Full resolution (JPEG) - On this page / på denna sida - A Survey of the Possible Effects upon A. C. Networks Supplying Television Receivers in Residential Districts, by Sture Colding
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Fig. 1. The flux $ and the currents (iu im, ib) in a d.c.
magnetized single phase transformer.
may entail further voltage drops, disturbing the
performance of the TV receivers.
However, the argumentation is not correct. It is
not permitted to adopt a kind of superposition
theory on non-linear circuits.
If we consider the transformer flux the induced
voltage ui, input voltage u± load current z&, primary
load current z\ and magnetizing current im, we will
have the following course, shown in fig. 1, where
the voltage drop is exaggerated to increase the
distinctness.
The wave-form of the tension u± of the supplying
network will not be altered. <f> induces u„ that is
normally slightly less than zzj,. When if> is taken out
from the secondaries, $ must be changed a little,
because it is the only connecting link between the
primary and the secondary winding. As a result the
numerical value of A7 • d <I>/dt, i.e. zz,-, must decrease
a little during the interval of t2—tlf fig. 1.
Because ib can have one direction only the flux
displacement from the off-load flux also will take
place in one direction only. The actual flux
immediately is displaced to a position where its
peak values on the side, to which the displacement
has occurred, are followed by an increased exciting
current. These additional exciting current pulses
have the same sign as ib, whose corresponding value
on the primary side has the opposite sign. The
additional pulses of im will balance the load pulses ix.
No d.c. component of z\ will remain, according to
the general theory of electrical circuits. The two
surfaces of the two pulses (exactly spoken:
corrected with a slight alteration of the normal exciting
current) are namely of the same magnitude.
The most important conclusion is that the
augmentation of im originates from ib which is transferred
to the primary side in a normal manner (z\). This
gives rise to the balancing current pulses of about
the same magnitude (d.c. value) and not to a greatly
increased magnetizing current which is obtained
under magnetic amplifier conditions. The theory of
magnetic amplifiers is not applicable here.
The damping constant of the primary circuit is
increased when the magnetizing peaks occur.
Therefore 4> is limited in this direction. The voltage drop
z’m • in the primary circuit replaces the flux to a
position near the off-load curve.
Fig. 2 shows the current pulses in a single-phase
transformer. The area of the upper peaks (im) has
the same value as the area of the primary load peaks.
The curve below shows ib. It should be noted, that
the phase displacement between zzt and the
additional pulses of tfn is about 90°. This contributes to
the fact that the additional copper losses are very
small. Certainly the reactive power is increased with
a measurable value, but this is of no practical
importance. Most distribution transformers are connected
in delta-star. The saturation phenomena are here
a little more complicated.
Generally spoken, the augmentation of the iron
losses are of most interest. These are approximately
proportional to (Bmax)2 in the actual area. Certainly
B is displaced to a little higher value in one
direction, where the losses are increased, but the opposite
peak values are smaller than in the case without a
d.c. component on the secondary side. Roughly
spoken — at least if the impedance of the primary
circuit is low — $ maintains a good sinus wave in spite
of the fact that the peaks of im may be increased
five or ten times.
Laboratory tests
Laboratory tests have verified the theory above.
As samples two transformers were used, one with
cold-rolled steel sheets and the other with hot-rolled
steel sheets. With respect to the additional losses no
differences between the two samples could be
noticed.
The transformer was connected during the test to
the ordinary 6.3 kV supply network. To the secon-
Fig. 2. Currents in a single phase transformer feeding a
half-wave rectifier with condenser input to the
filter. Above the primary current, below the
secondary current.
.146 ELTEKNIK 1959
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