While .. awaiting actual nameplates and numbers from the OP, here are some "possibilities".
Presume that in order to minimize conversion work:
- the OEM 1942 gearbox was utilized,
- that a new pulley was NOT fabbed,
- and that it had come from the same 10EE it is in, now
- and that 10EE had been one of the most common 2500 RPM capable 1942 Round-Dial. Or MAYBE a 4,000 RPM one?
- presume also the AC motor is comfortable at a 90 Hz (150%) VFD "overclock". , but not presently being pushed to the riskier 120 Hz (200%) overclock.
- ignore the 50 to 70 RPM "slip" for the moment, lets run simpler "close enough" numbers at "synchronous" or nominal motor shaft RPM.
- also for simplicity, let's take the pulley ratio at 1:1. They aren't, exactly, but "close enough" where a 3 HP large frame @ its nameplate max safe 2400 RPM was to deliver a Field-weakened spindle RPM of 2500.
Possible AC motors include:
-
two-pole 3-Phase, 3600 RPM nominal @ 60 Hz, 5400 RPM @ 90 Hz.. we need no go further. Even if seeking 4,000 OEM RPM, not 2500? He would already HAVE that. So the OP probably does NOT have a 2-pole motor.
-
four pole 3-Phase, 1800 RPM nominal @ 60 Hz, 2700 RPM @ 90 Hz, 3600 RPM @ 120 Hz.
Possible choice. Common, even. But unlikely in THIS case. A VFD tune-up and he would be already be above a 2,500 RPM OEM with OEM 2,500 RPM pulleys. IF the OEM "donor"
had been a 4,000 RPM machine and the same pulleys for that had been retained? 90 Hz would have put him above the 4,000 RPM spindle RPM as well.
Just a guess, but I
doubt he has a 4 Pole motor. Or he would not be complaining.
-
six pole 3-Phase, 1200 RPM nominal @ 60 Hz, 1800RPM @ 90 Hz, 2400 RPM @ 120 Hz.
THIS.. smoother than 4-pole - is a "not uncommon" selection. Even so, it should be able to reach OEM spindle RPM of 2500 with pulleys from a 2500 RPM host, or 4,000 RPM with pulleys from a 4,000 RPM host with no more than VFD tweaking.
Possible. If 6-pole it is? Just tweak the VFD.
Now .. the most probable culprit after an under-tuned 6-pole?
The smoothest at low RPM, rather UNCOMMON, but often CHEAP off eBay because the NEED for them is also uncommon and they do not sell as fast?
8-pole, 3-Phase motor.
- eight-pole 3-Phase, 900 RPM nominal @ 60 Hz, 1350 RPM @ 90 Hz, 1800 RPM if yah dare 120 Hz.
Yaz, Marathon "Black Max" inverter-duty (and not only) with dataplates the size of a cookie sheet listing all their options, limits, and "goodness"
can go waay faster.
But if yah HAD one of those, AND any decent VFD? It already WOULD be going waay faster, yah?
An 8-pole motor can be a challenge as to hitting OEM RPM with either set of OEM pulleys. Shop-fab motor/gearbox end pulley - and a BIG f**ker at that, 'coz yah can't make the spindle one a great deal smaller and still fit, and still carry useful power - is pretty much
unavoidable with an 8-pole motor.
UNLESS.. the person who did the original conversion was more into larger-diameter chuck and faceplate working, and NOT so much into smaller diameter higher-RPM collet-working.
If best performance at mid and low RPM were all they really wanted? IF HSS/Cobalt were good enough, carbides less commonly utilized and not a big deal?
Performance would have "been there", and smoothly, even when going slowly.
Should the OP swap to some other OEM pulleys? Or make new ones in calculated "custom" sizes"? The math was never hard. Metal can be purchased in large diameters.
All that can still be an AVOIDABLE pain in the ass?
So I might not bother. Lazy Iyam.
If your TIME is worth a damn? It could be faster, easier, not all that expensive... to
change the AC motor instead of messing with pulleys at all.
Given I have the spares to do it, no new out-of-pocket spend nor any fab work?
I'd go back to OEM Dee Cee. End of AC and its fragile, complicated, VFD problems, altogether.
Or.. even LESS work?
Just run what you GOT!