Re: Regenerative Braking?

"bsr3997@xxxxxxxxxxx" <bsr3997@xxxxxxxxxxx> wrote in
news:1142148085.831451.46910@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:


daestrom wrote:
"BobG" <bobgardner@xxxxxxx> wrote in message
news:1141842105.410326.270820@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
This sounds like snake oil. Lets say the battery bank is 48V. We're
driving along at 40mph, maybe humping 35V into the motor.... now
take foot off the gas... motor is outputting about 30V lets say....
to get this back into the battery, you need to hump it up above 48V
right? And to load down the motor to apply braking, you would need
a low impedance load to hook across the motor that would absorb
that 30V.... maybe a bank of boostcaps, but then you need a
separate dc to dc converter to get it out of the boostcap bank and
back into the batteries?


There are several ways to boost the output voltage of the
motor/generator while braking so that the energy can be reclaimed.
The obvious one is to increase the magnetic field strength. That
method has been used with regenerative braking systems in stationary
machinery for many years.

You seem to have *assumed* a particular type of motor (permanent
magnet perhaps?).

daestrom

Agreed. I work with machinery that provides regenerative braking down
to a full stop.

For maximally efficient automotive use, you need a regen brake with
enough wattage to make an emergency stop at speeds up to 85 mph. This
means being able to lock up the wheels and skid the car on dry pavement.
If the motor/generator can meet this requirement, it can be assumed that
100% of the car's kinetic energy is recovered to the battery tub, and
0.00% is lost to the friction brakes.

After that, it becomes necessary to invent a way for the ABS to control
the regen brake so that the driver doesn't lose control of the car in a
skid. But until the mmotor can skid the wheels, the need to add ABS is
only of ancillary interest.

Then the only purpose of the friction brakes is to prevent the car from
rolling, and those losses are negliglble compared to the losses involved
with using them in an emergency stop. Not to mention that such f-brakes
will be of a much lighter duty design than emergtency f-brakes, so
important material cost savings will result from installing those
cheaper "backups."

Friction brakes are provided for added safety and to
prevent motion when already stopped. These drives have an efficiency
of about 95%. Most of the current hybrid cars use small motors that
cannot slow the car as quickly as you often need to. Consider the
distance covered to accelerate a 3000 lb car from 0 to 60 mph with a
20 hp motor. Using that same motor as a generator will require the
same distance to stop the car. BTW a 40 hp car weighing about 2000
lbs accelerates to about 60 mpy in a quarter mile.

Isn't there some way to design an assymmetrical motor/generator with
some switching ciorcuitry that would reorganize series coils into
parallel coils, allowing the unit to generate energy faster when
stopping than energy is released when accellerating?

The driver doesn't *need* to skid his tires while accellerating from a
standing start, bue he sure as hell needs to skid them when stopping
suddenly. There MUST be a way around this dilemma.
.

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