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Heritage Institute of Technology
Subject: Basic Electrical Engineering (ELEC 1001)
Assignment-I (AC Fundamental & 3 phase system)
Section-A (AC Fundamental)
1. A circuit consists of a resistance of 6Ω, inductance of 0.4H and a variable
capacitance across 100V, 50Hz supply. Find out i) value of capacitance at
resonance, ii) voltage drop across capacitor, iii) Q factor.
2. A coil of resistance 10Ω and inductance of 0.02H is connected in series with
another coil of resistance 6Ω and inductance of 15mH across a 230V, 50Hz supply.
Calculate i) impedance of the circuit, ii) voltage drop across each coil, iii) the power
consumed in the circuit.
3. A two element series circuit consumes 700W of power and has power factor of
0.707 leading when energized by a voltage source of waveform v= 141.1 sin (314t
+ 300). Find out the circuit elements.
4. Derive the mathematical expression for r.m.s and average value of a sinusoidal
voltage v = vm sin wt.
5. A 20Ω resistor, a choke coil having some inductance and some resistance and a
capacitor are connected in series across a 25V variable frequency source. When
the frequency is 400 Hz, the current is maximized and its value is 0.5A and the
potential drop across the capacitor is 150V. Calculate the resistance and the
inductance of the choke coil and the capacitance of the capacitor.
6. A circuit receives 50A current at a power factor of 0.8 lagging from a 250V, 50 Hz
single phase supply. Calculate the capacitance of the capacitor which is required to
be connected across the circuit to make the power factor unity.
7. Derive the expression for the average real power delivered by a single phase
source with an e.m.f of e =1.414Emsin wt when the source current is i = 1.414Imsin
(wt-α).
8. Write a short note on a) power factor, b) foam factor, c) real and reactive power.
9. Write down the effect of frequency in a) RLC series, b) RLC parallel circuit.
10. Write a short note on band width.
11. What is Q factor? Derive the expression of it for series and parallel RLC circuit.
12. Why the phasor diagram is used in ac circuit. Draw and explain the phasor diagram
for series parallel RLC circuit.
13. Show that the power dissipated across an inductor and a capacitor is zero. And
also calculate the power dissipated across a resistor.
Section-B (3 phase system)
14. In a 3-phase 4-wire, balanced system, the current in each phase is 10 A. The
current through the neutral wire will be _________.
15. In a three phase balanced system, line voltage makes ________ angle with phase
voltages.
16. Which of the following in a four-wire system?
a) Delta with neutral b)Star with neutral c) Both delta & star d) Any combination of
4 wires.
17. What is a three-phase balanced A.C. system? Show that in a three-phase balanced
AC circuit, the sum of current in the neutral is zero.
18. Explain the principle of measurement of balanced 3-phase power by 2-Wattmeter
method. Draw the neat circuit & Phasor diagrams.
19. These equal impedances (6 + j8) Ώ are connected across a 400 V, 3-phase, 50 Hz
supply. Calculate - i) The line current & the phase current ii) Power factor iii) Active
& reactive drawn by load per phase.
20. If a balanced load is connected across a balanced supply, the sum of 3 phase
current is ____.
21. A balanced 3-phase star connected load is fed from a 208 volt, 3_phase supply.
Each leg of the load has a resistance of 35 Ώ. determine the power factor, the total
power, the phase currents & line currents of the system.
22. In the two wattmeter method of measuring 3 phase power, the two watt meters
indicate equal and opposite readings when load power factor angle is
60 degrees lagging b) 30 degrees leading c) 45 degrees leading d) 90 degrees
lagging
23. The load in each branch of a star connected 3 phase circuit consists of 10 Ω
resistance and 0.06 H inductance in series. The line voltage is 430 V, 50 Hz.
Calculate the phase voltage and the phase current.
24. Show the relationship between phase current and line current, phase voltage and
line voltage of a star as well as delta connected three phase system. (with phasor
diagram)
25. A star connected three phase load draws a current of 20 A at a lagging power
factor. of 0.3 from a balanced 440V, 50 Hz supply. Find the circuit elements in each
phase if the elements are connected in series.
26. The three phase voltage and current of a star connected load is 100V and 10A. The
power factor. of the load is 0.8 (lag). Assuming that the system is 3 wire, 3 phase
and the power is measured by two wattmeter, find the reading of the two
wattmeter.
27. In a balanced 3 phase 220 V circuit, the line current is 115.5 A. When the power is
measured by two wattmeter method one of the instruments reads 20Kw and
other is zero. What is the power factor of the load?
28. Two wattmeter are connected to measure the input to a balanced 3 phase circuit.
The readings of instruments are 2500 W and 1500 W respectively. Find the power
factor. of the circuit when (a) both reading are positive and (b) when the later
reading is obtained after reversing the current coil of the wattmeter.
29. A 3 phase 230V load has a power factor. of 0.7. Two wattmeter are used to
measure power which shows the input to be 10KW. Find the reading of each
wattmeter.
30. Three similar coils each having series resistance of 25 Ώ and capacitance 150 µF
are connected in star to a 3 phase 400V supply. Find line current, power factor,
total kVA and kW.