Dr. D.Y. PATIL INSTITUTE OF ENGINEERING, MANAGEMENT AND

RESEARCHAKURDI, PUNE-411044

BASIC ELECTRICAL

ENGINEERINGTheory Question Bank

SPPU Paper Pattern(Most Likely)

Subject : Basic Electrical Engineering.

Time : Two Hours Maximum Marks:50

N.B.:- i) Answer Q no. 1 or Q no. 2, Q no. 3 or Q no. 4, Q no.5 or Q no.6, Q no. 7 or Q no. 8.ii) Neat diagrams must be drawn wherever necessary.iii) Figures to the right indicate full marks.iv) Use of electronic pocket calculator and steam tables is allowedv) Assume suitable data, if necessary.

Q1. A)Numerical question on Unit 1. [6] B)Theoretical question on Unit2. [6]

OrQ2. A)Theoretical question on Unit 1. [6] B)Numerical question on Unit 2. [6]

Q3. A)Numerical question on Unit 3. [6] B)Theoretical question on Unit 4. [6]

OrQ4. A)Theoretical question on Unit 3. [6] B)Numerical question on Unit 4. [6]

Q5. A)Numerical question on Unit 5. [7] B)Theoretical question on Unit 5. [6]

OrQ6. A)Theoretical question on Unit 5. [6] B)Numerical question on Unit 5. [7]

Q7. A)Numerical question on Unit 6. [7] B)Theoretical question on Unit 6. [6]

OrQ8. A)Theoretical question on Unit 6. [6] B)Numerical question on Unit 6. [7]

SPPU Unit Wise Marks weightage –Theory exam

Subject : Basic Electrical Engineering

Unit no. Unit Name Total Marks

Unit 1 Elementary Concepts 6

Unit 2 Electromagnetism 6

Unit 3 Single phase transformer & Electrostatics 6

Unit 4 A.C. Fundamentals 6

Unit 5 Single Phase A.C. Circuits & Polyphase A.C. Circuits 13

Unit 6 D.C. Circuits 13

Unit 1 :Elementary ConceptsTheoretical Questions

Q1) Define temperature coefficient of resistance. State its unit & the factors on which it depends. (Dec14)

Q2)If ∝1 & ∝2 are RTCs of material at t1 0C & t2 0C respectively, then prove that:

i) ∝1∝2 = 1+ ∝1 (t2- t1) ¿α 2=

α 11+α 1 ( t 2−t 1 ) & hence obtain

1) α t=α0

1+α 0 t 2) α 0=

α t1−αt t

(Dec12,May14)

Q3)Define insulation resistance & derive expression for it of a cable. (Dec13,May13,May15)

Q4) Explain the effect of temperature on resistance of: i)metal conductors ii)insulators iii)alloys

(Dec15)

Q5)Explain the terms with respect to electrical quantities and state their units:i)Work ii) Energy iii)Power.

Numerical QuestionsResistance,RTC,Insulation Resistance

Q1) A coil of insulated copper wire has a resistance of 150Ω at 200C. When the coil is connected across a 240V supply, the current after several hours is 1.25A. Calculate the temperature of the coil, assuming the temperature coefficient of resistance of copper at 200C to be 0.0039 /0C

(May15) Q2) A piece of silver wire has a resistance of 3Ω. What will be the resistance of a manganin wire one-third the length & one-third the diameter that of silver? The resistivity of manganin is 30 times that of silver.

(May13)

Q3) A single core cable has its conductor diameter as 5 cm. The resistivity of conductor & insulator are 1.73 x 10-8 Ω-m & 8 x 1012 Ω-m respectively. Calculate resistance of conductor & insulation for a cable of 100 meter. (Dec13)

Q4)A heater coil, after running for several hours switched on, draws a current of 2 A from constant voltage 400V. If the temperature rise is 70oC, find the change in voltage required to maintain same current at the time of starting the heater coil at 0 oC is 0.0038/ oC.

Q5) A filament of 240 V of metal filament lamp is to be constructed from a wire having a diameter of 0.03mm and a resistivity of 4.3µΩ-cm.If the RTC of the filament material is 0.005 oC at 20 oC,what length of the filament is necessary for the lamp to dissipate 60W at a filament temperature of 2420 oC. Assume room temperature as 20 oC.

