Re: Direct Drive Revisited

[XURQ03A@prodigy.com ( LEE A HART)]

Todd Halverson wrote:
> My goal is is to complete the installation project in one winter's time.

Ambitious for a first-time EV, but possible. You'll get lots of help from
members of the EV list (though not all of it helps in the same direction :-)

For this short time frame, I'd suggest a simple modular design. Do the basics,
but plan to rip things out and redo them as you learn more. You don't have to
"get it right the first time". Just make it work for now, and fix it later
when you have more experience to judge what is "right" for you.

> The 4x4 would only be engaged occasionally...
> Not being engaged, would I lose any efficiency?

Yes. The extra weight and extra rotating parts still consume energy (not all
of them are uncoupled when the front hubs are disengaged). However, you are
again only talking about a 5% or so range penalty.

If you are thinking about experimenting with AC vs. DC drives, I might suggest
removing the transfer case and using separate motors and controllers for the
front and back axle. Put in a plain old DC system to the back wheels first.
Then put your experimental system to the front wheels. Now you have a perfect
setup to run one or the other drives to see how they compare. Or both at once
if you need 4WD.

This could also be used with two identical DC motors (see the other direct
drive posts).

> From your advice, ("10% hit") the automatic is gone!

You might as well check to see if it has a locking torque converter (some
Fords do). If it does, you might as well keep it for first tests.

> I am still confused. What is more efficient direct, thru a transmission,
> or AC? & by how much?

It's not that simple. What you care about is overall system efficiency, which
is the product of the efficiency of each piece. Charger x batteries x
controller x motor x transmission x differential etc. Then, the efficiency of
each piece changes, depending on load, speed, torque, temperature, etc.

Proponents of, say, AC motors will brag about their high efficiency, but fail
to mention that AC controllers are less efficient. Or that direct drive EVs
eliminate transmission losses, but at the expense of increasing motor losses.
Or that a PM motor (either DC or AC) has a high peak efficiency at its best
operating point, but is worse than the alternatives at its worst operating
point. Throw in variables for cost (more money improves performance) and
ingenuity (a clever designer can tweak up an otherwise mediocre system to
outperform one that is theoretically better). The result is a confusing babble
of voices, and no clear winners.

But, this is a strength of EVs. Even after playing with them for 100+ years,
nobody knows for sure what the "best" system is! That means there is still
room for the backyard inventor to discover useful things!

> Torque converters, sprag clutches that's mechanical stuff aren't they?

Luckily (for us EEs), there is an electrical equivalent for most mechanical devices.

            transmission = transformer (with fixed taps)
        torque converter = variac (variable transformer) but with
                           high resistance winding (i.e. lossy)
            sprag clutch = diode (locks up to pass torque one way, but
                           spins free the other)

So, a clutch and manual transmission is equivalent to a rheostat that can be
varied from open to shorted, feeding a transformer with several fixed taps,
selected by a big switch. An automatic transmission is the same thing, with
the rheostat replaced by a variac wound with high resistance wire (and some
logic to control it automatically.

> Lets talk! I have designed production 1/8th HP PM PWM motor controls...
> As I also stated to Lee Hart, I work at Miller Electric welding equipment.

That's a pretty close field. Both EVs and welders have to handle high power,
and be pretty tough and reliable. Though one thing occurs to me; welders don't
care much about efficiency.

> ...I'd like having something simple rather then duct taped together!

But duct tape *is* simple! Perhaps you meant "reliable" instead of simple?

>> Another advantage to AC is inherant regen, whereas it is complex to
>> implement DC regen.

> I was thinking of a boost converter... not that complex. What am I missing?

All motors can be used as generators (i.e. do regen). The problem is that some
are easier to use than others. The series DC motor (most commonly used in EVs)
happens to be one type that is hard to use as a generator. To put it in
electrical terms, it has a negative resistance as a generator, and so is
inherently unstable.

A sophisticated controller can compensate, and do regen with a series DC
motor. However, most series DC motor controllers are not built for it.

If you want simple reliable easy regen, use a shunt or PM DC motor. Then the
controller problems for regen become simple. Though using it as a traction
motor becomes harder (nothing is ever free :-)

Lee Hart                     If you would not be forgotten
4209 France Ave. N.          Soon as you are dead and rotten
Robbinsdale, MN 55422 USA    Either write things worth the reading
phone (612) 533-3226         Or do things worthy of the writing
e-mail XURQ03A@prodigy.com   (Ben Franklin, Poor Richard's Almanac)
Lee Hart                     If you would not be forgotten
4209 France Ave. N.          Soon as you are dead and rotten
Robbinsdale, MN 55422 USA    Either write things worth the reading
phone (612) 533-3226         Or do things worthy of the writing
e-mail XURQ03A@prodigy.com   (Ben Franklin, Poor Richard's Almanac)