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

Unity | |

0.5 leading | |

0.866 leading | |

0.707 lagging |

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

\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)

15 | |

20 | |

18 | |

22 |

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

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

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

30\Omega \pm 3\Omega | |

30\Omega \pm 0.9\Omega | |

3000\Omega \pm 90\Omega | |

3000\Omega \pm 3\Omega |

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}

\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

Schering bridge | |

Maxwell bridge | |

Kelvin bridge | |

Wien bridge |

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.

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 _______.

5.87 | |

2.35 | |

11.5 | |

9.2 |

There are 5 questions to complete.