active electromagnetic/inductive braking

Discussion in 'General Electronics Chat' started by magnet18, May 19, 2012.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    So I had an idea.
    Motors are made to spin by pushing magnets with magnetic fields created by coils.
    Some electric cars also recover power while braking by reversing the effect and creating a generator.
    Are there currently any systems that take it a step further, and continue to power the coils during braking, but with reverse phase or polarity or whatever in order to actually push against the magnets attached to the wheels and slow the vehicle (or anything attached to a motor) down?

    It wouldn't surprise me at all if there was, it's not a complicated idea, just wondering :)
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's perfectly feasible but in my (limited) experience with electric vehicles nobody does it. The main problem I think is that electric motors produce very high torques and even under normal motor braking they need systems to reduce the braking effect so as not to lose traction and screech the tyres.

    So they don't just short out the motor to stop the vehicle as even that would be too much braking, and driving the motor in reverse as you suggest would be even more of a braking effect than shorting out the motor is.
     
  3. dataman19

    Member

    Dec 26, 2009
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    Nice web site...
    ..
    To answer your question - Yes...
    Mag lev trains use the reverse magnet scheme, as do some light rail systems.
    ..
    But they "normally"dump the excess current into load resistor banks.
    ...
    Incidentally, Modern Locomotives use resistive braking, and switch to dynamic
    braking (where the field coils are fed back into the adjacent complimentary field coils in the locomotive exciter). This has its price though, at high speeds and extremely heavy loading the EMF can build to a point where it totally overcomes the field coils and can burn out the exciters - which is why there are governors and safety interlocks to assure that active dynamic braking is not applied).
    ...
    Then there is a whole range of Industrial motors that use active braking to radically slow the motor spin rate (as in some conveyor systems) - but these are extreme situations and the systems are expensive.
    ..
    I believe the Postal System uses a dynamically controlled package sorting system that uses this technology, as do3es FedEx and UPS.
    ...
    Dave
    Phoenix, AZ
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Induction motor braking via "plugging" is a well known hard braking method. Needs careful use.
     
  5. BSomer

    Member

    Dec 28, 2011
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    Are you referring to an "electronic dynamic brake"? Where a DC voltage is induced onto a set of windings of the motor.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    No - for instance "plugging" in a 3-phase induction motor involves the hot swapping of two stator phases so that a contra-rotating field is set up. It's a bit like a DOL start with the motor free-wheeling in the opposite direction. It creates substantial heat loading for the rotor [greater than that produced with standstill DOL starting] but decelerates the motor rapidly. The control system which does this would need a speed sensor feedback to indicate when the rotor has come to or is nearing standstill - so that you don't end up with the motor actually running in the wrong direction.
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    coolio, thanks for the info guys
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    BTW - Bsomer's comment / question in post #5 is perfectly logical as DC injection into a motor phase winding is also another means employed for induction motor braking.
     
  9. gerty

    AAC Fanatic!

    Aug 30, 2007
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    1 major problem I see with this, a bad connection/blown fuse could leave you without brakes:eek:
     
  10. dataman19

    Member

    Dec 26, 2009
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    No system should rely on "dynamic" or "active" braking. This is why they also have friction brakes.
    Diesel engines also have a neat braking system whereby the engines exhaust is fed into the intake (causing the engine to load up). Another version is where the exhaust is also closed off or restricted.
    ....
    t n k is correct in stating that "Plugging" needs careful attention. But..... it will certainly slow a motor down, and fast.
    Dave
    Phoenix, AZ
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Just to make aminor nitpick, this is common but it's not "applying reverse voltage" to a motor it is applying a controlled load. In almost every case I know of the controlled load for braking is LESS than a dead short, and even a dead short is less load than reversing the motor voltage which would be very violent and cause some nasty overcurrents. :)
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
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