Work ,Power ,EnergyQ1)Find the current drawn by a crane motor when raising a mass of 1000 kg through a height of 15 meters in 10 sec. The supply is 400 V dc gear efficiency is 0.6 & motor efficiency is 0.8 (Dec12)Q2)A pump driven by a DC electric motor lifts 1.5 m3 of water per minute to a height of 40 m. The pump has an efficiency of 90 % 7 the motor an efficiency of 80 %. Determine i) The power input to the motor, ii) the current taken from a 480 V supply. Assume the mass of one m3 of water is 1000 kg. (May14)

Q3)A motor pump set lifts 1200 m3 of water to a height of 15 meter per hour. If the efficiency of the motor is 80% & that of pumpis 75 %. Calculate the monthly bill of set if it is used 4 hrs per day for 30 days at a rate of Rs.5 / day. (Dec14)

Q4)A water pump lifts 50m3 of water per hour to a height of 20 m. The overall efficiency of water pump set is 0.60. Find monthly bill, when the pump is used 4 hours a day for a month of 30 days,at the rate of Rs.5/kWh. (Dec15)Q5)An electric furnace is used to melt aluminium. Initial temperature of the solid aluminium is 32 oC and its melting point is 680 oC. Specific heat capacity of aluminium is 0.95kJ/kg/ oC, and the heat required to melt 1 kg of aluminium at its melting point is 450kJ. If the input power drawn by the furnace is 20 kW and its overall efficiency is 60%, Find the mass of aluminium melted per hour.Q6) A bucket contains 20 litres of water at 20 oC.A 2.5kW immersion heater is used to raise the temperature of water to 95 oC.The overall efficiency of process is 90% and the specific heat capacity of water is 4200J/kg oC. Find the time required for the process. Also find cost of energy(electricity bill) for 365 days if the rate of energy is Rs. 3per unit.Q7) A diesel-electric generating set supplies an output of 50kW.The calorific value of fuel used is 12,500 kcal/kg. If the overall efficiency of the unit is 35%.

1. Calculate the mass of oil required per hour and 2. The electrical energy generated per ton of the fuel.

Unit 2 ElectromagnetismTheoretical Questions

Q1) Derive the expression for energy stored per unit volume in the magnetic field. (Dec12, May13,Dec13)Q2) With reference to a magnetic circuit, explain the terms:- i) Magnetic flux ii) Magneto motive force iii) Magnetic field intensity iv) Magnetic flux density v) Reluctance vi) Permeability of free space (May14)

Q3) Compare electric & magnetic circuit i.e explain similarities & dissimilarities between them.

(Dec14)Q4)What is dynamically induced e.m.f and statically induced e.m.f.? Derive their expression for its magnitude. (May15)Q5)Obtain an expression for coefficient of coupling between two coils placed on common magnetic field. (Dec15)

Numerical Questions

A-Series Magnetic CircuitQ1)An iron ring has a mean circumference of 180 cm. It carries a current of 1.5 amp. &has 600 turns of 180 cm. It carries a current of 1.5 amp. & has 600 turns of coil wound over it. The relative permeability of the iron is 1200. Calculate i) MMF ii) Field strength iii) Flux density (May13)Q2)An iron ring of mean circumferences of 50 cm has an airgap of 2 mm cut in it. It has circular cross section with area of 5 cm2. It carries a coil wound with 600 turns wound uniformly. The relative permeability of iron is 580. If the coil carries a current of 2 amp. find the flux in air gap. (Dec13)Q3) A coil of N turns is wound on a cast iron ring which has mean length of 50 cm & its cross section is of 4 cm diameter. The current flowing through the coil is 2 Amp which produces a flux

of 6 mWb in the air gap of 2 mm length. If the relative permeability of iron is 1000, Calculate no. of turns N. (May14)Q4)An iron ring with mean circumference of 140 cm& cross section of 12 cm2 is wound with 500 turns of wire. What is the relative permeability of the iron if exciting current of 2 amp flowing in coil, produces flux of 1.2 mWb? (Dec14)

Q5) A steel ring of mean diameter 50 cm is wound with 500 turns on it. A flux density of 1 Tesla is produced in the ring by mmf of 4000AT/m. Calculate magnetizing current. Also find the current when an air gap of 1 mm is cut in it, keeping B=1 Tesla in the ring. (Dec15)

B-Electromagnetic InductionQ1)If a coil of 150 turns carries a current of 10 A & flux linked with it 0.01 wb, then calculate the inductance of the coil. If the current is uniformly reversed in 0.1 sec., calculate the e.m.f induced. If a second coil of 100 turns is uniformly wound over the first coil, find the mutual inductance between the coils. (Dec12)

