Closed Delta 120/240V 3-phase service

krw wrote:


I would have had to go to Detroit to take the test. In the mid '70s it would have cost me about $1000 to make the trip, including a several day stay. That was why I walked away from broadcast work for 15 years. I was tired of patching junk with no parts budget, zero downtime and idiot station managers who thought they could tune a TV transmitter like their ham gear.
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And what do do if the peaks and dips didn't coincide properly?
Actually neutralising was more an exercise with older HF transmitters before they made more use of grounded-grid stages.
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Stuart Winsor

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VWWall wrote:

A tested out of the three year broadcast engineer course at Ft. Monmoth in 1972. I got the highest score on record at Ft. Knox, KY for my M.O.S. My final duty station was an AFRTS radio & TV station at Ft. Greely, AK.
I didn't get back into broadcast until the late '80s, when the station was responsible to the FCC. Most of my work was consulting on older equipment the young punks couldn't fix. I did move and rebuild a RCA TTU-25B UHF TV transmitter for a station in Destin Florida around 1990. Had lots of fun with the building inspector who had no idea what I was doing. His last words were, I'm getting a cease & desist order!!!... when I asked him what he knew about 10 KV armored HV wire.
I am 100% disabled these days, and no longer doing broadcast work. I miss it, but no one will hire someone with all my health problems. :(
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I spent 20 years working in HF broadcast for the BBC external services, then 7 years "domestic" - LF, MF, VHF and UHF TV then the last 10 years repairing Microwave link equipment. I retired April 2007.

Can't say I've heard of that one, however, checking a definition via google I think we would just have referred to "an internal flash-over" - not something that happened very often. Seemed more common in Vac capacitors which, without the protective circuits that valves usually had could sustain an internal arc for several seconds.
Take a look here:
http://www.bbceng.info/Operations/transmitter_ops/Reminiscences/Woofferton/Woofferton_2.htm
I was never at Wofferton, spending all my HF career at Daventry but we had the Marconi BD272s shown here in the first four pictures. The BY1144s were used as final and modulator valves and had the two-phase filament we were talking about. They were vapour (steam) cooled using the "steam down" arrangement. By the look of the pictures Wofferton didn't take as much care of their transmitters as we did!
(Note the safety "earthing wand" in some of the pictures)
When he speaks of needing 15mins for a wavechange however he is somewhat exaggerating - two people working as a team would take around 5mins to change all the coils and put all the settings on prior to powering and tuning. (Welding gautlets were used to handle the hot coil members)
The last picture, of Sender 83, was like the new transmitters installed at Daventry before I left. The Wofferton ones had a sort of pre-set tuning arrangement and operated by remote command from a computer system that ran the schedule. The Daventry ones (300kW) were a little later and fully auto-tuning over the entire band.
The frequency synthesizer providing the drive was commanded via its HPIB bus from the station computer control system. The Tx had what amounted to a frequency counter at it's input, which derived the address for some EPROMS, which stored coarse tune settings. Pneumatically operated switches operated to tap the tuning coils and stepper motors wound Vac caps to the correct place (HT was suppressed during this operation). Phase discriminators provided fine tune information once low HT had been applied.
They had very fast acting protection for the valves (tubes) to prevent internal damage.
It was tested by hanging about 5 feet of 5A fusewire from the HT (11kV) supply to a vacuum contactor. The contactor was closed putting a dead short across it and the protection was fast enough to prevent the fusewire from blowing. It was quite spectacular when it didn't :-)
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As an aside, I should have mentioned that the BD272s used two transformers to give a six phase supply (since that seems to be the topic under discussion) to two banks of six AR64 Mercury arc rectifiers.
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Stuart Winsor

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Stuart wrote:

