Lathe vibrations

[SillyOldDufferModerator @sillyoldduffer]

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

[duncan webster 1Participant @duncanwebster1]

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

[Phil WhitleyParticipant @philwhitley94135]

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.

[peak4Participant @peak4]

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

[EmgeeParticipant @emgee]

Posted by Phil Whitley on 17/12/2017 17:04:11:

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

Phil

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

Emgee

[Phil WhitleyParticipant @philwhitley94135]

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.

[Phil WhitleyParticipant @philwhitley94135]

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.

[Phil WhitleyParticipant @philwhitley94135]

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.

[peak4Participant @peak4]

Posted by Phil Whitley on 18/12/2017 21:17:47:

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.

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

[Mike PooleParticipant @mikepoole82104]

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

[Mike PooleParticipant @mikepoole82104]

Posted by SillyOldDuffer on 17/12/2017 17:23:10:

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

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

[Phil WhitleyParticipant @philwhitley94135]

Hi again Peak4, If that is what it says in the motor, I would go with that, although it does sorta seem to defy logic. If you consider the three phases of the supply with peaks and troughs 120deg apart, not only must the coils be connected in the correct order as you travel round the stator, but all the windings must be operating in the same way magnetically in order to create the flowing river of magnetism in the stator which the rotor chases, but never quite catches up to! I feel I haven't explained that very well, but I know what I am talking about, although it would not be unusual if no one else did. We used to have standards like BSI which were strictly adhered to for many years, and meant that whenever you pulled the lid on a motor you were met with the same, or at least a very similar set up. Today we are seeing Chinese made equipment with three phase motors that have been "converted" to single phase with a couple of capacitors, they are unbalanced, let out a hefty growl when running, and generally overheat. I come from an era when this would have been outlawed. We see today in the Grenfell report that the Building regulations have been deemed "not fit for purpose" with a wide range of fudged issues and regs that are totally open to interpretation, which really is not a regulation at all! Unfortunately I can report with some authority that the new IET regulations (formerly the IEE) regulations for electrical installations are no better, I am glad to be out of it all!

[Phil WhitleyParticipant @philwhitley94135]

Hi Mike Poole, Yes, possibly, shorted turns in one or more windings would unbalance the torque pull on the rotor, and cause noise, but also would cause overheating, and it is usually not long before overheating from shorted windings becomes a leakage to earth as the heat destroys the laquers insulation value, and all the fabled magic smoke escapes!. Easy way out is to get it tested on a normal 3 phase supply at a local motor repair shop. I am lucky, when my late father and I built what is now my workshop in the early seventies, I insisted on 3 phase, and as he had been a long time YEB employee, we got a good deal. very different today, recently bought a three phase TIG welder off a guy who had been using it on a generator as he had no 3 phase. The 11kV line passed right by his works, but they wanted £30,000 for a transformer and a bit of underground! He had bought a single phase Artec TIG for about £1500.

[Neil WyattModerator @neilwyatt]

Posted by Phil Whitley on 19/12/2017 20:15:23:

all the windings must be operating in the same way magnetically in order to create the flowing river of magnetism in the stator which the rotor chases, but never quite catches up to!

Reversing one coil should simply have the effect of reversing the motor.

Neil

[duncan webster 1Participant @duncanwebster1]

Posted by Neil Wyatt on 20/12/2017 15:28:40:

Posted by Phil Whitley on 19/12/2017 20:15:23:

all the windings must be operating in the same way magnetically in order to create the flowing river of magnetism in the stator which the rotor chases, but never quite catches up to!

Reversing one coil should simply have the effect of reversing the motor.

Neil

Neil I've sent you an e-mail via normal channel (ie not a pm)

[John HaineParticipant @johnhaine32865]

Posted by Neil Wyatt on 20/12/2017 15:28:40:

Posted by Phil Whitley on 19/12/2017 20:15:23:

all the windings must be operating in the same way magnetically in order to create the flowing river of magnetism in the stator which the rotor chases, but never quite catches up to!

Reversing one coil should simply have the effect of reversing the motor.

Neil

Neil, experimental evidence posted earlier in the thread proves that this is incorrect. The coils have to be connected in the right sense so all the mutual inductances couple correctly and the 3-phase electrical load presented by the motor is balanced. Once this is done, reversing a pair of connections of the 3-phase supply reverses the motor.

Interestingly, a 2 phase motor doesn't I think require this as the windings, being at 90 degrees, have no mutual coupling.

[Mike PooleParticipant @mikepoole82104]

Posted by Neil Wyatt on 20/12/2017 15:28:40:

Posted by Phil Whitley on 19/12/2017 20:15:23:

all the windings must be operating in the same way magnetically in order to create the flowing river of magnetism in the stator which the rotor chases, but never quite catches up to!

Reversing one coil should simply have the effect of reversing the motor.

