Lathe vibrations

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!

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.

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)

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.

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

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

"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.

Peak 4, Good post!

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

OK, I've learnt something

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

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

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!

Hi Duncan, good post, to reverse a three phase motor you swap over any two legs of the incoming supply. You can do this at the motor, or the starter or isolator, or even at the plug if the equipment is plugged in. One of the first jobs we did at a completed installation was to phase out all the machinery and the socket outlets to make sure that all the equipment ran in the right direction, and that any machine, plugged into any socket, would run correctly. Many's the time we got despatched to a farm because the farmer had bought a new grain auger, and the first time he came to use it, it went the wrong way. They soon learned to do ot themselves!

Hi Phil.

I live just around the corner from the Dales Factory. The remnents of the workforce move out before the end of this year and it is planned to demolish it and build houses on it.

I've fitted the motor but due to xmas duties, the jobs come to a halt.

I will update everybody when it's back up and running.

Merry Christmas to all, and all the best for 2018.

Best wishes

Peter

Hello Duncan I like your description. It is good practise to exchange any two incoming lines when reversing a three phase motor, and you will be successful every time. Exchanging connections on the motor terminal box you might be lucky on direct on line, but what about star/delta. I once went to a job where the customer has requested to have a motor direction of rotation reversed on a star delta starter/ motor, some one has rearranged the connection in the motor terminal box. The motor started OK on star but what a commotion on delta, it attempted to reverse. It took some time to work out what had happened and to reconnect the incoming lines in the correct manner. Stick with the lines and you always be successful. After a rewind we had a home made rotor mounted in a Perspex tube, the motor was connect up on a low voltage supply and you put the rotor inside the stator and noted which way it rotated, we also had a home made growler for locating stator winding faults. They wouldn't be permitted, now to dangerous. John

Peter Smith , Gristhorpe? I used to race cars there in the olden days, and my late father new Lenoard H Dale personally, as he worked for the YEB. Seasons greetings to all, Bah Humbug, drink port and forget

Phil!

Hi all

Happy new year

As promised an update on progress with the lathe vibration problem.

Removed the 3hp motor and fitted an abb 1.1 kw 240 volt 3 phase motor.

The vibration felt in the lathe is much reduced and easily liveable with, however when using a tipped tool, taking a decent cut in top gear with the inverter showing 80hz, the lathe slows, so 1.5 hp is not enough to cope with tipped tools.

So, as i'm reluctant to use old second hand motors with the inverter, i've been looking at 2.2kw (3hp) 240v foot mounted 3 phase motors. I understand some cheaper motors are poorer qualty than others. Given the time and effort put into this lathe I'm considering a crompton greeves motor but there coming out at £200 plus vat and carrage.

Can anybody give me advice on how good these motors are or recommend a better ( and preferably cheaper) product

Thanks Peter

Peter, you could see if one of the better known re-winders have any refurbished motors up for grabs.

e.g. Browill Rewinds near me; there's lots of others depending on where you live.

Bill