steve sawtelle via EV wrote:
So, as a thought exercise, do you think a motor-generator could be
designed to start these engines yet fit within the physical
constraints?
Hi Steve,
It's possible; the question is whether it would be practical (buildable;
affordable). It would take a pretty special high-tech motor.
A normal ICE starter is a series DC motor. Such motors can be built with
a very high power-to-weight ratio; but then the efficiency is low. The
low efficiency means the current is very high, and they overheat
quickly. The brushes are solid copper, to minimize resistance; but in
turn causing a very short life expectancy.
*Every* motor is also a generator: But series motors are hard to control
as generators. The above characteristics of a starting motor (low
efficiency, short life) also make it unsuitable as a generator that runs
all the time.
There are many applications that have combined the ICE starter and
generator into a single unit (aircraft, tractors, golf carts, lawn care
equipment). It this case, they designed it as a generator first; then
tacked on the ability to also use it as a starter.
A traditional generator is a shunt DC motor. The brushes are carbon for
long life. If it's going to used at high current (like for starting),
the brushes and commutator have to be much larger (making the motor
bigger). The field is separately powered via the voltage regulator. This
is an easy way to regulate the output. But a second series field is
required to make it work as a starter. Having to put *two* field coils
and a bigger commutator and brushes makes the unit bigger. Not as big as
a starter + generator; but it's close.
All this means I don't think you can fit an adequate starter/generator
in the space available for the generator alone with conventional techniques.
Motors with higher power-to-weight ratios generally use high-strength
magnets. They remove the need for the field coils; but also make it
harder to control. So you're going to need some high-power electronics
to control it.
Another problem with high power-to-weight motors is that (like ICEs)
they need to run at high RPM. This will mean a gearbox. That too is
going to take up space.
There may be an easy, cheap, clever solution; but right now I don't see
it. Maybe look at other approaches, like a spring starter?
Lee Hart
--
When something bad happens, you have three choices: You can let it
define you; let it destroy you; or you can let it strengthen you.
-- Theodor Seuss Geisel
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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