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Posted by daestrom on April 13, 2008, 12:40 pm
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> On Sat, 12 Apr 2008 12:52:00 -0700 (PDT), piccolopete@comcast.net wrote:
>
>>First time using Google groups. Hope this works.
>>
>>Anyway, I recently upgraded my recumbent trike to an electric vehicle
>>and want to take advantage of regenerative braking to extend the range
>>of the batteries. The motor is brushed, so it should act like a
>>generator during coast mode. I was thinking I could just use
>>something like an LM338 voltage regulator adjusted to about 28 volts
>>and have it switch on to provide power to the 24 volt batteries when I
>>turn off the motor. Comments, links to schematics, or suggestions are
>>welcome!
>
> My experience with many years of working with electric scooters and bikes
> of various
> sizes is that regen isn't worth the effort and cost. It basically doesn't
> have any
> material effect. The vehicle/rider is too light to store much energy plus
> very
> little time is spent braking vs coasting or accelerating.
>
> A brushed motor is particularly bad because the brush timing is wrong for
> generating.
> Much of the energy is dissipated as heat in the motor because the brushes
> are
> shorting commutator bars that have voltage between them. Commutation
> should occur at
> the neutral field point when the voltage between the bars under the brush
> is zero.
>
The ideal place for brushes in a motor or a generator is *always* the
neutral plane. If there is voltage between bars when the brush shorts
across them, you get sparking and burning of the bars. The problem is when
the neutral plane shifts with load.
The trouble here is if the motor doesn't have commutating poles. Depending
on the size of the machine, it's quite possible that it does. (seen 'em on
motors as small as 3 hp). If it does have commutating poles (also called
'interpoles') then the neutral plane won't shift very much at all between
motoring and generating and the unit will work fine both ways. I've worked
with units that can change mode from 500 kW motor to 500 kW generator with
the twist of a knob and run for hours either way.
Without commutating poles, then yes, the neutral plane will shift to one
side when motoring and in the opposite direction for generating and it is
difficult to get good operation in both modes. Slotted pole tips helps a
bit in this situation by having the pole tips (the part that 'flares' to the
side after rising up through the coil) saturated. This limits the field
distortion from armature reaction.
But the bigger question is how is the motor controlled. If it is currently
being driven with some form of PWM, then the generated voltage probably *is*
too low. If it were a field-controlled DC motor, it would be a different
story, just increase field strength when you want to slow down and the
generator-action will take power from the wheels and pump it into the
battery.
And of course regenerative braking, just like dynamic braking, cannot bring
the vehicle to a complete stop, nor hold it there. Mechanical brakes are
still needed.
daestrom
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