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Capacitors vs. batteries in Regenerative Braking Systems

Hi All,

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.

However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough to be of any use during
acceleration..

Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?

Thanks!
 
E

Eeyore

Jan 1, 1970
0
Hi All,

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.
Indeed.


However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough

Actually the *motor* uses the stored energy. The capacitor stores it. The reason
for using ultracaps for this is the rate at which energy is recovered during
braking is so significant that it might damage the battery if you tried to store
it there.

to be of any use during acceleration..

I think you mean during braking. There's no trouble using energy when
accelerating.

Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?

Motors and generators are very similar. A permanent magnet motor for example
*is* a generator by default. There's no big difference.

As for how a generator slows the vehicle down, it's simply removing the kinetic
energy of the vehicle and turning it into electrical energy which gets stored
for use later. Normal brakes just turn that kinetic energy into heat.

Graham
 
P

Phil Allison

Jan 1, 1970
0
<[email protected]>

** Groper alert !

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.

However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough to be of any use during
acceleration..

Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?



** Time to get yourself a small DC motor and do some experiments.

If you connect it to a battery via a SPDT switch, wired to short circuit the
motor in the off position - you will see how a motor can be used as a
break.

Then wire the switch so the motor connects to a low value resistor in the
off position.

The problem with motor voltage during breaking will then probably dawn on
you.



......... Phil
 
D

default

Jan 1, 1970
0
Hi All,

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.

However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough to be of any use during
acceleration..

Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?

Thanks!

You treat the back emf of the motor like a low voltage, but high
current source and step up the voltage and feed it back to the battery
- that's regenerative braking, for a DC motor.

An AC traction motor is a little different, but the same principle -
trigger a semiconductor to transfer energy back to the battery at the
right phase angle when decelerating.

Or use a variable displacement pump/motor/accumulator and use
hydraulics to recover energy.

Almost all systems use storage batteries - not capacitors. Capacitors
are good for relatively low energy storage and fast energy transfer.
Batteries are good for large energy and slow speed. You don't need
high speed for vehicle drive systems - unless you're doing something
cutting edge with "super caps."
 
<[email protected]>

** Groper alert !




** Time to get yourself a small DC motor and do some experiments.

If you connect it to a battery via a SPDT switch, wired to short circuit the
motor in the off position - you will see how a motor can be used as a
break.

Then wire the switch so the motor connects to a low value resistor in the
off position.

The problem with motor voltage during breaking will then probably dawn on
you.

........ Phil

It's spelled brake and braking. Unless you toss em hard enough.
 
L

Lionel

Jan 1, 1970
0
Hi All,

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.

However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough to be of any use during
acceleration..

While I have zero experience in these systems, I would've thought the
obvious approach would be to use capacitance in parallel with the
batteries for that reason. I fully admit that there may be problems
with this approach that I'm not aware of.
Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?

The motor doesn't actually 'switch' functions in the usual sense of
the word. Putting it very, very simply, what happens is that the coils
of wire on the armature cut the fields around the magents, & if you
put power into the coils, the resulting field around the armature
'pushes against' the magnets, & if you 'push' the armature through the
fields around the magnets, it induces power into the coils. The
important thing to remember is that both effects can happen *at the
same time*, eg; when you're powering a motor in a vehicle thats' going
up a hill, so gravity is 'pushing' the motor in the opposite
direction. This results in back-EMF that fights the power you're
putting into the motor. So if your vehicle is rolling forward via
inertia, but spinning the motor armature, you can collect that energy
& use it to charge your battery, & the act of tapping that power
'fights' the motion, thus acting as a brake.
You can demonstrate the concept yourself by taking a powerful motor
with permanent magnets (eg; any DC or stepper motor), & spinning the
shaft with your fingers while nothing is attached to the leads. You
then shorts the leads together & try it again, & you'll find that the
shaft is much harder to turn.
(This is a really fun & effective way to demonstrate these concepts
to students, BTW, & you can also prove that it generates power by
using a DC motor with a light bulb & an ammeter instead of a short
across the leads.)
 
Thanks everyone,

Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.

Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the manget, which in turn "brakes" the vehicle?
 
L

Lionel

Jan 1, 1970
0
Thanks everyone,

Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.

Thanks. :)
Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the manget, which in turn "brakes" the vehicle?

Um. Another way of putting it might be to say that you can convert the
potential energy of the vehicles motion into electricity by using it
to spin the motor shaft, & by taking that energy, you're slowing down
your vehicle. And bear in mind that by changing the amount of
electricity you're draining, you change the amount of breaking.

Yet another way of looking at it is to think of the forward motion of
a heavy vehicle as being like the spinning of a heavy flywheel. That
motion is form of energy, as is a manetic field in a coil of wire, or
a charge in a capacitor. You can add energy to any of those things, or
subtract energy from them, & (disregarding friction & other
inefficiencies) the energy is equivalent, regardless of the form it
takes, & can be transferred back & forth.
 
E

Eeyore

Jan 1, 1970
0
Thanks everyone,

Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.

Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the manget, which in turn "brakes" the vehicle?

It *does* 'fight' the magnet. That's what slows it down !

Graham
 
Thanks everyone,

Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.

Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the magnet, which in turn "brakes" the vehicle?

This probably isn't the right way to think about what is going on.

In a permanent magnet motor, the drive coils see a changing magnetic
field as the motor shaft rotates.

This generates a voltage - the "back EMF" - between the ends of the
coil. If you apply a higher voltage to drive current through the coil
against this "back EMF" you produce a torque roughly proportional to
this current which tends to make the shaft spin even faster, and you
are using the device as a motor to accelerate your vehicle.

If you let the "back EMF" drive current through the coil in the
opposite direction, into a battery, capacitor or resistor, you are
using the device as a generator to decelerate your vehicle. Again, the
decelerating torque produced is roughly proportional to current
circulating through the motor coil.

Any current circulating through the coils also generates a voltage
across the resistance of the coil, which generates heat - if you allow
the coils to get too hot they can heat the permanent magnets in the
motor above their Curie point, and they will cease to be permanent
magnets, so you do have to pay attention to the amount of current that
ends up circulating through the coils. The heating effet is
proportional to the current squared, so the polarity of the current
doesn't matter when you are calculating the heat being dissipated

In fact the voltage across any individual coil is an alternating
voltage that changes polarity a number of times as the shaft rotates
through 360 degrees. Permanent magnet DC motors include a mechanical
switching arrangement - the "commutator" - that rectifies this
alternating voltage into a direct voltage roughly proportional to the
speed at which the motor shaft rotates, which reverses polarity when
the direction of rotation is reversed.

If you want to follow the behaviour of the motor in more detail, you
have to start worrying about the inductance of the individaul motor
coils, which affects the rate at which the current through the
individual coils can change, and - even later - you can start worrying
about the extent to which the current through the coils induces a
magnetic field which can add to or subtract from the magnetic field
being produced by the permanent magnets.

When you get to this level, permanent magnet electric motors start
looking a lot like brushless DC motors (which use built-in electronic
swiches rather than mechanical commutators) and stepping motors (where
you are expected to supply the electronic switches).

Hope this helps.
 
J

Jan Panteltje

Jan 1, 1970
0
Hi All,

I've been reading about Regenerative braking systems and their use in
hybrid-engine setups. Most articles say something to the effect of,
'the electric motor can double as a generator to slow the vehicle and
return power to the battery'.

However, I've also read that Capacitors are the only thing capable of
using the stored energy quickly enough to be of any use during
acceleration..

Would someone please explain how regenerative braking actually works,
more specifically the part about the electric motor switching tasks
and becoming a generator, and how as a generator it can slow the
vehicle while storing energy created from the breaking?

Thanks!

If you have a DC electromotor, you can use it as generator too.
So if you turn it it will generate a voltage.
If you connect a load to it, then it will take more power to turn it.
If you connect an empty capacitor to it, via a diode, it will start
charging that cap.
It cannot discharge the cap if it stops because of that diode.
Now if you short that diode, then the energy in the capacitor will
cause the motor to turn again.
 
