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Thread: Hembrug lathe on the way.
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1st Feb 2017, 06:24 PM #16Diamond Member
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I don't know how expensive they are but could you get a 2 speed motor? 2/4P would get your top speed up and then use a VFD to get the speed slower. Why do you need high torque at low speed? what do you call low speed?
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1st Feb 2017, 06:46 PM #17Most Valued Member
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1st Feb 2017, 11:03 PM #18Most Valued Member
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I dont need 13 rpm but i am really going to be using this lathe to do all my screwcutting and I normally like to do it down at the slow end of things, the original 45 RPM would be great. A two speed motor would be good but it will only get me back the high end of things (two speed motors are normally 1440, 2880 i believe) as someone else said the 720/1440 motors are mich harder to come by it seems. Using a VFD will definitely do the job so ill just keep an eye out for a multispeed motor.
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2nd Feb 2017, 01:50 PM #19
My son-in-law is a motor rewinder, so I asked him how 3 speed motors are wound, he wasn't sure, the only 3 speed motors he has rewound in the past were crane motors which have extra windings for torque control. The dahlander wound 2 speed motors are as I described earlier, so, if you are shopping for a new motor, the preference would be for a 4 pole 8 pole dahlander wound motor, which is 720/1440 rpm, most 50 Hz motors will run up to 100 Hz, and that gives you the 2880 top speed, also worth remembering that high speeds usually mean small diameters, so you generally don't need as much power as you do at lower speeds.
Hope that all makes sense. A two speed 720/1440 motor with VFD would be your best choice. What little I know about Hembrug lathes, are that they are worth spending that bit extra, you will end up with a first class machine.
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2nd Feb 2017, 01:56 PM #20Most Valued Member
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RayG, thank you from the cockles of my weary heart, that is exactly the piece of terminology i needed to find the right motor. Im unsure how driving a two speed motor is going to go with a VFD but we shall see. Im assuming the motor speed can be changed in motion?
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2nd Feb 2017, 03:24 PM #21Most Valued Member
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PeteF went through this exercise with his Aciera, IIRC, and went to a single speed motor with VFD. I'm 90% sure you can't swap motor speeds on the fly if it's running via a VFD without letting the magic smoke out but I could be wrong.
If you look at the typical dual speed motors there's quite a bit of power/torque difference between the speeds. The motor I *don't* have for my baby mill was a 1.9kW/1.1kW motor at 1440/720 rpm.
So investigate fully as it might be necessary to wire some sort of interlock to prevent swapping motor speeds while it's running, which I do quite happily with my Arboga drill press.
FWIW some Harrison lathes and most Kearns S type horizontal boring mills have 3 speed 3 phase motors fitted. Good luck getting a used one though.
PDW
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2nd Feb 2017, 03:36 PM #22
The problem most people experience with dahlander wound motors is the conversion from 415 to 240, and end up doing what PeteF did and put in a single speed motor, however if you have 3 phase 415, it's a different ball game, just wire the VFD upstream of the motor switching, most VFD manufacturers recommend against downstream switching under load, I've never found it to be a problem, but best if you don't do it.
The problem is that as soon as the motor windings are disconnected from the drive, while the motor is spinning, during the time it takes the flux to decay, the motor acts as a generator, so a delay long enough for the flux to decay is needed before re-connecting to the drive output stages. If you stop the motor between changes, that's all you really need to do.
As PDW says the difference in torque between a 720 RPM motor driving a gearbox and a 1440 motor on a VFD is substantial.
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2nd Feb 2017, 05:44 PM #23Most Valued Member
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Sooooo - if I'm understanding you correctly (I do have 415V 3 phase) I can wire a VFD between wall socket & motor speed selection switch, power it all up and switch between high/low speed while the motor is spinning? And not blow up my VFD?
Because, if so, perhaps I can plan on ditching the Kopp variator from my Chipmaster after all. The Chippie has a 10:1 mechanical speed reduction and the Kopp gives another 10:1 (more or less) giving speeds from 30 to 3000 rpm. Mostly I can live without the really high speeds as I can't recall the last time I cranked the lathe over 2000 rpm.
If I have to stop the lathe motor first, that probably isn't a show-stopper either. It'd be pretty rare you'd want to instantly double (or halve) the spindle speed. The problem there might be remembering to stop the motor at the VFD before switching speeds. Interlock of some type.....
Mind you I'd still have to find a 4HP or better 2 speed motor, which might be a challenge.
