Evening Charles, apologies for the delay - had a few late nights at work
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I kept the three-speed switch as I wanted to have best-possible torque at low speeds (and the full Holbrook Experience) - the 4-6-8 pole motor gives more torque and usable power (twice as much, at least) in the low-speed (8pole) config' than running the VFD at a low frequency (although I can and do run down as low as 5Hz/2.2 spindle RPM for "jog" and a few special applications), its inductance in 8-pole chokes off the winding current at high frequencies (I go as high as 83 Hz to get the spindle up to 2500 RPM for short periods rather than the original 1500 max.). The Holbrook also has a bit of an inconveniently big gap between the two top speeds, a VFD allows a bit of fine-tuning and some clever stuff like near-constant surface speed for facing and parting (like the Big Boys have on their CNC machines), dynamic braking etc.
I use a configurable relay in the VFD to switch *off* an external fan mounted on the end of the motor when the VFD output gets *above* 30Hz to ensure it gets enough cooling airflow (and it cools it between runs - plus it reminds me I've left the power on!)
As for star points... the Holbrook's motor uses a Dahlander pole-switching setup for the 4 and 8 pole settings (which switches power to the middle of each phase winding of what would otherwise be a delta-winding and connects the "corners" into a "star" point versus just powering the corners) and a separate delta winding for 6-pole use - which is probably why its motor's getting on for the size of a dustbin while only making 3 HP! It takes some thought to get your head around (or mine, at least...)
The RPC is currently coming together (slowly, waiting on the proverbial Slow Boat from...), built on a budget:
5.5 KW (7.5HP) 3-phase 415v 4-pole motor off EvilBay for £70;
Oxford 180A oil-cooled welder (ancient, 240 and 415 tapped primary, 6.5 KVA *continuous* rating, peak 11 KVA - should do the job!) £19 off EvilBay;
32A type-C MCB for 240v input, assorted contactors, 9-13A motor overload (plugs into motor contactor),16A 3-phase MCB, 40A 3-phase RCD (EvilBay) between £5 and £15 each;
Handful of "RUN" rated capacitors for the motor start circuit (50, 100, 200, 300 uF 460 acv - to mix and match) £5-£8 each on EvilBay;
3 off on/off/lamp pushbuttons £1.99 each on EvilBay;
24v control transformer (for contactors and lamps) from my junk pile;
pair of 16A 3-phase sockets (junk pile);
small electrical cabinet with the door cut out and replaced with some Alupanel to take meters, sockets, switches etc., 2.5mm/4mm SY braid-covered cabling to hook bits up, all skip-dived at work
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lily-gilding: 3 off 500vac meters for phase-to-phase; 3 off 20Aac meters for the phases and a 50Aac meter for the input, £5 - £8 each, guess where; 24v panel lamps for fault indication £1.29 each....
Soooo - a couple of hundred quid, with careful shopping?
The motor and transformer are both a little bigger than I was expecting so they'll be living outside in a kennel built from some leftover Dexion shelves and angle!
The RPC is probably needlessly complicated, but not a patch on the VFD implementation on the lathe...
Dave H. (the other one)
(previously an electrical / electronics engineer by trade, in case you hadn't guessed, now moved on to photons, bits and picoseconds)