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green_is_now

1/4/2013 11:07 PM EST

The switches need to be able to handle both high current in one scenario and ...

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green_is_now

1/4/2013 11:02 PM EST

The problem is in all the IR and reactive losses of the matrix wiring and ...

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# Electric hub motor improves EV range: Part 1—Technology basics

## 4/28/2011 3:52 PM EDT

A vehicle has 60 cm external diameter wheels, so it moves about 2m for each wheel rotation. A 180 km/h (50 m/s, 112 mph) speed corresponds to 25 wheel turns per second. At each wheel turn, the 24 magnets are passing in front of each wire, alternatively north and south. The electric current has to be inverted in the wires for each polarity inversion, in order to keep the same direction of the driving forces (see previous figure). The current pulse periods corresponds to 1/12 of a wheel turn. At the maximum speed of 180 km/h, the minimum period is 3.33ms (300 Hz max), easily achievable with existing power MOSFETs.

When the space between magnets is in front of the wires (red for instance), the current in these wires has to be stopped, before being inverted. The “complementary” wires (green for instance) are in front of the magnets, with current flowing, in order to keep the torque constant. The phase difference (positive or negative) between red and green wires depends on the motion direction (forward or backward).

A magnetic field sensor (Hall-effect type, for instance) is joined to the axle to determine the relative position of the magnets and wires, and thus commands the current switching.

Each stack generates a driving torque, Γ = 0.27 * I

When the wheel runs at a "pulsation," ω, a counter-electromotive force (CEMF) = E, is generated on the wires:

E = 0.27 * ω
(E * I product is the electrical equivalent of the mechanical power Γ * ω)

At 180 km/h (ω = 2 * 3.14 * 25 = 157), E = 42V.

At a speed v (km/h), E(v) = 0.23 * v.

Electronic gearbox performance at varying speed

At start up, E = 0. The series resistance of the wires and the switches is the only limiting factor for the current in the stacks. The 12 stacks are connected in series, powered by a 130V battery.

A maximum of 28 switches are connected in series (a conservative approach, with one switch at each end of each stack). The switches between two stacks have a DC command, while the switches at the ends of the network are switching to the two poles of the battery, in order to invert the current in the net.

The 28 switches have a total series resistance of about 5.6 Ω (the conservative approach includes the series resistance of the battery and of the connections). The current flowing through the 12 stacks is about 20A, generating a maximum starting torque of 260 Nm (four wheel drive).

In order to control this torque, it is possible:
• To power only two wheels (torque divided by 2);
• To power only one network (red or green, torque divided by 2);
• Or control the duration of the current pulses (Pulse Width Modulation).

When the vehicle speeds up, the CEMF voltage increases, and reduces the flowing current:

I = (Vbat – E) / R

At 45 km/h (28 mph), the CEMF voltage on each stack is 10.5V, and thus on the 12 stacks in series, 126V—approaching the battery voltage value. The current flowing through the stacks starts to become negative for speeds faster than 45 km/h, and the motor acts as a generator.

The wheel motors also act as brakes. All the current generated by regenerative braking is used to charge the battery. No additional circuitry is needed.

In order to brake at speeds slower than 45 km/h, it is necessary to short circuit the stacks into a resistor. The braking energy is lost. Using it to recharge the battery would require additional circuitry, that may be worthless, as this energy is relatively small (0.5 * M * v2). [Ed. Note: Previous calculation for energy corrected from original posted text.]

Part 2 of this feature covers applications at higher, more practical speeds and manufacturability.

Roland Marbot is principal at EZ Consulting, marbot.roland@neuf.fr.

Bob Lacovara

4/29/2011 10:16 AM EDT

It's a nifty motor, and might find application in non-automotive applications. To read that some of the control circuitry now in use needs liquid cooling is rather discouraging: that's a lot of wasted energy. One other point: recovering braking energy isn't merely a matter of being able to lay hands on it at the motor. You have to do something with it, typically put it into your battery. Trouble is, batteries don't recharge in seconds, and some fraction of that energy is lost. Some type of supercap might be indicated.

Efried

5/4/2011 3:30 AM EDT

batteries might be designed for 2C-5C charging, supercaps failed so far to define their business model in transport - too bulky, too expensive

prabhakar_deosthali

4/29/2011 10:19 AM EDT

A very informative article on hub motors for EVs. I would have liked this article to contain some graphics - Torque speed waveforms and schematics showing how commutation is achieved. Some comparative charts showing the efficiency and range improvement over other similar motors will also add value to this article.

agk

4/30/2011 6:40 AM EDT

By reading this article my thoughts go back to the VCR 's head drum direct drive and capstan direct drive motors. The same design in a small size and lower power it was working there. But here this motor has to meet many more things as shock proof, water proof,dust proof,heat proof and carry atleast two times more than the specified weight.

elPresidente

5/2/2011 5:15 AM EDT

This seems awfully naive(full blame is on EET editors for accepting the piece), almost like it was written by a high school kid, and there's nothing novel or "nifty" about it for all you guys caught agasp in the comments section - do the math on the cost of a printed circuit motor with enough layers to be both mechanically and electromagnetically sound, appropriate layup Tg, and copper thickness and you very quickly have a \$8000 weak torque motor per axle.
The Maker community have been building axial flux alternators and generators (which is what this is) for well over a decade for wind power and have figured out how to bury windings ECONOMICALLY without the excessive cost of prepregs and laminating presses. Read Hugh Pigot's book if you want reality and cost-effectiveness versus fantasyland equation based nonsense.
The issue of flux saturation and reluctance in the air/epoxy gap also is not addressed in this math exercise, which will seriously limit the mag field contribution of the 1 Tesla magnets that are being assumed in force calculations.
To suggest fixing the axle to the frame of the vehicle to "simplify shock absorbers", is extremely naive as well as is the notion that much less than 10cm is a large enough axle stub to support a 1.5 tonne vehicle hitting a pothole...LOL this has the same shaft diameter as a 1/2HP furnace blower motor. Clearly, the author's never designed autmotive drivelines; apparently just a dreamer with a glass of wine and a fireplace and enough engineering knowledge to be reckless.

