The instrument used to measure
active power ‘P’ drawn by a load or circuit is called ‘wattmeter’. Three types
of wattmeter are in use. They are:
1.
Dynamometer type,
2.
Induction type, and
3.
Electrostatic type.
The most commonly used wattmeter
and available in labs are the dynamometer type. Although digital wattmeter are
also in use and are mainly found in industries.
Lets’ have a look into the Electrodynamometer type Wattmeter…
An electrodynamometer type
wattmeter has two coils; a fixed coil and a moving coil. The fixed coil is also
called the current coil (CC) since it carries the load current or a fraction of
it. The current coil, which is connected in series, is made up of thick wires
of few turns and is divided into two identical parts (as shown in the figure). The current coil is
divided into two to have a uniform magnetic field. The terminals of these fixed or
current coils are marked ‘M’ and ‘L’.
The second coil is movable and is
called the pressure coil (P.C.). It is located inside the current coil and is
made up of large number of turns of very fine wire. A very high resistance is
also sometimes added in series with the pressure coil (also called voltage
coil) which makes the resistance of pressure coil in kiloOhm range; usually 5,
10 or 20 kiloOhm. The pressure coil is connected in parallel to the load and
carries a definite very low value of current .The terminals of pressure coil
are marked ‘COM’ and ‘V’.
Fig 1 and 2: Two different views of Dynamometertype wattmeter.
Working of Electrodynamometer type Wattmeter:
The pressure coil or the moving
coil, which is suspended on a spindle, moves in between the two halves of the
fixed coil. The movement is due to the interaction of the magnetic fields of
the two coils; fixed and the moving. The controlling torque is provided by two fine
springs which also serves as leads to pass the current into the pressure coil. A
pointer is attached to the moving coil which directly indicates the value of
active power recorded by the wattmeter.
The deflection of the wattmeter is given by:
The deflection of the wattmeter is given by:
T = K . V . I. cos(phi)
where ‘K’
is a constant,
V and I are the r.m.s.
value of supply voltage and load current, and
‘phi’ is the phase difference between V and I.
Multiplying Factor of Electrodynamometer type Wattmeter:
Wattmeters usually have
selection facility i.e. one can select the range of voltage as well as current
of the wattmeter. Suppose we have a 2.5/5 A wattmeter and by properly connecting
the links on the wattmeter we can select either 2.5 A or 5 A capacity range.
Similarly, we can select the
voltage range also. Suppose we have a wattmeter with voltage range 75 V, 150
V, and 300 V. One can select any one voltage according to the voltage applied
to the circuit. Let’s make you more clear.
The voltage applied in the short circuit test of a single phase small transformer is very low, usually 10 – 20 V, so in this case we have to select the 75 V range. On the other hand, in the open circuit test of the same transformer normal rated voltage of 230 V is applied, hence we have to select the 300 V range.
Depending on the selection of voltage and current, we have to consider the ‘multiplying factor’ for further calculation. In simple, a ‘multiplying factor’ is a factor which is to be multiplied into the wattmeter reading to obtain the correct value of active power in the circuit.
An example for ‘multiplying factor’ is given below:
Current selected

Voltage selected


75 V

150 V

300 V


2.5 A

1

2

4

5 A

2

4

8

The figures in ‘bold’ are the
multiplying factors. For example, when we
select (connect to) 150 V and 2.5 A, the ‘multiplying factor’ is 2 and for a selection
of (connection to) 150 V and 2.5 A, the ‘multiplying factor’ is 4. Multiplying factor
for the same values of current and voltage may vary according to the
construction of the wattmeter.