Myford S7 VFD problem

[Glyn DaviesParticipant @glyndavies49417]

I have a Myford Super 7 lathe with a Newton Tesla AV750 pre-wired VFD and motor. Very happy with it, except – with the drive belt on the faster of the two motor pulley ratios, engaging the lathe countershaft clutch trips the VFD and the lathe stops. Even with the VFD speed setting way below maximum and easing the clutch lever in as gently as possible, it still trips and takes many minutes before it will restart.

Sometimes, pressing the stop button twice and then the start button brings the motor back to life, other times it takes quite some minutes before it will re-start, even if the power is turned off and back on. Newton Tesla’s solution was to advise me not to use the clutch when in the high speed motor pulley range.

Do other S7 users have this issue? Can I drill the casing pop rivets and get to the VFD and alter some parameter that is set too sensitively and causing the trip? If so , how?

Thanks

[john fletcher 1Participant @johnfletcher1]

I too have a Super 7 with a VFD, and after reading about your predicament, I went in my workshop and ran the lathe, no problem at all, top speed 2030 or near enough. Well regarding access to internals I don’t think you should have to do, its access to the programming /setting code you need and what I have seen they are buttons and an external display. Recently I had to buy a new inverter as the 30 year old pre – owned one expired. My replacement cost £68 from PRC as I think many others with fancy name titles may also originates from there as well. With much appreciated help from a member of this forum I was able to program the device myself. I made the remote, wired using SY or is it CY and the remote is screened colours wires, all mounted in a sheet steel box with plenty of ventilation. I hope you soon get things sorted.  John

[peak4Participant @peak4]

Have a word with Newton Tesla before anything else, I think you’ll find them very helpful.
Is the main spindle correctly adjusted and free to turn, with no tension on the drive belt to it?
Is it still free to turn when fully tensioned?
Bill

[Harry WilkesParticipant @harrywilkes58467]

Agree with Bill and maybe just needs the setting ‘ tweaked ‘, i don’t have the same VFD but my S& runs fine on any pulley arrangement.

H

[bernard towersParticipant @bernardtowers37738]

I have the same setup as you John and dare I put into print that mine is ok. Is there any stiffness in the mechanicals of the lathe?

[Robert Atkinson 2Participant @robertatkinson2]

John,
Sounds like a conservative VFD setting. Are you saying that the Newton Tesla enclosure is pop-riveted closed with no access for servicing?
If that’s the case it’s very unusual.

Robert.

[Glyn Davies]

On 29 January 2025 at 14:03 Glyn Davies Said:

… ..Newton Tesla AV750.. … ..with the drive belt on the faster of the two motor pulley ratios, engaging the lathe countershaft clutch trips the VFD and the lathe stops. Even with the VFD speed setting way below maximum and easing the clutch lever in as gently as possible, it still trips and takes many minutes before it will restart.. …

 

… ..Newton Tesla’s solution was to advise me not to use the clutch when in the high speed motor pulley range.. …

..That doesn’t sound very helpful to me – ?

[John HaineParticipant @johnhaine32865]

Yes the NT packages are wholly enclosed and pop riveted closed. No accessible user settings.  Opening the case invalidates the warranty.  Mine has never tripped but I’ll try your procedure tonight to see what happens.  Since I fitted it I do use the clutch less and less, no real need for it.

[peak4Participant @peak4]

Strange, I missed your initial comment Re NT suggesting not using the clutch.
Does it behave if you let the speed ramp up with the clutch engaged?
Mine is on a Warco 720, essentially a S7 clone, but with a taper roller headstock, so little bearing drag.
Hence my suggestion to check for a free running spindle; my real Super 7 did prefer warming up a bit before running at full tilt in a very cold workshop.
I always use the clutch, (bronze cone type) gently feathering not dumping, regardless of speed selection, and have never had a problem.
I did have one tripping an ELCB, but that was a component, which had aged out of spec, and they dealt with it very efficiently.

Bill

[John HaineParticipant @johnhaine32865]

Mine’s ok.

[Glyn DaviesParticipant @glyndavies49417]

Thanks John – looks like I have a duff VFD unit. The headstock bearings are fine and correctly adjusted.

[Rod ClemettParticipant @rodclemett60985]

Hi Glyn,

There was a video posted by ‘The English Hobby Machinist’ on YouTube recently, about his experience of fitting a NT system to his Myford.

He experienced a similar problem with the clutch – so unless that’s your own youtube channel, there’s at least two of you!

I was very close to ordering one of these systems, but I’m not keen on risking an ‘upgrade’ that disables some of the existing functionality of the lathe.

https://youtu.be/YQYKQYAWXZI?feature=shared

Clem

[Glyn DaviesParticipant @glyndavies49417]

Thanks Rod – I found that video and was about to post a link when your post appeared. I bought my NT package two years ago and the video seems to be June ish last year. So the tripping issue has been around a while and not unique to mine. I assume the VFD monitors motor current and trips if the current exceeds a certain threshold. If I knew what VFD is in that pop rivetted box, I’d get a manual and see about changing the threshold. What is infuriating is that the remedial action to reset the trip seems to vary. Sometimes, pressing the stop twice as soon as it trips resets it and the motor will start; at other times I have to wait ages before it will restart.

[duncan webster 1Participant @duncanwebster1]

Ring NT and ask them, I’ve always found them very helpful.

As stopping and starting the motor frquently isn’t an issue with a vfd, I’m not sure why this reduces functionality.

[John HaineParticipant @johnhaine32865]

I believe that NT mainly fit Mitsubishi inverters.

