Myford inverter & top speed

Hello all, could I ask some advice please regarding my Myford S7

Currently I run the machine using a Transwave inverter kit.

I have a 4 pole motor running at 1370 RPM at 50hz

Running the motor at 70hz and using a 63mm pulley on the motor to the large diameter pulley on the clutch gives me a chuck speed of 1250rpm.

I would like to get this up to more of the plate speed which is 2150rpm.

If I raise the motor to 120hz I can achieve this speed but I would imagine this is not good for the motor.

Can I ask what others have done with reference their pulley sizes ?

Thanks in advance.

Your motor wont mind at all running at 120hz indeed I wouldn’t worry up to 150-160 hz

At 50hz a 2 pole motor would have a nominal speed of 3000rpm less slip percentage which may be about 5% so true rpm about 2850. The bearings used in 2 pole or 4 pole motors will more than likely be the same so running your motor at the same speed as a 2 pole motor should not cause any issues. There will probably be the same rotor in a 2 pole and 4 pole motor as well so that will also not be an issue. Add in the fact that motors with this nominal speed of 3000rpm may be driven, in some situations, by an over-running load to higher rpm then the manufacturer will have taken this into account as well. The motor will not disintegrate at 3001rpm. If there is a possibility the motors will be used with 60hz supplies then the nominal speed and possible over-speeds will also go up by 20%. This all matches Ian's comment about not worrying about running at 150-160hz. The biggest issue will be that if you increase frequency the inductance effect of the coils will reduce the current that can be pushed through the motor so don't expect the same torque that you get at 50hz. Chances are though that high speeds are wanted for small diameters so torque should not be too much of an issue.

Martin C

I run a 1Hp 4 pole motor on my Super 7, I bodged a pulley onto the motor to run the lathe so I could modify the standard 2 speed primary drive pulley to fit the new motor, I've found that with the final drive belt on one down from the highest speed the inverter and the two primary drive ratios do pretty much everything necessary.

Speed wise +1 on the comments above, I run my motor up to 3000rpm and have had no issues in over 15 years

Regards
Nick

Thanks all the reply's

I have also spoken to Transwave who basically said it will be ok, just be aware of the braking and rapid deceleration ,

Thanks all.

Without any calcs and not knowing the exact set up, are your pulleys about the same size? If so, up the motor pulley by about 50% and that will do it. If your pulleys are different sizes, perhaps swapping them might get you closer?

It is always better to run your motor as close to its design as possible, if you want the best from it. The motor should be capable of starting the load at the correct gearing.

As far as braking is concerned, there is a risk of a screwed-on chuck unwinding if the deceleration is forced. Depends on the inertia of the chuck and workpiece. Too late if it has already landed on your toes.

A further question might be: is the chuck rated for those sort of rpm’s?

.

ndiy ... You express admirable concern regarding the chuck rating; but, as per Brian's post ^^^, unless the lathe is fitted with the wrong chuck ... all should be well.

Please see **LINK**

https://www.myford.co.uk/acatalog/SUPER-7-PLATE-FOR-SPINDLE-SPEEDS-1619.html

MichaelG.

Has anyone actually had a chuck unscrew itself from a Myford lathe. Its so often referred to, but I have never spoken to a person who has actually witness the event. I have had a Super 7 for more than 30 years and it's never happened to me and I do run the lathe in reverse especially when screw cutting. The lathe is fitted with a very early inverter from RS and on reverse I wind up the speed and has been well so far. John

I assume the anti-unscrewing set screw on the Myford Connoisseur is put there for a reason..

Neil

Not a myford but had lots of occasions of chucks unscrewing off graduate woodturning lathes when a large lump of wood didn’t want to stop as quickly as the motor wanted to stop.

Even with the vfd set for a slow slow down it still happens ...I do have grubs fitted for occasions when I definitely dont want it to happen

Thread on a graduate is 1.5” 6tpi

Not a myford, but similar to one made by them at one time. Chuck started to unscrew when the speed was reduced too rapidly. Not going fast, so a quick increase in speed sorted the problem. It was in back-gear at the time. I am more careful, when reducing the spindle speed, since.

So, yes, it can happen. One u-toober made a brake for his lathe spindle (which sports a screwed on chuck). After a warning to be cautious, the device has never been seen in action again.

I used to occasonally get a 5 1/4" chuck to unscrew if i didnt put a cut on in normal direction first. Likwise starting up in reverse. Very rarely had chuck unscrew turning opposite direction and a cut on.
Right pain to get off after usually lock solid requring other means to undo.

With variable speed you would be using a soft start.

Never had a chuck unscrew when stopping the motor but had a back plate mounted ER collet chuck unscrew when using the clutch which concentrated the mind somewhat . . . 🙁