Lathe vibrations

A friend has a Colchester lathe, fitted with a three phase Delta connected motor powered via an inverter. In former times when the lathe was powered via a converter and the motor connected in Star with a pilot motor running he says he didn't have any vibrations and every thing worked well. Some time later going modern, he bought an inverter for speed control, then he says vibrations started. My friend has dismantled the motor to check the bearings, all OK, he did the same to the head stock( he tells me these are very expensive) all OK. I recall some one HERE in the past suggesting altering the inverter control frequency upwards to reduce the noise of the motor. Your practical suggestions would be much appreciated. John

Most VFD manufacturers will confirm that changing the switching frequency upwards can reduce the perceived noise.

It is possible to overdo it so I would suggest that you read up on your particular make/model.

If you slip the belt off the motor is it noisy in itself? A winding fault can make a motor run roughly, check the connections from when it was changed from star to delta. Check the winding tails are tight on the connector posts. The modulation frequency usually just changes the sound from the motor and choosing a higher one if the option is available can reduce the noise. Most modern VFD drives produce a reasonable wave shape so they are pretty quiet. Some early VFD used a very poor wave shape and the harmonics caused a lot of noise and heating of the motor.

Mike

Is it a Colchester "Chipmaster" ? I seem to recall them having some form of varispeed drive that likes a smooth power input. ​ I may be wrong it has been many years since I saw one

I have a Chipmaster, with a fully functioning variator the motor of which is 3ph driven through a VFD. No "noise" or vibrations with this setup.

Grum

If it's a two-speed motor, they apparently don't like inverters.

Hi All

Just joined the forum after my friend started this thread on my behalf.

The Invertor was purchased from ebay (last year) and was sold as a 2.2 KW VAT20 GE U20N 2K25. It looks identical to a Teco MiniCom E2-203-HF 2.2KW which direct drives sell (but much cheaper. I have in the past purchased a Teco Invertor from Direct drives and apart from the name I cannot find any difference, even the manual instructions and settings are the same.

I have carefully wired up the invertor, and have (amongst other setting changes) changed the F12 carrier frenquency settings across the whole range. F1,2,3,4, and 6 make the motor whistle,additionally in these settings and the other settings, the motor has a slight rumble and vibration which can be felt throughout the lathe. Also the turned finish both with tipped tools and sharp hss is poor.The Lathe is a 1977 Colchester Student, and the motor is a 3hp AEI Kapax. In desperation I have changed the bearings (waist of time), windings all measured the same and were like new. This is the second invertor fitted to this lathe the previous one also made the motor whistle but it was much worse and was only useable in the highest Carrier Frequency setting. The vibration is evident with the drive belt disconnected. Increasing the HZ up to approx. 75 is where the motor is happiest. Any Ideas?

It's not possible to wire them up incorrectly. If you connect one of the windings backwards, the motor will simply spin the wrong way. In which case you simply reverse ANY one of the windings.

Vibrations will be mechanical in origin eg imbalanced motor, countershaft etc. Try running the motor with the gearbox disengaged ("between gears", assuming the machine works that way. That way most of the rotating parts will be rotating.

Murray

With the advantage of seeing Peter's post, it does sound like the motor might be slightly out of balance.

Neil

The six wires from the motor windings should be marked so that each winding can be identified and the start and finish of each winding. Some are labelled with a colour and a tracer or bands to indicate start and finish, some are labelled U1 U2, V1 V2 etc. The star connection is easy as the 2s are usually connected together to make the star point. If there are no markings then it gets more difficult. There are plenty of diagrams of the 6 post connector block used on motors showing how the windings should be arranged. I would have a look at how your motor is wired from the point of view of the winding connections, the incoming supply does not matter as this only affects the direction the motor will run in. The delta connection should be U1-W2, U2-V1, V2-W1. If you can give more details or pictures it would help to see what's going on.

Mike

Alas it isn't that simple as Mike's post above confirms, as the windings on a 3 phase motor have mutual coupling between them and the couplings all have to be in the same sense for electrical balance. If the windings are marked or their ends brought out to terminals it is easier, but if not you need to identify the starts electrically. Some where on this forum someone described how to do this, I will have a hunt.