PDW
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2nd Feb 2017, 06:35 PM #24
Correct, over the last 20 or so years I've done probably 100+ pump control systems that run 5 or more pumps off a single VFD and switch from VFD to DOL contactors on the fly, we have never zapped a VFD in all that time, but there are some tricks in the control software. As well as mechanical interlocks stopping the VFD output bus being switched onto the DOL bus.
First you need a delay after the DOL contactor opens, to let the motor flux decay, usually 250 -400 ms is more than enough. Second if going from DOL to VFD, yhe VFD need to be configured for spin start rather than ramp start and the acceleration cranked up high for a few seconds. The spin start allows the drive to search from Max Hz downwards rather than from Min Hz upwards and locks in quicker.
On the lathe, you can do whatever you like downstream of the VFD if it's not running, the issue is not what the VFD is doing its what the motor is doing at that instant you re-connect. Of course if the motor is stopped there is no problem.
Even if the motor is still running and spindle still spinning, you can still switch in the VFD if you do the tricks outlined above.
When you go above 22kw then we use soft starters rather than DOL contactors,
Ray
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2nd Feb 2017, 07:41 PM #25Diamond Member
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2nd Feb 2017, 08:23 PM #26Senior Member
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No VFD, changing motors for me. My Elliot Omnitool is, 3 phase 48 to 4000 all steel chucks, 50 mm spindle bore. Lights dim.
cheers
Bruce
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2nd Feb 2017, 09:25 PM #27Most Valued Member
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That'd be a decent sized drill bit, all right. Biggest I have is 2" IIRC.
It's kind of interesting that the various lathe manufacturers took different approaches to getting a wide range of speeds with appropriate amounts of torque. The problem a lot of home shop people or those with limited money to throw have is replicating the speed range if for some reason the original system has gone tits-up (or just gone).
The Monarch graft of a big servo to the back gearing seemed to be a really good approach, but spendy. I've eyed off some of the industrial sewing machine servo motor systems wondering if I could use one, but they're not really powerful enough. I know nothing about the industrial servo gear so haven't even attempted to look.
Point is that I think it's a shame to take a machine like the Hembrug and not be able to use the full as-designed speed range, even if you don't really need it.
PDW
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3rd Feb 2017, 09:07 AM #28Most Valued Member
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I do have some form tools i use regularly like 7/16 external radius and 1/2" ext radius but i have a larger lathe for that, I really want this for its full spread of metric pitches, and maybe modul, if i find anything that ever uses them.
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3rd Feb 2017, 12:54 PM #29
If I recall correctly GQ was going to do a servo conversion on his Hembrug, I recall something about an electronic lead screw? If you were going down that track in future with the Monarch I would be looking for a big brushless DC, I picked up a couple on grays last year, with the idea of doing a retrofit, the price was right for a Kollmorgan 6sm but the downside is I'm still looking for the right driver, one will come along sooner or later.
http://www.graysonline.com/lot/0024-...-4a?redirect=0
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3rd Feb 2017, 02:16 PM #30Most Valued Member
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Since what I have done came up in this thread I'll clarify exactly what the situation was.
My Aciera is a milling machine fitted with at 3 phase 2 speed motor. Since I don't have 3 phase at the moment in the workshop, I run all of my machines through inverter drives (ie VFDs). There's a number of reasons for that beyond the fact I can effectively change single phase to 3 phase BTW, and I can foresee myself always doing that in future regardless of the availability of 3 phase (though if I had it I'd use 3 phase VFDs). The soft start and motor protection functions are just two attractions, quite apart from the ability to easily change speeds on the fly (and the latter I will normally use on the lathe, especially when parting).
With the two speed motor I decided to use just the high speed windings, run it through a VFD, and electronically control the speed. So on my control panel I have; high speed, variable speed, and low speed. The high and low speeds behave precisely as if the motor was wired as a two speed motor, and I pre-programmed the speeds to match the original motor speeds. So all of the speed charts on the machine in reference to the pulley selection and speeds read correctly. That is the way I'll normally use the machine, and when working I'll look up the appropriate speed for the cutter and select it with the pulley, just as if there was no VFD there. However it still has a soft-start/stop and I really like that. I can't recall what I have that start/stop cycle set to time wise. On my grinders the cycles are quite slow so there's no chance to spin a wheel. The variable speed is of course controlled by a potentiometer but I rarely use it. If I bothered to put a tacho on the machine I'd possibly use the variable function more, and I guess if I had infinite time I would, but it's just not that important and I use the machine as it was originally designed. Changing pulleys is extremely fast on that machine.
