Re: Production EVs

[Bob MacDowell <bobmacd@netcom.com>]

bdube@boulder.nist.gov (Bill Dube') writes:

>    The stall torque on a series-wound DC motor is enourmous. If you were
>able to stop the wheels from turning somehow, something would have to "give"
>in the driveline. Perhaps it would be the clutch. It might be the
>differential, but the motor *will* turn. 

True, DC motors have enormous torque.  But brakes in good trim have
enormous stopping power. 

The front brakes are usually much stronger than the rears, mainly because
a car in hard braking will "nose forward" and put most of its weight on
the front wheels.  On a car with traditional 60/40 balance, it can be 
doing as much as 90% of the braking.  

So on a front-drive car, the front discs *alone* should be able to stop
the motor, with some minor assist from the rears.  And on a rear-drive 
car, the torque that gets past the rear brakes should be easily annulled 
by the fronts, perhaps with some wheel scuff and drifting as they fight 
each other.  But it should be possible to control the vehicle enough to
aim it out of harm's way.   

The other thought is this:  some people don't like the idea of a contactor
on the throttle, so that you hear a "clunk" every time you step on the 
gas.  Ok, how 'bout two other places to put that contactor control:

1.  On the ignition switch.  Just tie that contactor into "Hot in Run"
or the (surely now unused) "Hot in Run/Start".   Then in case of runaway,
you'd do what any ICE driver would do: turn off the ignition! 

2.  On the "Neutral" switch ("Park/Neutral" on an automatic) -- this 
would be an added safety, so that if you were power-shifting and had 
trouble getting it back into gear, the motor would not run away. 

About the possibility of a main contactor getting stuck: gross overdesign
is not practical in every aspect of EV design, but the main contactor is
a really good place for it :-) 

-Bob