Re: Wheel motors - too good to be true?


Phil Hobbs wrote:
dagmargoodboat@xxxxxxxxx wrote:
Paul E. Schoen wrote:

The following is from a "Voltage Forum":
http://tinyurl.com/ogvr7

I checked the company website:
http://pmlflightlink.com/motors/wheelmotors.html

and found specs for their wheelmotors which seem reasonable enough
http://pmlflightlink.com/pdfs/eWheel.pdf


[hybrid car specs snipped]

Refreshing to see a car maker touting actual performance advantages,
rather than the fact that they'll pay for part of the gas their vehicle
will guzzle.


(Much of the data above is predicted and is dependent upon the terrain,
weather conditions, driving style etc. PML has used its best endeavours to
present accurate information, but accepts no liability for any errors
and/or omissions).


Okay, so it's not *actual* performance yet, but on the right track.
Present "weak" hybrids are little more than a conventional drivetrain
with an electric booster. Direct-drive would let one drop the tranny,
clutch, differential, and weight and mechanical losses of same. Also,
with per-wheel drive the tiny on-board internal combustion engine,
de-coupled from the road, can then be run at optimum rpm and
torque...much more efficient.

The 21KWH LiPoly battery looks like a nasty wear item -- last time I
looked, LiIon cells were about $0.50 / WH.

Cheers,
James Arthur


Motors tend to have torque proportional to their volume. There's a
maximum feasible value of the B field, around 1 tesla. As you increase
the diameter, you win linearly because the torque is force X distance,
and linearly again because you can pack more wires into the
circumference of the rotor. In addition, you win torque linearly by
increasing the length. Multiplying out, you get torque proportional to
volume.

That means that the only way to get high torque out of a small motor is
to run it at high speed and gear it down. Disadvantage #1 for wheel
motors.

But the above wouldn't preclude a short motor of large diameter--a
pancake motor--or a toroidal-shaped motor. The large diameter would
provide torque, and the short and/or hollow structure would save
weight.

I confess to being motor-ignorant and mostly taking the PML
Flightlink people at their word. They claim 4.2Kw (5.6hp) continuous
power for their model EW25/30, with a mass of 8.5Kg. Given the
phenomenon of 300g CD-ROM motors run at 400W-to-1.3Kw, e.g.
http://www.flyelectric.ukgateway.net/croc.htm, the PML Flightlink
figures didn't seem outrageous.

Disadvantage #2 is that they have terribly high unsprung mass,
which makes for very poor handling, especially over bumps.

8.5Kg *is* a chunk to add to the unsprung mass, but there are some
savings too--don't forget to subtract the brake disc/drum,
calipers/shoes (which PML eliminates), and axles/linkages. (Dunno how
much these amount to--Google's s/n ratio was horrible.)

It would all seem to come down to how much power one *really*
needs.[1]

Cheers,
James Arthur

[1] I measure my Acura as needing 14 hp (10.4 KW) cruising at 60 mph
(96 Kph). It has a 113 hp (84 Kw) engine--about 8x oversized--to
accommodate acceleration. In turn, I calculate roughly 68 hp (51 KW)
needed to accelerate 0-60 mph in 10 leisurely seconds, so at least 54
hp (40 Kw) reserve is needed in bursts. Electric motors should be good
in this respect, having high peak capability.

Torque-wise my car produces 13.7 N-m peak @ 5,500 rpm, versus the
EW25/30's rated 42 N-m @ stall (continuous), 168 N-m @ stall, peak
(30sec), per motor.

Finally, my Acura's EPA-rated for 30 highway MPG, but I average 37,
for a calculated overall gasoline-to-locomotion efficiency of 21%.

.

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