Step 4: The Stator: Obtaining, Care, and Feeding
For the past 4 pages I've said "stator stator stator stator". What IS the stator, and where do you get one? The stator is the number one most painful specialized industrial component to acquire for a motor build, generally speaking, and is usually what you end up designing your power system around just because you have one and by Robot Jesus you are GOING to use it.
The stator is difficult to just "make" because it requires the stacking and fitting of many layers of very thin, electrically insulated steel sheets. Not just any "steel sheet" either - no Home Depot galvanized roof patches here. Motor steel is called "electrical steel" or "transformer steel" and are special alloys that contain high silicon. This enhances the magnetic characteristics of the steel and reduces its conductivity.
So why does it have to be laminated - and especially insulated ones? This is due to the phenomenon of eddy currents. The short story is that moving magnets over conductive materials cause the material to dampen the magnet's motion. In a motor, that means your motor is trying to brake as hard as it's trying to go. Those eddy currents get turned right into heat. If you take the method that most new motor builders go:
"Well, I'll just cut it out of some thick steel plate or a block or something - I have a milling machine, it'll work, right?"
It will, but you'll make a heater that occasionally twitches, rather than a motor which heats up as it runs.
Having laminated, low conductivity sheets of material means that the eddy currents are neutralized to a large degree. For low speed motors, this "eddy current loss" or "core loss" can be negligible. For high speed motors, it can eat up as much as 15 to 20% of the power.
So where do I get a stator?
This will be the only "how to get" section that's not in the Resources page, because you generally don't just go and get one.
Because they require the punching, stacking, and otherwise processing of hundreds of little steel sheets, stators tend to be designed once and then mass produced by the thousands. This mass production is why they are hard to get new if you are a hobbyist or motor hacker.
Fortunately, the appliances and implements that these thousands of mass produced stators end up in are commonly available secondhand, for free, or as scrap.
Laser copiers and printers
My #1 favorite source for stators, as they tend to get junked by the dozen as departments and institutions get new equipment. Canon, HP, Xerox, and Ricoh tabletop copiers tend to be rich in 12 tooth stators in the 50 to 55mm range. In this case, older and bigger is always better. Project RazEr's motor came from a gigantic (floorstanding, needs-its-own-room-in-the-office style) laser copier, which not only yielded the one large motor, but several smaller AC motors and a bucket of gears, shafts, and pulleys. Printing equipment is always a good bet for electromechanical components, though new units tend to use stepper motors, which are not suitable for conversion.
The largest copier motors I have seen (before they enter the realm of AC induction) have 70mm stators.
These things show up for free all the time on Craigslist, or free stuff drives at institutions. Electronic recycling stations are also worth a call.
Junky old DC and AC motors
Old motors with burned windings or worn out bearings get thrown out all the time. DC motors are hit-or-miss. DC motor armatures tend to get designed with odd numbers of teeth because the lack of symmetry contributes to smoothness. While stators with tooth numbers that are an odd multiple of 3 can be turned into motors, they cannot use the LRK winding.
Because DC motor armatures spin internally, they have teeth that project outwards, which makes them ideal for BLDC conversion if the tooth count is correct.
AC induction motors and especially AC three phase motors are usually good bets for useful iron, except they tend to be conventionally shaped - that is, rotor on the inside, stator on the outside. We want the opposite, but if you just want a motor, this is a good place to start.
"Junky old motor" includes "junky old kitchen appliances", which often use a variant of the brushed DC motor called a universal motor. These tend to have 12, 18, or 24 tooth armatures, especially large multispeed blenders, usually under 50mm diameter.
Buy one
You know how I said you can't buy them? I lied. Hobbyists have recently become such a large market that a few companies actually make stock stators that are empty of windings and already surface coated to accept your own.
For the widest selection, see GoBrushless' motor stators. Check out the 65mm, 18 slot one!
For the monetarily endowed, many shops specialize in short-run and prototype lamination cores, including the aptly-named ProtoLam. Be ware - just one stator made to your design can cost several hundred dollars, but if you're just totally obsessed with rolling your own, the resource is available.
How large of a stator do I need?
The killer question.
Remember the torque equation
T = 4 * m * N * B * L * R * i
For most reasonable operating conditions, you can consider:
T to be a design goal. A goal for acceleration or hill climbing both require minimum force-at-ground figures, which translates to a torque at the motor.
