Need parts for custom brushless DC motor experiment

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Well-known member
Sep 30, 2020
This is a bit of an oddball request, but, since Steve's audience is a collection of geeks, I figured I'd try it.

I have been a fan of alternate energy for decades. I wish to get the parts for building my own brushless DC motor (BLDC) like those used in drones, but bigger than most. I need to be able to do my own windings so I can customize things like wire size, turns, wiring pattern, etc. I will then do experiments as to the power of the motor, speed, and use as a generator, etc. I have no machine shop, so I need ready made parts (and knowledge). I'm looking at an outrunner configuration with probably 50 mm + diameter and 12, 24, or 36 teeth (or coils). The number of magnet poles can be slightly less than, equal to, or slightly more than the number of teeth. Don't know about other parameters but I'm thinking about a Kv of 100 or less. None of this is set in stone. I've found some sources for stators (the part with the coils), but have not found the parts for matching frames and rotors. Yes, I could take apart an existing motor, but big ones tend to be expensive, and hard to disassemble without damage.

For those of you that might be interested, here are some resources I've already found.

The Moderator of this thread is Ron, but it's not me.

My upcoming week is very busy so I may not be as active here on the forum. But, all help is appreciated and I'll reply when I can.

Hi all, I thought I'd post an update on this thread. I've been doing a lot more research on these BLDC motors. I've learned enough to be dangerous :cool: and I thought I'd share. I'm very much a newbie at this, and certainly don't know enough to really explain things. But maybe I can at least explain what I'm posting. If some of you know more than me, please share and be gentle if I mess up.

I am trying to identify the basic structure of a BLDC that I may build if I can get, and afford, the parts. Two of the most critical attributes are the number of slots in the stator (stationary part with coils) and the number of magnets, or poles in the rotor.

I found this article that describes the trade offs involved.

Based on this, I've concluded there are three factors I want to initially keep in mind.

The ratio of slots to poles or Q, accounting for a 3 phase motor, is one. Poles always increase in increments of 2. For a 3 phase motor, slots always increase in increments of 3.

As far as I can tell, the formula for this is Q=Slots/Poles/3. This relates to how much surface area of the magnets is adjacent to how much surface area of the slots' teeth. He says to keep Q >= .25.

So, as an example, if there are 12 slots and 16 poles, designated 12N16P, then
Q=12/16/3=.25 If the poles increase any higher, then Q drops below .25.

He has a chart for this in the article.

So, my first criteria is that Q >= .25.

The second criteria is the number of cogging steps per turn. As the magnets rotate past the iron teeth of the stator, the magnets attract to the iron. This resists rotation. This creates vibration. If you're manually turning the motor as a generator, each cogging point is where it will resist the motion.

I have arbitrarily decided that I want 90 or more cogging steps per turn, or 1 every 4 degrees. According to the article, the higher the number the better, and the smoother the rotation.

The third criteria is the winding factor. According to the article, the winding factor is a number between 0 and 1 which represents the fraction of the armature current which is used to produce torque.

You want this as high as possible. As a practical matter, it stays under 1. I've decided I want the winding factor to be above .9.

He has a chart for this. This chart shows the winding factor for slot and pole combinations which also meet the Q >= .25 criteria.

So, I wanted to determine what slot and pole combinations would meet these three criteria.

I used this motor calculator.

I'm looking for green items on this chart. Other colors include Integer-slot winding, Fractional-slot winding, and Unbalanced winding. Green means Concentrated winding, which is what I want. This means I'm winding the wire around each slot's tooth.

The controls for this calculator identify the middle row and column. But you can read lower numbers than those.

Slots are listed on the left and poles are listed on the top.

I set the controls so I can see the row for 3 slots. The maximum number of poles to keep Q>=.25 is 4. Neither 3N2P nor 3N4P have a winding factor above .9, so I pass them by.

I look at the row for 6 slots. The maximum number of poles to keep Q>=.25 is 8. Neither 6N4P nor 6N8P have a winding factor above .9, so I pass them by.

I look at the row for 9 slots. The maximum number of poles to keep Q>=.25 is 12. Both 9N8P and 9N10P have a winding factor above .9.

Now I go to this calculator and plug in these numbers.

For 9N8P, I get 72 cogging steps and .945 winding factor.

I reject that because 72 cogging steps is too low.

For 9N10P, I get 90 cogging steps and .945 winding factor. I add this to a chart of possible configurations that meet the Q>=.25, 90+ cogging steps, and .9+ winding factor.

Using these procedures, I check every possible combination.

Here are all the combinations which meet these criteria from 3-33 slots.


So, as far as I can tell, motors of all these configurations should run with reasonable torque due to Q, reasonable efficiency due to wiring factor, and reasonable vibration due to cogging steps.

Now, I just have to figure out which one to build and how to get parts.

Hope this helps anyone who's interested in the topic.

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