Download 09-WATTMETER

Measurement
Dr.Othman
Eman barakat
Renada yaseen
WATTMETER
An instrument for measuring the electric
power in watts of any given circuit.
Basic Principle of Analogue Wattmeter
A wattmeter has two coils that multiply
volts times amps.
One coil is wired
as an ammeter,
the other as
voltmeter.
The movable
coil carries a
needle which
moves over a
suitable
marked scale.
Types Of Wattmeter
WATTMETER
ELECTRODYNAMIC
WATTMETER
ELECTRODYNAMO
METER
ELECTRONIC
WATTMETER
Electrodynamic
It consist of a pair of fixed coils, known as
current coils and movable coil known as the
potential coil.
The current coil of the wattmeter is connected
in series with the circuit (load), and the voltage
coil is connected across the line.
When line current
flows through the
current coil of a
wattmeter, a field is
set up around the
coil. The strength of
this field is in phase
with and
proportional to the
line current
The potential coil
provides current
proportional to the
voltage.
This coil has a highvalue resistor connected
in series with it to reduce
the current that flows
through it and make the
voltage-coil circuit of the
meter as purely resistive
as possible. As a result,
current in the voltage
circuit is practically in
phase with line voltage.
.
ELECTRONIC WATTMETER
Electronic wattmeter are used for direct,
small power measurements or for power
measurements at frequencies beyond the
range of other types of wattmeter.
Methods of connecting
wattmeter
Three-Wattmeter Method
Two-Wattmeter Method
One-Wattmeter Method
Three-Wattmeter Method
3 wattmeter's are connected
in such a way that each has its
current coil in one line and its
potential coil between that line
and some common point.
±
R
W1
I1
Z
±
1
W2
N
B
Z
Z
3
3
±
X
I2
Y
±
W3
±
I3
±
Total power : w1 + w2 + w3
If neutral wire available, the common point
X should be the neutral wire.
Note: Total power does not depends upon
whether the load is balanced or not
Two-Wattmeter Method
Current coils of two
wattmeter's are connected in
any two lines and the
potential coil of each joined
to the third line
±
R
W1
I1
Z
±
1
N
B
Z
Z
3
3
I2
Y
±
W2
±
I3
±
Total power : w1 + w2
If the neutral wire is available, it
should carry no current.
One-wattmeter method
If the load (3-phse Y or is balanced,
the power in any phase can be
measured by a single wattmeter.
W
R
IR
Z
N
Z
B
VYB
Z
IY
Y
IB
The current coil is connected in any
one line and the pressure coil is
connected in any one line and the
pressure coil is connected
alternately between this and the
other two lines.