thousands of lines of flux in the core which cross the air gap. Roughly the proportion of such leakage to main flux is on the order of the relative permeability of the core, like 1:1000 or so.

major



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Re: End turns bad?

Post by Buk___ » Mar 21, 2018 11:01 pm

major wrote:
Mar 21, 2018 9:05 pm
Below there is flux map of a magnetic circuit with an air gap on the right (green line) and a 4 turn coil on the left providing excitation (mmf)....
Okay. Let's explore that.
  1. In the image below, the first flux map is my reproduction of the image you posted as close as I could get without taking extraordinary measures. The top middle inset just reproduces the first few values on the legend 3x the size for readability, and shows the maximum flux density in the map is ~0.5Tesla.
    The bottom right inset is a close-up of the gap and fringing flux.
    .
  2. In the second flux map, I've reduced that gap from ~5mm to a more realistic 1mm. You'll notice that the overall flux density remains the same, but the fringing is vastly reduced.

    The other thing to notice is that (almost) all of the flux lines produced by the coil on the left side make it right around the circuit to the gap on the right. And even those lines ditching the low reluctance path for the air are (almost all) still linking around the gap.

    In a motor, they would still be cross-linking between the stator and rotor to produce torque.
    .
  3. In the third flux map, I've replaced one "jaw", with a magnet. The gap remains the same. The top middle inset shows that the maximum flux density has jumped from ~.5T, to over 1.2T; and the color map shows how much greater the flux within the whole circuit is.

    (The current flowing in the coil is the same 20A in all cases.)

    Note also that the fringing has been reduced to negligible levels; and even the 4 strays the peek out beyond the rectilinear limits of the gap, would still be linking stator core to rotor magnet and generating flux.
    .
There is more to come, but too much to add to a single image.

FringingFlux(reduced).jpg
FringingFlux(reduced).jpg (168.92 KiB) Viewed 281 times

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