[BC] AC Voltage Drop

[PeterH]

On Jun 29, 2009, at 10:51 PM, Milton R. Holladay Jr. wrote:

> IIRC, there is a wye secondary and a delta secondary which wind up  
> being 60
> degrees different in phase such that, combined,  there are 12  
> pulses per
> cycle resulting from the 6 sets of rectifiers. The ripple is so low  
> that no
> choke is required on the input of the fillter capacitor bank.
> I've heard them called 6 phase and 12 phase. Ftitz Bauer used it on  
> his
> FB-5J and Collins adopted it also, as it worked especially well on  
> AM xmtrs
> due to the low source impedance/very good regulation  of the non- 
> choke input
> filter power supply accommodating the pulsating load of an AM xmtr.

The primary is almost always Æ (the device most probably cannot see  
the difference between a Y feeder and a Æ feeder, anyway, because  
only three phase conductors are provided to the utilization  
equipment) [ * ].

The secondary is always Y, but a special variant of Y is called a Zig- 
Zag.

The pre-Zig-Zag transformers were either:

1) three-phase full-wave ("six-phase"),

2) two-phase full-wave ("four-phase"; RCA 5 kW and 10 kW AMs,  
predominantly, the primaries were connected in a "Scott-T", so there  
were three input phases and "four" output phases), or

while the post-Zig-Zag transformers were:

3) three-phase full-wave Zig-Zag ("twelve-phase").

I suppose there could have been an additional type:

4) two-phase full-wave Zig-Zag ("eight phase"), but I've never seen  
one of these (might have been a good option for 25 or 50 kW AM  
transmitters, however).

With almost any Æ-Y transformer, there is an inherent 30 degree phase  
displacement between the primary and the secondary.

The transformer name plate will show this displacement as a dotted  
line from one phase to the virtual center (Æ) or the actual center  
(Y) on both the primary and the secondary diagrams.

It is indeed possible to order, at significant extra cost, a  
transformer which has zero phase displacement between the primary and  
the secondary.

This is required, say, for utility switching stations where groups of  
transformers may be placed in parallel connection to increase the  
load capacity of a transformer bank.

You may parallel several transformers all of which have 30 degree  
phase displacement or several transformers all of which have zero  
degree phase displacement, but you may not parallel transformers with  
differing phase displacements.

Zig-Zag is "well-known in the art", and I doubt that a transmitter  
PSU which featured a Zig-Zag transformer is patentable, unless there  
was some other unique features which were "new and inventive", such  
as a new overload detection means, most probably in the neutral, and  
tripping upon a detected imbalance in the phases, which gets  
reflected as a spike in the "zero-sequence" current in that  
transformer's neutral conductor.

[ * ] At my last employer, a manufacturer of large mainframe computer  
systems, our feeders were 208 volts 415 Hz. These were Y feeders on  
account of the internal PSU design. Because the load was, variously,  
70 KVA or 140 KVA, and the switchers operated at such a low  
frequency, the harmonics were significant, and the neutral conductor  
was required to be one size larger than the phase conductors. The  
conduit ... these were all "hard wired" devices ... was required to  
be aluminum as the magnetic field around the conductors was significant.

Lest one laugh at a mainframe computer which requires 140 KVA (about  
what a plate-modulated 50 kW AM transmitter would require at 100  
percent modulation), consider that there is essentially zero power  
output from a computer. Most of the input power is lost as heat.

Additionally, and I'm fairly certain there will be more than a few  
laughs on this particular point, for that 140 KVA input you got a  
mainframe computer with all of EIGHT MEGABYTES of main storage. Yes,  
8 Megabytes, NOT 8 gigabytes.

Hell, back then the largest disk drives had a capacity of only 300  
megabytes, and it would be another decade and two generations later  
before the first mainframe disk drive had a capacity which was 1  
gigabytes or more (and by then the most common sizes were 1, 2 and 3  
GB). Another half-decade would get us to 9 gigabytes, and 27  
gigabytes followed a few years later.