Q2)If a coil of 150 turns is linked with a flux of 0.01 wb when carrying a current of 109 A, then calculate the induced emf: (i)If this current is uniformly reversed in 0.1 second.(ii)If this current is interrupted in 0.05 second. (May15)

UNIT 3 :Single Phase Transformer & ElectrostaticsTheoretical Questions

A-TransformerQ1) Derive an expression of e.m.f. induced in a single phase transformer. (May13,May15)

Q2) Compare Core type & Shell type transformer construction. (Dec13)Q3) Draw neat sketches to show core type & shell type single phase transformer. State:

i) Losses occurring in the transformer on load.ii) Their location causes, whether constant or variable.iii) Factors on which they depend (May14)

Q4)Write a short note on Auto Transformer.

Q.5) Write a short note on Direct Loading Method of single phase Transformer.

B-ElectrostaticsQ1) Obtain the expression for capacoitance of composite capacitor having 3- dielectric materials. (Dec15)

Q2) Derive the expression for energy stored in capacitor (Dec12,Dec14)

Q3) Define following terms:i)Electric fluxii)Electric flux densityiii)Electric field strengthiv)Absolute Permittivityv)Relative permittivityvi)Dielectric Strength.

vi) Electric potential

Q.4 Describe the charding and Discharging of Capacitor

Numerical Questions

A-Transformer

Q1) A 30 KVA, 6000/200 V,50 hz single phase transformer has an iron loss of 500 watt. Its primary & secondary winding resistance are 6Ω & 0.02 Ω respectively. Find its efficiency on full load at unity p.f. (Dec12)

Q2) An 80 kVA,3200/400 V, 50 Hz, single phase transformer has 111 turns on secondary. Calculate i) no.of turns on primary ii) Secondary full load current iii) C/S area of the core if the maximum flux density is 1.2 tesla. (May13)

Q3) A 25 kVA, 2200/220 V50Hz single phase transformer has a primary resistance of 1.8 Ω & a secondary resistance of 0.02 Ω. Calculate the transformer efficiency at half load 0.8 lagging p.f. The iron losses are 1000 watt. (Dec13)

Q4)A transformer is rated at 100kVA. At full load, its copper loss is 1200 W & its iron loss is 960 W. Calculate:

i) Efficiency at full load, 0.8 power factorii) Load KVA at which maximum efficiency will occur.iii) Maximum efficiency at 0.85 power factor. (May14)

Q5) A 600 kVA transformer has iron losses of 4kW & half load copper losses are 2kW. Calculate the efficiency of transformer at i) half load 0.8 p.f. lagging & ii) full load 0.8 p.f. lagging. (Dec14)

Q6) A single phase 4 kVA transformer has 400 turns on its primary & 1000 secondary turns. The net cross sectional area of the core is 60 cm2. When the primary winding is connected to 500V,50Hz supply, Calculate:

i) The max. value of flux density in the core.ii) The voltage induced in the secondary winding &iii) The secondary full load current (May15)

Q7) A 40 kVA, 2200/220 V, 50 Hz, 1-ph transformer has an iron loss of 250 W. The resistance of low & high voltage windings are 0.005 Ω & 0.5 Ω respectively. Calculate efficiency at full load & load p.f. = 0.8 lagging. (Dec15)

B-ElectrostaticsQ1) A series of combination having R= 2M Ω & C = 0.01 µF is connected across the DC source of 50 V. Determine the capacitor voltage & charging current after 0.02 s & 0.06 s. (Dec14)

Q2)Two capacitors 2µF and 4µF are connected in 1.parallel 2. Series across a 100V dc source. Determine:1. Energy stored in each capacitor and 2. Equivalent capacitance of their combination.

Unit 4 :AC FundamentalsTheoretical Questions

Q1) State and derive the expression for the average value of sinusoidally varying alternating quantity. (Dec12,Dec13,Dec15)

Q2) Derive an expression for instantaneous current and power consumed when voltage of V=Vm sin(ωt) is applied to pure inductance alone. Also draw the phasor diagram. (May15,May14)

Q3)Define r.m.s. value and derive an expression of r.m.s. value of a sinusoidally varying current. (May13,Dec 14)

Q4)Define the following terms:i)Waveform ii)Cycle iii)Instantaneous value iv)Time period v)Frequency vi)Amplitudev)Form Factor vi)Peak Factor