N to be the primary variable you can control. This is mildly coupled to i, which is dependent on your battery voltage.
R and L are the parameters set by your stator. In a way, m is also determined by your stator - after all, it has a fixed number of teeth that have to be divisible by 3 for this type of motor.
B is is the strength of the permanent magnetic field that the stator acts upon, set by your magnet strength (and a mechanical factor to be discussed)
Clearly this is a multivariate optimization problem. If you have a choice of how large your stator can be, the answer is the largest. The more L and R you can pack into the expression, the less N and i you need. Remember that motor current i is the biggest contributor to heating and efficiency loss.
If your L and R are already set because you have a pulled stator and want to use it, then the only realistic variables you can fiddle are N and B.
The stator is difficult to just "make" because it requires the stacking and fitting of many layers of very thin, electrically insulated steel sheets. Not just any "steel sheet" either - no Home Depot galvanized roof patches here. Motor steel is called "electrical steel" or "transformer steel" and are special alloys that contain high silicon. This enhances the magnetic characteristics of the steel and reduces its conductivity.
So why does it have to be laminated - and especially insulated ones? This is due to the phenomenon of eddy currents. The short story is that moving magnets over conductive materials cause the material to dampen the magnet's motion. In a motor, that means your motor is trying to brake as hard as it's trying to go. Those eddy currents get turned right into heat. If you take the method that most new motor builders go:
"Well, I'll just cut it out of some thick steel plate or a block or something - I have a milling machine, it'll work, right?"
It will, but you'll make a heater that occasionally twitches, rather than a motor which heats up as it runs.
Having laminated, low conductivity sheets of material means that the eddy currents are neutralized to a large degree. For low speed motors, this "eddy current loss" or "core loss" can be negligible. For high speed motors, it can eat up as much as 15 to 20% of the power.
So where do I get a stator?
This will be the only "how to get" section that's not in the Resources page, because you generally don't just go and get one.
Because they require the punching, stacking, and otherwise processing of hundreds of little steel sheets, stators tend to be designed once and then mass produced by the thousands. This mass production is why they are hard to get new if you are a hobbyist or motor hacker.
Fortunately, the appliances and implements that these thousands of mass produced stators end up in are commonly available secondhand, for free, or as scrap.
Laser copiers and printers
My #1 favorite source for stators, as they tend to get junked by the dozen as departments and institutions get new equipment. Canon, HP, Xerox, and Ricoh tabletop copiers tend to be rich in 12 tooth stators in the 50 to 55mm range. In this case, older and bigger is always better. Project RazEr's motor came from a gigantic (floorstanding, needs-its-own-room-in-the-office style) laser copier, which not only yielded the one large motor, but several smaller AC motors and a bucket of gears, shafts, and pulleys. Printing equipment is always a good bet for electromechanical components, though new units tend to use stepper motors, which are not suitable for conversion.
The largest copier motors I have seen (before they enter the realm of AC induction) have 70mm stators.
These things show up for free all the time on Craigslist, or free stuff drives at institutions. Electronic recycling stations are also worth a call.
Junky old DC and AC motors
Old motors with burned windings or worn out bearings get thrown out all the time. DC motors are hit-or-miss. DC motor armatures tend to get designed with odd numbers of teeth because the lack of symmetry contributes to smoothness. While stators with tooth numbers that are an odd multiple of 3 can be turned into motors, they cannot use the LRK winding.
Because DC motor armatures spin internally, they have teeth that project outwards, which makes them ideal for BLDC conversion if the tooth count is correct.
AC induction motors and especially AC three phase motors are usually good bets for useful iron, except they tend to be conventionally shaped - that is, rotor on the inside, stator on the outside. We want the opposite, but if you just want a motor, this is a good place to start.
"Junky old motor" includes "junky old kitchen appliances", which often use a variant of the brushed DC motor called a universal motor. These tend to have 12, 18, or 24 tooth armatures, especially large multispeed blenders, usually under 50mm diameter.
Buy one
You know how I said you can't buy them? I lied. Hobbyists have recently become such a large market that a few companies actually make stock stators that are empty of windings and already surface coated to accept your own.
For the widest selection, see GoBrushless' motor stators. Check out the 65mm, 18 slot one!
