The lathe does have a reversing clutch so it is always started unloaded.
A "pilot motor" to use the old term, or "idler", because it's shaft needs to NOT be doing any "work", (some are cut-off and covered or were never there) is a bit of a weird duck.
As an RPC, it begins life looking for all the world like any other 3-Phase motor with a MASSIVELY greater starting "inrush" current than it would need once running.. and generating "CEMF".
Double-trouble is that we have to start them off single phase.. with TWO phases absent. Enter the start cap or "pony" motor.
Then.. we get it up off its at-rest knees.. and run it as a special case of rotary transformer ... or watcha might call an auto-generator!
The bugger is STEALING the CEMF to push the relative phases around, coupling the CEMF from what it DOES have out of the wall - into the missing phase to derive the "generated" leg.
Enter the UNloaded L&S final-drive motor, de-clutched.
The windings and Iron in the at-rest condition STILL want a huge slug of current to get moving.
But we don't have as MUCH of the powerco grid back of the RPC as we would have off raw utility mains. The RPC is a limiter as well as an enabler.
A really demanding start can actually flip the RPC idler into instantly reversing its direction of rotation, so much energy is involved.
(At least) one of the commercial RPC makers has (or DID have) a chart on their website that categorized loads by how hard they were likely to be to start and how much to oversize their RPC HP rating vs the load HP rating.
Our common 1.5 times so often used on PM is only the beginning.
Some types of loads need a
far higher multiple for it to work.
The starter pops in cold days with higher speeds,
A "static" phase converter does no "conversion". It simply uses capacitors to trick a 3 Phase load motor into a "third-ass", not even "half-ass" version of its own RPC whilst
also trying to move the shaft load. Can't DO both with caps alone.
It is running on ONE rock-solid phase by robbing from the upstream input source to put enough energy via caacitance into where the TWO, not ONE,
missing phases
should have been.
So the motor does not QUITE fall off its perch to sit locked-rotor and just hum.
That works OK up to around 31 percent of rated load. The theoretical 33% left when the other 66% were never THERE to begin with, plus modest loss.
Then it falls off it's perch into the petrified birdshit in the bottom of the birdcage with uneven heating and progressively rougher operation - until a protective device interrupts the torture. Or it eventually fails a cap and kills the load motor. DAMHIKT. Cazeneuve. Previous owner. "Big Name" static converter. BFD. Never liked Euro-wimp motors in the first place!
The RPC has it limits as well, But up around 91 percent of load-motor nameplate. never quite 100% because even if the Voltage on the "generated" leg has been pumped-up with extra capacitance, it remains a "generated" leg - dependent on
petty theft from its siblings for its tucker.
It can never be as "stiff" under load as native 3-Phase with
all legs being fed
whole food, equally, and
directly.
Steelman-Haas reconfiguration, Peter was proud to point out could push the modified "SELF" converting load-motor ABOVE 100% of unmodified nameplate HP.
True enough as far as it went. Done with re-arranged WINDINGS, not "just" caps, the effect is as if three single-phase motors of one-third the total rating, each, were crammed into the same housing, just staggered equally around the same shaft.
Not so widely mentioned was that the price was paid in heavier power draw than a proper 3-Phase source fed UNmodified 3-Phase motor needed for the same output HP.
That rig trades efficiency for convenience and space-saving IOW.
TANSTAAFL
But an RPC at least gets an AFFORDABLE lunch, and at low/no age-degradation nor maintenance hassle!
Best we can expect. Nearly always good enough.
Why fight that when it is gonna cost yah more money, not less money?
My 10 HP Phase-Perfect? Around
four thousand bucks, bought brand new.
And the MAKER sez - right in the manual - to give it new caps at three-year intervals!
WTF? Average VFD needs new caps only at about SEVEN years out, and the "great ones" will do 12 years and "might"
double that.
The RPC? Caps could be good for 20 years. It can range to 28 max idler HP, but a P-P has easily double its nameplate for starting loads, so not all that different, "real world".
And I only have 30 or 40% of the cost of the Phase-Perfect in it, even WITH the complex four-idler rig. Folks do 10 or 15 HP all the time with used idlers for under $400. Ten percent of theP-P cost. Sometimes LESS. Freight is the big variable.
All idler motors here wer bought NEW or "NOS", average about $200-or btter in freight, too, BTW. And all but the one Brazilian Weg are monster-heavy Reliance "Duty Master" uber-durables, too. Triple the mass of the Weg, even at lesser HP.
RPC?
"Best we can expect. Nearly always good enough."