Lathe/VFD/DRO issue

I have an issue in that when I start my lathe the readouts start incrementing without me doing any machining. Could it be an earth issue? I know all my wiring is correctfor the lathe to run with the vfd.

Roger

Could be earthing or interference issue. Do the DROs and VFD both have screened cables? Could you post a wiring diagram showing the main connection points especially where the earths run?

Hi, Rodger,

Does the DRO appear to increment normally if you turn off the VFD, and rotate the lathe manually? - if that appears OK, then I would suggest that the problem is caused by the VFD - you probably know they can produce high levels of interference, so this would suggest a problem with the VFD screening/connections.

Keith

What lathe, VFD and DRO?

Are they all properly CE / UKCA marked?

Did this just start happening having previously worked OK or has never worked?

Was the VFD and DRO fitted to the lathe when new?

I'l now make some assumptions, You have just fitted the DRO or VFD and now have a problem. It is likely that emissions from the VFD are affecting the DRO. This does not automatically mean the VFD is at fault, the whole system must work together. However if the VFD is not fitted in a metal cabinet with a filter(s) and screened cable it is probably producing emissions above accepted limits.
The DRO might be overly sensitive to interference or not.
Answer the questions an I can make suggestions.

Robert.

OK, both have screened cables and the dro works normally with the vfd switched off, also with it on but motor not running.

vfd connections are as below:-

and the motor is earther inside the box

DRO screened cables:_

These are both plugged into the ring main in the workshop.

I have a mill with the same make DROs but single phase motor and don not have the same issues.

I don't know if it is earthing issues or interference from the motor but it's not my skill set so would appreciate any help with this.

Thanks

Roger

The vfd & DROs were purchased from Vevor UK so I 'assume as they are from a UK warehouse' that they are CE marked (legally)

The lathe is a 1937 Denham Junior MkII so not fitted from new, other info requested can be seen in my previous post.

Roger

Hi Roger

Some makers suggest connecting the screen to earth at both vfd and the motor, others say only at the vfd so check what your maker suggests.

Get ready for those who will tell you the cables should be terminated in a gland plate, no doubt a good idea but not related to your interference problem.

Emgee

I have to admit I've not tied in the screen to earth and will do so, I have no doubt after following some of the previous discussions in the forums I will be slammed by some for not following current regs whatever they are now but really hope they restrain themselves and focus on the easiest methods for me to get my DROs reading correctly

Roger

Edited By Rockingdodge on 21/06/2023 18:53:41

Do you have a suitable EMI Filter between the mains power and the VFD?

I have heard of folk not having one fitted experiencing all sorts of odd effects with other connected equipment, especially cameras, tv's etc.

If the screening does not sort the issue it could be worth a try but I would refer this to someone far more knowledgeable with these things than I !

I fitted one as it was strongly suggested to me and when my VFD arrived (A siemens unit) the instructions for the model I had said it had to have one.

Spark eating crocodiles and loom tangle weevils, its all witch craft I tell you, witch craft!

EDIT.. Sorry, I see Robert already pointed out the filter.. 

Edited By Taf_Pembs on 21/06/2023 20:20:22

Do the DRO reader head cables run parallel to the motor cable from the vfd?

If you disconnect each DRO reader head in turn does it still happen?

Where is the DRO display situated in relation to the VFD and motor?

The motor cable is running in the opposite direction to where the DRO cables are.

I'll try that when I'm in the workshop tomorrow.

The display is approx 18" (1650mm) away but at the same height.

Rodger, If all seems OK with the motor not running, then its likely the problem lies with the connection from the VFD to the motor, or the VFD/container grounding. If the motor is not running, then the VFD is not producing the "modified" drive sinewave outputs (actually composed of square waves). The edges of these square waves are usually what produces the interfering harmonics, so if the motor is not running, then any interference is much reduced.

Make sure the earth(s) you mentioned are made, then check that the motor cable (which appears to be screened in your photos), is firmly connected, and that one or both ends of this screen are also grounded. If that doesn't have any effect, then I would try re-routing the motor cable. These are the simple possibilities, and must be eliminated before moving on to more laborious checks, otherwise you could do a lot of work for nothing.

Thanks Keith, I will do this in baby steps, I know very well from debugging code both in my ex-working life and hobby work that the worse thing to do is make multiple changes, always bites you where it hurts.

Roger

Edited By Rockingdodge on 21/06/2023 22:02:35

Hi Rodger,

The problem is the VFD.

You need to connect the screens of the mains and motor cables to ground. This ideally needs to be done for the full 360 degrees of the screen, not just a pigtail. Ideally this is done with a metallic cable gland at the wall of the metal box the VFD is in. Or SHOULD be in. as an alternative some VFD suppliers sell a metal plate that screws to the drive and has separate metallic clamp for mains, motor and control cable screens and earth terminals.

You are wrong to assume that VEVOR areapproved because you boght it from their UK webstore. It must have been a while ago as you will find that currently they sell NO electrical equipment from their UK webstore. This is because it was being stopped at the ports and destroyed or sent back for non-compliance. My personal view is it is dangerous junk.
Apart from that NO VFD of this style can correctly be CE or UKCA marked because they are a component, not a finished product. They need enclosures, filters (some are built in) fuses, cable strain-reliefs (or properly rated connectors) and controls. Your installation does not meet basic safety requirements.

As a minimum you need a proper filter on the mains input and earthed screens as above. To meet accepted safety standards vhe VFD must be in a box. Ideally this shoud be metallic but as a minimum it must have a bare metal plate with the filter and VFD mounted on it.

Robert.

Emgee - you hit the nail on the head! - well done.

My personal view is it is dangerous junk.

Robert hit the nail even more squarely on the head. A lot is junk and some is potentially dangerous. I believe all their stock likely falls into a non-compliant category in some way - whether it be safety, conformity, QA or other short-comings.

Roger

Last time we had an issue like this it was a dodgy earth.

The mill dro not being affected when you run the lathe at least says the interference is not in the mains.

Try running the VFD through an extension lead plugged into the mill point. If that's still the same then your installation is bad, if it's OK your ring main earth has a break.

My DRO has an external earth point on it, if your does have you used it? I can see a brass screw head? near the mains lead and no earth lead.

The mains lead 'looks like' screened cable, but has holes in the braid big enough to put your head through and will leak like a sieve unless there is also a foil screen we can't see.

The VFD is in a plastic case so no screen.

Terminating the cables via a gland plate has a little known ability to earth all those required cable screens in one go.

In the end you may have to move the VFD further away or buy it a tin box to shut it up

Keith, Emgee

Robert, NDIY, I assume that you have purchased these vfds from Vevor or had one in your hands in order that you can make a considered opinion as to their quality and fit for purpose'

I stated in an earlier post that 'I have no doubt after following some of the previous discussions in the forums I will be slammed by some for not following current regs whatever they are now but really hope they restrain themselves and focus on the easiest methods for me to get my DROs reading correctly'

This still applies so either help me find a fix or just butt out, I have no time, patience or money to throw away what has worked perfectly without the DROs when I'm getting old enough not to worry if I'm going to wake up in the morning and find I'm now dead!!!

Rant now over. Now considered help please not wrong choices etc.

OK now I've 'temporary tied in the screen to the 2 earths and has not made any difference.

Putting the vfd into a metal box would mean I would not have access to the controls so what next guys?

Roger

So now try Roberts other suggestions of earthing the screening of the cables and fitting a filter if your VFD does not have one.

You, as are many others are using the control panel that is intended for commissioning and service functions as the main means of control. The control of he VFD should be from a properly installed panel with robust buttons and a quality potentiometer if required.

Mike