With all that said, I have NEVER seen or felt a time when the torque on low speed was insufficient for the work I was doing. The power of the original motor for the size of the machine likely never causes that to happen. Being able to creep the spindle around very slowly is a very handy function sometimes when looking for runout etc. But clearly very little power is required.
However, like I said I have many VFDs here, including on my lathes. In contrast to the mill I always use the variable speed function, and have a tacho I really need to put on as it will be handy. But I'm more familiar with lathes compared to mills, so can normally tell if my speeds are correct just from experience and too many errors in the "trial and error" learning curve. I do like lathes! With the lathe running normally, even in the most favourable torque position, then it's possible to stall the motor on heavy cuts and very low speeds. BUT that lathe has a back-gear, and in back gear no, it's just not likely to seriously stall it. You'd have to be a complete moron to be whinging about that sort of thing. Sure if you were trying to jamb a massive drill bit in really hard while being way under the recommended cutting speeds, you could probably stall it. Unfortunately idiots are all around us! Keep in mind, that is a fractional horsepower motor bolted to a glorified noodle!
My suggestion, for what it's worth, is fitting a VFD is a complete no-brainer. If you're at all concerned about the torque at extremely low RPM, then sell the 2 speed motor originally fitted and buy a larger 3 phase motor. I've seen decent sized, but marginally "shop soiled" motors out the back of H&F for example, and they're very cheap. Play your cards right and it may even be possible to sell the 2 speed motor and exchange it for a larger single speed motor, and not cost a cent.
As far as live switching a motor between speeds, no you can't do it and I know no VFD manufacturer who would support their product if they discovered that was done. Indeed in the front of every drive manual I've read they go to great lengths to say NEVER to disconnect the motor with the drive running. If you've done that in the past and got away with it, great! But it's like driving your car into a tree at 60 kph, having the airbags go off, and because you didn't die each time, then declaring there's nothing wrong with doing that! As Ray pointed out, there are definitely ways to get around that issue if you have a specific need to do so. But I personally can't see any good reason you would want to be doing this with a machine tool. VFDs are designed to be permanently connected to the motor they are controlling. Period. They will be tuned to that motor (either manually or automatically) and expect the motor to behave in an expected manner. That may not mean the earth will stop rotating on its axis if something in the motor changes, but some people seem to be forgetting these are motor drives, not cheapie single to 3 phase converters. So it's safest to treat them like they're designed to be used; as motor controllers.
I know electronics are for some people the work of the devil, and they don't want to know anything about it. So attached is a link that I think explains what happens with a collapsing magnetic force from an inductive load. A motor is really just a series of inductors, and if disconnected, as the magnetic force collapses it will produce quite a high voltage spike (sometimes called "back EMF" but that's technically something a little different, but run with that if you like). That voltage spike can be quite a high voltage, and we're all familiar with ignition coils in cars. Don't believe me, hang on the the end of an ignition lead and argue the point (excuse the pun). In itself that's bad enough, but the main issue is if you quickly reconnect the drive while that that field is collapsing, there is a very good chance it will blow up your drive. That's why Ray has a time delay in his need to switch in this condition. A possible scenario would be you decide to change speeds on the fly (no idea why) then at the instant you flick the switch you change your mind and flick back. If you do it quickly enough that field is still collapsing and you're up for another few hundred bucks for a new drive.
Anyway here's the link on that. https://www.youtube.com/watch?v=_tTzsEtIUJQ
The other issue, probably not as relevant, I don't know, is that the drives aren't intended to be run open circuit. They output a square wave, and square waves really aren't nice animals. They can cause all manner of interference and nasties around the place. But when the motor is attached to the drive, the motor coils will modify the square wave to a sine wave (and that's all part of the brains of the controller and their tuning), and all is well in the world again. I would also expect any decent drive to detect a fault if it saw an open circuit, and shut down with an error message. One of the primary functions of these in industry is to protect motors, and they will shut down for all manner of perceived faults. I can't say I've ever personally tried it, but I haven't tried driving into a tree either, just that's what I would expect the drive to do.
I would be curious to see any information from any drive manufacturer where they condone the live switching of motors downstream of the drive. There's just so many compelling reasons NOT to do it, I can't for the life of me see any good reason anyone would want to be doing that in this type of application anyway. For special applications like Ray's, fine, but that's not controlling a machine tool.
Hope that helps.
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