How effective is VFD braking?
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    Default How effective is VFD braking?

    Iíve got an old Feeler lathe and its spindle friction brake is no longer working.

    Iím thinking that an inexpensive 1 hp VFD can perform braking with an external braking resistor.

    Can a VFD stop the spindle in a reasonable time? I know this is a ďit dependsĒ question, but those of you who use a VFD to brake a motor (on any machine), how quickly does it stop? Approx how many revolutions?

    Thanks.

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    I have a VFD with braking resistor on my 13x40. It stops in like .25 - .50 seconds with a 6" chuck and 1,200 RPM.

    Regards.

    Mike

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    We have a VFD set up on our Mori 20x80 Lathe that can stop the spindle just as fast as the manual brake. You need to make sure that you provide a dynamic braking resistor that has a resistance value that matches the minimum resistance recommended by the VFD manufacturer.

    VFD install on Mori MR2000 Lathe

    We repurposed an old UniDrive Classic . . . that drive is close to 20 years old and still working like new.

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    A VFD can be programmed to stop pretty fast, especially if you fit a braking resistor.

    That lathe isn't going to have anything come unscrewed in a rapid stop.

    But why not just fix the spindle friction brake?

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    You likely cannot really count on how many revolutions it might stop in.

    You have choices. (maybe)

    Regular braking is capable of stopping just as fast as it starts, assuming the brake resistor etc allows that and can take it. Stopping is just acceleration in the opposite direction, and the VFD can supply the same current in either direction, giving the same torque.

    DC braking is very effective, needs no resistor, is hard on the motor if done much, and can even provide some semblance of "holding". While it cannot actually hold like a mechanical brake it can provide a pretty effective drag.

    Some VFDs do provide DC braking. There may be limits on when and how much is provided, and not all that do provide it will provide DC braking current at zero speed ("holding").

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    @JST: So there are 2 kinds of VFD braking? :: Resistor braking and DC braking? Could we call these passive and active braking? This opens another option. Iíll explore this. Thanks.

    @Joe Gwinn: The replacement brake device is US$900 and labor is probably the same to R & R it (itís cast as a part of the motor base, not easily removed). VFDs are as little as $150 plus $50 for a resistor and a few $100s to set up.

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    Quote Originally Posted by Billbo View Post
    Iíve got an old Feeler lathe and its spindle friction brake is no longer working.

    Iím thinking that an inexpensive 1 hp VFD can perform braking with an external braking resistor.

    Can a VFD stop the spindle in a reasonable time? I know this is a ďit dependsĒ question, but those of you who use a VFD to brake a motor (on any machine), how quickly does it stop? Approx how many revolutions?

    Thanks.
    Go Ogle"friction material suppliers"

    Rebuild your existing spindle brake.

    If only because it is "already there".

    Also the right fit to the task. Already.

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    Quote Originally Posted by thermite View Post
    Go Ogle"friction material suppliers"

    Rebuild your existing spindle brake.

    If only because it is "already there".

    Also the right fit to the task. Already.
    Not an economically viable choice. VFD is 1/10 the cost of labor to repair the brake.

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    I repaired the brake on my HLV AND added vfd braking resistor. It stops very quickly on the VFD brake but the friction brake is not nearly as effective. I have two VFD's with DC braking (the HLV also has it) and both of them default to 0.5s (when enabled) with a maximum of 5 seconds I think, along with a maximum frequency it gets invoked at (which is not very high). I think the DC brake is simply used to stop the motor when the spindle speed had dropped so low that the braking resistor's effectiveness is minimal.

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    Quote Originally Posted by Billbo View Post
    Not an economically viable choice. VFD is 1/10 the cost of labor to repair the brake.
    Is the Feeler a Hardinge copy? Then you are talking about a little wine bottle sized cork brake pad, actuating arm, solenoid, and control contactor in the main cabinet. All of that is easy to fix.