Numerical Questions

Q1) A pure inductance L=0.1 H is connected across voltage of v= 200 sin 314 t. Find the rms value of current, instantaneous and average power. Write down expression for current. (Dec12)

Q2)An inductive coil having negligible resistance and 0.1H inductance is connected across a 200V,50 Hz supply. Find i)Inductive reactance ii) RMS value of current drawn. State equations for voltage &current. (Dec13)

Q3)The equation for the alternating current is given by i=100 sin 100πt. Find the time taken by current to reach i)30Amp and ii)50 Amp for the first time. (Dec14)

Q4) A 50μF capacitor is connected across a single phase 230V,50Hz supply. Calculate 1) The reactance offered by the capacitor 2) The maximum current 3) The rms value of the current drawn by the capacitor. (May13)

Q5) The expression of the alternating current is given by i=5.48 sin ωt. Calculate:i)average value ii) rms value of the current iii) power consumed if the current is passed through a resistance of 10Ω. (May15)

Q6) A 50 Hz sinusoidal voltage has rms value of 200 V. At t=0, the instantaneous value is positive and half of its maximum value. Write down expression for voltage and sketch the waveform. (Dec15)

Unit 5 :Single Phase & Polyphase A.C. CircuitsTheoretical Questions

A-Single Phase A.C. Circuits

i) Single Phase Series A.C. CircuitsQ1) Derive expression for current drawn and power consumed by a circuit consisting of R and C connected in series across v=Vm sin ωt supply. (Dec12)

Q2) Obtain expression for power ,if x= vmsin ωt is applied to R-L series circuit. (Dec15)

Or

Q3) Sketch the voltage ,current & power waveforms for R-L series circuit. State the equations of v, i & p. (Dec14)

Q4) Draw and explain phasor diagram of an RLC series circuit, when:

i) XC > XL ii) XC< XL iii) XC = XL ( May15,May13)

Q5) For a single phase ac circuit , the applied voltage is v=Vm sin ωt and current drawn is i= Im sin (ωt-ϕ). Derive expression for average power. Draw waveform of voltage, current and instantaneous power over one cycle of voltage. (May14)Q6) What is series resonance? Obtain the equation for resonant frequency. (Dec15)

ii) Single Phase Parallel A.C. CircuitsQ1) What is admittance of AC circuit? What are its two components? State unit of these quantities. How the admittance is expressed in rectangular & polar form. (May15)

Q2)Define: 1)Impedance 2) Admittance. Sketch Impedance & Admittance triangle. (Dec13)

B-Polyphase A.C. CircuitsQ1) Derive an expression for line current in terms of phase current for three phase delta connected balanced load, with phasor diagram, connected across three phase supply. (Dec12,May13)Q2) Derive an expression for line voltage in terms of phase voltage for 3 phase star connected balanced load, with phasor diagram across the three phase supply. (May14)

Q3) Explain advantages of three phase over single phase system. (Dec 14)

Numerical Questions

A-Single Phase A.C. Circuits

i) Single Phase Series A.C. CircuitsQ1) A series R-L circuit consist of resistance of 3 Ω and inductance of 0.0106 H connected across v=141 sin ωt,60 Hz supply. Write down expression for instantaneous current drawn by circuit. Calculate average power dissipated and p.f. of the circuit. (Dec12)

Q2) An impedance Z1=(100+j0)Ω is connected in series with another impedance Z2=(50+j80)Ω. The circuit is connected to a single phase 230 V,50 Hz supply. Calculate: i) Current drawn by the circuit ii) power consumed by whole circuit iii) circuit power factor (May15)Q3)The voltage and current in simple series circuit are given by V=150∠30 °∧I=2∠−15° .If the supply frequency is 50 Hz. Determine impedance, resistance, reactance and power consumed by the circuit. (May13)

Q4) A series circuit, consist of resistance of 10 Ω and inductance of 0.1 H, connected across one phase 50 Hz AC supply. If the voltage across resistance is 50 V. Calculate i)Voltage drop across inductance and ii) supply voltage (Dec 14)

Q5)A circuit consisting of resistance of 20Ω and inductance of 0.1H in series is connected across single phase 220V,50Hz supply. Calculate i)current drawn ii)p.f. iii)power of circuit (Dec13)

Q6) A coil of 2Ω resisitance and 0.01 H inductance is connected in series with a capacitor across 200V supply. What must be capacitance in order that maximum current is drawn at frequency of 50 Hz. Find also voltage across capacitance. (Dec13)

