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Lathe Drive Motors, 1ph or 3ph?

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Ian S C	02/05/2010 12:06:04
7468 forum posts 230 photos	Just don't over speed the chuck, they can be burst. Ian S C
Circlip	02/05/2010 12:43:52
1723 forum posts	Nicely pointed out IanSC "Forgive them lord for they haven't a b****y clue" Regards Ian.
KWIL	02/05/2010 15:53:29
3681 forum posts 70 photos	Steve, Car crankshafts are usually pressure lubricated.
Steve Garnett	02/05/2010 20:49:11
837 forum posts 27 photos	I know - that's why I said "Given a sufficient supply of lubricant..." I've never found the need to spin a lathe chuck at 6000 rpm, I must admit, but the point about chuck busting is timely - probably a rather more cogent reason for not doing it, in fact. Even though it's easily possible with an inverter to get a lathe running at somewhere near this sort of speed, there are now several good reasons mentioned for not doing it, I'd say. Edited By Steve Garnett on 02/05/2010 20:50:49
Peter Gain	31/05/2010 22:29:38
103 forum posts	Hi All, Thanks for the replies & advice re-motor phases. It seems that 3ph is beneficial but not for the reasons that I at first considered. Having carefully read your replies, I will carry on with the existing motor but change to 3ph later. I agree with Wallace, the modern Tyco motors are not a patch on the old Cromptons. Regards, Peter Gain.
NJH	31/05/2010 23:45:09
2314 forum posts 139 photos	I also have reconsidered the change to 3 phase for the time being but, following comments made here, I have changed the primary drive belt to a linked belt which I purchased from Tony Griffiths (at Lathes.co.uk). Ordered one day - arrived the next and at £30 a great value improvement . Even at the top speed the lathe now runs smoothly and quietly . Thanks for the tip guys . This of course leaves some spare room in the approaching birthday list ------- now I wonder...........!! Regards Norman Edited By NJH on 31/05/2010 23:45:48
WALLACE	02/06/2010 20:35:26
304 forum posts 17 photos	Possibly final point (!). If you're short of the readies or just too mean like me, it's not too difficult to convert a delta 440 volt motor to a star 220. I've done several old Brook Crompton types and they work just fine. A long hard look at the windings on the opposite end of the armature to where the tails come out should show you where all windings are joined together - then it's just a matter of very carefully stripping back the insulation / cloth sleaving to get to the wire. These can then be soldered onto new tails and brought out to the connection plate. It's important to do this properly - I always use several layers of heat shrink sleaving to cover the joins and tie everything up with string to match the original. Then I'll spend a few days liberally coating everything with varnish. I used M4 brass bolts as studs on the paxolin connection plate with solder tags on the tails. Just to make sure everything was ok, I checked everything out with a megga - might be worth getting someone to do this for you if you don't have one as it puts 400-500 volts across everything so it really gives the insulation a good test - a multimeter powered by a 9 volt battery doesn't ! WALLACE.
