The AC Induction Motor Ebike Project

Please check for shorted laminations.
I have found large variations in new drill bits, for example the ones from ACE wont drill out a stripped happytime motor mount ( beer can metal).
The ones from lowes ,Cobalt, work fine but are so brittle they will shatter.

If you dont have a meg a ohm meter , or megger for short .
To test for shorts in the finished motor.
Any motor will be able to be tested with a growler.
Shades of the 1950's.
http://en.wikipedia.org/wiki/Growler_(electrical_device)

As far as drilling, that's already done and I did buy a new set of drill bits just to be sure I was starting with fresh ones. What I didn't have was a drill press, so it's eyeballed and roughly done.

Fiberglass epoxy resin works really well as an insulation material. It's a little bit of work to do, so an automatic motor building process would not make sense because of the labor cost, but the results are good for an individual project.

The magnet wire I will use is rated for 200C and the stock motor wires were rated for 40C... so I'll be as prepared as one can be for withstanding the higher temperatures that I'm expecting to create. Once the wires are in place I'll coat them with a layer of fiberglass resin to give added support and prevent vibration from being a problem. (on brushed motors the vibration is the main cause of shorts developing)

I figure that with the Six Phase power and the fact that I'm going to drive the motor really hard that I should be able to get the full 1 hp I need from it. However, I will only know when I get there. (the motor was rated as 1/3 hp)

Writing The Software

Before you write the software you need to study the strategies that others have used before. My past work "career" was as a C, C++, Java programmer during the dot.com era back in the last millenium, so I'm pretty good about reading code and figuring out the basic ideas being used. Put simply the code is all:

Interrupt Driven

...in other words all the timing for when things will happen as far as driving the MOSFETS is all based on interrupts that are set to go off at specified times. This might sound diffiicult, but it actually makes the job easier to do.

The strategy of software design goes like this:

Create a lookup table for the wave shape you want to execute.

Find the place in the lookup table that corresponds to the phase MOSFET you want to control (at this instant) and execute that action.

Reset the timers so that the next interrupt is triggered at the right time so that you can do this all over again at the next increment.

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What's great about this is that you alter the frequency not by having to recalculate the lookup table, but by "shrinking time" in the interrupt calls. The wave shape from the lookup table isn't changed, but the time is. The maximum frequency then corresponds to the point when the "spring" shaped waveform is completely compressed. (no time gaps left between interrupts)

The nice thing about this strategy is that the waveforms are in a lookup table and so there is no calculation going on. However, from reading the Chorus Motors knowledge base it becomes clear that when you swap in different waveforms you can improve performance. The "first step" to getting into the AC motor performance "big time" is to use the Third Harmonic Injection which is a trick that boosts voltage by 15% with no heating or losses. This is something you want at low frequencies, but don't really need at high ones, so you might swap waveforms depending on the frequency you are at. This insertion ends up with a waveform that is "Flat Topped" and some methods take the idea literally and completely flatten the curve.

With software... control methods are practically infinite...

Third Harmonic Injection

http://www.chorusmotors.com/technology/MeshconDerivation.pdf

This means that for a given fixed inverter output voltage, the change between h=1 and h=3 operation results in a change in the voltage applied to the windings by a factor of about 1/5. Another way of looking at this is that for a given desired winding drive voltage, the change to third harmonic raises the necessary inverter output voltage by a factor of 5. For a given fixed winding current and voltage, the power delivered to the winding is fixed. Since the inverter output voltage is being changed, the inverter output current must vary in an inverse fashion. This means that if a particular harmonic change will raise the required inverter output voltage, it will lower the required inverter output current. The above example is of particular benefit for improved overload capability during low speed operation.

For low speed operation, the inverter is operated with h=3 harmonic output. This raises the terminal voltage presented to the inverter output for a given synchronous speed, and means that for a given inverter limited output current, the winding current can be about 5 times larger than the winding current with h=1 harmonic. In this operational state, the base speed of the motor is effectively lowered by the same factor of 5, since the inverter voltage limit will be reached at a much lower synchronous speed. For high speed operation, the inverter is operated with h=1 harmonic order, which lowers the necessary inverter output voltage and enables full saturation at high speed without exceeding the inverter voltage limit.

The Meshcon system is particularly suitable for starter/generator applications, in which a single rotating machine has low speed, high torque motoring requirements to start an internal combustion engine, followed by high speed, low torque generation requirements after the engine starts. Other applications include inverter-limited traction systems, in which full power may be delivered both at low and high speeds, without needing high current switching elements for the low speed operation. The Meshcon system permits the full inverter capabilities to be used both at high and low speeds.



A Factor of Five !

