Lathe vibrations

Hi Brian

I checked the rotor assembly balance as in desperation i changed the motor bearings. They were very good and a good fit in the housings. With no play when assembled and the rotor spin freely by hand. I split open the old bearings and the were also in good condition.

Thanks for your thoughts

Peter

Just keep an eye on the motor temperature when running at reduced speeds, the 1.5hp one that is.

Good Morning Peter

Yes, torque boost will affect motor vibrations regardless of speed Which is why I said to make it constant.

If you have the option on your inverter to make the torque boost constant or even completely disable it I remain confident that this will solve your problem.

The motor is constantly trying to respond to any changes in load characteristics. .So, no bel) - smooth running (inverter not trying to do anything)

Refit belt-,motor now has varying load (gear selected, power feed' oil viscosity, tool resistance) Inverter is now 'chasing' load and 'hunting for perfect setting. This 'hunting is constantly changing causing the vibration. If you can disable the inverter trying to do this it will all run smoothly.

This scenario has been reported many times in the past on lathes of all kinds with problems ranging from strange patterns on finished work to motors sounding like badly set up Diesel engines!

IKB

IKB - that’s interesting... that’s a good point that you’ve raised. I can see the reasoning behind your thoughts/experience. I’ve never had to ‘play’ about with torque setting in the applications I’ve had experience with but well worth committing to memory..

John

Hi IKB

Thanks for your response, the torque control setting was set at 0, so i reset it at 3, tried the lathe, then 6, then 7 then maximum, 10. All made no difference to the the vibration which can be felt wherever you touch the lathe or even the motor body. It isn't a rumble like i found when reversing the winding wires, but is more a tremour. I think i understand your explination, the fault is evident with or without the drive belt fitted and does not change if you take a cut.

As you can probably gather, i'm becoming fed up to say the least of this lathe, i'm quite reasonable as a self taught engineer and only use it for fun, however having to tape the finished work piece to get a decent finish makes me want to look elsewere for a hobby.

Thanks again for your thoughts

Peter

As the motor used to run smoothly on a phase converter (presumably the type that uses inductors and capacitors to generate pseudo-3 phase), but seems to vibrate when driven from both of 2 different inverters; and you have verified that the winding's are in the correct sense; the only conclusion can be I think that the motor doesn't like running with an inverter. Apparently older motors may not be suitable, I don't know why, in my experience both the older motors I have used were fine. So the only option seems to be to use a motor that does!

You say that the 1.5 hp motor you have runs quite smoothly on an (both?) inverters, so the question that you asked is whether you could use that for the lathe? Well there are three reasons why you might not.

One: is it the right speed? To keep to the same speed ranges it would be best for the motor to run at the same nominal speed - the AEI motor was probably 4 pole ~1450 rpm, so if the new motor is the same that's OK.

Two issues with power: is the lathe so big that just its inherent friction absorbs too much power? Well, I doubt it somehow if it fits a standard sized workshop. And: how much power is needed for the cuts you are likely to do? Unless you are removing industrial production amounts of metal I would have thought that for amateur work 1.5 HP is quite enough.

According to my Brooks motor book, the connections for Delta running should be A1>B2 B1>C2 C1>A2. The windings must be oriented correctly, which is why they are marked with 1 and 2 indicating the correct (winding direction) connection. there would be no point in this method of marking if reversing one set of coils reversed the motor, it does not, reversing two of the incoming supply lines reverses the motor. The Motor is a dual voltage motor which runs on 380/415 connected in star, and 220/240 when connected in delta. motor noise can be caused by loose windings or loose stator or rotor laminations, but given the motor was running correctly, my feeling is that it is incorrectly connected. Reconnect it as above and report back please! AEI motors from the seventies did have a bit of a bad name, but it WAS running ok, so its got to be incorrectly connected, or there is a "dirty" phase from the inverter.

Phil

Hi Phil and all

Thanks for your input, i won't repeat myself as your ideas have been tested and results noted earlier in this thread.

This afternoon, i removed the motor and wired it up simply sat on the garage floor. The vibrations felt throughout the lathe were now felt in the body of the electric motor. So it appears this motor does not like my inverters.

The 1.5 hp abb motor, shaft height, shaft diameter and key size are the same as the 3hp AEI motor, so no guessing my next move!

I'll keep you informed.

Regards

Peter

OK, thanks for that, when I read the previous posts it showed another connection set, which was incorrect. A1>B2 B1>C2 C1>A2 and incoming lines to each junction for Delta is the only pattern that will work The diagram above does NOT use this pattern, and is incorrect.. You may well have trouble starting the lathe in higher speeds, with a smaller motor, unless you have a clutch.

Phil

Could Peter's symptoms be explained by an insulation failure? Maybe the motor's insulation was marginal and the inverter took it over the edge. Might explain why nothing Peter has tried fixed the problem.

I don't know how a 3-phase motor behaves with shorted turns or when a coil leaks to ground. Would two of the coils working OK whilst the third flashes over intermittently cause a rumble? I can imagine it upsetting a VFD.

And if shorted turns or a ground fault are a possibility how might Peter test for them?

Dave

I once converetd an RJH Ferret cutter grinder to work off single phase. I rang the motor manufacturers, and they advised that old motors do not like vfds, something to do with insulation. I did it with capacitors as shown in the MAP book, worked well, but could this be what has happened to OP's motor as it worked on a converter which produces a pseudo anaolgue 3 phase rather than the digital you get from a vfd.