    The plastic hand wheel is what I use sometimes. Press fingers into the inside of the wheel and there is no danger whatsoever.
    Your decision to go with a VFD will require direct connection to the motor and reconfiguring the electrical cabinet and in my opinion it's not worth it just for
    alternate brake.

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    Quote Originally Posted by Billbo View Post
    @JST: So there are 2 kinds of VFD braking? :: Resistor braking and DC braking? Could we call these passive and active braking? This opens another option. Iíll explore this. Thanks.

    @Joe Gwinn: The replacement brake device is US$900 and labor is probably the same to R & R it (itís cast as a part of the motor base, not easily removed). VFDs are as little as $150 plus $50 for a resistor and a few $100s to set up.

    It's not "resistor braking". It is really just doing counter-torque that slows the motor. In the process, the energy has to go somewhere, and the "somewhere" is raising the voltage on the DC bus, because the process essentially is "generating output" because the effective applied VFD voltage is less than the back EMF*. The energy "output" comes from the energy stored in the rotating parts. The resistor is just a place to "send" that excess energy, by discharging the bus into the resistor. At lower rates of slowing, the resistor may not be needed, the energy used by the VFD is enough of an "energy sink".

    DC braking is different in where it sends the energy. By putting a DC current in the windings, a large current is generated in the rotor bars. The energy ends up dissipated as heat in the resistance of the rotor bars, The rotor is not as good at getting rid of heat as the resistor, which may have a heatsink and fan, so it is tougher on the motor if done very much. it can also be a fairly violent slowing. And the rotor loses heat less well as it slows, you can really cook it if you overdo the DC braking.

    You can see that action if you just put a reasonable current through the windings of a motor, just one winding is enough, and then try to turn the shaft. There will be a significant drag, and it will be more the faster you try to turn the shaft.

    * The chopping action of the VFD waveform acts similar to a "boost converter" and "pumps up" the DC bus.

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    Quote Originally Posted by Billbo View Post
    Not an economically viable choice. VFD is 1/10 the cost of labor to repair the brake.
    Engaging a Chinook to haul-home a damaged Huey-bird didn't take a great deal of labour, EITHER.

    You have saner options?

    Not as if it was the massive brakes on a Caterpiggle D9G 'dozer, now, is it?

    I've got a 6 hoss "Four Quadrant" DC Drive on a 3 hoss 10EE that is happy with overrides to stop the bugger right quick - or shake a can of paint.

    Even so.. the Cazeneuve's all-mechanical brake is handier, all-around.

    No more complicated than a foot-bar, linkage to pull the free end of a length of re-purposed Vee belt cutoff - t'other end clamped down, into a tight wrap - or NOT - around about a quarter of the circumference of a pulley.

    Flat belt cut-off can work exactly the same way as Vee. It ain't no 747 on a short runway.

    Toe on that bar, your spindle is near-as-dammit "locked" for twisty-tasking, such as changing threaded goods or simply preventing movement on D1-3 keying, chuck key torque-ing, winding-up nose-cranker 5C collet goods, actioning RubberFlex impact hammer rimwheel, or Sjogren geared-wheel collet closers.

    Need varying degrees of braking aggressiveness or quick re-positioning to the next keyhole? Just move the toes according, same as driving of parking a motorcar.

    Even works exactly the same with an E-Stop tripped... or no power applied to begin with.

    Try any of THAT with a Vee Eff Dee?



    If you want a VFD, put a VFD on it.

    Your shop, your rules, your budget. No excuses needed. They CAN add some neat and handy features for relatively small money.

    But you'll not "justify" a VFD on the basis of braking.

    Ever.

    That is one thing they do NOT do as well as "all of the above" as to other options.

    No foul.

    It is simply in the basic nature of the way they get their input power served-up vs a direct on-the-grid INHERENTLY bothway-power-flow "4Q" DC Drive.

    Fully regenerative DC drive can utilize half of North America as the ultimate "braking resistor" pack, free of charge.

