Re: (ET) E20 motor at 48V

["David Roden" <etpost drmm net>]

Road EV hobbyists are used to the idea of abusing their traction motors. 
They routinely run their motors at 25% to 100% overvoltage.  Sometimes the 
drag racers go even higher than that (and pay the price).  There's a huge 
body of experience with many, many people who have done this over dozens 
of 
years and hundreds of vehicles, so the most popular motors' absolute 
limits 
(where the commutator fireballs!) are well known.

They can get away with this because they use forklift motors and variants 
thereon.  Forklifts are designed to run 3 shifts a day (with pack 
swapping) 
for 20 years or so - an operating life of about 175,000 hours.  A road EV 
might run for 4,000 hours at most (figuring 150,000 miles at 40 mph), so a 
practice that reduces motor life by 90% isn't going to slow them down 
much.  


We have no idea how overbuilt an ET motor might be, but I don't think it's 
too likely that it was intended to be run 24 hours a day for 20 years.  

Mark Hanson ran the drive motor on his E15 at 48v for years, using his own 
homemade controller, with no apparent harm.  Note, however, that he didn't 
apply any field weakening.  I seem to recall him telling me that 48v 
without 
field weakening produced about the same motor current as 36v with field 
weakening.  

Mark used a separate controller to reduce the 48v to 36v for the 
attachments. That's an important consideration.  I don't think the PM 
motors 
on the mower would be very happy at 48v.  

FWIW, the Alltrax ET controller, which I'm pretty sure was really 
explicitly 
designed for the E15 motor (though it may work OK with other ET motors), 
does have field weakening.  Last I heard FW was not enabled by default, 
but 
could be turned on in the software (the "turbo" button).  Its control is 
more sophisticated than the original GE design (duh), but the intent of 
its 
FW is about the same - to provide higher top speed under light load, but 
to 
maintain torque with heavy loads.  

For light load applications I don't think there'd be any harm in running 
at 
48 volts.  I probably wouldn't try it if my machine were used for for 
plowing, dozing, or towing.  I don't really see much benefit though.  Flat-
out in D2 seems plenty fast to me.  Especially with the entertaining and 
invigorating brakes GE fitted, how much faster do you really want to go?  


David Roden - Akron, Ohio, USA

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