[ausev] Dual electric motor questions

[Chris Robison]

On Thu, 2008-01-31 at 15:30 -0500, Michael Bonard wrote:

> I am looking for improving weight, reliability, and efficiency by 
> getting rid of the gearbox and differential: I would install two 
> electric motors (one on each front wheel). I would drive both motors in 
> parallel from the same DC-DC converter output. I am also looking for 
> using cogged belts to connect the motors to the wheels shafts.

A better term for the device is a "controller". A DC motor controller is
a buck type DC-DC converter that's intended specifically for motor
control, i.e. they usually don't have inductors on the output, and
depend on the inductance of the motor.  In the EV world, the term DC-DC
converter is used for the device that replaces the alternator and
provides 14VDC for accessories and lighting.


> 1. How will the motors torque behave in a tight turn? Since the wheel 
> which is inside the turn will rotate slower than the outside wheel, the 
> inside motor will deliver a higher torque to this wheel while the outer 
> wheel will be driven with a lower torque..  I hope that his torque 

Through it's possible that one motor will draw slightly higher current
than the other due to the small difference in back-EMF between the
motors, I think the motors will share current pretty closely. I don't
have direct experience with this situation exactly, and I don't know
anyone who does (partial exception below) but seriously think the
difference in torque will be barely measurable. I think a limited-slip
differential running at high speed through a turn would provide a lot
more difference in torque.

For what it's worth, the Tango (http://commutercars.com) uses one motor
for each rear wheel. The RPM difference is lower than yours would be due
to the car's narrower body, so it's not a great comparison.

> 2. How will the DC to DC converter react to a dual motor load? Hopefully 
> it should not mind if I am not exceeding its rated amperes, but again I 
> do not have any confirmation.

This is very commonly done for other reasons. Series-to-parallel
switching is popular for high-performance or no-transmission
applications. Mechanically the motors can be configured any way you
want, and electrically the motors are switched between series (better
low-speed torque) and parallel (better high-speed torque). The Zilla
controllers will do this switching for you, if you order that feature.

The controller's job is to limit current. It will not allow you to draw
more than its current limit. Consider this confirmed.


> 3. Is there any member who could advise me on the cogged belt idea? I 
> think that it is lightweight, relatively inexpensive, very simple, 
> reliable, and allows a 3:1 or more "gear" ratio. I did some research and 
> found that today we have motorcycles and ultra light aircraft using 
> cogged belts passing 100 HP at 3000 rpm. This could be very promising..

Horsepower is not a meaningful rating in this case, though it's often
used because it's roughly comparable between combustion engines of a
particular type (between 4-stroke gasoline engines, for example). The
real problem is torque. For a given amount of horsepower, electric
motors have much higher torque. It's the torque that breaks things.

Belts have been used for transmitting torque in 4-wheel conversions, but
the examples I've seen are with the input to a transmission, lower
torque than you're talking about. Doing output direct to the wheels is
going to be really stressful on the belts. If you're really looking to
improve reliability, I would consider avoiding belts in this situation.

Because you won't have a gearbox to multiply torque at the low end,
you'll be driving around effectively in 4th gear all the time. You'll
need a lot more torque from your motors. Having two of them helps, but
I'm not sure if you'll need more than this or not (e.g. higher current
controller, larger motors).  

You may at least want a lower gear ratio than 3:1.  Assuming a 20" tire
on a compact car, that gives you a 120mph top speed at 6000rpm. Your
motor would be running at 2500rpm at 50mph, pulling a lot of current and
getting hot. With a 4:1 ratio, you'll be at 3400rpm at 50mph, and your
top speed will be 90mph. Of course, at this point you'll doing quite a
bit of reduction, and your driven cog will be really big. With a 5-inch
cog on the motor, a 4:1 ratio will make your driven cog 20" in diameter
-- the same size as the wheel!  If your motor cog is 3" (more likely to
damage your belt), your driven cog will be 12". A bit more reasonable.

I think it will be important to find the tensile strength and the shear
strength per tooth of the cog belts you intend to use. You might find
that you're better off with chain (despite the enclosure it will
require, and preferably with an oil sump).

Final thought...  How heavy is the transmission you want to remove? A
second 8-inch motor will add over 100 pounds.

  --chris