Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 03:23 PM 9/29/2006, you wrote:
Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx> Jim: I have not been following this thread so pardon me if my comments are redundant. I believe that your "3 phase converter" is the type where you are using an idling motor(shown on your sketch) to allow one to drive a 3-phase motor. It is my understanding that this type of circuit will temporarily run a 3-phase motor up to a point somewhat below its 3-phase rating. It is also my understanding that the circuit loads(currents)will be seriously out of balance. As a result, it is not uncommon to eventually burn up the motor(not the idling one) when this circuit is used.
Not necessarily. This is a pretty standard rotary converter. The key things are to make sure that the idler motor is rated high enough. Consider it as Wye/Star connected for a moment (even though actually use them wired in delta).. the power going OUT of two of the legs (the 120deg and 240deg shifted phases) is entirely supplied by the power going IN one leg. So, if the LOAD motor is drawing, say, 20 Amps on each phase, your rotary converter (IDLER) will have 40 Amps going in on its supply, and you'll be drawing 60Amps total from the (single phase) line
Now, in reality, it's not quite that bad, because once the LOAD motor come up to speed, it acts as a rotary converter itself. In fact, this is a real problem with fusing/overcurrent protection for three phase motors. Once running, you can disconnect one of the phases and it happly keeps running as a single phase motor, but dissipating a LOT more heat in the one remaining winding. How much more? If the load is the same, 9 times more (3 times RMS current) which may be where the "burning up the motor" thought comes from.
The rotor in the motor will also get hotter (since it's part of the "transformer"). In a rotary converter this is less of an issue because there's no load on the motor, so there's less heat dissipated in the rotor to begin with.
Now, if you're driving something other than another induction motor with the rotary converter (e.g. a 6 pulse rectifier), I'm not sure how the converter will work, especially if the load current isn't sinusoidal.
But heck, that's what experimenting is for.