Help with VFD problems and 3 phase electrics

Hi, I'm new to the site but already finding it a font of useful knowledge and discussion.

I am seeking some experience with the following issue I have.

My setup is on a Boxford AUD 4.5" lathe (similar to under driven Southbend)

The motor is a TECO 0.75Khz (1HP) 3 phase, 4 pole 230v AC driven unit

https://docs.rs-online.com/7405/0900766b8120451f.pdf

fed by a 230v AC supply via a Chinese TECO E2 VFD

(https://elektromotor-online.nl/wp-content/uploads/2016/10/E2_ManualEnglishV11.pdf)

equipped with remote Fwd/Stop/Reverse switch with single turn 0-10v resistance Potentiometer (POT).

The lathe has been running perfectly until about a year ago when I noticed a decrease in the torque of the lathe, when working on pieces I would notice the spindle slowing down on cutting.

I haven't needed to use the lathe for about a year and it has been stored under cover in my outdoor workshop. I went to use it a couple of days ago and gave everything a spring clean and checked power etc and switched on.

The lathe ran in both forwards and reverse although I noticed that a couple of times I had to nudge the chuck to get it to run forwards. I also noticed that the POT would cut out part-way through the full range. I remove the POT cover and tested the POT and found that it had a defective central terminal (flexing and losing contact) but, could get it to respond across the full range if the terminal was pressed. I have replaced the POT with a new one of the same rating and tested the new POT and it works normally.

However, on switching on the lathe, only the reverse works and still with poor torque.

I have checked the frequency rise on the VFD LED display during POT adjustment and it records the Hz changing as expected between 0 and 50hz as programmed when switched in both directions on the remote.

I have checked the output voltage of all three output terminals from the VFD whilst switched on to max 50Hz running and am getting inconsistent values across all three terminals. Some report 230v some 140v and 160v. These voltages vary between the terminals.

Should I get steady 230v reading at each terminal and see a corresponding voltage decrease as the POT is turned down?

My suspicions are that the VFD has bitten the dust

Sorry if this is a bit of a long-winded post but would like to try and narrow this down a bit further before committing money out on the wrong parts for repair.

Thanks for reading

George

Just replace the VFD and see if it works. Outdoor storage is often very hard on electronics.

This would be more appropriately posted here -> https://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/

yea...storing a lathe outdoors is not gonna get you a lot of sympathy on the electrical or mechanical fronts.

It's kept in a proper garage out of the elements

George Cooper said:

It's kept in a proper garage out of the elements

Click to expand...

Condensation is still hard on electronics not in a suitable enclosure.

Do you own a voltmeter of reasonably good parentage?

Its a variable frequency drive, not a variable voltage drive. Your phase to phase voltages should all be the same. Some Chinese drives last a while, some don't work out of the box and most fall somewhere in-between. A junk 3hp VFD is $100

In the rare occasion I use a cheapo VFD I buy 3, 1 to use and the other 2 hoping 1 of them works for when the first one dies

If you are going to buy a new VFD - I personally think that the Control-Techniques drive with manuals written in the Queen's english are hard to beat . . . too bad the brits sold out although the brand is now owned by a high quality manufacturer out of Japan with engineering still done in England.

Joe Gwinn said:

Condensation is still hard on electronics not in a suitable enclosure.

Do you own a voltmeter of reasonably good parentage?

Click to expand...

Yes, I do have a good parentage multimeter. My limited understanding of 3 phase led me to believe that all terminals should stay at 230v and not change when the frequency changes. I tested the 3 output terminals noting fluctuating voltages across all three.

Thanks

As so eloquently stated by Hardplates it not a variable voltage drive but variable frequency. Think pulse wave modulation (PWM). It is changing frequency not voltage BIG difference.
If you have an O-scope and are familiar with how it works when measuring voltages; hook it up and you can see the waveforms change in frequency and not amplitude. Sometimes a good visualization is worth a thousand words (At least that is what the wife tells me.) Be sure to use a differential probe on your scope, unless you are hell bent on blowing it up. On the majority of scopes the ground on the front of scope is directly connected to the ground on the chassis. Which is connected to ground lug on the scopes power plug...…...hence damage to scope and possible you.
Apologies if you knew all this; but safety is most important.

Yeah, what Doug said above. Your meter is calibrated to read the RMS (root mean square, basically means average) voltage at 60 hz. The modulation frequency of the VFD, while it can differ and many quality units allow you to change it, is usually above 20khz. With that said reading the output volts with a standard VOM is meaningless.

Remove the mechanical load from the motor and try to run it. Take note of its operation, noises, etc. especially at low speed. While there could be an output transistor failing to turn on, most likely it isn't. Output transistors almost always fail shorted.

Good VFDs will have current limit capabilities which will allow the motor to operate overloaded at reduced speed. Not a good way of stating it, but basically when the current limit is reached that's all the torque you're gonna get. Better VFDs will time this event and shut the motor down when the time limit has expired.

As for the potentiometer get yourself a good, sealed, wire wound type of the same ohmic measure and linear taper. You can usually get by with a higher resistance version without issue. Don't go lower. Run of the mill carbon pots have a carbon trace printed onto a substrate that will wear away in short order causing intermittent performance. The tiniest amount of abrasive dust will accelerate that wear.