A six phase rectifier reduces the AC current through the filter caps, as well as the total capacitance needed to achieve the desired ripple voltage. The lower the AC current, the less chance of a capacitor trying to go into LEO.
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Yup
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| Stuart wrote:
|> |>> A Scott-T is used to get two phase from three phase. There are very few |>> two phase requirements any more. |> |> Often used for the filament supplies in the modulator valves on large AM |> transmitters. Two seperate filaments fed from the two different phases to |> reduce hum caused by modulation of the anode current due to variation of |> the emmision at mains frequency. | | Yep. That's one of the very few. I worked as a "broadcast engineer" | whilst going to college. I still have my Radio Telephone First Class | license. | | There used to be a few two phase motors, but they're almost extinct.
You could run that from your three phase power if you wire things up to get the right phase angles AND the right voltages. If a motor is wound so one phase is 240 and the other is 208, that should be happy on either 240DCT or Scott-T. But such a motor would not be run very easily on a Wye system without some additional winding tricks, and that would negate any economics of 2 windings.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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| snipped-for-privacy@ipal.net wrote: | |> I'm sure when A/C systems started showing up, with many using 3-phase, they |> were among the first to get them down in the very hot south. |> |> I wonder how many of those big+little transformer setups were Scott-T instead. | | A Scott-T is used to get two phase from three phase. There are very few | two phase requirements any more.
It can produce the equivalent of 240 delta when connected appropriately:
* * * * / \ / \ | / / \ / \ | / / \ / \ | / *-------* *---*---* *---*---* *---*---*
240D 240DCT Scott-T 240VCT
What you need is 208 volts at 90 degrees, one end connected to the neutral which is the center tap of the 120/240 single phase. The effective voltage phasing gives the same relative voltage as the 240 delta. You get a 90 degree phase angle when one primary is connected L-L and the other is connected L-N where the latter L is different (e.g. A-B and C-N).
| Some erroneously call the 240/120V system "two phase" when it's really a | single phase with a center tap. (Or two 120V windings connected in series.)
Personally, I would have preferred to use the term "phase" for each of the different vectors around the reference (generally the grounded conductor). But that's not how things worked out. So I need to use some other term. Often "pole" is used, but that can get confused with those big wooden sticks that hold wires up high. I'd say "vector" but no one would understand.
| The last time I encountered a true Scott-T was in my exam for | Professional Engineer, back in 1952.
I've found Scott-T dry-type transformers online.
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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----------------------------
wrote:

In a strictly technical viewpoint one can say that for an N phase star the phase voltages are 360/N degrees apart so that the 3 wire "Edison system" satisfies this criterion just as 3 phase, 4 phase,.... do. All with N hots and a neutral -i.e. star connection. With balanced loads the neutral current is 0 in all these cases. That is the justification for calling it 2 phase.
However, in Europe the 2-phase designation is used rather than 3-wire single phase because of the above viewpoint. (Of course they don't use it there). In North America 2 phase implies two windings in quadrature (and for balanced loads there is a neutral current). The system was started by Edison for DC circuits and somehow, along with the "Edison" name, the alternative single phase center tapped nomenclature came into use.
So, while not normally called that in North America, it is quite justifiably called (without error) a 2 phase system -at least on the secondary of a single phase transformer.
I remember the Scott T back in the 50's but never saw one that I can remember. I do remember the more common open delta transformers but usually the transformers were equal in size.
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Don Kelly wrote:

Then the two phase system is actually four phase. There are two equal voltages 360/4 degrees apart, each of which could be split with a center tap. It has been known as two phase wherever it was used.
Calling the 240/120V common system "two phase" may not be technically in error but it confuses common usage in the United States.

