(ET) AC for the ET?

["David Roden (Akron OH USA)" <roden ald net>]

> If I had all the money and time needed it sure would be cool to replace
>  the
> dc motors with AC motors, high voltage battery pack and a three phase
> inverter and controller. 

It's hard for me to see that AC motors offer many clear advantages for 
something like a lawn tractor.  This from a guy who has a road EV with an 
AC induction motor and controller -- but if I'd had to pay full price 
(buying it brand new), I never could have afforded it.

AC drive for EVs is sexy.  The motors can be sealed and still dissipate 
heat nicely.  They can run at extremely high speeds (good thing, because 
they often have to be geared way down to get good torque).  The torque 
band usually looks flatter than a DC motor's (but that's really a 
function of controller design), and they usually have regenerative 
braking (again, a function of controller design -- they're so complex 
already that adding regen isn't a big step).  

But while the motors are elegantly simple, the controllers are several 
times as expensive and complex.  I think I've read that the typical 3-
phase induction motor controller is about equivalent to ~six~ DC motor 
controllers.  The high voltages required mean expensive, shorter-lived 
battery packs.

Cost-no-object road EV prototypes like the GM EV1 use AC motors.  But GM 
loses big money on every EV1 -- that's the real reason they don't sell or 
lease them in significant numbers.  They can't afford to. 

On the other hand, there are tens of thousands, perhaps hundreds of 
thousands of practical, affordable electric vehicles in daily use all 
over the world.  Nearly all of them have DC motors.  These industrial 
vehicles -- forklifts, burden carriers, and golf cars -- use DC motors, 
PWM controllers (sometimes SCRs!) and a few big, burly lead batteries 
because that proven combination gives ample torque, good reliability, low 
cost, and cheap, simple maintenance.  


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1974 Honda Civic EV 96vdc
1970 GE Elec-trak E15 36vdc