DYNAMOMETER TYPE WATTMETER
Following figure shows the dynamometer wattmeter for
measuring the power. If two coils are connected such that the
current proportional to the load voltage flowing through one coil
and the current, proportional to the load current flows through
other coil, then the meter can be directly calibrated in watts. This
is true because the indication depends on the product of two
magnetic fields. The strength of the magnetic fields depends on
the values of the current flowing through the coils.
Ranges:i.
ii.
Current circuit 0.25 to 200 A without employing Current
Transformers.
Potential circuit 5 to 750 V without employing Potential
Transformers.
Advantages:i.
ii.
Since deflecting torque is proportional to the true power in
both the cases, i.e., dc and ac; and the instrument is spring
controlled, the scale of the instrument is uniform.
High degree of accuracy can be obtained by careful design,
hence these are used as standard for calibration purposes.
Disadvantages:i.
ii.
iii.
iv.
As there is no iron core in the coils, the field is very weak.
Hence a fairly large number of ampere turns are required
for the fixed as well as the moving coil. This makes the
moving system heavy, as well as there exists a significant
amount of power loss in the instrument.
A heavy moving element makes the torque-weight ratio
small resulting in large frictional error.
At low power factor, the error caused by the inductance of
pressure coil is very serious.
The reading of the instrument is affected by stray fields
acting on the moving coil.
INDUCTION TYPE WATTMETER
In all induction meters, we have two fluxes which are produced
by two different alternating currents on a metallic disc. Due to
alternating fluxes, there is an induced emf, the emf produces at
one point, as shown in the figure given below, interacts with the
alternating current of the other side resulting in production of
torque. Similarly, the emf produced at point two interacts with
the alternating current at point 1, resulting in a torque which is
in opposite direction. Due to these two torques produced in two
different directions, the metallic disc rotates. This is the basic
principle of working of Induction Wattmeter.
P1, P2  Points 1 & 2 respectively
I1 , I2  Currents 1 & 2 respectively
Td1 = - Td2
Ranges:i.
ii.
Current circuit – upto 100 A without employing Current
Transformer.
Potential Circuit – upto 750 V without employing Potential
Transformer.
Advantages :Compared to Dynamometer type wattmeter, this wattmeter has a
strong construction, long and uniform scale, free from effects of
stray field and has good damping.
Disadvantages :i.
ii.
iii.
iv.
The wattmeter is less accurate. It is accurate only at stated
frequency and temperature.
Weight of moving system compared to dynamometer
wattmeter is higher.
This wattmeter cannot be used on ac system.
Power consumption, as compared to dynamometer
wattmeter, is higher.
ELECTROSTATIC TYPE WATTMETER
These watt meters are used for measurement of very small
amount of power, where the voltage is high and power factor is
low. This type of wattmeter is also used to measure the dielectric
loss in cables on alternating voltage and for calibration of
wattmeters and energy meters. It consists of a quadrant
electrometer used with a non inductive resistor R as shown in the
figure below.
Advantages:i.
ii.
iii.
Electrostatic wattmeter is a precision instrument and
should be used as such.
It is free from errors on account of waveforms, frequency
and eddy currents.
It has a very small working torque.
Thus, judging the above wattmeters, we can
conclude that Dynamometer type wattmeter is
the most suitable wattmeter amongst all. So, the
electrician
must
install
Dynamometer
type
wattmeter in the industry.