DC injection Braking of AC motors

Discussion in 'General Electronics Chat' started by hemp, Jan 28, 2010.

  1. hemp

    Thread Starter New Member

    Jan 28, 2010
    Dear All,

    Can anybody help me with circuit for DC injection Braking for 3 phase AC motors or with any application notes??????

  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    I've built a few of these.

    The simplest system is to add an off-delay timer to the 'run' contactor and a 'brake' contactor mechanically interlocked to the 'run' one.

    The coil on 'brake' connects via a normally closed contact on 'run' and a normally open on the timer.

    That setup brings the brake contactor in for a fixed time each time the run one releases.

    For the DC power, I typically use 3 x 500VA toroidal transformers, 230V primary to 2x 24V secondary. These secondaries go to 6 x 35A bridge rectifiers.
    (Primaries wired in 'star' and star point to neutral, for UK 415V 3 phase supply).

    The outputs of the bridge recifiers are all linked to a pair of fuses and from there in to two ways of the brake contactor. the other side of the contactor ways goes to two phases of the motor (on the output of the Run contactor).

    That setup is for an 80 Amp motor. It puts about 150A DC to the motor and brings a motor, gearbox and cutting tool from (eg.) 900 RPM to a dead stop in 3-4 seconds.

    For smaller motors, scale down the size of the transformers etc.

    If using more than one transformer / rectifier, make all link wires from transformers to rectifiers the same length, and make the wires from the recs to the common DC terminals (ie. on the fuseholders) the same length.

    This equalises the current draw from each transformer winding.

    The transformers are over-run to give the brake current, but as long as the stop cycle does not repeat often enough to cause overheating this is not a problem.

    Fuses and interlocking are critical!
    If this is anything to do with stopping power tools etc. where there is possibility of injury, ensure the overall system complies with all the regulations and is properly self-checking against jammed contactors and so on.

    (And be careful working with high voltages & currents).
  3. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    Excellent description. We used a similar system except we only sent DC to two lines of the motor (1 transformer, 1 bridge) and stopped 100HP, 7200RPM direct drive spindles on a milling machine. Spindles were capable of running at 3600 or 7200 RPM. Two timers were used. One for low speed an one for high speed allowing different stopping times. Transformers used strap copper rather than wire in the secondary.
  4. hemp

    Thread Starter New Member

    Jan 28, 2010
    Hi Robert Jenkins,

    Thanks for the description. I tried to tranlate the description onto schematic. pls correct me if im wrong.
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    it's near but not quite; it is difficult to follow from a description.

    The transformers I used have two completely separate secondary windings, with each winding connected to a bridge rec.
    ie. these:

    (Doing it as you show would give the full winding voltage rather than half. You could use just two diodes from the ends of the tapped winding out to positive and the center tap to negative, if you prefer tapped windings).

    All the DC outputs from the bridge rectifiers connect to the same two fuses.

    The Run and Brake contactors both have normally open power contacts.

    The timers I use are the pneumatic add-on block style that clip to the contactor; this type:
    And these for the Run and Brake contactors, fitted with a mechanical interlock kit to ensure they cannot both pull in at the same time:

    You would also need some extra auxiliary contact add-on blocks for electrical interlocking.

    The timer fits on the 'run' contactor, the only power that goes through the timer N/O contact is the coil feed to the Brake contactor. (It also goes through a N/C auxiliary contact on the Run contactor itself.)

    The DC from the brake circuit fuses goes to two power contacts on the Brake contactor and on to two phases of the motor, the third phase is left disconnected while it is braking.

    Hope this makes it a bit clearer,