B+ supply???

[N8ECR]

I am slowly working on a old RCA BTA-5H convertion to the 160 meter band.  I have some consideration to do about the B+ supply.  First off it oringinally was 3 phase and the plate tranny is two transformers wire in a 4 phase Scott curcuit, using thyratrons ,(it is be rewired to single phase) I have disconnected the thyratron stuff, and willl use the two plate transformers separated and each running into a pair rectifier tubes, then rectified B+ leads on the rectifier filiment transformers CT's tied together; if this works out???   The big thing I am wondering about is the filtering lash up....  The filter choke is originally in the center tap of the plate trans(B- lead). running thru some resistor "thinggie" ;that looks like a giant selenium rectifier, and the filter caps in the B+ lead.  Can I leave this in this arrangment??? or change it???   if left in this arrangement will the supply act like a Capacitance input , or choke input...?   What you techies think here.  Mike/N8ECR

[Steve - WB3HUZ]

Got a schematic you can post?

[John K5PRO]

Three phase rectification systems have lower ripple, so the filtering L and C's are a lot smaller than the values needed for single phase rectification, for the same ripple level. Keep that in mind.

[Opcom]

The filter choke can be put in the negative lead. It's done to avoid having to have a super-insulated choke. Is the resistor thing across the choke?

Is the left-hand diagram using two transformers what you have? Is one transformer bigger than the other one? If so, the bigger one can be used to make do. I don't know a way to use both because the teaser transformer has a primary with less turns than the main transformer. A possible other issue is the voltage rating of the primaries. I don't know if it is delta or wye. Are they for 208 between them or have you tried 120 on each? The Right-hand diagram would be what I would try if my guesses about the transmitter are right.

Did you disuse the thyratrons due to wierdo circuit issues? I think they are regulators, maybe the B+ would be high without them. I might have a NOS dual 240V 28A variac for that. It's got a handwheel! But you might not need to worry about that once you put in a bigger choke, the voltage could be lower on single phase.

[Opcom]

BTW -how many KV you need for the caps? You will have to add some, probably..

[k4kyv]

I'm not all that familiar with three phase circuits.  What is the "teaser" circuit and what is its purpose?  I thought 3 phase would use a transformer with 3 identical windings in the primary and 3 in the secondary, with one end of each secondary tied together to a common ground point, or as in this circuit, the filter choke, and the other end of each winding going to the anode of a rectifier.  Or three separate transformers could be used as well.  The common point of the primaries would go to neutral, and the other end of each primary winding would go to one of the phases.

I would assume that A, B and C are the 3 phases and N is the neutral, but why is neutral connected to a tap on the primary of the teaser transformer and not to the common point between all 3 transformer primaries?  And why is the teaser transformer secondary midtapped with two rectifiers and not just a single winding with one rectifier like the other two windings on the "main" transformer?

[N8ECR]

Yes one of the transformer in the vault has a center tap provided on the primary, and is used in the 3 phase hook up, however I don't know if they call one a teaser, or not... they are both the same size, and look alot like those that Elctro Engineering use to make, you know with 2 coils on the core.  One is different in a way only that there is not as many taps on the primary  for ie... 208/215/220/230 ect...  This transformer maybe what you are calling a teaser...however all the secondarys run out to a thyratron, with the exception of the ct's which are connected, and go thru the filter reactor... yes more capacitance will be needed of course...I have an extra reactor, and capacitance will be doubled frome 8 to about 16mfd, or more if needed.

Now if I use the thyratrons and lift the grid connection, should I then wire them to the nearest filiment lead or take it back to the ct of the filiment transformer.  or forget it altogether and go get some 575A's or 872's

I don't yet know what the final B+ will end up being, but it should still be usable in some way, can't run this thing full strap, the pole pig outside woulld jump of the pole and run away.  But thinikng about it; would make a great slop bucket crusher. see ya 75......

Thanks for the help....MikeN8ECR

[Opcom]

I'm not all that familiar with three phase circuits.  What is the "teaser" circuit and what is its purpose?  I thought 3 phase would use a transformer with 3 identical windings in the primary and 3 in the secondary, with one end of each secondary tied together to a common ground point, or as in this circuit, the filter choke, and the other end of each winding going to the anode of a rectifier.  Or three separate transformers could be used as well.  The common point of the primaries would go to neutral, and the other end of each primary winding would go to one of the phases.

I would assume that A, B and C are the 3 phases and N is the neutral, but why is neutral connected to a tap on the primary of the teaser transformer and not to the common point between all 3 transformer primaries?  And why is the teaser transformer secondary midtapped with two rectifiers and not just a single winding with one rectifier like the other two windings on the "main" transformer?

In 3-phase input and output, the windings could usually be on one combination or three separate transformers. This is using a 3 to 2 phase conversion, but due to the center taps you get four phases. In this special "Scott" circuit, two or three transformers are used. One combined transformer can't be used because the wiring of the coils is to make a different phase relationship of the magnetic flux, that is it is not possible? to have a 3-phase and 2-phase magnetic flux in the same core at the same frequency. This said I do not know the power supply of the transmitter in question, but only know of this transformer setup in general.

"Teaser" is the name given to it in one of the engineering textbooks, "AC Machines" 2nd ed. Liwschitz-Garik and Whipple. I'm not sure if it's an industry-standard name or 'jargon'. Looking at the diagram it adds the vectors of the B and C phases to the A phase in such a way that the output of its secondary is (supposedly) 90 degrees from the output of the secondary of the main transformer. Speaking of that one, it add the vectors of the B and C phases to make one phase (0 degrees).

