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Thread: VFD Braking Resistors - another question.

  1. #11
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    I installed a bicycle disc brake on my motor pulley and setup a pedal actuator with the vfd set to freewheel stop. Normal stop I just lightly press the pedal to actuatte the stop microswitch and the motr coasts to a stop.
    Helder Ferreira
    Setķbal, Portugal

  2. #12
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    With many VFD you can program them to use dynamic braking (resistor or external braking unit) AND foot brake. When you press the foot brake it overrides the dynamic braking and simply freewheels (coasts) to a stop under mechanical brake control.

    My prior big lathe had an electro-maganetic brake - superb, and stops on a dime (unlike a vfd without DC injection). My current lathe has a foot pedal bar. I like the later but miss the electric brake. When I get around to converting this lathe to VFD, I'm going to implement both, and if I can get a vfd that will allow it, add DC injection at the last part of the dynamic braking cycle to make for a positive stop. The trick is to turn OFF the DC injection when rpm falls to zero, not just inject for defined fixed period. And... thermistor sensing (inside the motor) via the VFD to avoid overheating of the motor.

  3. #13
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    If you can set an over-ride, then obviously there is no issue, you never have both at one time.

    DC injection is stupidly effective at high speeds. At lower speeds it becomes less effective, but despite common warnings, it is pretty effective even at speeds that you can turn the motor just with your hand on the shaft. Try it sometime, and you will see.

    the one thing it really does not do, though, is a "positive stop". Zero speed is zero braking.

    And the DC that is injected is generally not enough to do much heating. The heating is actually of the rotor conductors, which heats the rotor, and indirectly, the stator and frame. I have never even tried to see what FLA as DC would do in terms of braking.... certainly a heck of a lot... Even a few hundred milliamps of DC does a very significant job of braking.
    Last edited by J Tiers; 04-16-2018 at 10:31 PM.
    1601

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  4. #14
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    Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.

    My usage is all on various (three) Hitachi VFD. Hitachi specifically recommends that you use a motor-mounted Thermistor in conjunction with DC braking, so they are concerned. The problem I ran into was that it was not possible to define that the DC injection finished at zero rpm, only for a defined time and braking time varied, so often the rotor was "locked" for a short (or long if programmed as such) time after stopping. Maybe the latest VFD have more control.


    For the rest of the audience,
    http://electrical-engineering-portal...-braking-motor

    http://www.machinedesign.com/technologies/give-me-brake

    https://en.wikipedia.org/wiki/DC_injection_braking

    Toshiba note for DC injection implementation

    https://www.toshiba.com/tic/datafile...aking_6551.pdf
    Last edited by lakeside53; 04-16-2018 at 11:40 PM.

  5. #15
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    Quote Originally Posted by lakeside53 View Post
    Hmm... if you have a VFD available, play with DC injection. Unlike dynamic braking, DC injection is a superb at low speeds (and yes, can be very aggressive at any speed) - a very positive gear slamming stop - and it literally locks the rotor in place if left applied (bad idea). I'd have been happy if it "gave up" at zero speed, but without feedback it doesn't know what zero is.
    This is where different manufacturer literature (and probably actual implementations) seem to differ. (and I can't try it out on my VFD's as ABB DTC drives have only dynamic braking and flux braking(what yaskawa probably calls high slip braking))

    Yaskawa is talking about less than 100% nominal torque during dc braking
    https://www.plantservices.com/assets..._SlipBreak.pdf

    But Google finds me for example this:



    So the most of the dc injection braking torque is available at relatively low speed, around 10-20% nominal and full speed braking torque is much less.

  6. #16
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    I see the graphs but have no idea what they mean. What is a unit? And who runs a mill motor at 10-20% That's like 6-12hz for a 60hz motor.

    When I'm on low pulley and back gear, the spindle is turning ~80 rpm but the motor is still turning at 1755 rpm at 60hz. How are all the guys power tapping on YT getting a quick stop? That's what I'd like to know. The graphs don't help me.

  7. #17
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    The graph actually does make sense...... I had to remember the basics here.....because it seems a bit counter-intuitive.

    When the RPM is high, the "slip frequency" of the DC braking is high, and the rotor current is less due to inductance of the cage conductors being a higher impedance at that frequency. That reduces braking.

    At the same time, the voltage goes up, so the curve is not linear.

    As the motor slows, the generated voltage is less, but the impedance is also dropping. At some point there is a maximum current point. Below that, the voltage is low, and the current again drops off.

    However, don't let it fool you, the effect of DC braking can be pretty violent from high speed even so. And it can be very noticeable at low RPM also.
    1601

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    Hashim Khan

  8. #18
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    On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

    It would seem that using the same combination on a lathe would work well there too.

    Dan
    Measure twice. Cut once. Weld. Repeat.
    ( Welding solves many problems.)

  9. #19
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    Quote Originally Posted by danlb View Post
    On a semi related note, my hybrid car uses regenerative braking. It uses a similar concept to brake by using the electric motor as a generator. The generated DC goes back to the battery pack. The deceleration is very predictable right down to the last few MPH. That's when it switches over to mechanical disc brakes. From what I read, they did not go with DC injection for that last part because of overheating fears.

    It would seem that using the same combination on a lathe would work well there too.

    Dan
    With the VFD that is in the car, they can program a very good braking curve. Standard VFD braking is not a heat issue because it is basically just running the motor in a normal way.

    DC injection is really "eddy current braking", that induces currents in the rotor conductors. That energy goes into I^2 * R losses, and ends up as heat in the rotor. If you beat on a motor hard enough, you could actually melt the aluminum conductors. It would take a bit of doing, but is possible.
    1601

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    Hashim Khan

  10. #20
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    I found it interesting that the Toshiba link I provided mentioned using DC injection it to drive moisture from the motor.
    Last edited by lakeside53; 04-17-2018 at 10:36 PM.

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