John Olsen	05/07/2010 02:07:12
1294 forum posts 108 photos 1 articles	I see this thread has been idle for a while, but I would like to add a couple of thoughts. Firstly the comment about "never have to change a belt again" with a speed controller is not necessarily true. If you need to do heavy turning at low speeds then it still pays to change the belt or to use back gear. If you are ripping something down to size and want to maximise metal removal rate, then you want the motor running at maximum power, which will of course be with the speed set to the highest. Then adjust the belt and back gear to give the correct cutting speed. While a Myford is only a light lathe, it can remove metal at a significant rate when set up correctly, and you can save quite a bit of time. I'm not going to quote actual depths of feed etc since that is going to vary a lot with the circumstances, and obviously some sensitivity needs to be used. The main advantage of the 3 phase motor would be with the speed control. I find it good for screwcutting, slow when it matters then a quick reverse back to the start. Secondly, I've just done the conversion to poly V belts as described by Neil Hemingway back in 1983 on my ML7. The description was for a Super 7, but the idea can be adapted for the ML7 without too much trouble. I have found this very worthwhile. The poly-V belt does not slip as readily as a V belt, and runs much quieter. The pulleys also don't get hot, as they did with the ordinary V belt. Incidently I have a Unimat 3 as well, which I converted to toothed belt drive about 25 years back. The use of the toothed belts does not seem to lead to any patterning on the job. Any patterns I have ever noticed are due to the tool getting dull and chattering beginning. These small lathes will teach you to notice the difference between a sharp and a dull tool! regards John
John Haine	05/07/2010 08:35:12
5563 forum posts 322 photos	Norman, was it Power Twist belting that you bought please? - John
NJH	05/07/2010 10:08:08
2314 forum posts 139 photos	Hi John Yes it was - I will mail further details direct. Norman Edited By NJH on 05/07/2010 10:33:24
_Paul_	05/07/2010 19:38:20
543 forum posts 31 photos	@ Wallace Saving a few quid really appeals to a Yorkshireman :D I have a couple of 400/440v motors I wouldnt mind converting to 220v do you have a circuit diagram of this conversion operation or perhaps even a couple of pictures regards Paul
John Olsen	06/07/2010 07:30:36
1294 forum posts 108 photos 1 articles	Paul, take a look at the terminals as many motors have provision for changing them from star to delta built in. The give away is if they have six terminals. When it is strapped in star, three of these will be linked together, and you should find that there are actually three links provided, even though there are only two gaps to cross. That lets you rearrange the connections into delta, eg the three straps join the ends of a paif of coils each. If you are lucky there may even be a little diagram of how to do it. I could provide a picture of how it is done on the one I just did, but others may differ. If there are only three terminals, then the star point is buried in the windings. A motor rewinder could do it, and you could yourself if you were very careful. But it is not so easy, you have to find the join, separate the three windings, and then provide each one with a safe terminal. regards John
William Roberts	07/07/2010 18:59:52
27 forum posts	Hi All, A good thread this, we have had all the advantages of 3ph but I noticed no one said wether it is cheaper or dearer to run 3ph than 1ph. Regards Bill
KWIL	07/07/2010 19:17:36
3681 forum posts 70 photos	Probably not a lot in it, if this cost is a problem to you then this is the wrong hobby.
peter walton	07/07/2010 21:24:32
84 forum posts	One other main advantage of 3 phase is instant reverse when screw cutting not something easily done on single phase. As any rotation left on the motor when you flip the switch then it will still motor in the same direction or as I found out blow the capacitor! Peter
Keith Long	07/07/2010 21:33:07
883 forum posts 11 photos	Hi Peter I wouldn't bank on instant reverse with a 3 phase motor - there's such a thing as inertia. Keith
John Olsen	08/07/2010 01:59:45
1294 forum posts 108 photos 1 articles	The power meter will measure the actual power used, to a pretty good sort of accuracy. The three phase motor is likely to be a bit more efficient. But there won't be enough in it for you to notice any difference. Of course there is a bit more capital outlay for three phase, the motor might be a little cheaper but you will have to buy the controller. There is a maximum recommended speed for the Myford Spindles, I can't recall the exact figures although something like 2000 rpm for the Super 7 sounds familiar. The ML 7 is not rated to go as fast. About thirty years ago I was involved with motor speed controllers at University, and did a project involving the design of an invertor. They had looked into the characteristics of the motors then on the market, and found that they were as supplied adequately balanced for running at up to 10,000 rpm. (eg three times normal speed for a two pole motor.) However, eddy current losses made it undesirable to go quite that fast. They did find that running at twice the normal frequency was no problem. Of course for best results you would want to set up the voltage/ frequency