As regards the smaller motor, I'd be a bit generous with ramp settings, my 8" 4 jaw used to cause the unit to trip on shut down as the slow down current was too high. The Colchester probably has a brake, so you don't need motor braking at all.

Edited By duncan webster on 17/12/2017 19:46:44

I think that the VFD would kick out if this was the case, they are usually pretty sensitive to wiring or motor faults. It is not unknown for some older motors to suffer insulation problems if used at high frequency on a VFD. Only way to test for a ground fault is to use a megger type high voltage tester between the windings and the chassis of the motor, again, it is possible, but I think unlikely. insulation faults usually follow an overheating event, or result from mechanical damage to the laquer on the windings, damp etc.

Phil and Dave, I've limited experience with inverters, just having the one running at the moment on my Centec, but with a couple more ready to wire up when I get the chance.

The easiest way for me to check out windings is to use a "megga" (or megohm meter), mine can be set up to measure high resistances, at 240, 500, or 1000v (i.e. to seek winding to earth, or winding to winding faults)

It can also be set up to measure very low resistances (i.e. earth continuity, and in this case coil winding continuity)

The combination of the two readings should help point to any faults in the motor, other than winding polarity reversals. It won't help deduce whether a motor will work with a digital inverter happily, even if it tests OK

N.B I'm not a qualified sparky, though I have produced the odd spark or two over the years.

I've just been out to the workshop and taken the cover off a brand new motor, bought to go on the Dore-Westbury mill; I've not had chance to fit it yet. The photos below show the interior terminal strip and the cover plate which shows where to fit the bus bars for 415v Star configuration, or 240v Delta config. As you can see, it's presently wired for Star, so I'll need to re-jig it for my 240v output inverter.

In this case, rather than A~B~C the windings are labelled U~V~W. so the following, and my previous diagram may be leading you the wrong way up the garden path.

The top row of bus bars are configured to give a star point, commoning all the coil-2 terminals together, with 415v 3 phase to each coil-1 terminal.

They are numbered left to right out of alphabetic sequence W2~U2~V2 but that doesn't matter for Star as they are commoned together. (It's to make it easier to re-config for Delta)

The bottom row are labelled in sequence U1~V1~W1. As you can see, winding U ~ red, V ~ Brown & W ~ Black

The "1" terminations have even been dabbed with blue paint/lacquer.

Now for 240v Delta, the bus bars are oriented North/South, as per the diagram in the lid below. (We're only concerned with the left hand two diagrams, as the RH ones are for single phase.)

Thus the common points are, from Left to Right;

U1 ~ W2 assuming U=A, V=B & W=C I think this equates to A1 ~ C2

V1 ~ U2 """"""""""""""""""""""""""""""""""""""""""""""""""""" B1 ~ A2

W1 ~ V2 """""""""""""""""""""""""""""""""""""""""""""""""""""C1 ~ B2

Which is what I think my previous diagram shows,

N.B  I may be in error assuming an A~B~C motor is equivalent to U~V~W one, which I accept may well not be the case.

Does your cover plate have a similar diagram, if so, make sure you follow that in case AEI use different terminology for their windings; I can't find anything on the net about their config.

Good Luck

Bill

Edited By peak4 on 17/12/2017 21:02:16

I think you will find those connections are used for running a star delta motor DOL when starting current allows.

Emgee

No such animal as a star/delta motor, star/delta starting is a method of starting any three phase motor by extending the ends of the three windings to a star delta starter, using a six core cable instead of the normal three core. it is the starter which configures them in star for starting, and delta for running. Any three phase motor can be started this way. A1>B2 B1>C2 C1>A2 is the connection system used to convert any star connected motor into a delta connected motor. Star/delta starting is used for motors which are over about 10Hp, or start on load.

Peak 4, yes you can assume UVW is equivalent to ABC. It is the numbers that give the windings the correct rotaitonal order and direction around the stator so you would connect U1>V2 V1>W2 W1>U2 for delta.

Just checked the Brooks bible again, I sorta see what they are getting at. They are calling a three phase motor which can be connected in star, or delta, or star delta started "a star/delta motor" This could be because I am a bit old school ( I retired this year) and tend to accept that they are all like this, which of course, nowadays, they aint. Some manufacturers, to save money(?), produce motors which are permanently connected in star, by soldering three of the coil ends together and burying the connection in the windings. In the trade, we used to call these motors "crap"! because it is bit more difficult to test these winding in situ because you cannot separate the three windings electrically and also because of the rate at which they failed in industrial settings. I have soundly thrashed myself for this uncharacteristic mistake. I shall also spend one hour on the naughty step.

Phil, Not wishing to fall out with anyone, but the makers of this particular motor (Elektrim SH71-4B) would appear to disagree with you; when the motor is connected for 240v 3 phase running, they show the following, both on the motor itself, and their instruction manual, which can be downloaded from the link above.

U1 ~ W2 V1 ~ U2 W1 ~ V2

The OP seems to have resolved his issue by fitting an alternative motor now.
Obviously I can't say, for certain, that Peter's motor should follow suit, but presumably it says on his motor cover plate.

Regards

Bill

p.s. looks like we were typing at the same time, but I took longer sorting out the relevant link.

Edited By peak4 on 18/12/2017 22:01:54

A motor for star delta starting would need to be 415v in delta and about 690v in star. A motor made for 415v in star would not be suitable for star delta starting but they are very useful for running on single phase input VFDs as the delta configuration is 230v ish.

Mike

A bridge meggar is the tool for checking the value of the windings, a multimeter is not the tool for the job. If you have a major failure a multimeter may be ok but your eyes and nose will probably give you a bit of a clue.

Mike