roldan

5/7/2011 8:37 AM EDT

I would very much like to be a naive high school kid. Unfortunately, I am a retired electronics engineer, with 40 years of background in the industry, acting now as an independent consultant. Almost twenty patents filed. Six presentations at international conferences. One invited paper at ISSCC. My career is behind me.
To be fully honnest, when I tried to patent this concept, I found that this was already done almost twenty years ago. There are at least two naive high school kids dreaming in the world. But, apparently, the other one failed in promoting the concept.
I think, but you can disagree, that it can be very beneficial for the whole industry, and not only the automotive one. For an independent consultant, the automotive industry is too big. I'm more interested in other applications, looking for attractive small niches. But, if I can convince the automotive industry to go in this direction, this should help for developing smaller businesses.
I would enjoy replying to technical questions. In your comment, I can't see any requesting an answer. Please, consider the concept, and not the implementation details.

davealle007

5/11/2011 10:42 AM EDT

Thats kinda scarey, just anyone that knows enough to be dangerous can post all this with the editor of EE times to go ahead and let it happen. I guess thats the downside of this kind of forum. The good side is that someone like roldan speaks up to reprove it, but is he right?

WarlockofOz

6/5/2011 11:27 PM EDT

I think you are getting confused between cm and mm. A 10cm(100mm) axil will, depending on what it is made from will easily hand up to 5 x 1.5 tons. I've lifted components up to 30 tons on pins that thick.

elPresidente

5/2/2011 5:16 AM EDT

And, as any HIGH SCHOOL kid can tell you, it's 1/2 m*v*v for energy, which is VERY significant energy as anyone who's hit a wall at 50km/h can tell you....you don't dump it into a resistor, especially for a vehicle carrying limited stored energy on board to start with...streetcars used to do that one 75 years ago to heat the cabin. We are supposed to be cooling the planet, not heating it with printed circuit motor braking systems or failing to recycle it for later use in moving the vehicle.
Has anyone done the math for the commutation "switches" in this wundermotor? For a 20A "starting torque" you are dissipating 400*5.6 or 2.25kW PER MOTOR, or 10kW of power in the switches for 4WD. Now add in your winding resistance. It gets a lot worse when you are using the 'sports car' 30A example that was calculated in a prior page - almost DOUBLE. The Chevy Volt only carries 16kW of batteries....
Thanks for the laughs - can't wait to see part deux. A new low, EET; a new low.

Efried

5/4/2011 3:36 AM EDT

Why is the mass per weight so bad with current electric motors?
Please allow for some innovation - even in early stage- My guess is that electric mobility will fail with current low RPM motor designs.

Efried

5/4/2011 3:36 AM EDT

mass per power of course kg/kW

astrayelmgod

5/3/2011 9:33 PM EDT

0.5mm looks like about 14 oz copper. My VERY limited experience with making coils out of PC boards was with 4 oz copper, and even that was a lot more expensive than making it the usual way. So, what is the attracttion for using PC stock?

BOY Dela Pena

5/5/2011 9:23 AM EDT

The design looks very good and will have some other usage, The main thing is how much it will cost and how you can have more detailed information.

Thank you,

Godofredo Dela Pena

DrQuine

5/9/2011 6:37 PM EDT

Two years ago "EE Times" reported that "Daimler bus combines hub motors, fuel cells" (http://www.eetimes.com/electronics-news/4197774/Daimler-bus-combines-hub-motors-fuel-cells) so it would appear that the deployment problems for hub motors are not insurmountable. It sounds like one challenge will relate to car handling characteristics having additional mass that will be moving with the wheels (rather than the vehicle body). This is probably less of an issue for a bus. Another design issue will be to ensure that the hub mounted motors are designed to tolerate exposure to the elements near the wheels rather than being protected in the engine compartment.

6/25/2011 9:13 PM EDT

Readers are directed to two sites:

http://www.technologyreview.com/energy/21666/

http://www.launchpnt.com/portfolio/aerospace/uav-electric-propulsion/

It doesn't look like Exro weathered the recession very well since I haven't seen anything from them lately, but my original analysis of their approach/patent (switched coils to change torque characteristics) led me to believe that the engineering was sound. Likewise for LaunchPoint.

7/5/2011 1:33 PM EDT

Has anyone heard of BionX, www.bionx.ca? They make hub motors used in boost systems for bicycles and velomobiles. Their systems go up to 500 watts and are priced around \$2,000 with a control system using regenerative braking, http://www.youtube.com/watch?v=eSFE151tRdM. Is the system described here better or cheaper than the BionX system?
I want to mass produce velomobiles with boost systems controlled by I-pads, and could use the help of some smart guys like you.

green_is_now

1/4/2013 11:02 PM EST

The problem is in all the IR and reactive losses of the matrix wiring and switches, and there cost

green_is_now

1/4/2013 11:07 PM EST

The switches need to be able to handle both high current in one scenario and high voltage in the other extreem.
So the semiconductors size and cost go throught the roof, add the IR loss gain for needed worst case voltage needed, making this not feasible quickly.