New Holbrook Lathe to replace my Boxford

John

Generally when choosing a motor for VFD conversion you are better off going for a 6 pole, 900 nominal RPM, one as this gives better torque at the lower speeds. Or even 8 pole if you don't plan to go fast or your machine large. In a practical world high speed is generally associated with small jobs done with small tools so power loss at the high end is rarely an issue. Its down the bottom where you are swinging large jobs that you want full power.

These days teh price gap between new 4 pole and 6 pole motors has narrowed considerably so 6 pole is economically viable if you decide to go for a new motor.

I'd certainly never choose a 4 pole motor to replace a two speed one if using a VFD to get the required speed range.

The aforementioned Holbrooks, like many high end British machine tools of the era, used a 3 speed compound motor. Basically a two speed motor giving the low and meduim speed range with a single speed motor built on the same shaft for high speed. Wonderfully engineered but monstrously large and heavy. Rewinding costs are basically "take out a mortgage" so pretty much anything is cheaper.

Clive

Congratulations Joe,

Define 'runout' - how are you measuring it? No Holbrook should have 4 tenths runout in the spindle and especially not a plain bearing machine. Also, spindle runout should not be adjustable in the bearing. Spindle PLAY, yes, but not runout.

I agree with Clive on 6 pole motors on VFDs.

And don't forget VFDs allow you to increase the frequency and speed not just reduce it. An "inverter" rated motor may run at several times the nominal 50 Hz speed. The limit is normally the mechanics of the bearings roator and fan. The 1/2 HP Gamak 6 pole motor I bought for the myford has a rated 50Hz speed of 910 RPM (no-load 1000) is rated to tun at up to neary 4 times its nominal frequency,195 Hz or 3500 RPM. Torque will drop off and eventually power too. Clearly however this 6 pole motor can happly run at the nominal speed of a 4 Pole, around 1450-1500 RPM so there is no loss of speed and a gain in reduced torque ripple.
Most "normal" motors are rated at 50/60 Hz so automatically can run at + 20% (~1200 RPM) within their rating and any modern 6 pole motor should handle the 75 Hz required to speed match a 4 pole motor speed.

Robert G8RPI.

When the museum was left a Tom Senior light vertical mill in a legacy, I did two things to it at first. The spindle was changed to R8 from MT2 and the 1/2hp single phase motor was changed to a 1hp 6 pole three phase one. This is powered from single phase mains via a Schneider Altivar 12 programmed to run between 450 and 1350 rpm,(25-75 Hz) with remote control. Together with the 4 speed belt drive, the spindle speeds have a good range. Top speed as set is 3000rpm, and could be 4000rpm with only 5 minutes added programming which is easy because of the excellent "quick start guide" from The inverter Drive Supermarket, which also makes the wiring a piece of cake, none of the whinging about not being able to understand how to set it up so common on this forum.

Edited By old mart on 30/11/2022 18:20:52

Joe,

Run this test on your Holbrook. This is from when I measured a South Bend 10L spindle. You should have no movement to see.

**LINK**

Joe, went missing 5 years ago? No sorry he has an album dated 2020.

Tony

Edited By Tony Pratt 1 on 30/11/2022 19:52:47

Damn I never twigged that it was a resurrected thread

Mike you would be better off to post your enquiry in a new thread with a relevant heading. As you can see, it has already been lost in the mists of time in this old thread.