doesn't this look suspiciously just like an elctric motor?I can't get this at all!

[Alistair Hosie]

Alistair is baffled I have several invertors convertors both rotary but they don't look this simple how could it work?Baffled again LOL

http://www.ebay.co.uk/itm/ROTARY-3-PHASE-CONVERTER-converts-single-to-3-phase-Ideal-for-lathe-drill-etc-/161169416652

[DFMiller]

Alistair,
Looks similar the one I have. I am running my 3 phase mill off it from Single phase.

Dave

[paulj84003]

It no dobt is a 3ph motor. I make my own phasers. I am using a 3 hp 3 phase motor as a rotary converter. currently The largest 3ph motor I need to run is 1 1/2hp and the phaser does that quite well.

[andywander]

A rotary phase converter is simply a 3-phase motor and as little as one capacitor. Some may also include a starting capacitor and a switch.

[_Paul_]

Looking at the box sat atop the motor I can see a couple of capacitor fixings, I have a similar setup, there are plans all over the web for this type of RPC.

Paul

[davidwdyer]

That's what I run my 3 phase machines with. Not identical but similar. Just a three phase motor running on 2 phases with a starting capacitor.

[J Tiers]

Alistair:

Remember that an electric motor generates a voltage as it turns. That voltage is very close to the incoming mains voltage, and teh small difference allows enough current to supply the needed power. The voltage is known as "back EMF".

A motor used as a "rotary phase converter" (RPC) uses that fact. Even though there is no input mains power on the third wire, the motor still generates the "back EMF" on that wire. Due to the mechanical position of the "poles" in a motor, this "back EMF" is in nearly perfect phase relationship to the incoming power, and so can provide the needed third output for three phase power.

When that third output wire is used with the two input wires, the result is good, usable, three phase power.

[Lu47Dan]

It no dobt is a 3ph motor. I make my own phasers. I am using a 3 hp 3 phase motor as a rotary converter. currently The largest 3ph motor I need to run is 1 1/2hp and the phaser does that quite well.

Piker, my homemade RPC is 30Hp
Dan.

[isaac338]

The only thing that's suspicious is the claim that it converts 240V-1ph to 415V-3ph - I don't think you can do that without a transformer. Probably just converts 240V-1ph to 240V-3ph.

[The Artful Bodger]

The only thing that's suspicious is the claim that it converts 240V-1ph to 415V-3ph - I don't think you can do that without a transformer. Probably just converts 240V-1ph to 240V-3ph.

Maybe, but if it is a dual voltage motor it can be wye connected as for high voltage with 240V applied across half of one leg which will act as the primary of an auto transformer causing 480V or so to appear between that leg and wye neutral point.

[PStechPaul]

If the 240 VAC is applied to a phase-neutral winding, the phase-to-phase voltage will be 240*sqrt(3) = 415 VAC. But normally the single phase is connected across two phases, with a capacitor to shift the phase to the third leg. If this RPC was purpose-built, it could have a special winding pattern that could boost the voltage. It may also be a 230/460 dual voltage motor, and you could apply external single phase voltage to the center taps and get double the output voltage, but at a much reduced power.

[isaac338]

Maybe, but if it is a dual voltage motor it can be wye connected as for high voltage with 240V applied across half of one leg which will act as the primary of an auto transformer causing 480V or so to appear between that leg and wye neutral point.

Neat. My 3-phase knowledge is limited, clearly. Thanks for pointing that out.

[PStechPaul]

That is true in theory, but most motors are wye connected for 480 or 240 phase-to-phase, which is 277 or 138 volts phase-to-neutral. So, if you put 240 volts on the center tap (half leg), it will be overvoltage and probably saturate and draw lots of current at 60 Hz. However, it could be connected across two center taps, which would be OK. The 415V as claimed would most likely be the result of the sqrt(3) phase voltage relationship.

[The Artful Bodger]

Bear in mind that this machine is advertised for sale in the UK where supply voltages and practices may not be the same as elsewhere.

[PStechPaul]

Since most of Europe (and possibly also Australia) use 50 Hz, equipment is typically rated at 50/60 of its US voltage, so 480 at 60 Hz would be 400 VAC at 50 Hz. AFAIK, the standard European voltage is 220 VAC line-neutral, which would be 381V phase-phase. The actual voltage can vary about 5%, so 380-420 would be within specification. With common use of VFDs and wide-input switching power supplies, the line voltage and frequency of the mains is not as critical as it once was.