Electrical and Electronic Measurements


Question 1
A 3-phase, star-connected, balanced load is supplied from a 3-phase, 400V (rms), balanced voltage source with phase sequence R-Y-B, as shown in the figure. If the wattmeter reading is -400 \mathrm{~W} and the line current is \mathrm{I}_{\mathrm{R}}=2 \mathrm{~A}(\mathrm{rms}), then the power factor of the load per phase is

A
Unity
B
0.5 leading
C
0.866 leading
D
0.707 lagging
GATE EE 2023      Measurement of Energy and Power
Question 1 Explanation: 
By observation of wattmeter connection, the wattmeter read reactive power
\because Wattmeter reading,
W=Y_{Y B} I_{R} (Angle between V_{Y B} and I_{R}.

Phasor diagram :

\therefore \quad W=V_{\mathrm{L}} \mathrm{I}_{\mathrm{L}} \cos \left(90^{\circ}-\beta\right)
=V_{L} I_{L} \sin \phi

Given: \quad W=-400 \mathrm{~W}
\therefore \quad-400=400 \times 2 \times \sin \phi
\Rightarrow \quad \phi=-30^{\circ}

Thus, P.F. =\cos \phi=0.866 leading
Question 2
Two balanced three-phase loads, as shown in the figure, are connected to a 100\sqrt{3}V , three-phase, 50 Hz main supply. Given Z_1=(18+j24)\Omega and Z_2=(6+j8)\Omega . The ammeter reading, in amperes, is _______. (round off to nearest integer)

A
15
B
20
C
18
D
22
GATE EE 2022      Galvanometers, Voltmeters and Ammeters
Question 2 Explanation: 
First perform delta to star conversion we know, for balanced load
Z_{star}=\frac{Z_{delta}}{3}=\frac{18+j24}{3}=6+j8\Omega
Draw the per phase diagram:

Z_{eq}=(6+j8)||(6+j8)=(3+j8)\Omega =5\angle 53.13^{\circ} \Omega
Therefore, Meter reading, I=\frac{100}{5}=20A


Question 3
A balanced Wheatstone bridge ABCD has the following arm resistances:
R_{AB}=1k\Omega \pm 2.1%; R_{BC}=100\Omega \pm 0.5%, R_{CD} is an unknown resistance; R_{DA}=300\Omega \pm 0.4%; . The value of R_{CD} and its accuracy is
A
30\Omega \pm 3\Omega
B
30\Omega \pm 0.9\Omega
C
3000\Omega \pm 90\Omega
D
3000\Omega \pm 3\Omega
GATE EE 2022      A.C. Bridges
Question 3 Explanation: 
The condition for balanced bridge
\begin{aligned} R_{AB}R_{CD}&=R_{DA}R_{BC} \\ R_{CD} &=\frac{300 \times 100}{1000}=30\Omega \\ %Error &=\pm (2.1+0.5+0.4)=\pm 3% \\ \therefore \; R_{CD}&=30\pm 30 \times \frac{3}{100}=30\pm 0.9\Omega \end{aligned}
Question 4
Inductance is measured by
A
Schering bridge
B
Maxwell bridge
C
Kelvin bridge
D
Wien bridge
GATE EE 2021      A.C. Bridges
Question 4 Explanation: 
Maxwell's bridge is used for measurement of inductance.
Wein's bridge is used for measurement of frequency
Kelvin's bridge is used for measurement of low value of resistance.
Schering bridge is used for measurement of capacitance, dilectric loss and permittivity etc.
Question 5
A non-ideal Si-based pn junction diode is tested by sweeping the bias applied across its terminals from -5 V to +5 V. The effective thermal voltage, V_T, for the diode is measured to be (29\pm2) mV. The resolution of the voltage source in the measurement range is 1 mV. The percentage uncertainty (rounded off to 2 decimal plates) in the measured current at a bias voltage of 0.02 V is _______.
A
5.87
B
2.35
C
11.5
D
9.2
GATE EE 2020      Characteristics of Instruments and Measurement Systems




There are 5 questions to complete.