Q7) An emf given by V=100 sin100πt is impressed across a circuit consisting of resistance of 40Ω in series with 100μF capacitor and 0.25H inductor. Determine: (i)rms value of current ii) power consumed iii)power factor. (May14)

ii) Single Phase Parallel A.C. CircuitsQ1) A resistance of 20Ω and coil of inducatance 31.8 mH and negligible resitance, are connected in parallel across 230V, 50 Hz supply.Find i) Currents drawn ii) p.f. and iii) Power consumed by the circuit. (Dec12)

Q2) Two circuits having impedances Z1= 8+j6 Ω and Z2=5+j10 Ω are connected in parallel across 200V, 50Hz, 1-ph a.c. supply. Calculate: i) current drawn by each circuit. ii) total current. iii)p.f. of whole circuit (Dec15)

Q3) Two circuits, the impedance of which are given by Z1=(10+j15)Ω and Z2=(6-j8)Ω are connected in parallel across an AC supply. If the total current is 15 A, what are the branch current drawn by two circuits. (May 13)

Q4)A coil having resistance 50 Ω and inductance of 0.02 H is connected in parallel with a capacitor of 35μF across a single phase 200V, 50Hz supply.Calculate branch currents and total current drawn by the circuit. (Dec14)

B-Polyphase A.C. CircuitsQ1) Three identical impedances each of 6+j8 are connected in star across 3-ph, 400V, 50Hz ac supply. Calculate: i) line current ii) active power iii) reactive power (Dec15)

Q2)A delta connected balanced load across a 400 V, 3 phase supply consist of three identical impedances, each equal to (15+j12)Ω. Find the line current, active power and reactive power. (May15) Q3)Three coils each of 5Ω resistance and 6 Ω inductive reactance are connected in delta across 3 phase,440V,50Hz,A.C. supply. Calculate current drawn, pf of system and power consumed by circuit. (Dec 13)

Q4) Three identical coils, each having resistance of 15Ω and inductance of 0.03H are connected in delta across a 3phase 400V ,50Hz supply. Calculate i)phase current ii)line current iii) power consumed. (May14)

Unit 6 :DC CircuitsTheoretical Questions

Q1) Derive the expression/formula to convert delta connected network into its equivalent star connected network. (Dec15,May-15,May13,Dec14,May14)

Q2) Derive the expression/formula to convert star connected network into its equivalent delta connected network. (Dec12)

Q3) State and explain Thevenin theorem and explain the steps to find Rth ,Vth and Ith

(Dec12,May15,May14)

Q4) State and explain Superposition Theorem. (May15,Dec13)

Q5) State and explain Kirchoff’s Laws. (May13,Dec14,Dec13)

Numerical Questions

Q1) Find applying Kirchhoff’s laws currents in three voltage sources in the network as shown in Fig. 1 (Dec12)

Fig. 1Q2)Apply Superposition Theorem to calculate current flowing in 2Ω resistance for the network shown in Fig.1 (Dec12)Q3)Apply Thevenin’s theorem to calculate current drawn by 8Ω resistor of the circuit in Fig.2

(May13)

Fig. 2Q4)Calculate current flowing in 8Ω resistance for the circuit shown in Fig.2 applying Superposition theorem. (May13)Q5)Calculate the equivalent resistance between terminals X and Y for the circuit shown in Fig. 3 (Dec 13)

Fig 3Q6)Apply Thevenin’s theorem to calculate current flowing in branch A-B of Fig. 4. (Dec13)

Fig.4Q7)Apply Kirchhoff’s Laws to calculate current drawn by 4Ω resistance for Fig.5 (May 14)

Fig.5

Q8)Apply Superposition theorem to calculate current flowing in 3Ω resistance in Fig 6(May14)

Fig.6Q9) Apply Thevenin’s theorem to calculate current flowing in 10Ω resistance,for Fig.7(Dec14)

Fig.7Q10)Apply Superposition theorem to calculate current flowing in 10Ω resistance [Fig7] (Dec14)

Q11)Using Kirchhoff’s law, find the current flowing through 6Ω resistance in Fig.8 (May15)

Fig.8

Q12)Using Thevenin’s theorem, find current flowing through 6Ω resistance in Fig.9 (May15)

Fig.9Q13) Using simple series-parallel combination,find RBC for Fig.10. (Dec15)

Fig.10

Q14)Using Mesh/Loop analysis, calculate current flowing through RBC for Fig. 11. (Dec 15)

Fig.11