Downloaded From : www.EasyEngineering.

net

Type 6: Wattmeter

For Concept, refer to Measurement K-Notes, Electro-mechanical Instruments

Point to remember:
The power reading of a wattmeter is equal to product of voltage across Potential Coil and
Current through the Current Coil and the cosine of angle between them. These all quantities
can calculated from the phasor diagrams.
Sample Problem 6:

ww
A single-phase load is connected between R and Y terminals of a 415 V, symmetrical, 3-phase,
4-wire system with phase sequence RYB. A wattmeter is connected in the system as shown in

w.E
figure. The power factor of the load is 0.8 lagging. The wattmeter will read?
(A) −795 W (B) −597 W (C) +597 W (D) +795 W

asy
Solution: (B) is correct option
In the figure
VRY = 415300
415 En
VBN=
3
1200
gin
Current in current coil

Ic 
VRY

415300
 4.15  6.870
eer
 power factor=0.8 
Z 10036.87
415
0 
ing 0
Cos=0.8  =36.87 

Power  VI * 
3
1200  4.156.870  994.3126.870
.ne
Reading of wattmeter P=994.3  cos(126.870 )
=994.3  -0.60
=-597 W
t
Unsolved Problems :

Q.1 The resistance of two coils of a Watt meter are 0.01 and 1000  respectively and both
are non-inductive. The load current is 20 A and voltage applied to the load is 30 V. Find the
error in the readings for two methods of connection
(A) 0.15% high, 0.67% high (B) 0.15% low, 0.67% low
(C) 0.15% high, 0.67% low (D) 0.15% low, 0.67% low

12

Downloaded From : www.EasyEngineering.net

 Downloaded From : www.EasyEngineering.net

Q.2 The current coil of dynamometer wattmeter is connected to 30 V DC source in series with
a 8 resistor. The potential circuit is connected through an ideal rectifier in series with a 50
Hz source of 120 V. The inductance of pressure coil circuit and current coil resistance are
negligible. Reading of the wattmeter is
(A) 282.84 W (B) 405 W (C) 202.57 W (D) None
Q.3 A voltage: 100 sin t + 40 cos (3t - 30) + 50 sin (5t + 45) volts is applied to the
pressure circuit of a wattmeter and through the current coil is passed a current of
8 sint + 6 cos(5t - 120) amps. The readings of wattmeter is?
(A) 939 W (B) 539 W (C) 439 W (D) 1039 W

ww
Q.4 The power in a 3- circuit is measured with the help of 2 wattmeter. The readings of one
of wattmeter is positive and that of other is negative. The magnitude of readings is different.

w.E
It can be concluded that the power factor of the circuit is
(A) Unity (B) zero (lagging)
(C) 0.5 (lagging)
asy (D) less than 0.5 (lagging)
Q.5 In a dynamometer wattmeter the moving coil has 500 turns of mean diameter 30mm.

En
Find the angle between the axis of the field and moving coil, if the flux density produced by
field coil is 15x10-3 wb/m2. The current in moving coil is 0.05 A and the power factor is 0.866
and the torque produced is 229.5x10-6 N.m
(A) 0 (B) 70
gin (C) 80 (D) 90
Q.6 Consider the following data for the circuit shown below eer
Ammeter: Resistance 0.2 reading 5A
Voltmeter: Resistance 2K reading 200V ing
Wattmeter: Current coil resistance 0.2
Pressure coil resistance 2 K .ne
Load: power factor =1
The reading of wattmeter is
(A) 980W (B) 1030W
t
(C) 1005W (D) 1010W
Q.7 A certain circuit takes 10 A at 200 V the power absorbed is 1000 W .If the wattmeter’s
current coil has a resistance of 0.15  and its pressure coil a resistance of 5000  and an
inductance of 0.3 H. The Error due to resistance of two coil of the Wattmeter, if the pressure
coil of the meter connected on the load side
(A) 15 W (B) 8 W (C) 11 W (D) 13 W
Q.8 The line to line voltage to the 3-phase, 50Hz AC circuit shown in figure is 100V rms.
Assuming that the phase sequence is RYB the wattmeter readings would be?
(A)W1=500W, W2=1000W (B) W1=0W, W2=1000W
(C)W1=1000W, W2=0W (D) W1=1000W, W2=500W

13

Downloaded From : www.EasyEngineering.net