Download 02-Wattmeter

A wattmeter
is an electro dynamo – meter
instrument used to measure the power in a
given circuit .
this type of meter has two coils,one is fixed
and the other is free to move .
Here, a pointer is attached to the movable coil and
the deflection is propotional to the repulsive force
generated from the currents moving through the two
coils .
Here , the movement is similar to the darsonval
movement .
When the line current flows through the current
coil , a field is set around the coil . the strength of
this field is proportial to the current and in phase
with it .
Note:
the high value of the resistance connected in series
with the potential coil makes that potential circuit
as purely as possible .
Hence :
the part in the potential circuit practically in
phase with the line voltage .
therefore , when the voltage is applied to the
potential circuit . the circuit is propotional to
and in phase with the line voltage .
Note:
The mechanical movement causes a deflection that is
propotional to the voltage value of the power .
Notice that one end of each coil is usually marked
with a ( + ) sign while the other is ( - ) . here an
upscale reading is obtained if a ( + ) current is flowing
in to the ( + ) end of the current coil while the ( + )
terminal of the potential coil is ( + ) WRT to the end .
Example:
Solution:
Using the mesh analysis analysis:
I = ( 11.18 ) / 153.4 A
P = (100)*(11.18)*cos(O-1534)
= -1000W
Using the down scale deflection for a ( + ) reading.