Something like this...

http://www.aliexpress.com/store/product/New-5N-M-1-5KW-3000RPM-110ST-AC-Servo-Motor-110ST-M05030-Matched-Servo-Driver/314742_760988112.html

As said the power would not be constant, the torque would be though.
You do have approx 3x constant torque for short periods, how long would depend on the drive as well as the motor but it would not be long, more for acceleration really.

So a similar Kw servo  and an AC Induction motor via VFD, the servo would be better but not as good as the induction motor via the gearing, reason being the induction motor would always be at the rated Kw no matter the spindle speed.

I have a 3.5Kw Servo  on the beaver mill and it is fine without back gear but much smaller and I would have my doubts, suppose it all depends what you will be doing.

Hood

I was comparing kW and Nm from AC servo to AC motor.

I thought 6Nm was 6Nm regardless but the servo would do 6Nm at 100rpm and the same at 3000rpm whereas the AC motor would ONLY be 6Nm at 1450 or whatever it was rated at - am i wrong? Obviously not grasped the whole power - torque thingy yet;)

Most of my stuff is done without back gear, only really used it when running the big fly-cutter but don't use it now as not building steam engines anymore.

I tried one of my normal jobs with the mechanical control up high and the motor down low - pretty much the worst case scenario and it worked fine so I should guess that a 1.8kw servo would be better as no de-rating and less transmission losses.

Ok the servo will have a continuous torque rating, 5Nm in the one you linked to, and 15Nm peak.
The Kw will only be at rated speed however, slow the rpm down and the Kw will drop.

It is exactly the same with an induction motor and a VFD when looking at the Kw but the Torque will also drop so the servo is better.

Now what you have at the moment is a VFD running an Induction motor but you are just using the VFD as a phase converter as you only vary the speed via the pulleys.
That means the motor is always running full speed so the Kw will always be the rated Kw. The torque however will vary as you adjust the pulleys, drop the spindle speed from 1440 to 720 and the torque has increased by two, Go the opposite was and run at 2880 and the torque will have halved.



I made a spreadsheet up a few years back to work out torque or Kw  of a Servo when I only knew one or the other along with the rated RPM, I have attached it below.

Hood

Will check spreadsheet tomorrow, thanks

But isn't torque what i need here?

Surely it does not matter if it takes 1kw to create 5Nm or 3kw - the torque is what does the cutting and kw are what is required to create that torque???

Kind of hard to explain but you need a certain torque to do something,  but you also need a certain power to do it at the speed required.
Probably not a good explanation, will try and think of a better way to explain but don't hold your breath

Hood

Ok look here
http://www.kennametal.com/en/resources/engineering-calculators/holemaking-calculators/tapping-torque-and-horsepower.html
You will see the torque required for tapping and you will also see the power required to do it at the surface speed required by the tool.

Hood

A simplistic generalisation is electric motors produce reasonably constant torque across their rated speed.

With varispeed, your 5Nm motor will result in a varied level of torque at the spindle. At say 145rpm spindle, and the motor running at 1450rpm, you have a 10:1 ratio giving you 50Nm torque at the spindle. Going the opposite way, and running the spindle at 4350rpm you have a 1:3 ratio, so you only have 1.67Nm torque at the spindle. With varispeed, you always have the full motor power available at the spindle.

Now if you go direct drive, you only ever have 5Nm available at the spindle. You will only get full power at full rated speed. Half the speed, you only get half the power, but still 5Nm. Quarter the rated speed, quarter the power but still 5Nm.
This is why most VMCs have such high power spindles. If they didn't, they simply wouldn't have enough torque for tapping, or large cutters/drills

Thanks Hood, that calculator rammed it home, seems with a 5Nm motor it would only be able to power-tap to M8 in steel, in reality thats not too bad as I probably would not expect it to do anymore than that on the original head, I always hand-tapped the big stuff.

Now if you go direct drive, you only ever have 5Nm available at the spindle.

Got it.

You will only get full power at full rated speed. Half the speed, you only get half the power, but still 5Nm. Quarter the rated speed, quarter the power but still 5Nm.

This is where my brain fogs-over again - does the lower power matter when cutting or does it just mean less electricity is used to do the work?

If my calculator says i need 4Nm to do a task at 100rpm and the motor is rated 6Nm at 3000rpm - it will do the task easily yes?

This is why most VMCs have such high power spindles. If they didn't, they simply wouldn't have enough torque for tapping, or large cutters/drills

That bit makes sense now, thanks.

Moving on, it would seem if i want to keep all my (theoretical) power, I would need a 3.8kw / 15Nm servo - this is possible BUT I would not have the power to reliably run it along with the table servos etc at full whack the spindle would need 14A alone - back to square one but lessons learnt and no money spent, thanks guys.

So it seems my choices are to use the original vari-speed head and fix the worn out bushes, or try and find a step pulley head which so far has been unsuccessful.

With a step pulley, Mach would know what pulley it was in from the last run and the VFD could give a useable speed range from G-code, my little macro tweak from earlier would tell me what pulley to switch to depending on code.

OR

Use the (repaired) vari-speed head as a step pulley - mark the speed dial 1 - 2 - 3 and instead of changing pulleys, just twiddle the knob to that setting and again use the VFD - this sounds like a reasonable idea to me??

What you reckon....

Have a search over on the home shop machinist forum, as I seem to remember John S(tevenson) posting about overhauling his POS Bridgeport varispeed head a while back.