Pretty much every highway-capable OEM EV company has tried, and
rejected, the hub motor design concept. Every one!
Hub motors sound absolutely marvelous to anyone that has not gone down
that path before. (Like management that have not full thought out the
issues.) Then, once you actually have built a prototype, the plethora of
deal-killer issues become glaringly apparent.
1) >>> Unsprung weight <<
Any addition to the weight of the wheel assembly makes handling
suffer, and traction suffer. It also makes the ride more harsh. Not a
deal-killer, but a serious engineering challenge. This issue alone is
not enough to tip the balance away from the possible advantages of a hub
motor design.
2) >>> Uncommanded adverse wheel torque <<
This is the biggest single issue for an OEM passenger vehicle. If
(actually _when_) one of the wheels suddenly stops or reverses on the
highway, the passengers are doomed. ENORMOUS legal liability.
There are a host of reasons why one hub motor wheel would exert an
adverse and sudden torque. When this happens, the vehicle will hurl
itself uncontrollably into the oncoming traffic, half of the time. The
other half of the time, it will hurl itself off the road into the ditch,
a bollard, a tree, or off a bridge. With a PM motor, you simply have to
short the stator in one motor for this to happen. There are a host of
other faults that will result in this sort of failure.
Yes, it might be possible to address every possible adverse torque
fault scenario, but it is very time consuming and prohibitively
expensive. This issue typically torpedoes the design.
3) >>> Harsh motor environment <<
The hub motor feels every bump and railroad crossing full force
with only the tire for cushioning. It also gets every torque pulse from
these bumps with no damping. (Think of wheel hop.) These severe torque
peaks are really hard on the mechanical portions of the motor, but also
translate into voltage spikes and current spikes to the inverter. HUGE
engineering and maintenance headaches. (Also see above about uncommanded
adverse torque, which can often be the result of these failures.)
>>> Move the hub motors inboard?? <<
This is the fix for issues #1 and #3, but doesn't address issue #2.
Also, if you are going to use half shaft axles, and mount the
motors to the chassis, why not simply combine the two reduction
gearboxes into one, and use a conventional differential. Which is the
standard configuration for pretty much all OEM EV's. :-)
Bill D.
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