Neil

Well at least it's the right time of year to say "oh no it doesn't" as I mentioned in an earlier post in this thread I have always believed the polarity of the coils is important so off to the workshop I went to test my belief. The result was that the direction did reverse but the motor now ran slower and with a distinct vibration, it was not happy. Manufacturers uniquely identify all the ends to aid correct wiring. I know some manufacturers guarantee that a motor will turn in a certain direction, SEW eurodrive often supply their motors with a plug and socket terminal box so that a spare motor can be just plugged in and you are back in production. Changing direction is usually achieved by swapping two of the incoming phases but if an inverter is fitted then often there is a parameter to effect this.

Mike

[peak4Participant @peak4]

OK Folks, there's any number of videos on Youtube which explain the principal of "Rotational Magnetic Fields"

Inevitably some are better than others, and some longer than others.

Here's a couple of the longer ones,

so make sure you know how to fast forward videos ;

The first explains reasonable well how the generated field rotates, the speed effect of the number of poles and a few other bits.

Start off by getting your head round his explanation of poles and pole pairs, then you can keep fast forwarding a bit to the diagrams from about 22 minutes, where it becomes obvious that what you are seeing is analogous to a N-S pole bar magnet rotating smoothly either clockwise or anti-clockwise.

If one reverses a winding's polarity, as opposed to a phase supply, the N-S,N-S,N-S,N-S,N-S,N-S,etc suddenly becomes N-S,N-S,S-N,N-S,N-S,S-N etc, which clearly isn't conducive to either full mechanical power output or smooth running.

Video-1

When you've got your head round that, have a look at another video in the series, Motor Connection Diagrams, regarding star/delta etc, but please note that most of the discussion in our thread here revolves around a motor running in either 415v Star config, or 240v delta config, rather than using star to start the motor and then switching to delta for when it's then rotating.

Video-2

He also has one on inverters etc, which I've not seen yet

Video-3

Edited By peak4 on 20/12/2017 20:40:07

Edited By peak4 on 20/12/2017 20:40:34

[Phil WhitleyParticipant @philwhitley94135]

"The result was that the direction did reverse but the motor now ran slower and with a distinct vibration, it was not happy. "

Quite correct Mike, we used to call this "Growling" as that is often what it sounded like. you get a very similar sound if a phase is off or low. You usually find that if started a number of times , the actual direction the motor runs in is random, sometimes clock, sometimes anticlock.

[Phil WhitleyParticipant @philwhitley94135]

Peak 4, Good post!

[Mike PooleParticipant @mikepoole82104]

I did a spell in our motor winding shop as an apprentice and after winding a few motors I got a bit blasé about the process, I managed to reverse one coil out of the set that become U V or W in a finished motor, it did not run well. It was standard practice to test run the motor before the final lacing and shellac dip. It would have been a wretched job to unpick a shellaced and baked motor although many people have picked the star point out of a finished motor the risk of doing some harm must always be present. The coating on the wire is very robust and to make the internal connections we twisted the wires together and flashed them with an oxy acetylene flame which formed a very neat copper weld, a sleeve of systoflex and then lacing it all together completed the job ready for dipping in shellac. Small motors have small gauge wire which is easy to feed into the stator slots but higher horsepower motors use thicker wire and if you do not wind tidy coils they will never go into the stator slots. To ensure that you have clearance to fit the rotor a rubber mallet may help to form the coils to clear of the rotor. I found motor winding most enjoyable but although a few dozen was good fun I can imagine that a few thousand might begin to lose my interest.

Mike

[Neil WyattModerator @neilwyatt]

OK, I've learnt something

[john fletcher 1Participant @johnfletcher1]

When I first posted the story about Peter's motor, never did I ever think I would start such an interest in 3 phase motors. If readers go back to page one they will see that I had already described incorrect motor connections and the result. Anyway I'm sure readers have learn a lot. Not sure of the state of progress regarding Pete's vibrations. Happy Christmas to all. John

[duncan webster 1Participant @duncanwebster1]

Like Neil and several others I've always thought that reversing one phase of a 3 phase motor would just reverse the motor, with no other effects. However I'm wrong. I done some sums which show convincingly that you have to get the windings the right way round. Swapping the mains connections to 2 delta points certainly does reverse the motor, reversing a phase looks like a bad idea. The red, yellow and blue lines show the magnetic field progressing against rotor angle. First picture is nice and smooth, second shows yellow phase reversed. OOPS. Yes it will run, but I bet it's lumpy

[Phil WhitleyParticipant @philwhitley94135]

Systoflex! there's a blast from the past Mike. Funnily enough the firm I did my apprenticeship with(Jeffersons of Driffield East Yorks) is still going, but is now almost exclusively a motor and pump shop, whereas when I was there we were industrial contractors, and all our rewind work went to Dale Electric of Filey, Now Dale power Solutions Ltd. Jeffersons was originally the Driffield branch of………….Dale electric. Round and round it goes!

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