I'm also under the impression that "inverter duty" means more cooling fins and design considerations so it won't fry itself running at lower speeds, but I'm not certain of that just something I've seen with a few different motors out in the wild.
Would a 460V motor connected to 230 as an RPC generate similar voltages/amperages on L3 as a 230 volt motor would?
You have to read the datasheet from the manufacturer because inverter duty can mean more than a few things.
It may have an internal slip ring to short the rotor to the frame, it might have insulated bearings instead.
Usually they advertize the constant torque as a function of rpm, below some rpm or percentage of nominal rpm you have to reduce the output torque because the fan isn't blowing as much air.
I have a totally enclosed 5 hp motor (s.f 1.25) configured for 132 volts delta; at 120vac single phase it wastes 200 watts and pulls 11.5 amps from the line.
At 138 volts which corresponds to 240 vac nominal "line" voltage it wastes 258 watts iirc and pulls 15 amps from single phase line.
From memory the amps at half line voltage were more like 6 amps and the watts probably 100.
The motor starts to saturate at 120 vac out of 132. What this means is as an RPC idler the third leg will be closer to true three phase at 120vac than it will at 132 volts. this is a 10% reduction in voltage from 230vac nominal, and its a 13% reduction from the nominal 240v that most people have across the country.
This "saturation" at 120/132 is also the same percentage I have found with half a dozen other induction motors.
I have a 2 hp synchronous motor that I manufactured from a 2 hp marathon pump motor, it too saturates at about 90% of nameplate, and 86% of nominal line voltage. It needs about 20 watts to energize the rotor coils to deliver 120/240vac 60 hz nominal output in double or single delta (corresponding to 208/415 wye, a 10% reduction from nameplate volts).
Anyhow:
My 5 hp 4 pole motor.. with no run caps. yes, it will start my 1/2 hp bench grinder as an rpc idler (both configured for 132v delta)
But once its started the line current on the third, generated leg, is close to zero.
3, 3.2, 0.2 amps under load.
The whole setup actually consumes less power if I disconnect the third leg.
Voltages at the 5 hp idler unloaded are 120, 116, 110 volts.
My 2 hp synchronous motor starts the grinder up twice as fast (when used as an rpc idler) and it delivers
.8, 1, and .5 amps to the grinder under load.
1.6, 2, 1 amp under a heavier load. (these numbers from memory i have all this data elsewhere)
voltages are more like 120, 119.5, 119 under load.
There are several things going on with RPC idlers at reduced voltage:
They will work fine at half voltage however they will be similar in performance to a motor that is about 1/3rd the nameplate hp.
This is mostly due to the electrical resistance of the windings are electrically similar to a motor of 1/4th nameplate hp.
However smaller motors are less efficient.
But the larger motor running at a reduced voltage should deliver better power at the third leg because the core is not magnetically saturated, so this is why i'm willing to speculate it will be similar to a motor of 1/3rd nameplate hp.
But perhaps more importantly the larger motor at reduced voltage will probably consume less no load watts than a motor of 1/3rd nameplate operating at full voltage.
Someone who has two motors that are the same ratio could verify.
I could test this myself I have another 2 hp 3 phase motor, but most of you are interested in much larger motors and a single digit hp motor is not very interesting when 10 hp motors will have half the electrical losses (92% efficient instead of 80%)
When you push the third generated leg up with capacitors.. i cannot speculate on whether the added voltage is generating any real torque in the driven motor. i don't believe it is.