The neutral point is shown to illustrate that the teaser transformer primary winding is not the same turns ratio as the main transformer windings. The teaser winding is trapped at 86% in order to properly add the vectors so the magnitudes are correct (more ugly math). The 86% tap, as I understand it, can be there at the neutral point for a WYE input or can be at the high side there for a Delta input. There is a pretty good explanation of the circuit here:
http://en.wikipedia.org/wiki/Scott-T_transformer

The teaser output has the phases 90 and 270 degrees by virtue of the center tap, there being 180 degrees between them like any transformer. The bottom transformer is shown as two transformers, but in the transmitter is question, it is one. It makes the phases 0 and 180 degrees.

[Opcom]

Yes one of the transformer in the vault has a center tap provided on the primary, and is used in the 3 phase hook up, however I don't know if they call one a teaser, or not... they are both the same size, and look alot like those that Elctro Engineering use to make, you know with 2 coils on the core.  One is different in a way only that there is not as many taps on the primary  for ie... 208/215/220/230 ect...  This transformer maybe what you are calling a teaser...however all the secondarys run out to a thyratron, with the exception of the ct's which are connected, and go thru the filter reactor... yes more capacitance will be needed of course...I have an extra reactor, and capacitance will be doubled frome 8 to about 16mfd, or more if needed.

Now if I use the thyratrons and lift the grid connection, should I then wire them to the nearest filiment lead or take it back to the ct of the filiment transformer.  or forget it altogether and go get some 575A's or 872's

I don't yet know what the final B+ will end up being, but it should still be usable in some way, can't run this thing full strap, the pole pig outside woulld jump of the pole and run away.  But thinikng about it; would make a great slop bucket crusher. see ya 75......

Thanks for the help....MikeN8ECR

It is interesting that they are the same size. I have only worked with one of these Scott circuits and the teaser was a bit smaller. Maybe it was just the particular construction.

If you use the thyratrons, then you will have to drive the grids or they won't fire and give you B+. I was only thinking that if the rig has a regulated B+ circuit using thyratrons, that would be a great thing to keep. If it's a lot of hassle, I swear by 575A's and 673's. The older 575 has the same ratings but a nice big pear shaped globe. What kind of thyratrons are they? It would be interesting to know the type.

The reason I suggested using the main transformer alone is that on single phase power, the secondary voltage from it will be different (lower) than from the teaser since the teaser has a steeper turns ratio (again not knowing the transmitter, only the circuit). The result might be that only two (teaser) rectifiers conduct until you really throw a load on it, at which point the other two (main) would conduct.

If you can use both, then you might need a way to adjust the input voltage on one of them so that the HV AC is equal from both. The taps might be helpful in accomplishing this since 230*.86=197 which is not too far from 208. Another point to be looked at is that if the main transformer wants 208V 3-phase per leg, that is more or less the same as 115V to neutral, per leg. Your new neutral would be the primary center tap of the main transformer. Same for the Teaser but it would want only 99VAC across its primary to make the same secondary voltage.

[Opcom]

Here is an example of a thyratron regulated DC supply. It's interesting because the design is very simple yet it is scalable.
This is from the Radio Handbook, 13th edition 1951.
The entire article is in a zipped PDF here:
http://www.bunkerofdoom.com/lit/thyratron_reg_dc.zip

[k4kyv]

In 3-phase input and output, the windings could usually be on one combination or three separate transformers. This is using a 3 to 2 phase conversion, but due to the center taps you get four phases. In this special "Scott" circuit, two or three transformers are used. One combined transformer can't be used because the wiring of the coils is to make a different phase relationship of the magnetic flux, that is it is not possible? to have a 3-phase and 2-phase magnetic flux in the same core at the same frequency.

OK I now understand what they are doing, but I still don't see the point in a transmitter power supply, where the ultimate output is to be direct current, of converting 3-phase power to two-phase and then rectifying the two phase to produce DC.  I can see where two-phase a.c. might be needed for running a certain type of motor, but in the DC power supply of that transmitter, wouldn't it have been simpler to just use the standard 3-phase rectifier circuit with three rectifier diodes?

Nikola Tesla's original polyphase power system was based on simple to build two-phase components. However, as transmission distances increased, the more transmission line efficient three-phase system became more prominent. Both 2-φ and 3-φ components coexisted for a number of years and the Scott-T transformer connection allowed them to be interconnected.

This said I do not know the power supply of the transmitter in question, but only know of this transformer setup in general.

[Opcom]

In 3-phase input and output, the windings could usually be on one combination or three separate transformers. This is using a 3 to 2 phase conversion, but due to the center taps you get four phases. In this special "Scott" circuit, two or three transformers are used. One combined transformer can't be used because the wiring of the coils is to make a different phase relationship of the magnetic flux, that is it is not possible? to have a 3-phase and 2-phase magnetic flux in the same core at the same frequency.

OK I now understand what they are doing, but I still don't see the point in a transmitter power supply, where the ultimate output is to be direct current, of converting 3-phase power to two-phase and then rectifying the two phase to produce DC.  I can see where two-phase a.c. might be needed for running a certain type of motor, but in the DC power supply of that transmitter, wouldn't it have been simpler to just use the standard 3-phase rectifier circuit with three rectifier diodes?

Nikola Tesla's original polyphase power system was based on simple to build two-phase components. However, as transmission distances increased, the more transmission line efficient three-phase system became more prominent. Both 2-φ and 3-φ components coexisted for a number of years and the Scott-T transformer connection allowed them to be interconnected.

This said I do not know the power supply of the transmitter in question, but only know of this transformer setup in general.

Pinching pennies on Thyratrons? Not likely since two transformers probably cost more than one.  No savings on filters either due to 240Hz instead of 360Hz to filter. There had to be a very good reason for going to the trouble, either technical or $. How about the maximum operating frequency of the thyratrons? Are they mercury vapor?