Single phase or three phase?

[pstemari]

Dunno what he meant, but the static phase converter mentioned that only powered two phases after startup is pretty lame.

The voltage you get from an RPC is skewed relative to ground. That can occasionally cause problems, especially if auxillary circuitry picks off a single phase to derive a 120VAC to neutral. With an RPC, that works for two of the three phases, but the phase created by the RPC (the "wild leg") runs at 208VAC relative to ground.

VFDs don't have that problem, but because they use pulse-width modulation to simulate a sine wave, they do generate a lot of RF noise.

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[682bear]

My understanding of it is that an RPC creates 2 phases... you have one phase already, which is the L1-L2 phase (where L3 is the 'wild leg') and L2-L3 and L1-L3 are the 2 phases generated by the RPC...

The installation manual states that you can get reliable 120v current by either connecting to L1 and ground or L2 and ground (again, where L3 is the wild leg).

That may be what was meant, IDK, but IMO, saying it is 'fake' 3 phase is sort of like saying my gasoline generator creates 'fake' electricity...

Not trying to be argumentative... I'm no expert and I'm trying to learn what I can about the subject...

-Bear

 

[projectnut]

Please explain the meaning of 'fake 3 phase' in regards to an RPC...

-Bear

I believe he was referring to a static converter, not a rotary converter. A static converter generates a temporary 3rd phase using a series of capacitors. The third phase is only generated during the start up. Once the motor is spinning the third phase drops out and the motor continues to run on single phase. A motor running with a static converter only generates approximately 2/3 of it's rated horsepower.

 

[JPMacG]

If you are purchasing a small hobbyist type lathe and you think that some day you might want to sell it, then I think you should get single phase.

 

[MrWhoopee]

"Fake 3 phase". A basic rotary phase converter (no additional electronics) generates the third leg, but it doesn't generate full voltage on that leg.

 

[markba633csi]

The source impedance characteristics are different on the generated leg vs the two utility power legs- fixed motor loads usually don't care when
the 3rd leg is "tuned" to give the proper voltage
Mark
I guess you could call the 3rd leg "fake" for that reason

 

[wrmiller]

I’m thinking about purchasing a new Lathe and it is available in both single and three phase.In my shop I have single phase power only.Is it better to use a three phase unit with a phase converter or just order single phase unit and be done with it?
Thanks

If you are not interested in variable spindle speed, go with the single phase and plug it into the wall.

The only reason I went with 3-phase machines (lathe and mill) was because I wanted variable speed and a VFD gives me that without having to buy a much more expensive DVS-type machine.

 

[Silverbullet]

I vote three phase if you can afford it. The electric bill will also show a savings over single phase. You should weigh the differences and go with what you can do. My first garage shop had one outlet in it ,ad thirty ft from the house. I bought 10-3 uf feeder wire and hand dug the trench from the basement to the shop . Wired 220 and ran an RPC to my lathe and Bridgeport. Worked everyday for a month and it only raised the bill $15.00 .
I was happy with that just starting out doing work and lowering cost .

 

[pstemari]

My understanding of it is that an RPC creates 2 phases... you have one phase already, which is the L1-L2 phase (where L3 is the 'wild leg') and L2-L3 and L1-L3 are the 2 phases generated by the RPC...

The installation manual states that you can get reliable 120v current by either connecting to L1 and ground or L2 and ground (again, where L3 is the wild leg).

Right. L1 and L2 are the original two phases, and L3 is generated. Any pair of phases gives you 240 VAC, L1 or L2 to neutral gives you 120VAC, and L3 to neutral gives you 208VAC.

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[682bear]

Right. L1 and L2 are the original two phases, and L3 is generated. Any pair of phases gives you 240 VAC, L1 or L2 to neutral gives you 120VAC, and L3 to neutral gives you 208VAC.

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Umm... L1, L2, and L3 are just 'legs'... the 'phases' are dependant on the relationship between 2 of the legs, thus L1-L2, L2-L3, and L1-L3 are the 3 phases. Am I correct on this?

Otherwise, what we call a single phase motor would actually be 2 phase, with an L1 and L2 power wire...

Or am I completely backwards?

-Bear