    Also already taking up money and space OUTSIDE my shop's walls.

    Even so? I prefer those mechanical brakes. Clutches, too, when and where I have the option.

    "Control freak", does that make me, then?

    ME?

    Whom, ever, wudda thunk it!


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    I have installed numerous VFDs on lathes that did not have a mechanical brake, and also many that do. The VFD on smaller lathes as others have indicated can stop the spindle in ~1 second, so is advantageous in many settings where a manual brake is less convenient and less predictable. There are limitations as you gain more momentum/energy (bigger chucks/higher RPM) in the system as to the braking time and in some cases it will trigger an over voltage buss error which will trip some VFDs into a free run mode/coast to stop, not good. Many low cost VFDs do not support an external braking resistor which can significantly effect the stopping time. Better VFDs support a number of different program variants which will allow maximum electronic braking without triggering an over voltage error. DC injection is usually used at lower Hz range, and more effective with high momentum systems coming to a halt. At least in my experience on lathes in the 12-16" swing I do not routinely use it.

    As far as stopping rate/revolutions (getting back to your original question) if you figure say 60 RPM with 1 second braking then you are talking ~1 revolution, 600 RPM ~10 revolutions. The one significant factor when using a VFD is the stopping predictability/position, I use an electronic trigger stop system as limit stops in many of my systems,the chuck/DRO position will repeat to +/-0.0002" in most cases per pass given the same speed/feed. I have done many manual brake lathe VFD systems, in almost all cases the brake is used as an emergency stop or to statically hold the chuck, otherwise the electronic braking of the VFD is used. I also provide different selectable acceleration/deceleration rates, so under routine use you are not banging the lathe to a stop. E-Stop always overrides this for the maximum braking. So a few things to consider.

    So hopefully this addresses your questions as to the viability and predictability of using a VFD for lathe braking. Regardless of what others say, it is up to you to decide the cost/benefit to your application. The added benefit is adjustable speed on the fly if you do not have a vari-speed mechanical system. There is an added complexity/cost of the VFD install and control safety systems, it is not an inexpensive proposition to do a proper install.

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    Quote Originally Posted by mksj View Post
    So hopefully this addresses your questions as to the viability and predictability of using a VFD for lathe braking.
    Well no. It only confuses the choice for a smallholder with ONE machine.

    We have, "right here on PM" covered the heavier INDUSTRIAL situations already.

    There's tons of those out there on utility-mains true 3-Phase with serious money invested in serious VFD selection and application engineering to share multiple VFD in a "cell" or "local grid" off their DC bus.

    Two things are key:

    -first, there is MONEY to be saved in engineering all that. We are talking conveyor lines, winders, all sorts of process industry that is run 24 x 7 for years on-end where even the power bill, better utilized than otherwise is serious coin.

    - Hard to do that in even a fair-sized machine shop. Few other loads to put into any sort of sharing pool. Too much need of random use or independent use, too few the opportunities to synchronize operations and the power they need.

    First, scratch off regeneration for braking.

    Second: A small Feeler lathe where the OP sez the VFD he needs is CHEAPER than one TENTH the labor of repairing, not "inventing", an already existing brake?

    That doesn't sound like it even justifies the higher cost of a VFD that can utilize external braking resistors plus the cost of buying and implementing and safely mounting the resistor pak so it HAS them to use.

    What he'd have "right out of the box" is only what his local capacitor bank can absorb inside the - he did say inexpensive? - VFD.

    Not much there on a small and low-cost VFD already stressed to convert 1-P to 3-Phase, is there?

    What you can do with a VFD is not what HE can expect "right out of the box". Poor braking and/or faulting and free coasting is what he can reasonably expect.

    Unless he spends more money on a better VFD and/or does more engineering.

    His call if he wants to do that, but TANSTAAFL.

    "Cheaper than 1/10th the labour to fix the brake" it will no longer be.

