Russ Sciville wrote:
Is it just me or is the idea of using an automatic transmission with an
electric motor more than a little silly?
Not at all. An automatic transmission is useful, especially in a budget
EV conversion.
It is completely practical to build EVs with *no* transmission.
Essentially all purpose-built EVs have no transmission. You use a larger
motor and controller to make up for the lack of a transmission. It needs
enough RPM to reach the highest speed you will need, and enough torque
to climb the biggest hill without a transmission. It turns out that the
overall cost and weight are lower this way.
But in an EV conversion, the transmission is already there, so it's
"free". In many cases, it's integrated into the differential
(transaxle), so you can't remove it even if you wanted to. Having it
allows you to use a smaller (cheaper) electric motor and controller; one
that doesn't have enough speed and torque to cover your entire operating
range.
An automatic transmission protects against "driver error". Many people
today don't know how to drive with a manual transmission; they would
burn out the clutch, or destroy the motor from lugging it (excessive
current) or excessive RPM.
An automatic lets you idle the traction motor, and run all the usual
accessories from it with belts (alternator, air conditioner, power
steering pump, vacuum pump, etc.) This is a cheap and easy way to avoid
buying new electric versions of all these.
Most of the extra losses in an automatic transmission come from the
torque converter. But most modern automatics have locking torque
converters. When it is locked, efficiency is essentially the same as a
manual transmission, and only 5-10% worse than no transmission at all.
Noting that the transmission oil will get hot enough to heat the
cabin confirms it.
I think Roger meant that you may need to *heat* the automatic
transmission fluid when there is no ICE engine heat. Once the torque
converter is locked, the transmission doesn't produce much heat. The
transmission and its are designed to operate the best when hot; not cold.
Surely using AC motors is the way to go as all the manufacturers use them for a
reason and they are readily available these days as are the inverters.
AC happens to be fashionable at the moment. But DC works just as well.
Both are completely suitable for transmissionless EVs. Trains have used
them for 50+ years without transmissions, and they certainly need
tremendous pulling power.
--
There are few industries with more BS than the battery industry.
Elon Musk
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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