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EVguru
01-06-2010, 03:20 PM
My Harrison L5 is finally all back together and the headstock is now oil tight.

I took the motor apart and located and seperated the star point to re-wire the motor for 240V. With the new inverter temporarily hooked up and after correcting the base frequency setting (400Hz resulted in just 50V on the motor and no torque) it ran up nicely.

Part of the reason for going with the inverter was to raise the 750rpm top speed. Harrsion sold 1500 and 2000rpm versions and I've determined that that it was only the motor and/or belt drive that was different. After running the lathe at 750rpm for a couple of hours to make sure I hadn't over-preloaded the spindle bearings, I began pushing the speed up. 100Hz and 1500rpm was no problem and so I went on to 134Hz and 2000rpm. With temperatures just above freezing this is getting near the limit of the motor/inverter, but I'm unlikely to be needing too much torque when machining at this elevated rpm and hopefully the workshop will be a bit warmer when fully comissioned.

rkepler
01-06-2010, 03:54 PM
Part of the reason for going with the inverter was to raise the 750rpm top speed. Harrsion sold 1500 and 2000rpm versions and I've determined that that it was only the motor and/or belt drive that was different. After running the lathe at 750rpm for a couple of hours to make sure I hadn't over-preloaded the spindle bearings, I began pushing the speed up. 100Hz and 1500rpm was no problem and so I went on to 134Hz and 2000rpm. With temperatures just above freezing this is getting near the limit of the motor/inverter, but I'm unlikely to be needing too much torque when machining at this elevated rpm and hopefully the workshop will be a bit warmer when fully comissioned.

While the lathe bearings and such can handle the higher speed I'd advise some caution on an older motor. Often these won't be real happy at more than double their rated speed.

EVguru
01-06-2010, 05:02 PM
While the lathe bearings and such can handle the higher speed I'd advise some caution on an older motor. Often these won't be real happy at more than double their rated speed.

I'm often asked by classic bike owners whether they should have an unleaded conversion done on their cylinder heads.

"Might as well wear out what you've got now"

I'm taking the same approach to the motor.

John Stevenson
01-06-2010, 05:30 PM
Motors are cheap enough :rolleyes: :p

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rkepler
01-06-2010, 05:32 PM
Motors are cheap enough :rolleyes: :p

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It's not the motor failing, it's the sudden stop for everything attached to it. If just the bearings go it's not a real problem, if the rotor fails that's another and real problem.

John Stevenson
01-06-2010, 06:18 PM
It's not the motor failing, it's the sudden stop for everything attached to it. If just the bearings go it's not a real problem, if the rotor fails that's another and real problem.

As many know on here I do a lot of electric motor repairs and I can't remember the last time I saw a rotor that let go.

Rotors are 'iron laminations that have an alloy 'conductor' cast around the laminations and then machined off.

Early rotor designs have the slots in the laminations stamped as dovetails so the cast conductor can't fly out. Modern rotor design has closed slots so the conductor has no access to the outside and the OD of the rotor is a 'solid' stack of laminations.

Also bear in mind that the high frequency motors built for router spindle heads like the 12,000 rpm Wadkin Spindle Moulders share the same rotors as the standard 2 and 4 pole standard motors.

So if they won't let go at 12,000 rpm they won't let go at 3,500 rpm.

What does limit how much you can overspeed a motor is the stator design. Older motors have a lot of iron and copper in them and these can get eggbound to use the correct technical term and what happens is that as the Hz increases the revs actually drop, the motor sounds like it's lost a phase and it gets hot.

Another problem with high Hz is that it creates vibration in the stator [ sorry don't know the correct term ] and one of the common results is that the binding round the windings cracks and breaks away with the loops dropping into the motor and even causing them to jam.

Modern motors don't suffer from this because they now use two pack resins instead of the old fashioned shellac.

If you have any worries about over speeding and the motor will allow it then get the stator re-baked with modern resins.

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