Rotary Phase Converter – Part 2

I’ve sadly used the big Keiyo Seiki lathe only a handful of times since getting it, as what passed for my rotary phase converter (pull cord, 10HP idler and a big disconnect switch) left a great deal to be desired.  Not just in terms of ease-of-use, but also in terms of voltage balancing, and exposed wires just waiting for errant chips to come into contact.  Getting the rotary phase converter past the pull cord stage has taken a good deal of time because it’s somewhat of a pain, and I’m not entirely certain of what I’m doing (which is probably why it’s somewhat of a pain).  Given that 240VAC will provide some impressive sparks and smoke (not to mention affecting one’s nervous system in a negative manner should the conductance of Homo sapiens come into play), I’ve approached further development of the unit with a good deal of caution.

My primary guide has been Jim Hanrahan’s tutorial, which I’ve referred to continually while adding indicator lamps and pushbuttons to the system.  At this point, I still only have the 10HP idler in use, with the 7.5HP idler still to be added.  With the two idler motors on the bottom shelf, I’ve been adding the various controls to the second shelf (the top shelf of the cart will be a good spot to keep the dividing head and tilting vises for the mill).  Here’s a breakdown of what the system looks like right now:

A) The unfinished control panel is a rather flimsy piece of sheet metal – I’ll need to stiffen it up before I mount it permanently.  On the left of the panel (top of the photo) are two indicator lamps – one for the incoming 240VAC and one for the 120VAC from the transformer (I know, I could have run a neutral line from the breaker panel and gotten my 120VAC that way, but this is how I decided to run things).  The center modules consist of a start button (which has two contact sets – one connects to the motor starter, the other connects to the starter capacitors.  Ideally, I’d use another contactor for the starter caps, but the pushbutton contacts are rated for 10 Amps, and they should last a good long while), a stop button, and an indicator lamp.  The empty holes will contain the start and stop buttons and indicator lamp for the 7.5HP idler once I get it wired in.  The pushbuttons and lamps are from Surplus Center.

B) Motor starter for the 10HP idler, purchased from Igor Chudov.

C) 200mF 370VAC run capacitors from Surplus Center.  Note that one lead on each is disconnected – more on this later.

D) Bank of ten 64mF 220VAC start capacitors, also from Surplus Center.

E) Step-up/Step-Down Autotransformer from Jameco.

F) Power distribution board, consisting of a few terminal bars from the hardware store mounted on a piece of polycarbonate.

G) Motor starter for the 7.5HP idler (currently unused, and it seems to smoke a bit when I power the coil).

I’ve needed to get the lathe going in order to kick out some custom paintball gun parts for my friend Blue Fish, so recently I fired up the RPC for some actual use.  Jim Hanrahan’s guide seemed to indicate that there’s simply no substitute for measuring volts/amps of an RPC setup to dial in the system with regards to capacitance (the 200uF run caps were simply a guess as to the needed value – that, and large run caps aren’t easy to come by, so I thought I’d start on the high end).  With the beefy run caps in place, I fired up the system to see what voltage and amperage I had on the 10HP idler without it powering any equipment:

Unloaded Amps:

Red 0.2 – Blk 22.3 – Wht 16.5

Unloaded Volts:

Red/Blk 280 – Red/Wht 315 – Blk/Wht 233

Wow, the voltage differences are pretty severe – how do the values change when I’m actually using the lathe?

Loaded (lathe running at 600 RPM) Amps

Red 7.6 – Blk 21.3 – Wht 12.8

Loaded (lathe running at 600 RPM) Volts

Red/Blk 261 – Red/Wht 297 – Blk/Wht 234

Things look a little better when the system is actually in use rather than standing by, idling.  Still, the noise from the idler is excessive – rather than the quiet purr I recall, it growls continuously and vibrates the whole cart.  Even more, the housing of the motor was HOT after use.  Something was certainly not right, so I disconnected the run capacitors and tried again.

Unloaded Amps

Red 0.2 – Blk 13.1 – Wht 12.1

Unloaded Volts

Red/Blk 212 – Red/Wht 209 – Blk/Wht 233

Loaded (lathe running at 600 RPM) Amps

Red 3.3 – Blk 12.4 – Wht 14

Loaded (lathe running at 600 RPM) Volts

Red/Blk 203 – Red/Wht 211 – Blk/Wht 211

Wow, what a difference!  Running with no run capacitors whatsoever seems to result in a much better voltage distribution.  What’s more, the idler is much quieter and stayed cool while running.  This is a pretty clear indication that I should just leave the run caps out of the circuit, and the voltage and current may even out even more once I add the 7.5HP idler in.

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