For the monetarily endowed, many shops specialize in short-run and prototype lamination cores, including the aptly-named ProtoLam. Be ware - just one stator made to your design can cost several hundred dollars, but if you're just totally obsessed with rolling your own, the resource is available.
How large of a stator do I need?
The killer question.
Remember the torque equation
T = 4 * m * N * B * L * R * i
For most reasonable operating conditions, you can consider:
T to be a design goal. A goal for acceleration or hill climbing both require minimum force-at-ground figures, which translates to a torque at the motor.
N to be the primary variable you can control. This is mildly coupled to i, which is dependent on your battery voltage.
R and L are the parameters set by your stator. In a way, m is also determined by your stator - after all, it has a fixed number of teeth that have to be divisible by 3 for this type of motor.
B is is the strength of the permanent magnetic field that the stator acts upon, set by your magnet strength (and a mechanical factor to be discussed)
Clearly this is a multivariate optimization problem. If you have a choice of how large your stator can be, the answer is the largest. The more L and R you can pack into the expression, the less N and i you need. Remember that motor current i is the biggest contributor to heating and efficiency loss.
If your L and R are already set because you have a pulled stator and want to use it, then the only realistic variables you can fiddle are N and B.
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What I'm thinking is laminating very thin steel sheets with very thin sheets of silicon to mimic the effects you outline... Or maybe just laminate the steel with a silicon adhesive...
There'd probably be a minimum size beyond which you'd lose too much efficiency, but would larger, slower-turning, motors be feasible?
Thinking about it now, anything thin enough would probably not be available in small-enough quantities to be affordable for the DIY'er (yet, anyway).
So, we're back to the adhesive - and you'd probably need 100% pure silicon (like aquarium sealant is, I think).
Maybe an idea for someone else to tinker with?
It's a common mistake.
I believe the idea is to make the sheets of an iron-silicon alloy as the silicon decreases the electrical conduction within each sheet while the sheets are coated with some kind of coating (the perfect sort of thing to coat something with) to prevent conduction between the sheets. you could use silicone, but I think saran wrap or wax paper would work just as well if you didn't have any lacquer, shellac, or spray paint.
But if you did use extremely thin sheets of steel and insulated them from each other I'm sure it would still work much, much better than a block of steel.
You can get something called "shim stock" in thicknesses down to 1 or 2 thousandths of an inch (0.02 to 0.04 mm) at an auto parts store or from a machinists supply.
That's about half the thickness of a piece of paper. they're pretty flimsy so if you went this route you'd probably want to put thicker sheets on either side to maintain the shape.
Now all you have to do is make the poles/spokes/teeth. We're talking about hundreds of sheets for one motor. You can cut the stuff with scissors but we're talking about hundreds of sheets for one motor! I don't know how many times you'd have to sharpen them.
If you have access to a laser cutter you're set.
Otherwise I'm thinking a whole bunch of squares with a hole in the center. Two big end blocks and a bolt down the center. Then to the lathe and the end mill or better yet a wire EDM machine.
BTW shim stock is also available in brass, copper, plastic and possibly aluminum. None of these will do. It has to be steel. Iron would be better but I've never heard of iron shim stock.
From my experience the epoxy makes a very good insulating layer around basicly everything it touches and with a fine to small grain iron powder could get a fairly decent density of Iron in the mixture.
Using this aproach you could actually cast the stator with a small mould and a basic vacume rig. Advanced, but not impossible for a DIY person.
I am curious to hear any criticisms of the idea.
Did a bit more research and there are some uni's doing research on sintered rotors for high speed electric motors where eddy currents become a bigger loss. They have developed some cast materials but these seem to be mostly rare earth sintered materials that are much harder to fabricate individually but might be an option in a decade or three if the idea catches on and the rotors start to get mass produced.
And you're right about the "hundreds of sheets" - another reason (or hundreds of them!) the average back-yard'er would have a horrible time...
Still, it was an idea that might have been worth pursuing, and at least I've learnt from the discussion.
Thanks again.
I've just been looking for some information that would be helpfull, but at the same time - understandable. That's just what I need! =)
Just one question- from my high school days I remember that eddy currents are a function of size- therefore the lamination etc. Why not use iron (best magnet) filings set in epoxy - either as laminates or one single block. I suppose laminates would be easier.
Do you think it'll work?