The Scott-T requires a special transformer, tapped to provide equal quadrature voltages from the equal three phase inputs. I've never seen such a transformer used for any other purpose. They were used to supply motors with two equal windings in quadrature, which was a balanced load, usually on a four wire feed.
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I don't know if it is official, but a lot of utilities, engineers, and electricians call it "split-phase" in the USA. The term seems to eliminate some of the ambiguity of "two-phase".
Someone could claim that the 208V coming into an apartment is "two phase" since by one definition, you have two of the three phases available at the panel, but electrically it just resolves into single phase.
Then there is this interesting circuit.
H N H
| | | | |- switch | | | | |____120V lamp____|______120V lamp_____|
If the two hots are connected to a single phase 120/240 split phase circuit, the behavior of the lamps is different when the switch is closed, then when the two hots are connected to a 120/208 V. supply from a 3 phase service (such as for a large condo or apartment building).
Beachcomber
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CORRECTION - That should say, the behavior of the lamps is different on the two different types of service when the switch is "OPEN".
Sorry, my bad...
Beachcomber
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| |>Then there is this interesting circuit. |> |>H N H |> |>| | | |>| |- switch | |>| | | |>|____120V lamp____|______120V lamp_____| |> |> |>If the two hots are connected to a single phase 120/240 split phase |>circuit, the behavior of the lamps is different when the switch is |>closed, then when the two hots are connected to a 120/208 V. supply |>from a 3 phase service (such as for a large condo or apartment |>building). |> |>Beachcomber |> | CORRECTION - That should say, the behavior of the lamps is different | on the two different types of service when the switch is "OPEN". | | Sorry, my bad...
Actually, I misread your diagram. I have a normal fixed space font and you apparently have a variable space font. You can never reliably draw a diagram in text with such a font because there is no standard for the spacing of such fonts. Only someone using exactly the same font can see exactly what you drew.
If you have any way to switch to a fixed space font like "Courier" when editing a posting, that would be best because it will display as you create it with any fixed space font (all characters are the same width and the spacing you create is maintained the same).
So it seems what you drew is a "switched open neutral". Well, what can I say but: you're not supposed to allow having an open neutral. Clearly the exact behaviour of an open neutral varies by system configuration.
Try a 120 volt delta-vee system :-)
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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| |>Calling the 240/120V common system "two phase" may not be technically in |>error but it confuses common usage in the United States. |>> | | I don't know if it is official, but a lot of utilities, engineers, and | electricians call it "split-phase" in the USA. The term seems to | eliminate some of the ambiguity of "two-phase".
So I could call the setup I described earlier as "three split-phase"?
| Someone could claim that the 208V coming into an apartment is "two | phase" since by one definition, you have two of the three phases | available at the panel, but electrically it just resolves into single | phase.
You could derive three phase from it with 2 transformers that have a 120 volt primary and an isolated 120 volt secondary. Wire up one transformer primary to one 120 volt supply L-N. Wire up the other to the other 120 volt supply L-N. Now wire the secondaries in series with one of them having a reversed phase. Connect the end of the other one to neutral, and you end up with 120 volts at a phase angle 120 degrees away from the two supply phases. The resultant diagram would look like:
A * \ / \ \ / \ N C / / B
A-N and B-N are original supply connections, so wiring things this way is ultimately an autotransformer. C-N is the newly derived phase that completes the three phase system. The power factor on the two supply phases will be awful, depending on how much that derived phase is used.
It could also be done NOT as an autotransformer with transformers that have 2 120 volt secondaries. Many step-up/down transformers might have dual 120 volt primaries and secondaries allowing them to be wired as 120 and/or 240 volt isolation transformers. Wire primaries in parallel for 120 volts, and wire secondaries arranged as above to do the same trick as an isolation transformer.
| Then there is this interesting circuit. | | H N H | | | | | | | |- switch | | | | | | |____120V lamp____|______120V lamp_____| | | | If the two hots are connected to a single phase 120/240 split phase | circuit, the behavior of the lamps is different when the switch is | closed, then when the two hots are connected to a 120/208 V. supply | from a 3 phase service (such as for a large condo or apartment | building).
Yes, that is true. This is also why the NEC has rules like 310.15(B)(4)(b).
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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Don Kelly wrote:

I just found a good reference:
http://www.electrical-contractor.net/Forums/ubbthreads.php/ubb/showflat/Forum/15/topic/000055/Number/0/site_id/1
http://preview.tinyurl.com/3bxtgx
There seems to be a lot of misinformation about this now almost extinct circuit. The need for an 87%, (sqrt3/2), tap is unique. One reference said it was still in use in Philadelphia.
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On 12 Feb 2008 21:09:22 GMT, snipped-for-privacy@ipal.net wrote:

It alters the voltage between the phases if you have a L/L across the missing leg it will drag down that voltage
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snipped-for-privacy@ipal.net writes:

I would assume no effect, as long as the insulation was rated for 208VAC to ground vs. 120VAC to ground, and you don't overload the possibly small high leg transformer. The insulation won't be an issue for anything designed for European use where either leg may be hot, since either leg must handle 240V. One overload problem is when something is connected across the missing leg of an open delta system. The transformers have to deal with a higher VA per delivered watt, plus more copper losses. Combine this with a small high leg transformer and you may see voltage sag.
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On Wed, 13 Feb 2008 16:48:16 +0000 (UTC) Michael Moroney
| snipped-for-privacy@ipal.net writes: | |>What I wonder is how 240 volt L-L loads would do with either the closed or |>open 240D/120 systems, when connected between the high-leg and either pole |>of the 120/240 side. Consider the simple 240 volt water heater, assuming |>it treats both wires as hot, which it must in USA single phase systems. | | I would assume no effect, as long as the insulation was rated for 208VAC | to ground vs. 120VAC to ground, and you don't overload the possibly small | high leg transformer. The insulation won't be an issue for anything | designed for European use where either leg may be hot, since either leg | must handle 240V. One overload problem is when something is connected | across the missing leg of an open delta system. The transformers have to | deal with a higher VA per delivered watt, plus more copper losses. | Combine this with a small high leg transformer and you may see voltage | sag.
Would a computer overload it?
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snipped-for-privacy@ipal.net writes:

What do you mean by "computer"? A PC or a big-assed old mainframe? I seriously doubt a PC with a 400W power supply will overload a high leg transformer of a few kVA.
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