    If even it really was, when fairly evaluated. About which there is "reasonable doubt" anyway.

    Not as if it were a 1930's Baruffaldi gone unobtanium after all.

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    Quote Originally Posted by thermite View Post
    But you'll not "justify" a VFD on the basis of braking.
    As others have mentioned the brake on this lathe is pretty simple. Not to put too fine a point on it,
    if one cannot fix that so it works well, then there's little chance a braking resistor equipped VFD could
    be successfully installed on it.

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    Quote Originally Posted by thermite View Post
    Well no. It only confuses the choice for a smallholder with ONE machine.

    We have, "right here on PM" covered the heavier INDUSTRIAL situations already.

    There's tons of those out there on utility-mains true 3-Phase with serious money invested in serious VFD selection and application engineering to share multiple VFD in a "cell" or "local grid" off their DC bus.

    Two things are key:

    -first, there is MONEY to be saved in engineering all that. We are talking conveyor lines, winders, all sorts of process industry that is run 24 x 7 for years on-end where even the power bill, better utilized than otherwise is serious coin..............
    Evidently you only like to pontificate endless nonsense according to the world of "Thermite", nor did you read the last line of my post. He already indicated that he felt the cost of the brake repair would be much greater. Why don't you stick the the question he asked and provide something constructive and tangible, and let the OP decide what he wants to do. I have done hundreds of small lathe VFD systems, they can be done relatively inexpensively and there are numerous other benefits that can be achieved. He is not asking about mega machine VFDs in this situation, nor dealing with a 10ee lathe.

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    Quote Originally Posted by mksj View Post
    Evidently you only like to pontificate endless nonsense according to the world of "Thermite", nor did you read the last line of my post. He already indicated that he felt the cost of the brake repair would be much greater. Why don't you stick the the question he asked and provide something constructive and tangible, and let the OP decide what he wants to do. I have done hundreds of small lathe VFD systems, they can be done relatively inexpensively and there are numerous other benefits that can be achieved. He is not asking about mega machine VFDs in this situation, nor dealing with a 10ee lathe.
    Not MY fault he is right more often than wrong, so why not? Tell yah a secret as to WHY? The old fossil CHEATS. It ain't "his" knowledge. He reads the manufacturer's MANUALS. They're FREE for the download, after all.

    When you claim performance figures waaay better than a dozen major makers the PM community uses a lot of - and often discusses - better than figures printed right in the maker's OWN specs?

    That ain't "magic smoke" yer blowin' up our collective virtual arses.



    Guess you were too busy firing up yer smoke generator to have the time to READ what I and several others have posted that WAS "constructive and tangible" - even economically sound?

    "Evidently" you didn't pay a lick of attention to a VFD that is meant to cost ONE TENTH that of a simple mechanical brake repair?

    18 dollars is that? Or only NINE bucks? Under twenty dollars, that "1/10" is.
    USED "Huanyang" alleged-VFD price level, and only if it is KNOWN failed already?

    Or was he planning to hire a neurosurgeon with a whole operating theatre of support medicos to fix the silly brake?

    Brake cork assembly for Feeler FTL-618EM (L-6206/6206A) Lathes – Eisen Machinery Inc

    How hard can it be to glue new cork onto the existing metal holder? Figure it might take all of an hour? TWO hours? Figure salvaged cord-in recycled tire doormat might be even better than natural cork? Surely grips well enough to stop a Peterbilt even on steel plate. See also Huarachi sandals. Ga-ron-tee yah natural cork will char and burn easier than tire-tread.

    Helluva lot less energy stored in a Feeler than you have in your "VFD RULES" crusade, yah?


    As said ..yah WANT a VFD? Just BUY it for what it IS good at. Fix the brake as well.

    Don't try to rationalize the VFD on "avoidance" of neglected-component maintenance NOT side-issues.

