Electronic Wattmeter
Chapter 2
N
Fig 2.6b Measuring power
in
an unbalanced star-connected load
2.7 Measuring Systems
with Low Power Factor
It
is
possible with conditions of low power factor or
high
crest
factor that only a small deflection is obtainable without
overloading. This is a direct result of the real power being
low
in
the circuit and thus it is unreasonable to expect a large
deflection.
In
practice low power factors are commonly
encountered when measuring, for example, the iron loss of
transformers.
In
this case the magnetising current
may
cause
overloading of the wattmeter when it is displaying a very
small
deflection corresponding to a small iron loss. This difficulty
may
be
overcome
by
the use of power factor correction capacitors.
In
theory the use of such capacitors to 'tune' the transformer
inductance does not affect the power reading but
it
does
reduce
the supply current enabling a more sensitive current range
to
be
used without overloading the wattmeter, thereby providing a
more significant deflection.
The capacitor used
as
shown
in
fig 2.7 can
be,
for convenience,
a decade box of suitable voltage rating and value. The loss
angle of most commercial capacitor boxes is
so
small that
it
is
insignificant compared with normal power transformer loss
angles
and
so
requires
no
correction.
The value of the capacitors is not critical,
it
being adjusted
on
test
to
extinguish the current overload indication.
1 6
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