Some considerations on AC Motor braking resistors

Discussion in 'Electrical Engineering' started by Ignacio Simón Yarza, Jul 18, 2005.

  1. Hello,
    I will pipoint some issues that may clarify the facts:

    1) VFD FOC techniques try to get equivalent behaviour to inverse braking but
    in such a way that differential speed between stator and rotor magnetic
    fields is controlled. A lagged field which makes rotor speed slow down is
    produced on stator windings. But then, Where does the kinetic energy goes?
    2) These rotor and stator magnetic fields result in two differential
    voltages. The difference between them yields a voltage vector which
    generates a current flowing back into the VFD. This is an energy returning
    path which accounts for the dissipation of kinetic energy on the rotor axle.
    3) The additional point to be clarified is why using this braking technology
    instead of using a magnetic field acting proportionally to axle speed?? just
    for the same reason for which VFD are used instead of direct feeding of
    motor. As FOC theories allow treating AC motors as if they were DC
    control-prone motors, so does VFD braking. In case Thyristor bridge is well
    dimensioned, any braking curve may be obtained. On the contrary, DC braking
    is cheaper but on its basic development it only allows a damped braking,
    with a counter-force proportional to speed, and a mechanical brake works
    always at the end of the braking cycle.
    Best Regards.
    Ignacio Simón Yarza.
    Mech eng
    Electronic and automatism eng

    "Bob" <> escribió en el mensaje
    > Actually, most VFDs do both. Dynamic Braking, i.e. using the braking
    > resistors, is a good way to bring the motor down CLOSE to a stop, but
    > as the motor slows, the energy in it becomes less andless, so the
    > braking energy that can go into the resistors becomes less as well. So
    > at some point, the law of diminishing return takes over, and you have
    > to finish the job using DC injection braking.
    > On Wed, 6 Jul 2005 18:47:57 +0100, "Ted Rubberford"
    > <> wrote:
    >>"Fred" <> wrote in message
    >>> Correct me if I'm wrong but I believe that AC motors are braked by
    >>> introducing a DC current across the stator windings??
    >>> Fred

    >>Read the post again boy, "motors controlled by a VFD"
    >>Teddy Rubberford.
    >>Bizarre Bugger...
    >>> <> wrote in message
    >>> news:...
    >>>> The only place for motor energy to go is into the load or into the
    >>>> resistor. To slow the motor the drive the energy has to go somwhere,
    >>>> its only path is back through the IGBT or similar device back to the
    >>>> DC bus, where a resistor bleeds off the excess energy to keep the DC
    >>>> voltage under control.
    >>>> The AC/DC converter on the front end is different for a regenerative
    >>>> AC drive. SCRs vs. diodes. It lets you shut off or otherwise control
    >>>> the incoming power to control the braking.
    >>>> On Tue, 05 Jul 2005 23:58:49 GMT, "Dave" <> wrote:
    >>>>>Hello, can someone give me a explanation on how braking resistors work
    >>>>>motors controlled by a VFD?
    >>>>>Thanks, Dave


    Ignacio Simón Yarza, Jul 18, 2005
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