RichD wrote:
> On Feb 5, John Bailey <john_bai...@[EMAIL PROTECTED]
> wrote:
>> By the way, the problem is getting the power IN to the capacitor as
>> well as getting it out! Of course you can get nearly all power out of
>> the capacitor, its how to avoid putting half of it into a
>> resistor--sooner or later.
>
> Interesting. It is a schoolboy level proof.
>
> But the automotive circuit does not conform
> to the models discussed. The car battery is not
> charging/discharging a capacitor, but an
> electric motor/generator, which resembles a
> R-L circuit with a back emf (i.e. another battery).
>
> However, it might still partially apply... how to
> model charging a non-ideal battery? Does it
> look capacitive?
Dynamic braking makes use of both the generated EMF and the
motor inductance. The R is just one of the unavoidable
losses. Assuming the motion generated EMF is less than the
storage voltage, the braking controller shorts the motor,
briefly, allowing the motion generated EMF to drive a
current ramp up to some upper limit. The rate of that ramp
is dependent on the motion generated EMF and the motor
inductance. Then the short opens and allows an inductively
generated EMF to add to the motion generated EMF, to drive
that peak current back into the storage battery (or
capacitor).
The motor current then ramps down at a rate dependent on the
inductance and the difference between the storage device
voltage and the motion generated EMF. Once the current
falls to a lower limit, the short is reapplied and another
current peak is produced. The motor braking torque is
pro****tional to the average current during this cycle, which
happens many times per second, and the upper and lower
current limits are chosen by the controller to produce the
commanded braking torque. This works till the motion
generated EMF falls too low to achieve the the desired peak
current when the motor is shorted. Then mechanical brakes
have to take over to finish the stop and hold the vehicle still.
Note that there is no intentional loss in this process. The
switch across the motor is ideally either a short (zero
ohms) or and open (infinity ohms), and real switches come
satisfyingly close to these two states. The largest loss is
usually the motor winding resistance, which can be quite
low. Capacitors take and release energy more efficiently
than batteries (which exhibit a little hysteresis effect
when the energy flow changes directions), but with more
voltage change. I think vehicles now in production, charge
the batteries, directly with dynamic braking, but I would be
happy to learn that I am wrong on this.
--
Regards,
John Popelish
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