Single-phase to Two-phase and Three-phase

Back by popular demand is an old post (since lost in the conversion to the new software) for a method of converting single-phase to two-phase AND three-phase using a single idler in an RPC system.

This method simulates a Scott-T, as do most RPCs of the balanced type, but in addition to, or instead of three-phase, this method produces two-phase, as is required by machinery which was formerly installed in Buffalo/Niagara Falls, NY/Niagara Falls, Ont and Philadelphia/Camden/Reading.




Three-phase is taken off as usual, from L1/A, B and L2/C.

Two-phase is taken off in a new and inventive way, from L1/Y1 and L2/Y2 and X1 and N/X2.

An autotransformer is used to get the 0.867 to 1.0 "boost" in the X1-X2 phase.

This is one of the very few cases where N is required in an RPC system.

Thank you for reposting that. It might be advantageous for me to make one of these as my new shop will certainly need three phase and it is at least possible that one or more of my 2 phase machines (there are four of these in another, stored shop for which I long ago madea single to two phase converter, and I am picking up one new one of these 2 phase machines, a direct drive table saw--that I'll probably never use) could be useful in the new shop.

Of course I should admit that I don't know how to read diagrams such as yours. I will--some time--try to find out in my old electrical textbooks (my father's college books, from the late 1930's) I do recall that the Scott system is noted in a least one of these.

There is almost a one-to-one correspondence between the "phasor diagrams", above, and the schematic diagrams which implement the function.

I'll try to put together a proper schematic diagram shortly.

Here ...



... is a small version of the basic schematic.

Here ...

http://img.photobucket.com/albums/v390/peterh5322/One_to_Two_and_Three.jpg

... is a larger version of the same.

Not shown is the usual "3-wire control" and its control station, but the expected, and required overloads are shown as are the two poles of the main contactor.

A step-up of the X-phase is required for balanced two-phase. The correct ratio is 208:32, but 240:32 or 240:48 could be used, too.

This combined RPC can supply two-phase or three-phase, or both two-phase and three-phase, up to the capacity of the RPC system.

peterh5322 said:

This method simulates a Scott-T, as do most RPCs of the balanced type, but in addition to, or instead of three-phase, this method produces two-phase, as is required by machinery which was formerly installed in Buffalo/Niagara Falls, NY/Niagara Falls, Ont and Philadelphia/Camden/Reading.

Click to expand...

Good Heavens, I can't believe that anyone else has even heard of the Scott-T as used in Philadelphia! I had a project at a previous employer to instrument most of the LV distribution system in Philadelphia but it never materialized. I learned a lot about the 2-phase system, how it works, how it's protected, etc. "Pilot Wire" differential relaying, etc. The prototype hardware is still in place but disconnected - at least it was 2 years ago.

There's still some 2-phase in the New York City area, at least it's shown on my phase maps. Some 25 cycle, too.

Thank you for bringing back a host of interesting memories!

"I can't believe that anyone else has even heard of the Scott-T as used in Philadelphia"

The Scott connection has traditionally been used in these cases:

1) old-work two-phase utility feeder, supplying a new-work three-phase customer,

2) new-work three-phase utility feeder, supplying an old-work two-phase customer,

3) three-phase to three-phase voltage conversions, where the primary is always ∆, and the secondary is either ∆ or Y, with a neutral being possible only in the Y case.

Many present-day utility three-phase transformers of 333 KVA (one-third MVA) and smaller are so-called T-transformers, wherein the dual transformer is contained within the same housing, thereby saving space and possibly the cost of a vault. These transformers have lower "per unit" impedance, and better regulation than other economical alternatives. Alas, T-transformers are not compatible with certain "technical" loads, such as solid-state broadcast transmitters.

The Scott connection finds applications in instrumentation, wherein a small, precision three-phase motor is driven by a two-phase source. Usually, these are part of servomechanisms.

As the circuit which includes the neutral, namely X1-X2, includes capacitors, and any capacitors must be on the load side of a motor controller, this is one of the very few cases where the neutral must also be switched along with the lines.

I will be revising the schematic to incorporate switching the neutral.

Peter, thank you for your updates, and Fasto, thank you for your memories. As I've said before I may try this for my new hobby shop, as I have--besides 3 phase machines planned for the shop--two phase machines in an old shop that I might bring down.

My four two phase machines--lathe, drill, bolt cutter, pedestal grinder--cames from a Philadelphi iron works. I don't know the power requirements of the shop either at its old location (in downtown Philadelphia where I first saw these and many other machines in the mid 1960's) or at the shop's new location (still in Philadelphia out of the downtown however) that it moved to circa 1970 or so. I'd certainly guess that in both locations the shop used both 2 and 3 phase machines. That's only a guess, however, as all the machines I recall were 2 phase. I assume that newer machines in the shop--a variety of steel saws, mostly--were 3 phase. How they were getting 2 phase (and, probably, 3 phase) I don't know, though it was clear that the original configuration in the first location that I was in (that they'd been in since before the introuction of electricity, running the machines with a steam engine) was line 2 phase.

I've just looked at a 1951 appraisal of the first location and see that there is a listing for a

Westinghouse 30 KVA multiple unit dust tight capacitor assembly 2/60/220V
style 58Y759

and also (in another shop area)

1 5KVA phase Eisler transformer 3 to 2 phase installed

Am I correct in thinking--I may well not be--that the first entry may indicate two phase into the building and that the second indicates 3 phase being converted to power two phase machines?

