No subject

11.3. A.F. TRANSFORMERS

 The effective frequency band of an a.f. transformer is limited at
the low end by shunt inductance and at the high end by leakage or
series inductance. Shunt inductance, so called because it acts as a
by-pass to the transformer load impedance, is simply the inductance
of the primary. It should be measured at a low frequency, to ensure
that the effect of stray capacitance is negligible; 50 c/s is usually
suitable. Working conditions should be reproduced as nearly as
possible during measurement, as regards amplitude of a.c. and d.c.
(if any). Suitable methods are described in Secs. 9.18 to 9.22.
 Leakage inductance is relatively small, and is a measure of the
extent to which the primary is not completely coupled to the
secondary. The equivalent total leakage inductance referred to the
primary is the inductance measured on the primary side when the
secondary is shortcircuited. Although it affects the high frequency
end, there is no reason why it should not he measured at the same low
frequency as the shunt inductance, except perhaps that its reactance
is then rather small.
 Self-capacitance is an important parameter of intervalve
transformers, but these are now rarely used; in output transformers
it often plays only a minor part. But it may have to be considered
when the output transformer is in a negative-feedback loop,
especially since it is increased by the means adopted in the better
transformers to reduce leakage inductance—the interleaving of primary
and secondary windings. With this arrangement one may not be safe in
assuming that the stray capacitance is equivalent to a constant
capacitance in shunt with the primary and independent of frequency,
and instead of trying to measure it as such it is better to measure
the phase shift (Sec. 10.7).
 The efficiency of an output transformer can be measured by means of
an output meter and a.f. signal source. The source should be adjusted
to give a suitable amount of power, measured first by the output
meter connected directly to it, and then through the transformer. If
the transformer ratio is n : 1 and its optimum load resistance R, the
resistance of the output meter should be as near as possible to n*n*R
for the first reading and R for the second. The efficiency is the
ratio of the second reading to the first, and should be measured at
the lowest, highest, and one or more intermediate frequencies.
Alternatively this power ratio can be expressed in dB (Table 14.17)
and is then called the transformer loss. For this method it is
necessary for the output-meter transformer not only to have the
appropriate pair of ratios but also reasonably equal efficiencies at
these ratios, at all frequencies concerned. If there is any doubt
about this—or if an output power meter is lacking—the measurement can
be made by a valve voltmeter or even c.r.o. across the appropriate
resistances.
 An important characteristic is non-linearity, reckoned as the amount
of distortion caused under specified conditions by a specified signal
amplitude, or alternatively the maximum signal power for a specified
amount of distortion. Perhaps the most useful information, covering
both of these, is a curve of distortion against amplitude or power.
Methods are the same as for a.f. amplifiers; see Secs. 11.12 to
11.16.
 It is of course most essential that working conditions or their
equivalents are reproduced—the same amount of d.c. and the same
impedances connected to the windings.

 Ref: "Harmonic Distortion in A.F. Transformers", by N. Partridge.
(W.E.), Sept. to Nov. 1942.


----- Original Message -----
From: "Murray Grandy" <mgrandy at TELUS.NET>


> Does anyone know of test equipment that will determine the
impedance of
> output transformers. Did Heath make any such thing. I have a bunch
of
> unmarked transformers and would like to know what they are. I'm
talking
> about the primary, primarily. Hi Hi.
> Murray

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