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New (to me) Lathe - Smart & Brown 1024

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hopefuldave:
Thanks Charles, I'll have to check the wallet... A few other projects going on at the mo', bought a Substantial oldish milling machine a few weeks ago and I'm mid-way through buying bits for and building a 240-415 rotary convertor to power it - the Holbrook's fitted with a VFD hacked to give 415 from 240 in, simple mod' on an older VFD (ABB ACS-300 series) but I ran into the same issue with a 3-speed motor, I *think* the work-around is successful (so far) though!

When converting your Harrison, something to ensure is that the VFD output's disabled in some way if/while switching motor windings - the Holbrook has a microswitch on the speed-change lever (originally dropped out a main contactor) that I've used to a) disconnect the fwd/reverse switch's common feed and b) switch between stop/start configurations so it goes into "freewheel and brake" / "boost" stop / start mode with a longer freewheel interval (1 second, to allow time to move the lever!) by toggling the "parameter set 1/2" input, plus I've added a trio of homebrew load reactors between VFD and speed-change switching (the "Frankenstein Switch") and 1kV MOV over-voltage devices across the motor windings to limit inductive spikes - I simulated it all in LabView and got the results I was looking for, works (so far...).

Some more modern VFDs have a "gate block" input that effectively isolates their outputs (putting the motor in freewheel), may do the necessary but I'd still be wary of inductive spikes from switching windings, and the "gate block" would need to operate before the speed-change switch (the Holbrook has a "gate" on the speed-change lever, moving out of the detents operates the microswitch).

I found the Holbrook (model C13 from about 1955) on EvilBay a few years ago to replace my previous Challenger AG, it's been a bit of a labour of love! ALL the existing control gear had to come out (would have done it anyway as the relays had 415v coils, so 415 to all the switchgear - ouch!) and I'm still looking for a few parts for it and making some others - it cost me a lot less than a secondhand mini-lathe at the time, though, probably because it's not a "name" machine to many and it looked, frankly, Horrible* - it cleaned up a treat, but in the process I discovered a  few things (mostly made of Unobtainium) needing attention or making :)

Once the new old mill's up and running I have a Cunning Plan (thanks Baldrick) to add a fourth control shaft and gubbins for feed/leadscrew reverse, something the Holbrook Model T and H have but which was missed out on the Model C, inexplicably, although the space and most of the necessary parts are there anyway! A helpful lad at work is going to 3D print transparent plastic mock-ups so I can see where I've gone wrong before I start casting and machining parts... Lending him a few hand tools to modify his Airsoft armoury and giving Clear Instructions has worked out well :)

Dave H. (the other one)

* Painted with either an old carpet or a dead dog, compacted swarf and coolant hiding sound metal...

Charles:
Thanks for the info, a couple of questions; possibly silly ones as motor theory has never been one of my strong subjects...

Since the VFD gives infinite speed control, what is the advantage of using the original multi-speed motor windings? in other words why not connect directly to the fastest winding and let the VFD handle the slower speeds? And, if your VFD has been hacked to give 415v out, does this mean that the motor is wired star? if so, what is the motors neutral/centre of star connected to?

There's an ABB ACS-300 on eBay at the moment for $600  :bugeye: don't think I'll be using one of those when the generic Chinese ones can be had for £80!

I would be interested to know a little more about you RPC, cost/complexity etc, as this would allow the use of the original switchgear and potentially save a major rewiring exercise.

Bluechip:
3-Ph. star connected motors don't use a neutral.

The star point is not generally brought out as such. It's just the ends of the three windings that get connected together.

Pic. gives the general idea,  just swiped off the net.

Delta = Mesh, Star = Wye. Different names for the same thing.

D.

Charles:
Bluechip - thanks that makes sense, i guess a motor is inherently a balanced load, so doesn't need a neutral..

hermetic:
You need to remember that if you slow the motor down too much, the cooling will be comprimised and you may need to put an  extra cooling fan to blow on the motor. VFD's can damage windings at high frequencies especially if the windings are not encapsulated in resin, if they are, no problem. Far better to use a 240v to 415 volt inverter, and change speeds with the gearbox!
Phil

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