Gap between magnet and coils on an Axial motor

Joined
Feb 21, 2020
Messages
21
Folks,

What you guys have to say about gap between magnets on a axial motor...same with the coils? what is the general rule of thumb?
minimize the gap? keep it between a certain range?
 
There is a lot of good information about this and many other necessary things in threads like APL's motor design thread(s); I highly recommend reading that entire thread. :)

https://www.endless-sphere.com/forums/viewtopic.php?f=30&t=97860

A list of all posts in that thread that discuss gaps
https://www.endless-sphere.com/forums/search.php?keywords=gap&t=97860&sf=msgonly



There are also other axial-motor threads with good info in them, in this subforum.
 
I've been trying to find the links and info to answer this right, but it's spread out through different PC's and the 60 page axial thread.

I think what you are referring to is coil pitch, span, pole pitch, span, and their related coil and pole arcs. There is a lot of YouTube videos on it once you use those words in the search. I haven't found the one in particular that I wanted, but here's a link to get close.
https://www.youtube.com/watch?v=pfj4w2Jz3xA
https://www.youtube.com/watch?v=5pnbFmdi4Mc

Also go to Goggle Images and use those words to find what your looking for, that's what I usually do first.

Page 36; https://endless-sphere.com/forums/viewtopic.php?f=30&t=97860&p=1526164&hilit=coil+pitch#p1526164

Pole arc Pole pitch.png

There are a lot of youtube India based lectures on motor design too,.. they tend to look at it from a mathematical and geometric point of view, which is understandable, but not my strongest suit. Beautiful if you can understand it though.

Anyway, motors use 180 electrical degrees in a circle, and everybody else uses 360 mechanical degrees, so right off the bat it gets confusing.

Going from memory, and still looking for the links,.. you draw the diameter of the stator. Then draw in the number of coils you are
using with sides touching, plus one extra. Then remove the extra one and adjust the rest evenly,.. this should get close to the spacing needed. The spacing is also connected to different orders of harmonics, like the 5th and 7th and so on, and I see talk of 150 degrees for most modern motors, but for us noobs,.. not really on the radar 'yet'.

To get the core size you will need to know and add the thickness of the coils sides or copper fill, the resulting center hole will be the core size. Core tooth overhang, shoe, or lip spacing between cores is usually around 3 - 4mm or enough to get the wire through to wind the coils.

Magnets are a little different and can touch, as in most hub motors, but usually are spaced 3-4mm too. It gets more complicated if your after the greatest efficiency, and the pole arc winds up being around 75 percent of a full pole arc, or touching magnets.

All in all, we just look at what most motors are doing and copy it, but it certainly helps to know what is really going on. Hope this helps.
Motor designing from scratch makes you go down these roads,.. study these things for a week or so and you should have a much better handle on it.
 
APL,
I figure it out to determine the pole arc....the pole arc is a fraction of the pole pitch. The reason for that, as you already documented on your thread is to minimize cogging torque (undesirable condition at slow speeds) as well to minimize flux leakage to nearby magnets. I read couple articles that specify the optimum value for the pole arc is 68% of the pole pitch, but that % is only focused on reducing the cogging torque, which comes with a price, reduction on torque. The image below is a summary of what was found on that article. This was done on several BLDC motors but radial type....I'll assume same applies to axial motors.
PolePitch_Ratio.PNG
In my case and since the my magnet geometry is fixed (off the shelf) I'll be around 90% ratio. I can minimize the cogging torque by using other methods such as magnet skewing. Still needs to find out how the ratio will affect the flux leakage.

Have to investigate more about the gap in between the coil shoes as well
 
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