That's it. I have just written 12 Instructable pages without actually telling you how to build anything
. I think few can beat that...
This is only intended as a guide and primer on what you could do. I did not include directions on how to fabricate one specific motor because it assumes too much engineering knowledge to tell someone to follow my lead, at least in my opinion. In a future Instructable, I might go over the specifics of building RazEr's motor. But, in the interest of modularity, I elected to keep things separate this time.
Maybe you guys
can take up my slack by talking about how you made your
What I can do now, though, is put in a few fabrication notes for when you embark on your hub motor adventure.
� The "elevator pitch" in terms of motor design here is to stuff in the strongest magnets and the largest stator using as many turns of the largest wire running across the highest voltage battery you can get your hands on. Maximize ALL of N, R, L, i, and B. But wait, I thought earlier you said as few
turns as possible was the best? Not necessarily: I said that just enough turns to get a workable Km contributes to lower motor resistance. There is no need to constrain yourself to low turn numbers. In fact, high turn numbers running at high voltages are almost always better than low turns and high current!
� Use a good high temperature 24+ hour epoxy to glue the magnets in. Cheap hardware store 5 minute epoxy has inadequate time to set, and the chemical crosslinks are not nearly as strong. Thin laminating epoxy (for fiberglass and carbon fiber layup) is recommended, with a microsphere filler
. The filler shortens the working time of the epoxy, but causes it to be stronger and more tenacious.
� Speaking of gluing the magnets, you may notice that they have a tendency to snap towards eachother in your can. To avoid this, cut up some popsicle sticks into wedge shapes and push them into the gap to separate the magnets.
� GoBrushless' rotocalc also generates a magnet placement guide image. Print this out at full scale on a piece of paper and perform your magnet gluing over it.
� As long as you have machine access, make jigs and fixtures to help you glue the magnets. Try not to let them float as you're gluing.
� While on the subject of epoxy, sealing your motor windings with high temperature enamel or epoxy will keep them together (prevent unraveling or jiggling) and make them more heat resistant. Do this AFTER
you make sure your motor works and winding is correct.
� Never wind wires on a naked stator. The metal edges will pierce the magnet wire's thin enamel coating and result in a phase short to the core. You are bound to make more than one, so the phaes will short to eachother!
If you cannot avoid winding on a bare stator, liberally apply heatshrink or electrical tape to the inside corners of the stator, and wind carefully. If you create a short, you MUST
rewind that phase.
� Pull your wires tight. Loose windings are more likely to be damaged, and they are longer than they need to be, so your motor has extra resistance.
� Insulate, insulate, insulate. You have wire running past high speed rotating surfaces which will abrade the insulation if allowed to rub.
� Use a good, flexible wire. Silicone high strand count (HSC) wire, including the popular "Wet Noodle" from W.S. Deans, are the best choice.
� Use high quality hardware. On Razer's motor, I made the mistake of using stainless steel screws because they were cheap and already at the hardware store (instead of ordering high quality socket head cap screws). Bad mistake - they sheared and stripped one by one, leaving the motor wrecked.
A Note on Motor Control
BLDC motors can either be sensored
Sensored motors have Hall Effect sensors
which react to magnetic fields. There are at least three of them inside your average sensored motor, and they function as a very crude position encoder. A sensored motor controller reads the state of these sensors and correlates them to the position of the motor through a lookup table. It then outputs the proper voltage levels to the motor according to this state table. This is called Space Vector Modulation.
Yours Truly has build a fully hardware (logic chips, op amps, no microcontrollers) SVM motor commutator
for a class project. And it actually worked.
Sensorless motors are operated by controllers which sense back-EMF. Remember from the page about DC motors and their ability to be used as generators? Every time the brushless motor moves, it puts out a sinusoidal (or trapezoidal) waveform on its 3 connections. A smart controller can actually read these voltages and have an idea of which direction the motor is traveling. It can then sequence its output to "encourage" the motor to keep rotating, generating torque.
What is the difference? One has 3 more parts and the other doesn't?
Well yes, and...
� Sensorless motors cannot operate from standstill
unless the controller is very sophisticated. If the motor is not moving, the controller has no way of know where it is. There do exist controllers which can sense motor position based on the effect of the motor's magnets on the phase inductance. However, those are ungodly expensive and are a new industrial technology (which makes them even more expensive.
� Hence, if you keep your motor sensorless, you may find yourself kick-starting your vehicle.
� The vast majority of inexpensive R/C airplane motor controllers are sensorless.
� Sensored motors can operate from 0 speed, but require a controller that can read them. These tend to be more expensive than their sensorless brethren.
� Additionally, if you add sensors to your motor, you have to place them in the correct spots. Hall sensor placement is a quasi-nontrivial process that requires knowledge of the motor's electrical slot ratio.
Two popular Hall Sensor placements exist: 60 degrees and 120 degrees. I glean over this on my website, but the degrees refers to how many electrical
degrees apart the sensors are.
To place Hall sensors properly in your motor, you have to know how many electrical degrees each slot (or tooth) occupies:
°elec = 360 * p / t
where p = number of pole pairs. For a LRK motor, this is 7. Likewise, t
, the stator slot count, is 12.
For a LRK motor, the electrical degree of one slot is 210 degrees.
Now that you know the °elec of your motor, you can technically place the first sensor anywhere. Let's call this the "A" sensor. I have just wedged it between the Aa
winding of the first phase.
You must place the B sensor in a slot that is °elec ahead of sensor A. This may or may not actually end up in the middle of a slot, and it is an iterative process. Each slot is 210 electrical degrees, so start adding. Begin at 0 degrees, the position of sensor A. Keep track of the number of times you add, wrapping around 360 degrees for each result, until the result is equal to 120.
1) 0 + 210 = 210. No need to modulo 360. The number of additions is 1.
2) 210 + 210 = 420. Subtract 360. The result is 60. The number of additions is 2.
3) 60 + 210 = 270. No need to modulo 360. The number of additions is 3.
4) 270 + 210 = 480. Subtract 360. The result is 120. The number of additions is 4. You win.
Thus, sensor B should be 4 slots away from sensor A, and sensor C a further 4 slots away.
, in a LRK motor, a 120 degree hall sensor placement actually results in the sensors being physically 120 degrees apart. Isn't that awesome?
� Sensors complicate the wiring issue because you need at least five more wires: Logic power, ground, and the three outputs A, B, and C.
However, I believe that sensored motors (or the wacky inductive sensorless jiggymabob) are the best for small EVs. And EVs in general. They allow you to take full advantage of the massive torque capabilties of BLDC motors by using them at 0 speed!
DIY electric vehicles are fun and exciting, as well as a treasure trove of learning opportunities. Engineering your own motor
is no small feat, especially one destined to be operated in a vehicle of your own design.
Here's hoping that future regulations over the nascent electric vehicle industry and laws over their operation grant amnesty to, or even encourage, DIY mechanics, hobbyists, and experimenters.
The virtually rendered motor seen in the opening page is a motor for my next crazy EV project: Deathblades. I'm aiming to do what alot of people have been peer pressuring me to do, and drop RazEr's technology into some foot trolleys of certain head trauma. See my Youtube page for a snazzy animation
of how the hub motor goes together. If you've been confused by my thousand-word explanation, this should help clear it up!
If you've never seen RazEr in action, check out its test video here
I'll be updating, editing, and changing things around as I go, so if you see any glaring omissions or errors, absolutely point them out to me!
And good luck. See the next page for a list of resources!