What they seem to be doing is altering the "apparent" inductance (self dampening) of the motor. By choosing a waveform that accelerates the current initially and then slows it down at peak this means the peak is widened dramatically. They are in effect getting the benefits of a higher inductance type of behavior (normally the result of more turns of magnet wire) without having to actually rewire or switch anything. Torque expands without burning up the motor at low rpm because you never allow the momentum of the current to get out of hand. (the Third Harmonic acts to dampen the peak current)

For those into gasoline motor history this is a little like "tuned exhaust" or "headers". It's actually the fact that the exhaust pipes created a backward moving "standing wave" near the exhaust outlet that allowed the ports to be increased thus allowing more power. The Third Harmonic (and other harmonics) are like have custom pipes.

The difference is that with a "tuned exhaust" the optimization usually occurs at mid to high rpm, but with the Third Harmonic the optimization occurs at low rpm. So the benefit here is large at the lowest rpms. The Third Harmonic makes the motor pull hard down low. Really hard.

Many Flavors

There are more PWM "flavors" that you might load into the lookup table to run the motor. Here are two:

THIPWM - Third Harmonic Injection Pulse Width Modulation



...this is the "standard" Third Harmonic Injection mentioned in the last posting.


SAPWM - Saddle Pulse Width Modulation



...this adds some extra optimization above the simple Third Harmonic Injection:

"the SAPWM inverter has been improved in restraining harmonic current, reducing torque fluctuation and enhancing output torque." - Infineon AP16097 PWM for ACIM 2006

Third Harmonic Injection "Like" Six Phase Power

It becomes apparent that if you take a Three Phase machine and apply the Saddle PWM method to it you are "really" attempting to simulate a Six Phase machines.

Taking the SAPWM and inserting (gray) Six Phase curves:



...which either means that the Third Harmonic Injection is of no value for Six Phase power or it acts as a magnification effect on top of it. Maybe the Chorus machine finds the "sweet spot" somehow? ("tuned")

Cheap Ain't Bad

http://digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MC3PHACVPE-ND



Just because something is cheap ($8) doesn't mean that they skipped on their homework:

3-Phase Waveform Generation - The MC3PHAC generates six PWM signals which have been modulated with variable voltage and variable frequency information in order to control a 3-phase ac motor. A third harmonic signal has been superimposed on top of the fundamental motor frequency to achieve full bus voltage utilization. This results in a 15 percent increase in maximum output amplitude compared to pure sine wave modulation.

...given that the Third Harmonic Injection is "the thing" that you need to do to boost the Three Phase power and get reasonable performance the easy entry point on all this is to just buy the chip and run it.

Six Phase power is better... there are less eddy currents so less heat... but as a first pass it might be wise to get something operational at Three Phase and then look to upgrade later. Seldom does anything ever work perfectly the first time, so repeated efforts are the rule. It's possible to wire the motor so that you could decide on the phase by the way you connect the wires afterwards. (it's not necessary to wire everything with just one wire, you can wire each slot individually) Since I'll also be learning what magnet wire is going to work best (how thick, how many turns) for the motor windings the odds are that a few iterations will be needed before I dial it in.

Hmmmmm....

On the other hand the programmable chips are cheap too and once you have the control of the software you are free to make changes as needed.

A nice chart. This confirms the low end torque as being perfectly flat, but it's interesting how the power falls off in the higher rpms. This is probably an "actual" result verses "theoretical". In theory the power would stay flat once it peaks, but when the rpms get that high there are going to be losses that don't occur at lower rpm.

The fact that they are driving the motor to 14,000 rpm is amazing...

(standard AC Induction motors operate under 3,600 rpm)

Homework Completed

At this point I think I've actually "done my homework" on this project on the conceptual level. I've gone through all the factors that contribute to performance and have even prepped the motor by drilling out the grooves to prepare for rewinding.

...so after a month of study I now have "a clue" of what needs to be done.

The next step is to really ramp up the intellectual level so that I can do the software programming for this. Software is something that can be done all winter long... so there will be a point when the snow and ice have me trapped indoors and I'll get to it.

Today is a nice day... just not in the mood for a heavy challenge now...

An Interesting Product...

http://www.greenmotorsport.com/green_motorsport/products_and_services/3,1,388,17,12958.html



Make & Model / Name : GMS AC M1 Water cooled : Code : GMSM1-w-4tm90l-4

Type / description : 3 phase AC motor. A lightweight high performance water cooled electric motor designed for electric vehicles up to 500KG.

Motor Continuous Rating @ 45 Deg 18 KW (24 hp)

Peak 5 minute rating 27.5KW (36 hp)

Torque 45.5 NM @ 4000 rpm or 50NM at 3800 rpm (cal)

Weight of motor unit without coolant. 15.54KG (36 lbs)

Motor Fabrication Cast aluminium water jacket and chassis

Motor Voltage 48v optimised

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They are building this motor for things like Go Karts and Electric Motorcycles.

That's 36 lbs / 36 hp = 1.0 lb per hp

...too big for my project, but there's a trike over at Endless Sphere that could make use of this and save 15 lbs in the process. (he's using a 50 lb industrial motor.... not designed for EV's... and getting only 2 hp which means that's 2 / 24 = 8% of what is possible )