[ARC5] Bucking transformer configurations.

[Geoff]

Hmmm, gremlins must be active again (-;  Several mesages seem to go missing 
at times.
Ive removed your comments in case the message was rejected for length or too 
many recipients.

Here it is for the 2nd time.
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This should be required reading on the subject.

http://sound.westhost.com/articles/buck-xfmr.htm#s10

Using that same RS transformer mentioned earlier here is the result of some
indepth testing a couple of years ago.

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The transformer used was a NOS Radio Shack cat. no, 273-1352, rated for 12.6
VAC, 1.2 A output, 120 VAC input. It has a center tap, which was ignored for
the tests. The transformer was made in Taiwan and purchased new about 15-20
years ago. Except for current rating (and physical size), it is presumably
similar to the one cited by the OP.

It was decided to load the transformer to approximately 10% of its current
rating, and approximately 100% to explore its behavior at differing loads.
Ohmite "Brown Devil" wire-wound resistors were used as loads. Resistances
were measured with a Rubicon Wheatstone bridge.

A Weston 432 low range wattmeter was used to measure the no-load and 10%
load wattages. Another Weston 432 with higher ranges was used for the 100%
load tests. The input voltage was measured with a Sensitive Research model D
dynamometer voltmeter. The input current was measured with a SR model DYP-2
dynamometer volt-ammeter. Load current was measured with a Westinghouse PA-5
ammeter. Load voltage was measured with a Fluke 77 DMM.

In all cases, the input voltage was adjusted as required with an isolated
variac to 120 volts. In keeping with the OP, only the step-down
autotransformer and bucking configurations were considered; not the step-up
or boost connections. "Autotransformer connection" means that the primary
and secondary are connected in series across the AC line, with the output
taken from the junction of the primary and secondary, and the other side of
the AC line. "Bucking" means that the AC line is connected to the primary in
normal fashion; the load is placed in series with the secondary, and in
parallel with the primary. With either arrangement, phasing of the windings
determines whether the load will see a voltage higher or lower than the
line.

Results:

At 10% and 100% load, the autotransformer connection exhibited slightly
better power factor and efficiency, but worse voltage regulation than the
buck-boost connection. With a small power transformer of the type tested, it
would be better to use the autotransformer connection if maximum efficiency
is desired, and voltage requirements are only approximate. The bucking
connection should be used if maintaining a particular voltage difference is
of prime importance.

[Note: In all cases, the desired output voltage was 120-12.6 = 107.4 V.]

Autotransformer Data:
(At 120 volts input)

At no load, the input current was 0.040 A and the wattage was 2.4 W. This
yields a VA of 4.8 and a power factor of 50%. The output voltage was 112 V.

At 10% load (830 Ohm resistor), the input was 0.135 A, 15.5 W. This yields a
VA of 16.2 and a PF of 96%. The output was 108 V, 0.130 A, for an output VA
of 14.05. The efficiency (VA out / VA in)(100) was 87%. The regulation (V no
load - V load) / V no load was a 3.57% drop.

At 100% load (88 Ohm resistor), the input was 1.14 A, 133.5 W. The primary
VA was 136.8 and the PF was 98%. The output was 107.5 V, 1.22 A. The output
VA was 131.15, giving an efficiency of 98.24%. Regulation was a 4.02% drop.

Buck Transformer Data:
(At 120 volts input)

At no load, the input current was 0.057A, 3.2 W. This gives a VA of 6.84 and
a PF of 47%. The output voltage was 110.1 V.

At 10% load (830 Ohm resistor), the input current was 0.142 A, 15.2 W. The
VA was 17.04 and the PF 89%. The output voltage was 109.2 V, and the current
was 0.132 A. This gives a VA of 14.37. The efficiency was 84%, and
regulation was 0.82% drop.

At 100% load (88 Ohm resistor), the input current was 1.12 A, 130 W. The
primary VA was 134.4 and the PF was 97%. The output voltage was 106.7 V, and
the current was 1.21 A. The output VA was 129.1. Efficiency was 96% and
regulation was 3.09% drop.

Raw Transformer Data:

DC resistance of primary: 47.73 Ohms. Secondary: 0.65 Ohms.

No load current with secondary disconnected (@ 120 V): 0.056 A, 2.89 W.

Short circuit characteristics: For a secondary current of 1.2 A, an input
voltage of 18.4 V was required (3.62 watts).

Comment:

The textbook reference cited ("Magnetic Circuits and Transformers") contains
a chapter on autotransformers which concludes, reading between the lines,
that there are few general conclusions or formulas that can be derived. Much
has to do with the specific design and construction of a particular
transformer. In other words, your mileage may vary!