VFD DC injection and dynamic braking transistors/resistors discussion
Close
Login to Your Account
Likes Likes:  0
Results 1 to 9 of 9
  1. #1
    Join Date
    Mar 2005
    Location
    Northwest Indiana
    Posts
    4,216
    Post Thanks / Like
    Likes (Given)
    845
    Likes (Received)
    847

    Default VFD DC injection and dynamic braking transistors/resistors discussion

    So having recently picked up the 25 H.P. Baldor VFD from my other thread, I've been thoroughly reading the manual and discovered that a series 15H, "E0" drive has no dynamic braking hardware. It will do DC injection braking though. My thinking has been in the direction of just repairing and using the lathe's mechanical brake and not worrying about that.

    However, reading up on the subject has gotten me interested in how exactly it works. There are two terminals at the terminal block labeled 'B+' and 'B-' that I assumed were connected to the DC bus and that seems to be the case, as the voltage reported by the drive as 'DC Bus Voltage' and the voltage at those terminals is the same. There are also two terminals that are related to the braking labeled as 'D+' and 'D-' that I'm not sure about. All four are to be connected to the external braking transistor and resistor, along with their grounds of course.

    How hard would it be to create my own external unit just to do it for the experience? I'm fairly well versed in basic electronics and have created other circuits, repaired plenty of surface mount and through hole electronics, etc. Seems like it shouldn't be too complicated, to simplify it, just have to figure out how to drive the transistor so it activates the output to the resistor once the voltage on the DC bus hits 380V then shut it off after it drops down to 360ish.

  2. #2
    Join Date
    Dec 2003
    Country
    UNITED STATES
    State/Province
    Washington
    Posts
    4,712
    Post Thanks / Like
    Likes (Given)
    2223
    Likes (Received)
    4057

  3. #3
    Join Date
    Mar 2005
    Location
    Northwest Indiana
    Posts
    4,216
    Post Thanks / Like
    Likes (Given)
    845
    Likes (Received)
    847

    Default

    I could, but I wouldn't learn a whole lot... sort of the purpose of the exercise.

  4. #4
    Join Date
    Jun 2001
    Location
    St Louis
    Posts
    18,020
    Post Thanks / Like
    Likes (Given)
    1886
    Likes (Received)
    3037

    Default

    Quote Originally Posted by eKretz View Post
    So having recently picked up the 25 H.P. Baldor VFD from my other thread, I've been thoroughly reading the manual and discovered that a series 15H, "E0" drive has no dynamic braking hardware. It will do DC injection braking though. My thinking has been in the direction of just repairing and using the lathe's mechanical brake and not worrying about that.

    .
    You DO have the dynamic braking hardware.

    What you do NOT have is something to deal with the energy that the dynamic braking dumps back in he DC bus. I suppose that could be a difference without a distinction, but it actually is an issue. The resistor is not needed to DO the braking.... it is needed to avoid the braking causing a shutdown on fast stops or high inertia loads.

    The Bonitron, or their competitors, will cost you several times the amount that rolling your own can. But it is a "guaranteed solution".... I have used them, and they work very well.

  5. #5
    Join Date
    Mar 2005
    Location
    Northwest Indiana
    Posts
    4,216
    Post Thanks / Like
    Likes (Given)
    845
    Likes (Received)
    847

    Default

    No, I dont. The E0 drives have no internal braking transistor, it's an external-only on the E0, so you have to add it on. For dynamic braking you need the transistor to turn on only when the DC bus voltage gets above a certain level, then the energy gets sent to the resistor for dissipation. Afterward, the connection needs to be broken once the bus voltage drops back within normal limits. If I connect just a resistor it will be active all the time on this particular drive because there's nothing switching the connection.

  6. #6
    Join Date
    Dec 2003
    Country
    UNITED STATES
    State/Province
    Washington
    Posts
    4,712
    Post Thanks / Like
    Likes (Given)
    2223
    Likes (Received)
    4057

    Default

    The transistor acts as an analog switch by employing a PWM waveform. As such simply turning it on when the bus is high and then turning it off when the bus is low with a duty cycle measured in msec wont likely give good results and may screw up the inverter current regulator performance. DB resistor circuits generally have a switching frequency of 2 or more kHz with a variable pulse width duty cycle to keep DC bus level under control.

  7. #7
    Join Date
    Mar 2005
    Location
    Northwest Indiana
    Posts
    4,216
    Post Thanks / Like
    Likes (Given)
    845
    Likes (Received)
    847

    Default

    Ah, so it's a little more involved than I was thinking. Thanks for the clarification.

  8. #8
    Join Date
    Jun 2001
    Location
    St Louis
    Posts
    18,020
    Post Thanks / Like
    Likes (Given)
    1886
    Likes (Received)
    3037

    Default

    It IS possible to just switch a resistor in and out as needed, if you set the "off" and "on" points correctly. The PWM is slicker, of course.

    There is also the point that the resistor may need to be monitored, or the duty cycle set, so that it is not over-powered and damaged/"burnt up".

    Quote Originally Posted by eKretz View Post
    No, I dont. The E0 drives have no internal braking transistor, it's an external-only on the E0, so you have to add it on. For dynamic braking you need the transistor to turn on only when the DC bus voltage gets above a certain level, then the energy gets sent to the resistor for dissipation. Afterward, the connection needs to be broken once the bus voltage drops back within normal limits. If I connect just a resistor it will be active all the time on this particular drive because there's nothing switching the connection.

    Yes you do have it. The actual braking is done by the same parts that produce the motor currents to begin with.

    What you lack is the resistor and switch assembly to prevent the dynamic braking from "pumping up" the DC bus too high. For lower rates of braking, the inherent power use of the VFD can handle the braking energy without letting the DC bus get too high, and the braking needs no "dump resistor".

    That's the point... dynamic braking is just a change in the way the IGBTs are switched in time. It pulls the energy out of the motor, and puts it on the DC bus as "returned charge" on the capacitors (that raises voltage).

    In some systems that have multiple motors and VFDs, there is NO resistor, the braking energy from some motors/loads is used by other motors running different parts of the machine. And in still other systems, it is put back on the AC supply line, by a "bidirectional" inverter.

  9. #9
    Join Date
    Mar 2005
    Location
    Northwest Indiana
    Posts
    4,216
    Post Thanks / Like
    Likes (Given)
    845
    Likes (Received)
    847

    Default

    Yes I get all that. I am still missing hardware that is necessary to really use dynamic braking in my application. The resistor and switch are hardware... I do appreciate the elaboration though.


Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •