VFD Can they run a motor at 1.5 Hz - Page 2
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  1. #21
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    Quote Originally Posted by MattiJ View Post
    I'd classify ABB's DTC drives pretty much to "very fancy" category. Close to AC servo performance with induction motor. And besides it seems to be made in correct country

    On trade shows they used to have big motor with half meter lever directly on the shaft and you could try arm-wrestling with that "thing". Position hold at zero speed and full torque.
    Aye, but "big motor" is probably nowhere NEAR an option for Peter's needs, though!

    And have a care trying to "arm wrestle" with a large-frame, nominal 3 HP, nominal 23 or so ft-lb Reliance.

    Per Reliance' own White-Paper of some years ago, given sufficient voltage for it, locked-rotor or near-as-dammit has 90 seconds before it "wedges" the average commutator bar from thermal expansion. "slip" is a term not in the vocabulary for a brushed DC motor. Move the load, or die trying, unless protected, upstream.

    The bars on that old 10EE motor are way larger than "average", so 6 to as much as NINE times nameplate torque is theoretically possible for the the overload time I have set on the SSD. I do limit it to 24 A vs the nameplate's 12 and a bit, but still, that's also at a substantial over-voltage, Field as well as Armature.

    There may be sit the difference - 2 X nominal torque being dirt-common - that could have aided the original DC motor on the machine Peter is addressing to have had better ability to overcome inertia, (plus any stiction) from dead-rest to very slow RPM.

    Horses for courses.

    They can all make stuff rotate, just not in the same way in all circumstances.

  2. #22
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    Quote Originally Posted by MattiJ View Post
    I'd classify ABB's DTC drives pretty much to "very fancy" category. Close to AC servo performance with induction motor. And besides it seems to be made in correct country

    On trade shows they used to have big motor with half meter lever directly on the shaft and you could try arm-wrestling with that "thing". Position hold at zero speed and full torque.
    I saw that 25 years ago already
    But can you rotate that shaft 5 dgr and do the same ??? Just wondering

    Peter

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    Quote Originally Posted by Peter from Holland View Post
    I saw that 25 years ago already
    But can you rotate that shaft 5 dgr and do the same ??? Just wondering

    Peter
    AC "torque" motor, yes. No seriously discernable notchiness or cogging atall. Mind small ones.

    Bodine. Punched paper or Mylar high-speed tape tensioning management gadgetry, Comsat Labs contract, NASA application.

    Similar, Bodine again, document feeders, Autopen Signature machines, in-house.

    Not a lot of power involved, but acted like an infinite, and ADJUSTABLE coil spring, did a magical job of of controlling fast-moving tape, or slow, and intermittently cycling fanfold paper - tension, slew, tension, slew.

    Lift door closers, higher power, but usually have an overload timer, buzzer, and quit trying for another timer-set rest period. We simply current-limited for indefinite stall. Curves were published, of course. Still are:

    http://www.bodine-electric.com/Asp/P....asp?Context=7

    http://www.bodine-electric.com/Asp/P...Torque%20Motor

    Other folks would have larger capacity ones. Bodine Brothers never did do "big", only durable.

    That said, I still suspect you are into Servo territory, current state of costs, dynamic range of the need, & common technology. It is what most folks do, quite a long time now, after all.


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    Quote Originally Posted by Peter from Holland View Post
    I saw that 25 years ago already
    But can you rotate that shaft 5 dgr and do the same ??? Just wondering

    Peter
    Afaik can do that also but you would need position sensor. But don't hang on me over that, advanced vfd control is really not my cup of tea.

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    Well I bought the thing and we will see how far we get

    Peter

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    Quote Originally Posted by MattiJ View Post
    advanced vfd control is really not my cup of tea.
    Worth it for design-in of a product line, significant volume or a large machine-hall or process-plant network, I'd guess.

    Otherwise, sufficiently "advanced" VFD has what percentage of the market, and is the price-point even still attractive vs the least-costly of servo drivers?

    Not likely to be a "cheap" critter, is it?

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    Quote Originally Posted by Peter from Holland View Post
    Well I bought the thing and we will see how far we get

    Peter
    You saved me from temptation as the price was dirt cheap for ACS800 drive

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    Quote Originally Posted by Peter from Holland View Post
    Well I bought the thing and we will see how far we get

    Peter
    You probably already have the odd resolver / position sensor about?

    Not my cuppa - no need, but IIRC, VFD manuals indicate they do make possible some rather worthwhile gains.

    DC counterparts, 4Q drives, can enable a full order of magnitude improvement, some DC Drive makers, even more yet.

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    Quote Originally Posted by MattiJ View Post
    You saved me from temptation as the price was dirt cheap for ACS800 drive
    Thats what I imagened. In fact I posted the link only after I did notify the seller I was going to buy it

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    Quote Originally Posted by Peter from Holland View Post
    Thats what I imagened. In fact I posted the link only after I did notify the seller I was going to buy it
    At $335,854.80, I would have to wonder WTF the eighty cents was about, if not also the fifty-four bucks, but.. oh, wait.. Deckel guy, right? And what HP did you just grab?


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    Something up with your maths, to get 3000rpm out of a nominal 1400rpm at 50hz motor you only need just over 100hz depending on slip. A good 4 pole 3 phase motor will happily work north of 150hz, but bare in mind tourque drops off once you go over base speed with a AC motor.

    Sounds like you just need a better drive, but to get good low speed results you need some kinda vector control and if you want great speed control you pretty much have to goto some kinda servo style loop were motor movement is fed back into the power being given to the motor, weather that be a DC servo or a Ac servo or a induction motor.

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    An ACS800 can deliver relatively smooth torque at low frequencies with encoder feedback as can any AC Vector drive.

    With regard to servo positioning performance, the same is true and the best motor for the job is one that has similar inertia characteristics as the load.

    We have used AC Induction motors for feedrate axes on all manner of milling machines and they perform fine.

    However, in any application involving an induction motor with an inverter . . . if you want smooth torque at anything less than the slip speed of the motor, you require feedback on the motor shaft.

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    So we have to find us a puls generator then
    What would the slip speed of a 1400rpm 50hz 400volt 2.2 kw motor be BTW

    Peter

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    Quote Originally Posted by Peter from Holland View Post
    So we have to find a puls generator then
    What would the slip speed of a 1400rpm 50hz 400volt 2.2 kw motor be BTW

    Peter
    100 rpm . . .

    Synchronous speed for a 4-pole motor is 120 x Frequency in Hz / # of poles

    Slip speed = Synchronous speed - Rated speed

    Slip frequency = (synch_spd - rated_spd) / synch_spd x rated frequency

    Attempting to operate a motor using an open loop VFD at less than rated slip frequency without feedback is a crap shoot at best, especially when you have a variable load. The ACS800 does a decent job at lowere frequencies but my experience is that a closed loop (I.e. Encoder on the shaft) AC Vector drive does better.

    Also, when you say pulse generator, for proper FOC control, you need pulse and direction. This is more commonly described as an encoder with A quad B incremental signals or you can use some form of two channel sin/cos signal.

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    Quote Originally Posted by motion guru View Post
    ... you can use some form of two channel sin/cos signal.
    resolvers ? In 2017 ? Nooo, say it ain't so, Joe !

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    Quote Originally Posted by SeaMoss View Post
    resolvers ? In 2017 ? Nooo, say it ain't so, Joe !
    I wasn't thinking resolvers, but rather sin cos encoders . . . like this example -
    Sine/Cosine Absolute Encoders

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    Quote Originally Posted by motion guru View Post
    I wasn't thinking resolvers, but rather sin cos encoders . . .
    I was just kidding ... but you know, with some a-to-d converters ....

    More seriously, resolvers were pretty damned robust - coolant didn't phase them, they lasted a looong time. Is it just that they are analog that people don't like ? Seems like for some things they'd still be better ?

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    We use resolvers all the time in new systems where heat is an issue - they are robust as you say and hard to kill.

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    Quote Originally Posted by Peter from Holland View Post
    Did it turn smoothly at 1Hz or did it stutter I would imagen you could see the rotor jump from one magnetic field to the other every second
    Peter
    No I was amazed at how smooth it was. In fact I had dropped some steel chips on the motor housing. They would slowly stand up, tilt over, lay down, tilt back up, in a very smooth motion that was wild to see!

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    Quote Originally Posted by garyhlucas View Post
    No I was amazed at how smooth it was. In fact I had dropped some steel chips on the motor housing. They would slowly stand up, tilt over, lay down, tilt back up, in a very smooth motion that was wild to see!
    Well....don't forget that commercial motors are not like the simple ones illustrated - or even built - for a school science or physics demo.

    The fields DON'T "jump". One end of the baton being passed is still in the hand of the giver while the other has already been grasped by the receiver. They don't 'snap' it, IOW, they hand it over gradually.

    Think helical gears in constant mesh instead of a pawl of the sort that advances a shaper's cross feed, one "push" per stroke.

    The helical gear has discernable teeth one can count, but they hand-off smoothly, not abruptly.

    Even the involute shape of the tooth on a straight-cut spur gear has some of that 'goodness' vs the shaper feed.

    Serious AC motors are built to do that sort of hand-off - single-phase, most of all. They must do.

    DC Brushes contact more than one segment at a time, and the coils each goes to may be interleaved, not purely sequential as well. Similar interleaved and overlapped hand-off going on, more flexibility in the design of it. A DC motor's "timing" is from its own brushes, locally. It does not have to care about 50Hz, 60Hz. or VFD's selecting other frequencies.

    It switches its OWN reversals internally, and per specific design goals built into coils, brushes, and commutator.

    The DC motor Peter is trying to emulate would have used quite few devious tricks. The sort of AC motor to match those would need far more counterparts of its own to those than a VFD can bestow on any ordinary AC motor that has no hooks to "hang them on".

    Among those "tricks". "Fuzzy" definition of pole-count. The Reliance Type T used in a 10EE clearly and obviously has 4 poles. And then.. it has "interpoles". And two PAIR of brushes. 690 RPM "base" does not "compute" directly in the same way line Hz and pole-count do for AC. It is almost as if it had a pole-count that varied with RPM.

    Which, in fact, exist, too. Motors or generator heads that "print" magnetized areas ahead of themselves onto a rotor to adapt dynamically to RPM changes.

    AC motors have their bag of tricks as well. As with DC, we no longer as often see them because their "offspring" - steppers and servos - have taken over the market for anything "special" a simpler motor plus VFD cannot manage.

    This one looks to be such a "special need", hence the OEM's use of a specialized DC motor.

    One of which I would love to get my grubby paws on, BTW.

    It does not seem to have been an "ordinary" DC motor atall, and I have this mill that has a "pancake" AC motor powering the knee that I'd like to replace with Dee Cee.

    Mind "rapids" were done with gears and clutches, so critical it is not...

    Last edited by Monarchist; 08-22-2017 at 09:58 AM.

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