Step 4: The Stator: Obtaining, Care, and Feeding
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.
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.