Re: Regenerative Braking?


"John Schutkeker" <jschutkeker@xxxxxxxxxxxxxxxxxxxx> wrote in message news:Xns9788E358E67Clkajehoriuasldfjknak@xxxxxxxxxxxxxxxxx
"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.


Uh... 'lock up the wheels and skid' is not 'maximally efficient'. In fact, it is zero efficiency. All the energy goes into rubbing tires across pavement. If the wheels/motor-generator don't rotate, there is no energy recovery.

While it may be necessary to have such a braking ability, it is not the sort of braking that needs to be considered in the regenerative brake design. It seems clear that a backup friction brake will always be needed.

Although reverse torque can be developed in a motor to hold it stationary for maximum braking, that mode of operation wouldn't be 'regenerative'. In fact it would actually require more energy from the battery/fuel-cell to hold the wheel stationary while skidding.

If the regenerative braking is strong enough to cause wheel slippage on snow or wet pavement, that would seem the upper limit on regenerative braking needs. At this point it would need to be modulated (as you suggest below), and any further braking force can be supplied by the still necessary 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.

Sadly, I think the only solution is to keep a friction brake. To build a motor/generator that can require such a large amount of torque and only allow one or two revolutions would not be economical. The number of times that such a demand is actually made, compared to the increased cost of such an over-sized motor/generator just wouldn't make much sense.

Much has been made about regenerative braking, and lots of 'oohs' and 'aaahs' about how great an idea it is. But consider, a 1000kg car traveling 27 m/s only has 364.5 kJoules of kinetic energy. So if it were possible to *perfectly* recover all this energy, that's only about 101 watt-hours. If the car requires only a very modest 20 hp to travel at 'speed' over the road, those 101 watt-hours of energy would be equivalent to traveling an extra 24 seconds.

In 'stop and go' driving, let's say a 'soccer mom' commuting around town at 15 m/s, with 10 stops from home to little Johnny's soccer game. The kinetic energy at 15 m/s is 31.25 watt-hours, and perfectly recovered from 10 stops would be 312.5 watt-hours. If only 10 hp were needed to travel at this lower speed, those ten stops add up to an extra 151 seconds of driving time.

Since the average car takes quite a bit more than 20 hp to travel at 27 m/s, is regenerative braking really worth it? Only when it can be implemented very cheaply such as in battery powered vehicles.

daestrom

.

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