Dave, G8WRB, inquired about 3 phase HT supplies:
> I'm looking to design an HT power supply for a YC156 (3CPX5000A7)
>triode. One approach is the common single phase input, single phase
>transformer, lots of expensive high voltage capacitors.
> I've been considering another approach, which seems to me to be much
>simpler:- use a 3-phase supply, 3-phase transformer, and a bridge with 6
>diodes. This gives 4.2% RMS ripple, without any storage capacitors,
>compared with a totally unacceptable 48.2% for single phase bridge
>without any storage capacitor or choke.
Absolutely correct. Much lower ripple, and needs 6 instead of 4
diodes for a full wave bridge rectifier.
>I suspect the 3-phase transformer will cost more than a single phase
>one, as there are more windings, but this cost is probably more than
>offset by the cost savings in not having any electrolytic capacitors.
>The other potential advantage, is that in the event of an arc in the
>tube, there is far less stored enegy, so protection systems don't need
>to be as rugged. Assuming you have a 3 phase supply (which I do) it
>seems the obvious way to go.
Yes, 3 phase system is so much better if you have the AC feed for it.
You still need some capacitance to reduce that 4.2% ripple to a more
insignificant amount. But you can go from say, 20 uF to 4 uF with
similar residual ripple, for a choke input filter, and much smaller
choke to meet the critical inductance criterion for best regulation.
>However, somone once told me that such a system with no storage
>capacitance would destroy a transformer. Does anyone know if this is
>true ? What approaches are taken on large HT supplies for amplifers much
>bigger than amatuer ones? 3-phase would obviously be used, but do they
>use storage capacitors ??? Perhaps just a small amount would be enough,
>but I'm not sure why any would be needed.
Not a problem. I haven't heard that one!
I helped develop 3 and 5 kW Fm transmitters which were marketed, and
they used the same final, and the same PA cavity. The high power rig
had higher plate voltage. Other difference was single versus 3 phase
iron. The 3 phase was cheaper to built if I remember correctly. The
cost of the iron was offset by the cost of the bigger LC filter
needed for 1 phase HT supply. Also the inrush to the diodes and the
peak charging current through them is reduced if you have lower
capacitance filters - Another advantage that allows derating the
rectifier diodes.
If you really want to run it with low stored energy, get a second 3
phase transformer, and run one in Delta and one in Wye. Series of the
two secondaries (after each is rectified with diodes) of them results
in a 12 pulse ripple, with extremely low ripple as is.
>I once played around with the concept of using three single phase
>transformers on a 3-phase supply. This works fine, which does mean it
>would be possible to design an HT supply that worked with a 3-phase
>supply, or used the three tranformers in parallel to use it from a
>single phase supply. It would only take an hour or so to reconfigure the
>transformers to swap from single phase to 3-phase operation.
Thats a lot of iron to be packaging! It should work though. However,
the voltage may be different out of one versus the other. (phase to
phase voltage of the 3 phase versus end to end voltage of the
secondary winding of paralleled transformers).
73
John
K5PRO
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