Electronic Wattmeter
Chapter 2
2.6 Measuring
Power in
a Three-phase
System
Balanced system
Polyphase measurements invariably require connection of the
common terminal of the
EW604 to at least one live line. The
line
voltage must then not exceed 280V. For three-phase systems
this implies a maximum line voltage of
490V.
If
both the supply and the load are balanced and either a neutral
is
available
on
the supply or a star point
on
the load, the power
can
be
measured by connecting as
in
fig 2.4 and multiplying the
reading obtained by three.
EW604
3 phase
supply{----.:
8
::.._
y
~~-L~-.
a)
Supply neutral available
3 phase
JRB
supply
ty
b)
Star point of load available
Fig 2.4 Finding the power
in
a balanced load system
If the supply and load are balanced, both the power and the
power factor can
be
obtained from two readings using the
connections
of
fig
2.5. Two readings P
1
and P
2
are taken using
the two positions of the switch. The power is then given
by
P
1
+ P
2
,
and the power factor
is
1
2
p1
1--
1 + 3
p2
p1
1+-
p2
14
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