I am trying to understand 3 phase a little bit better. I was wondering, is there such a thing as a transformer with a single phase input, and three phase output? Is it possible to make one? (I'm not talking about phase converters that use electronics or a motor/genset combo, i'm just talking transformer here)
If you are strict about the system being a pure transformer, then you cannot build a single to three phase transformer. This is because you need some sort of phase shift somewhere to produce the necessary phasing difference.
The simplest single to three phase converter is an induction motor. Note a motor/generator combo, but just a single motor. You supply single phase power between two of the terminals, and extract your third leg from the other terminal. The single phase supply stores energy in the rotation of the rotor, and you get that back (with pretty good phase shift) from the other terminal. This is probably the closest thing to meet the description of a single to three phase transformer.
Once you have at least two phases, you can use transformers to combine the two phases in appropriate proportions to get any other phase count. Apparently three phase to 12 or 18 phase transformer sets were (are???) sometimes used to feed power to rectifier banks to produce DC with very low ripple.
#129838 - 09/21/0512:58 PMRe: Does this transformer exist
I like Winnie's descriptions using an Induction Motor. May sound strange to some, but the Induction Motor is a Transformer with a "Spinning Secondary", and as pointed out by Winnie, the Aux. Winding of a 1Ø Induction Motor may be used to create a Polyphase output from a Single Phase input (by incorporating the Aux. Winding's lead with the 2 leads of the A.C. Power System).
As Mr. Spock would say: Fascinating!!!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
#129840 - 09/22/0512:27 PMRe: Does this transformer exist
Thanks for the help. For some reason, I thought that if you could rotate a secondary out of phase, it would lead or lag, Instead, i hit me that it would decrease the amplitude...
Vout=Vsec*cosθ, where θ is the relative angle that you rotate the secondary.
The only way it would be possible without using moving parts would be to use 2 pieces of ferromagnetic material which are timed correctly behind and forward in the space-time continuum... or something like that
#129841 - 09/22/0502:59 PMRe: Does this transformer exist
That technique works pretty well to change from N phases to M phases, where N is greater than 1.
You simply use a wound rotor motor, with the stator wound for N phases, and the rotor wound for M phases, and lock the rotor in position.
In one of my worksites, we used a wound rotor induction machine, which looks exactly like the transformer that you describe, with a three phase primary and a three phase secondary. The rotor was anchored to a gear box that let us rotate it, but would hold it in whatever position it was placed. By changing the rotor position we would change the phase of the voltage induced in the rotor coils. The rotor coils were connected 'buck/boost' to the feed lines, giving us a nice variable voltage three phase output.
#129842 - 09/23/0506:18 PMRe: Does this transformer exist
May sound strange to some, but the Induction Motor is a Transformer with a "Spinning Secondary", and as pointed out by Winnie, the Aux. Winding of a 1Ø Induction Motor may be used to create a Polyphase output from a Single Phase input (by incorporating the Aux. Winding's lead with the 2 leads of the A.C. Power System). Scott35
To get the correct phase angle you’ll need a 3 phase induction motor as your ‘idler’ or rotary converter….
It does not need to be large. Typically a 2hp idler can support 7.5hp loads.