plate transformer specs

[W8ACR]

I need the secondary current rating of an RCA XT-3312 plate transformer. Primary is 105-120 VAC, secondary is 3680VCT. The ID plate says 216KVA, so I'm assuming it should be 2.16KVA, and I'm assuming that is the primary KVA rating. This thing is big, but not gigantic, probably weighs about 100 lb. I'm guessing that it produces about 1500-1600 VDC, so maybe it was used to power a pair of 810's?? Open frame construction, looks like 1930's or 1940's style.

Thanks, Ron

[Steve - K4HX]

Looks like the power transformer in RCA and GE 250 watt transmitters.

[KA2DZT]

I would think from the KVA rating that the current rating is about 1amp or a little over 1amp.

Fred

[KM1H]

Thats a lot of extra KVA for a 250W carrier and mod supply.

Using .9 X the AC as common in well built choke input supplies that leaves roughly 1650VDC.

Maybe a pair of 813's with triode mods for a 500W TX?

Carl

[Tom WA3KLR]

Running 24/7 in broadcast service does not allow the transformer designer to cheat like they do for low cost consumer products.  Also this one has to work at 50 Hertz, possibly allowing a margin to 47 Hertz for bouncing generators, hence the big size.

Assuming the 2160 VA is for the primary, the secondary rating would be 2000 VA.  With the 1840 V secondary this is 1.09 amps.  

LTSpiceIV simulation results:
With 2 full-wave rectifiers and choke filter I came up with 1608 VDC at 766 mA load to get a secondary current at the 2000 VA limit.  Aside from the temperature rise (or lack of in Amateur use), you need to remember the instantaneous magnetic limit of the core.

[KM1H]

Lets see here using nothing more involved than your fingers.

For a 500W TX even a poor design wont require more than 700W DC input.
For 300W of audio I'll give a bit extra so call it 600W DC input.

My fingers tell me thats a conservative 1300W CCS needed from that transformer and assuming the 2KVA secondary rating thats still a lot of left over capability, even at 50Hz which it happens to be built for so that should still be 2KVA.

[Opcom]

The entire RCA BTA-250L consumes 1825 watts @ 100% mod of its 250W carrier.
Its transformer has 7 terminals, that one has 8 and has different numbers.

Maybe that one is for a 500W transmitter considering the 2.16KVA rating for the plate alone - 2KW if the 90% efficiency rule of thumb is applied.

Even so a 500W transmitter's plates might consume 600W DC for the RF stage and 600W DC for the modulator. -1200W DC. not including large bleeders and voltage dividers used for supplying screen grids and/or other stages as was common.

A 1KW  TX would use more like 1200W DC plus the same for a modulator, but these guesses above are based on 80% RF efficiency and 50% modulator efficiency, so 2KVA might just be enough.

90% RF eff. plus 65% AF eff.
kw -
90% = 1000W RF
100%==1100W DC

65% = 500W AF
100% = 770W DC

1870W DC for an efficient 1KW TX. Does this make sense? Is it wrong?

[KE6DF]

The entire RCA BTA-250L consumes 1825 watts @ 100% mod of its 250W carrier.
Its transformer has 7 terminals, that one has 8 and has different numbers.

Maybe that one is for a 500W transmitter considering the 2.16KVA rating for the plate alone - 2KW if the 90% efficiency rule of thumb is applied.

Even so a 500W transmitter's plates might consume 600W DC for the RF stage and 600W DC for the modulator. -1200W DC. not including large bleeders and voltage dividers used for supplying screen grids and/or other stages as was common.

A 1KW  TX would use more like 1200W DC plus the same for a modulator, but these guesses above are based on 80% RF efficiency and 50% modulator efficiency, so 2KVA might just be enough.

90% RF eff. plus 65% AF eff.
kw -
90% = 1000W RF
100%==1100W DC

65% = 500W AF
100% = 770W DC

1870W DC for an efficient 1KW TX. Does this make sense? Is it wrong?

Back to the other disucssion we had on another thread.

The transformer KVA rating is based on total current drawn from the AC line.

You can't count on a power factor of 1.0, so if a 250W transformer draws 1850 watts, the KVA might be considerably above 1850 when power factor is considered.

Does anyone know the power factor of a typical broadcast transmitter?

Dave