    Can't sort a brake as SIMPLE as Hardinge and Feeler used? Best avoid WHEELBARROWS, then. You'll never pass the CDL driving test.

    Now .. can't change a cork, how d'yah then manage as many distinct but TERSE numeric codes as may be needed to set up a VFD with its keypad?

    Need fewer than a dozen? Truly. Or sometimes even less.

    But WHICH dozen out of around two HUNDRED?
    Last edited by thermite; 11-29-2019 at 02:39 AM.

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    Default Very effective IMO

    I have old Allen-Bradley 1305 and 1336 drives on my vertical mill, lathe, radial drill, and concrete power trowel, and I have dynamic braking AND DC Injection Braking programmed into all but the trowel, and they're incredibly effective. I move the lever to STOP, and they stop NOW!

    As others noted, dynamic braking is regenerative, so you'll need the braking resistor appropriate for your drive setup. once the motor gets down to a certain speed, dynamic braking falls off, and on my A-B's, they ramp up DC injection, which brings the spindle to a very firm stop.

    Dynamic braking curve has several parameter settings, and I don't recall what they are, but it allowed me to set the slow-down ramp speed from very minimal, to very aggressive.

    DC Injection Braking had level and duration adjustments. Once I had the dynamic braking adjustments set, I identified the DC injection level and time, and I've ignored them ever since, but when I flip it to STOP, it's an immediate thing.

    My 10EE has a D1-3 spindle nose, so the chuck cannot 'unthread', but I HAVE had some things threaded on that have become loosened up (but fortunately, not thrown) by the drive's ability to change speed rapidly.

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    We ran the electronic braking on one of our big American lathes (~24" swing). When first installed with the default settings it stopped the lathe so fast that the work was spinning in the chuck while braking; so yeah, it's pretty damned effective.

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    Quote Originally Posted by DaveKamp View Post
    I have old Allen-Bradley 1305 and 1336 drives on my vertical mill, lathe, radial drill, and concrete power trowel, and I have dynamic braking AND DC Injection Braking programmed into all but the trowel, and they're incredibly effective. I move the lever to STOP, and they stop NOW!

    As others noted, dynamic braking is regenerative, so you'll need the braking resistor appropriate for your drive setup. once the motor gets down to a certain speed, dynamic braking falls off, and on my A-B's, they ramp up DC injection, which brings the spindle to a very firm stop.

    Dynamic braking curve has several parameter settings, and I don't recall what they are, but it allowed me to set the slow-down ramp speed from very minimal, to very aggressive.

    DC Injection Braking had level and duration adjustments. Once I had the dynamic braking adjustments set, I identified the DC injection level and time, and I've ignored them ever since, but when I flip it to STOP, it's an immediate thing.

    My 10EE has a D1-3 spindle nose, so the chuck cannot 'unthread', but I HAVE had some things threaded on that have become loosened up (but fortunately, not thrown) by the drive's ability to change speed rapidly.
    Not to forget. You are dealing with seriously "veteran" and overbuilt VFD that were the Caterpiggle bulldozer of their era. Similar heavy-lifter, fully-featured VFD certainly exist yet, today, but.... They do not come cheap. Only serious industrial users with serious equipment end up applying them "from new".

    "The problem is..." VFD are nowhere near "all alike" in implementation. Only in concept.

    So then yah get the penny-dreadful dilletantes' expecting that their entry-level priced commodity or even loss-leader priced VFD can do all the same stuff as a top-end, top priced, heavy industrial model.

    Well it cannot.

    And you'd spend more trying to ADD what it didn't ship with than it would cost to buy an industrial-grade VFD that DID have all the good stuff and even covered it to specified capability under a decent warranty.

    4Q DC Drives have the advantage. Cheap or dear, they at least ALWAYS have the local grid to dump directly into.

    Downside? Decent machine-tool suitable DC Motors can cost the very Earth.

    TANSTAAFL

    Mechanical brake ain't a BAD idea even IF you have the best electricals, ever.


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