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Thread: VFD Braking Resistors - another question.

  1. #1
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    Default VFD Braking Resistors - another question.

    There was a recent thread on VFD braking resistors, but Ill start a new one.

    Im probably going to reveal a whole lot of ignorance of VFDs by asking this question, but here goes.

    Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Lets ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

    Ian.

  2. #2
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    That sounds like a question for the manufacturer.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  3. #3
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    Quote Originally Posted by IanPendle View Post
    There was a recent thread on VFD braking resistors, but I’ll start a new one.

    I’m probably going to reveal a whole lot of ignorance of VFD’s by asking this question, but here goes.

    Using a VFD, you can slow and then stop the motor by setting the ramp down duration, or use a braking resistor or use DC injection. Let’s ignore the DC injection method. If I set up the VFD to use a braking resistor (assuming I have set the parameters correctly for the breaking resistor use), does this override the ramp down settings? By this I mean, will it stop the motor by using the braking resistor only, or does it wait until the ramp down period has finished and then switch in the braking resistor?

    Ian.
    No and No.

    It will stop the motor according to set ramp "rate" by dumping the "braking" energy to the brake resistor. Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor.
    (Motor will act as a generator during dynamic braking and the generated energy has to go somewhere. Either brake resistor, backfed to utility network(only fancy expensive models) or used up by VFD losses and partly stored on DC bus capacitors)
    Last edited by MattiJ; 04-16-2018 at 04:09 AM.

  4. #4
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    Hello Matti,

    OK, understood.

    "......Without brake resistor its not possible to use as fast ramp as you would be able when braking with resistor......"

    That sentence makes it all clear.

    Thanks.

  5. #5
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    That's pretty much it- you need a resistor for either a heavy rotating mass, or to stop very quickly (or both.)

    For example, my little Logan and Sheldon both are set for sub-2-second ramp down. I don't need a braking resistor, they seem to tolerate those speeds just fine, even with decently sized chucks and workpieces.

    My big Springfield, however, if or when I convert that to VFD, will likely need a big braking resistor if I wanted to stop it under 2 seconds- especially if I had a big workpiece in it. But, if i dialed that back to a 3 or 4 or 5 second ramjp-down, said resistor likely wouldn't be needed.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  6. #6
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    My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

    If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.

  7. #7
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    DC injection works very well.

    The energy from the stop stays in the motor, it appears as heat in the rotor.*. so if you use it to stop very often, the rotor, and eventually the rest of the motor, will get considerably hotter than usual, which is generally bad.

    * During the stop, large currents are induced in the rotor, which is essentially a short circuited winding. The heat comes from the resistive loss in the rotor, due to the (small) resistance of the rotor "windings". The currents are larger than the normal running current.
    1601

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    Quote Originally Posted by CCWKen View Post
    My BP would error out on 2K rpm and above with a four second stop without a resistor. I just ignored it and pressed reset. With the resistor, I can't get below two seconds at 1200 rpm. I still have to tinker with the settings a little after understanding the language. By the way, my resistor didn't even feel warm to the touch after 7-8 timing experiments. But the weather has been cool lately.

    If DC Injection is so bad, why are all the major YT players using it? I see (and hear) it used a lot.
    Downsides of DC injection is possible overheating of the motor and limited braking torque. Good VFD +resistor set up correctly should be able to slow down the motor with dynamic braking in same time as it takes to speed it up = pretty fast.
    I think DC braking is perfectly fine for applications with low/limited inertia and limited amount of starts per hour. Like a mill or most lathes. High rpm lathe with large steel chuck could be pushing your luck.
    Dynamic braking excels when you have 60 ton load on a crane and you need to lower it 30 meters down.

  9. #9
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    Is there any problem using the manual brake to assist stopping at higher rpms or with a heavier tool such as the Suburban fly cutter? Or will that foul the VFD's logic?
    Last edited by CCWKen; 04-16-2018 at 01:43 PM. Reason: Corrcted the "fowl" language. :)

  10. #10
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    The VFD slows by reducing the frequency at the selected rate. That makes the motor "generate", because it is then turning above synchronous speed, and as a result it puts energy back onto the DC bus, raising the voltage, and removing the energy fom the rotating load, slowing it.

    All the VFD is doing is reducing frequency, and consequently, motor speed. If you use another means to slow the machine faster than the VFD does, the VFD may actually start to DRIVE the machine against the braking, which defeats the purpose.

    Use one or use the other.
    Last edited by J Tiers; 04-16-2018 at 12:27 PM.
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