On a side note the most highly valued electrical components of that shops were the 4" conduits filled with 4-4/0 cable, several hundred feet of these.

Thanks again.

25 cycle stuff

A little out of thread but There is still some 25 cycle (Hz) stuff in Keokuk, IA on the Mississippi river. The hydro plant there feeds a dedicated 25 cycle transmission line to St. Louis, MO where it is converted to 60 Hz, I think they said there was a little 25 cycle stuff in Keokuk as well.

"The hydro plant there feeds a dedicated 25 cycle transmission line to St. Louis, MO where it is converted to 60 Hz, I think they said there was a little 25 cycle stuff in Keokuk as well."

No doubt.

My family's former company, a limestone quarrying firm with operations at several Southern Illinois sites near St. Louis, operated from 25 Hz until the co-op abandoned it.

Thereafter, we generated our own 25 Hz using a pair of "Superior" diesels about the size of a house. One main generator, one alternate main generator.

After one of the "Superior" prime movers threw a rod and destroyed itself, we elected to convert to 60 Hz, which involved changing out nearly a thousand motors and their controls.

Here are updated schematics, showing the complete system, including its controls.

I have also annotated the original drawing, somewhat.

northernsinger said:

Peter, thank you for your updates, and Fasto, thank you for your memories.

Click to expand...

Sir, I thank you for the compliment as I have never thought myself old enough to have useful memories! (I am only 39!)

Regarding the 25Hz distribution in the New York City area, this was in the southern part of Brooklyn, presumably used by the Navy Yard. I have read that the 25Hz was preferred for usage in electric cranes, as the 4-pole and 6-pole motors ran at only 42% of their 60 Hz speed. This allowed for finer control in the days before VFD's were available. I believe the New York City subway also uses 25 Hz AC.

Incidentally New York City - and presumably other older cities - still have 600V DC distribution, used for old elevators and such. Today in New York City the DC is generated by rectifiers located nearby the points of load from the 440VAC 3-phase system; it was once supplied in the same manner as AC distribution. I saw an old DC watthour meter in a utility's museum once.
--
Aaron

>Here are updated schematics, showing the complete system, including its controls.

Peter, should the 3-phase starter be shown with normally open contacts?

--
Aaron

"Peter, should the 3-phase starter be shown with normally open contacts?"

Correct.

The schmatic diagram, above, has been updated to show the non-operating state. (The operating state, shown in previous diagrams, made more sense when the shown in the abstract, without the "3-wire" control system and the starting circuit).

Note that the contactor which is operated by the potential relay may be, and usually is, a multi-pole 50 amp contactor, with the required starting capacitance spread equally across all contact sets, up to 550 µF per contact set.

"I assume that newer machines in the shop--a variety of steel saws, mostly--were 3 phase. How they were getting 2 phase (and, probably, 3 phase) I don't know, though it was clear that the original configuration in the first location that I was in (that they'd been in since before the introuction of electricity, running the machines with a steam engine) was line 2 phase."

I'd assume that premises with two- and three-phase would have had two-phase installed first, and the three-phase came second.

Although two separate services are frowned upon, it is entirely conceivable that individual electric utilities bent the rules to minimize their cost, and the cost to the customers as well.

To that end, I could see an industrial premises where there was a two-phase service and a three-phase service, with a Scott transformer on the pole, outside the premises.

I know that certain premises in and around Camden were two- and three-phase, as RCA made radio transmitters there which were two-phase internally, but were provided with Scott connections on the plate transformer primary so that a three-phase utility could be accommodated.

These transmitters also featured so-called "four-phase" rectifiers, thyratrons in reality, and because of the expense of the phase-controlled power tubes, four tubes was significantly less expensive than the six tubes which would ordinarily be required.

Single-phase is not an option at the 5 kW and 10 kW power levels, the normal range for these transmitters.

Power reductions to 0.25, 0.5 and 1 kW, as for night operations, was possible, although this wasn't necessarily the least cost solution to reduced night power operations as you still had those 892R tubes in the modulator and Class C final, and those burn lots of power, not to mention lots of filament power.

Below 5 kW and above 10 kW the two-phase solution was not economical, as below 5 kW was expected to be single-phase while above 10 kW was required to be three-phase.

These two-phase RCA products were delivered in significant quantities in the immediate post-WW-II period, perhaps from 1947 to as late as 1955, after which time a change was made to tetrodes and six-phase rectifiers.

What would be required for a breaker panel for the generated 2 phase power? I have multiple machines that are 2 phase, and would like to find a way to easily manage all the wiring runs.

If the X leg has two 120V lines and the Y leg has one 240V and one neutral line, is there a "best practice" way to incorporate this in to one panel?

Would I need a 2 phase starter for my machines, or could I use a three phase with this configuration since one of the lines is neutral?

I spoke with Eric Lavelle a couple of weeks ago about him possibly building one of these, and am hopeing that between he and I, we will be able to come up with a working model of this.

Matt

"What would be required for a breaker panel for the generated 2 phase power?"

I would wire these machines as two-phase three-wire, using a three-phase fusible safety switch.

Four wires would be fed to each two-phase machine: X1, Y1, X2/Y2 and G.

The two-phase Monarch 10EEs used a three-pole magnetic motor starter wired in this way, and only after the starter were X1, Y1, X2 and Y2 split out as T1, T2, T3 and T4 at the motor-generator.

T4 was provided on the terminal block of all machines, but it was not used on three-phase 10EEs.