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Capacitors vs. batteries in Regenerative Braking Systems

Discussion in 'Electronic Design' started by [email protected], Mar 16, 2007.

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  1. Guest

    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

    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?

  2. Eeyore

    Eeyore Guest

    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.

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

    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.

  3. Phil Allison

    Phil Allison Guest


    ** 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

    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

    ......... Phil
  4. default

    default Guest

    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."
  5. Guest

    It's spelled brake and braking. Unless you toss em hard enough.
  6. Phil Allison

    Phil Allison Guest

    ** Blame MS Word spelcheka....

    ........ Phil
  7. Lionel

    Lionel Guest

    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.
    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.)
  8. Guest

    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?
  9. Lionel

    Lionel Guest

    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.

    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.
  10. Eeyore

    Eeyore Guest

    Where I live it's spelled (sic) spelt !

  11. Eeyore

    Eeyore Guest

    The author played no part in this ?

  12. Eeyore

    Eeyore Guest

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

  13. Guest

    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.
  14. 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.
  15. Rich Grise

    Rich Grise Guest

    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. ;-)

  16. Rich Grise

    Rich Grise Guest

    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!

  17. Guest

    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. ;)
  18. Guest

    Thanks again -- this is a really great group.

    This helps my understanding alot.



  19. Lionel

    Lionel Guest

    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. :^/
  20. Rich Grise

    Rich Grise Guest

    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

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