this is possibly not a relevant observation but if your considering changing the motor would you not just get a single phase motor as your running on single phase
As advised above i fitted the bars back and tested again without inverter connected. floats between 7.0 and 7.1 between phases it's sensitive as to where the shaft is you can get them all the same at certain times. I then connected back up to the inverter and ran it upto speed. Voltages exactly the same across all 3 and the Amps the same too without any load they were at 0.3amps each. Next step I'll connect it and check what they look like under load.
I'm also going to take to top off the gearbox and just check for anything daft it feels no different to when i refurbished, it spins nice and light in low gears getting harder as you move up. In high gear spinning the chuck feels free and smooth I can't feel anything untoward that would stop it getting upto high speed.
Quick update not fixed yet but Black Friday came up trumps with a new 1.1kw motor. I thought what the heck it will help get to the root of the problem one way or another and if it turns out to be the VFD then it’s a bonus as I have a motor for a belt grinder project
I've just realised I want one of those ratchet screwdrivers with 1/4 drive end, I've got a couple of normal ones,a stubby,a short T handle,a long T handle, T handle 1/4 drive ratchet but I don't have a normal handle ratchet 1/4 drive.
It was the motor, fitted the new one and changed the VFD to the new plate specs and bang it span up to 1500 RPM within a second. Totally different lathe now.
Need to patch in the new motors thermistor to the VFD, then tidy it all up. I'll change the gearbox oils too and tidy it up she's looking a bit of a mess as i haven't cleaned it since the restoration last year!
Well done on finding the problem. Sometimes everything tests out ok and you have to try and isolate the issue by swapping stuff, it’s not always possible but sometimes it’s the only way. My guess would be the motor might have showed some imbalance if you tested with an insulation tester/megger, if you get the chance of picking one up cheap they are handy tools for stuff like this but if it’s not something you do often then maybe a bit overkill.
Thanks @daleyd and thanks to everyone else who provided advice.
A megger has been on my watch lists at auctions for sometime, i'll continue to keep and eye out as like you say its one of those things thats handy when you need it. I have had troubleshooting in the past where it would have helped. Cant justify buying one new just need to keep and eye out.
Side note with the new motor the braking force is brutal, i need to back it off in the vfd settings tomorrow i don't want to but such stress on the lathe. I have the optional external resistor for the VFD which enables special regenerative braking duty but i think its a bit OTT for the lathe.
yes having another quick play this morning its clear that whilst the old motor was 0.9kw and this one is 1.1kw its far super in torque when performing heavy cuts there is no speed reduction at all. The VFD was constantly doing some trickery with the old one to keep the speed up when turning there is no sign its having to do that with this one at all.
wiring tidied, thermistor wired up, VFD braking and thermistor parameters set, lathe cleaned up and all the gearboxes had their oil changed, checked spindle bearing torque.