Found it! http://www.model-engineer.co.uk/forums/postings.asp?th=107561

Edited By John Haine on 12/12/2017 07:39:35

As John Haine, above states it's not that simple, I suggest readers read the article highlighted in John's post above.

Now a true story and my experience. Several years ago a friend was given an Italian made circular saw machine which had been used to cut aluminium for secondary double glazing. The motor had literally been ripped out of the housing, a motor with six unmarked wires poking out. My friend asked me to wire it up in Delta and to run the motor via a capacitor set up for a grinder. I traced out the three sets of coils, no problem, but when I attempted to run the motor it got hot very quick and ran very slow with a strange noise. I realised what I had done wrong and knew of the arrangement to which John refers, I had seen it done by pucker winders.I simply rearranged the coils connections until we got it correct, not a long job once you realise what is wrong. This was the first time I had experienced this fault as all other motors had markings on the coil ends. Out of interest I deliberately wired up a motor incorrectly and the fault clue appears to be slow speed, strange noise and motor getting warm if not hot. Better mark the leads out clearly, if possible. John

The original post said the vibration only occurred after the VFD conversion and changing from star to delta so it would seem likely it is an electrical problem. I have often encountered drives that make a mechanical racket but have an electrical problem.

Mike

Could it be that one phase from the VFD is pure sine wave from the mains, but the other two are raggedy square wave approximations? Possibly due to buying the smallest possible inverter? I reckon one needs about 50% spare capacity in an inverter, particularly a cheap chinese offering.....

Hi guys thanks for your response.

Just for info, when i stripped the electric motor i checked the rotor with the pulley attached upon a crankshaft balancing jig i made to balancce my motorbike crank. It appeared to be statically in balance. The vibration is evident in the lathe with the drive belt disconnected, as such only themotor spins. It has no problems with rotational speed or power ( heavy cuts can be taken without noticable reduction in chuck speed), as such i don't think a phase is low or down. The vibration is not a glaring one but if you place your hand on the tail stock, apron, base or head stock the vibration can be felt (with or wothout the drive belt on). My next step is to change the wiring fromthe invertor to the motor, and i have a spare 1.5hp 3 phase motor. I will connect this in place of the lathe motor but let it spin on the floor. If this one vibrates....... anybody want to buy a colchester lathe!

Well speaking as a professional engineer who has spent much of the last 20+ years in motor and inverter design, I'm not inclined to accept that I've been talking out of my arse all this time. If the motor really does behave differently with one phase reversed (beyond simply spinning in reverse), then there is something more fundamental going on, like it's not correctly wired up as the 3-phase motor we have been talking about.

I would guess that the most likely explanation is that it was a 2-speed motor (6 windings) and they haven't been correctly reconnected when changed to delta. You could spin it up on single phase (applied across 2 of the phases and given a kick to get it moving) and see if the 3rd phase voltage is what it should be.

This guff about "mutual inductance" is just that. A 3-phase motor has interaction between the windings but the phase of the individual windings plays no part in correct operation.

Murray

Nevertheless Murray, as the link I posted above and other research on the internet reveals, it does matter!  Like you I didn't think so until I looked into it.

Edited By John Haine on 12/12/2017 11:53:19

If I didn't matter then why do manufacturers go to the trouble to label the tails? Could be a big payout for someone in their ideas scheme if they can just use 3 colours instead of 6 or six unique identifiers.

Mike

Had to go out in the freezing workshop to try the theory. Motor connected to inverter runs very smooth, reversed U1 U2 motor runs in opposite direction but with a significant vibration, couldn't find my tacho but it looks slower. It looks like you're never too old to learn Murray, every day is a school day.

Mike

The motor is a single speed bog standard squirrel cage motor and the inverter I used is an old Mitsubishi U100 single phase input.

It sounds exactly like the symptoms described by the OP.

Edited By Mike Poole on 12/12/2017 21:52:15

Edited By Mike Poole on 12/12/2017 21:53:22