[Charles LamontParticipant @charleslamont71117]

I have a VFD on my Super 7 and use the clutch all the time. You should be able to use yours. Stop the motor just to get a rough vernier progress check on the job? No thank you. Better for the motor, and I don’t have to wait for it to ramp up and down. I suggest you tell NT their ‘solution’ is not good enough.

[Mark RandParticipant @markrand96270]

If the inverter can’t start the motor at full load and full speed, it’s undersized, misconfigured or defective. I’d have words with NT with a copy to trading standards…

[Neil LickfoldParticipant @neillickfold44316]

Sounds to me that there is something a miss with your VFD.  Clutch or no clutch, should not make any difference to the motor running. Even in the highest speed setting.

Ask if they can test your unit for you. A pain to unbolt etc, but there may just be a problem with your unit.

 

[PeteParticipant @pete41194]

One thing you might try Glyn, increasing the motor ramp up time by a second or two in your program settings. I suspect what your seeing is the same as I did with my BP clone mill at the highest speed setting on it’s step pulleys. With it, there’s a lot of mass to get rotating. And because of the increased torque the VFD was seeing above the factory presets on it’s highest spindle speed, mine would sometimes fault out.

But if it does that while using your lathes counter shaft clutch, that to me strongly indicates there’s some kind of excess drag somewhere within that clutch assembly. If so, then the extra torque to overcome that drag or friction might be why it’s tripping out. And it also sounds like that might be causing excess heat within the VFD or possibly within the motor and preventing an immediate restart sometimes. With a decent VFD, there’s a lot of self protection parameters programmed in to them, and any unusual faults could be a sign that somethings probably not correct.

One of my own examples with a non VFD controlled motor. Making a stupid mistake and using a far too heavy gear oil on my Emco lathes powered longitudinal feed many years ago resulted in cooking the motor because of that extra drag once it started to dry out and thicken a bit. That was an almost $800 lesson about why I should be following what the manufacturer recommended. So you might also want to check your own motor and VFD’s operating temperature after the lathe has been used for an hr. or so. If there is any extra drag between the drive motor and spindle, your motor and possibly the VFD might start getting a lot hotter than they should be. It’s also not impossible that everything is fine and the factory presets were just set a bit too conservative by mistake? But I’d still want to be very sure that mechanically, everything on your lathe is 100% correct first. Just in case, you haven’t made any adjustments to the lathes spindle bearing preload before this started happening?

[Glyn Davies]

On Glyn Davies Said:

<SNIP>

What is infuriating is that the remedial action to reset the trip seems to vary. Sometimes, pressing the stop twice as soon as it trips resets it and the motor will start; at other times I have to wait ages before it will restart.

This is due to the design of VFDs They convert AC mains to high voltage DC before converting back to variable frequency DC. The DC has fairly large storage capacitors at around 300V DC. The low voltage for the control electronics is derived from the high voltage DC. Depending on the stage of charge and load when the drive trips the capacitors can have enough charge on them to power the control electronics for several minutes. The electronics won’t reset until the voltage has decayed to a low level.

Fobert.

[John HaineParticipant @johnhaine32865]

Points from the above:

1. There’s evidence that the NT unit works fine under the OP’s scenario (my lathe).
2. Pointless to suggest that he dives in and reprograms the settings as the box is sealed and opening it invalidates the warranty (which he might need to rely on).
3. Myford lathes are unusual, I think, in having spindle clutches – usually the operator has to turn off the motor to stop the spindle.
4. The soft start/stop on the NT unit allows the motor to smoothly run up as many times as you like without harm.  The single phase motor which was standard fit was a clunky horrible unit and starting at least made a horrible clunk.

[noel shelleyParticipant @noelshelley55608]

Cold weather and the oil being thick ? Run it up for a few minutes to warm up the oil/bearings, does it still happen ? Has this just become apparent ? or has it always been so ? Correct oil ? Turning the spindle by hand may be easy but trying to run up to2000 not so. Noel.

[duncan webster 1Participant @duncanwebster1]

I suspect what causes the problem is the sudden change in motor speed when you engage the clutch causing a big current surge. Just give up on the clutch, it’s not needed with a vfd. The soft start/stop gives everything a much easier ride, and the chuck stops rotating more quickly, which avoids waiting, or the temptation to stop it with your hand. The only issue I’ve ever had with mine was when stopping the faceplate, I had the slowdown ramp set too high.

[Glyn DaviesParticipant @glyndavies49417]

Thanks for the replies. The state of charge of the DC capacitors affecting how long I wait before it will restart is good news – suggests that the unit isn’t faulty. If it is a Mitsubishi inverter, it looks like parameter 9, the motor rated current, might be set low and it looks straightforward to raise it slightly.

I just can’t quite bring myself to drill those pop rivets.

[David JuppParticipant @davidjupp51506]

Personal opinion only, treat as such.

These NT packages seem to be standard offer, so I’d have expected that the settings in the inverter were dumped in from a file, rather than entered individually.  That doesn’t entirely remove the possibility of an error being made, but does reduce it.  That the same package behaves OK for others is also a warning.

In an ideal world I’d like to check the internal fault log, to see what the trip code was.

There will be options beyond current overload that can trip a VFD, for example I squared t – which may be downrated below a certain fraction of notional motor speed (I’ve been caught out by that one).

Whilst connected to check the log, I’d probably also monitor the reported motor current to check it is a sensible value, and not somehow higher than expected.

It’s easy to make a change with good intent, that either doesn’t help, or just masks the real problem.

This assumes that it is the VFD that has tripped, not some other protective device.

[Author]

Posts