curve to double the voltage at double the frequency, and that is not normally feasible for us amateurs. It does then give you twice the normal rated power out of the motor, without significant increase in losses. That was with good quality motors, I don't know how the Chinese stuff would fare. If you do have the need for something to go fast, and here I am thinking more of tiny milling cutters rather than lathes, most controllers will let you program the maximum frequency up much higher. Since you can't also increase the voltage, the torque will fall off, but this is not likely to matter if you are trying to use a very small milling cutter. There will be a limit to the speed for your mill spindle bearings too, but this is likely to be higher than for the plain bearings in a Myford lathe. Most controllers won't give instant reverse, they have an acceleration/deceleration curve. (also programmable.) If you reverse too fast with a screw on chuck it will give you a nice surprise, so don't do this with a three phase motor on real three phase supply with a reversing switch. Also bear in mind that the turning loads will try to unscrew the chuck, so if you are using an upside down tool be careful. regards John
William Roberts	09/07/2010 19:09:32
27 forum posts	Thank you John thats all I asked, the cost is not a problem . Bill
WALLACE	13/07/2010 22:34:34
304 forum posts 17 photos	Hello Paul. It's a while since I converted a motor from 440 to 220 - so I don't take it as gospel ! I was going to write something for MEW a few years ago when I acquired a motor for a tool and cutter grinder - but it was wired up for both so I didn't have to do it ! Anyhow - I've only done this trick on the old Brook-Crompton motors. It's fairly straightforward to strip them down - the only tricky bit is to identify the sleave where the bottom of all 3 windings come together. The windings are segmented so there are other joins. If you look carefully, then go away and look again (!) you should see the only one where three wires go in. Carefully peel this one away - the wires are very brittle and everything's stuck together with varnish so take it easy. When the fabric sleave is removed, you should see the 3 ends of the windings soldered and twisted together. These will need to be seperated and joined individually to 3 insulated multicore wires ('tails') to take up to the junction box. Ideally these should be yellow, blue and red - but it doesn't really matter as the 3 windings are easy to identify afterwards. The thickness should be good enough to take 10 amps or so - use the existing ones as a guide.. The joins should obviously be soldered and then covered with several layers of heat shrink sleaving.Be very careful at this point - it's easy to break the coil windings. The joins and tails need to be tied down with string - as the other windings are. Then cover everything with varnish - I used some that I had in a tin in the garage giving it several coats over a week. Obviously the centre of the motor needs to be kept clear - the new tails should be tucked out of harms way so tno rotating parts rub against them when it's all put back together. Right - so hopefully you now have a motor with 3 sets of windings that need to be brought out to seperate connections at the terminal block. You might be lucky and find that the paxolin plate can take another 3 studs - if not, you'll need to get hold of some to make up a new one ! M4 or 4BA bolts are fine to make up the connection studs - make sure they're as far appart as possible from each other AND any metalwork ! The studs should be in two rows of three columns. I used solder tags to keep everything apart and neat - plus spring washers and loctite on the studs to make sure everything stays tight. The wiring from the tails should be as http://www.rotor.co.uk/technical_support/star-delta.pdf And that's pretty much it. ! If in doubt, get the motor checked by someone who knows what they're doing ! MAKE SURE THE CASE IS EARTHED PROPERLY BEFORE YOU TRY IT OUT FOR REAL !!!!! I used a insulation tester to make sure everything was ok - but it was a high voltage one (generally called a 'megga'). This puts about 500 volts across everything which is a way of making sure the motor will take mains voltages - if you test the insulation resistance with a bog standard multimeter it only puts about 9 volts across the insulation - which may be fine - but not so good at 220 ! Final point about inverters. Usually if you switch them into reverse, they'll ramp down, stop, then go backwards over a few seconds depending on how they're programmed (talking inverters here). I would NEVER just flip one of the windings round to do a reverse when it's rotating - the only thing that would be instantaneous would be the blowing up of the IGBT's in the inverter ! W. Edited By WALLACE on 13/07/2010 22:36:35
Stub Mandrel	14/07/2010 11:19:22
4318 forum posts 291 photos 1 articles	achieve 6000 rpm anyway. Given a sufficient supply of lubricant, I can't see any inherent reason for not running a plain bearing at this sort of speed though - on a similar sort of scale, isn't that exactly what happens with a car crankshaft? But only with forced lubrication! A better comparison is with wheel bearings, which (size for size) seem to be expected to handle far greater loads on vehicles than they ever see ona lathe. Neil W.

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