R

Rich Grise

Jan 1, 1970
0
Thanks. :)


Um. Another way of putting it might be to say that you can convert the
potential energy of the vehicles motion into electricity by using it
to spin the motor shaft, & by taking that energy, you're slowing down
your vehicle. And bear in mind that by changing the amount of
electricity you're draining, you change the amount of breaking.

If the car is moving, that's kinetic energy. If the car were sitting
still at the top of a hill, that's potential energy.

And it's "brake", not "break" - break is what the car does when you don't
use the brakes, and it hits a bridge abutment. ;-)

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Thanks everyone,

Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.

Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the manget, which in turn "brakes" the vehicle?

If you want to really graphically demonstrate this effect, get about
3 feet of insulated wire, a magnetic compass, and a bar magnet. Make
a loop of the wire (i.e., connect the ends to each other), but make
it kinda narrow and rectangular:


_________________________
| |
|_________________________|


Then, put the compass under one of the ends:

_________________________
| |
| (c)
|_________________________|

And pass the bar magnet near the other end:

_________________________
| |
(([magnet]))| (c)
|_________________________|

(the parens signify the magnetic field around the magnet)

and the compass will deflect. The magnet and wire on the left
is the generator, and the compass on the right is the motor.

If you move a wire through a magnetic field, it generates a
voltage. If you take the same wire and just let it lie there,
but pass some current through it, the wire will move. They're
exactly the same effect, just kinda like in opposite directions.

Hope This Helps!
Rich

Cheers!
Rich
 
Ok guys, I know.. silly error. However, if you check the original
post, brake is spelled correctly twice (including in the subject
header of the post and only spelled as "break" once. ;)
 
Thanks again -- this is a really great group.

This helps my understanding alot.

best,

Gregory

Thanks everyone,
Yea, obviously I know next to nothing about electromagnetics, but I
appreciate those of you who took the time to reply.
Let's see if I've got this correct:The force of the magnet inside the
motor is strong enough so that when power is drawn from the motor to
charge the battery, the momentum of the vehicle (which is turning the
drivetrain) -- that motion is not significant enough to fight against
the manget, which in turn "brakes" the vehicle?

If you want to really graphically demonstrate this effect, get about
3 feet of insulated wire, a magnetic compass, and a bar magnet. Make
a loop of the wire (i.e., connect the ends to each other), but make
it kinda narrow and rectangular:

_________________________
| |
|_________________________|

Then, put the compass under one of the ends:

_________________________
| |
| (c)
|_________________________|

And pass the bar magnet near the other end:

_________________________
| |
(([magnet]))| (c)
|_________________________|

(the parens signify the magnetic field around the magnet)

and the compass will deflect. The magnet and wire on the left
is the generator, and the compass on the right is the motor.

If you move a wire through a magnetic field, it generates a
voltage. If you take the same wire and just let it lie there,
but pass some current through it, the wire will move. They're
exactly the same effect, just kinda like in opposite directions.

Hope This Helps!
Rich

Cheers!
Rich
 
L

Lionel

Jan 1, 1970
0
If the car is moving, that's kinetic energy. If the car were sitting
still at the top of a hill, that's potential energy.

Yes, sloppy thinking on my part.
And it's "brake", not "break" - break is what the car does when you don't
use the brakes, and it hits a bridge abutment. ;-)

Ouch. I hate it when people use 'break' for 'brake', or 'loose' for
'lose'. You can sure tell that I wrote that post on Friday night,
after a few beers. :^/
 
R

Rich Grise

Jan 1, 1970
0
Yes, sloppy thinking on my part.


Ouch. I hate it when people use 'break' for 'brake', or 'loose' for
'lose'. You can sure tell that I wrote that post on Friday night,
after a few beers. :^/

No big deal - I was just feeling pedantic. ;-)

As far as how the regenerative braking goes, I'd opt for batteries that
can handle as fast of a charge as they will a discharge, which is,
admittedly, a pretty tall order for batteries - but something tells
me that to do it with capacitors would take about 15 tons of capacitors
and bus bars, in a package about the size of a city bus. =:-O

Cheers!
Rich
 
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