assignment-3 singal-phase transformers

VIDHYADEEP INSTITUTE OF ENGINEERING AND TECHNOLOGY

Vidhyadeep Campus, Anita (Kim), Ta. Olpad, Dist. Surat

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Subject Name: BEE Subject Code: 3110005 Sem: 1st 2019

Assignment-3_Singal-Phase Transformers

Q.1 If the efficiency of a transformer with very light load is represented by ‘n1’ and

efficiency of the same transformer with nearly full load is represented by ‘n2’.

Which of the following is generally true?

(A) n1 < n2 (B) n1 > n2 (C) n1 = n2 (D) None of these

Q.2 Voltage regulation of a transformer is positive and less than 100%. That means,

(A) Output terminal voltage increases with load

(B) Output terminal voltage decreases with load

(C) Output terminal voltage is not affected by the load

(D) Output terminal voltage is zero

Q.3 Transformer core is generally tightly clamped in order to:

(A) Reduce the noise in the transformer

(B) Reduce the copper loss

(C) Reduce iron loss

(D) Increase the rating of the transformer

Q.4 A single phase transformer is operating with no load. Winding resistances and

leakage reactances are negligible. Keeping the supply voltage constant, if the

frequency of the supply is increased, the iron losses of the transformer will:

(A) Increase (B) Decrease

(C) Remain the same (D) Increase or decrease

Q.5 By mistake, 230 Volt DC is applied on HV (High Voltage) winding of a 230/115

Volt, 50 Hz, single phase transformer for a long time period. Which of the following

is incorrect?

(A) Transformer HV winding will burn out

(B) There will be a very high voltage on LV winding

(C) There will be zero voltage on LV winding

(D) All of the above



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Q.6 Iron core of a single phase transformer is replaced with a wooden core of same

dimensions and rated voltage at rated frequency is applied on the primary winding.

Which of the following is correct?

(A) There will be very high voltage on secondary winding

(B) There will be rated voltage on the secondary winding

(C) There will be almost zero voltage on the secondary winding

(D) None of these

Q.7 Efficiency of a transformer on no load is expected to be:

(A) 100% (B) 96% (C) 0% (D) 99%

Q.8 Efficiency of transformers is higher than that of electric motors of similar rating

because:

(A) Friction and windage losses are only 10%

(B) Friction and windage losses of motors are very large

(C) Friction and windage losses are only 5%

(D) Friction and windage losses are almost zero

Q.9 Derive an EMF equation for transformer with usual notation.

Q.10 Explain the constructional details of transformer

Q.11 How eddy current losses can be minimized?

Q.12 What is transformation ratio?

Q.13 Draw phasor diagram for no load condition in transformer.

Q.14 Why transformer rating in KVA

Q.15 Which relay is used for the protection of transformer?

Q.16 Enlist various losses in transformer.

Q.17 Explain the concept of ideal transformer.

Q.18 Explain operating principle of transformer. What happens if D.C supply is given to

the transformer?

Subject Coordinator H.O.D. (Elect)



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Subject Name: BEE Subject Code: 3110005 Sem: 1st 2019

Assignment-3_Three-Phase Transformers

Q.1 The power factor of a transformer on no load is poor due to

a) open –circuited secondary

b) magnetizing reactance of transformer

c) low no-load current

d) low primary winding resistance.

Q.2 In order to reduce the hysteresis losses in transformer

a) core may be laminated

b) silicon steel may be used as core material

c) core may be impregnated with varnish

d) All of the above.

Q.3 Explain the constructional details of transformer




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Subject Name: BEE Subject Code: 3110005 Sem: 1st (2019)

Assignment-2_ Single Phase A.C. Circuits:

Q.1 Explain with the aid of a phasor diagram the phenomenon of resonance

in a circuit containing an inductor, a capacitor and a resistor in series.

Q.2 Voltage and current for a circuit with two pure elements in series are

expressed as follows:

v (t) = 170 sin (6280 t + π/3) volts

i (t) = 8.5 sin (6280 t + π/2) amps

Sketch the two waveforms. Determine: (i) the frequency (ii) power

factor stating its nature (iii) values of the elements.

Q.3 Add the following currents as waves and as vectors

i1 = 7 sin ωt and i2 = 10 sin (ωt + π/3)

Q.4 An inductive coil draws 10 A current and consume 1 KW power from a

200 V, 50 Hz, a. c. supply. Determine: (i) the impedance in cartesian

and polar form (ii) power factor (iii) reactive and apparent power.

Q.5 Two impedances given by Z1 = (10 + j5) and Z2 = (8 + j6) are connected in parallel

across a voltage of V = (200 +j0) volts. Calculate the circuit current, branch currents

and power factor of each branch. Sketch the vector diagram with vectors in

appropriate proportion.

Q.6 Explain the phenomena of resonance in a.c. parallel circuit. Derive the

mathematical expression of resonant frequency. Sketch the graphical

representation of parallel resonance.

Q.7 Two branches numbered ‘1’ and ‘2’ having impedances of

3 + j4 and 3 –j4 respectively are connected to a 230 Volt,

50 Hz rms source. Find out :

(i) The total current drawn from the source.

(ii) Power factor of that current.



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(iii) Draw the phasor diagram for I1, I2, the total current and

supply voltage.

Q.8 Give the comparison of series resonance and parallel resonance.

Q.9 Define the following for AC circuits :

[1] R.M.S. value

[2] Average value

[3] Power factor

Q.10 A resistance of 20 Ω, an inductance of 0.2 H and a capacitance of 100 μF are

connected in series across 220 V,50 Hz supply. Find

[1] Impedance

[2] Voltage across R,L & C

[3] Power factor & angle of lag

[4] Active & apparent power

Q.11 Prove the condition of resonance for R-L-C series AC circuit. Also analyze the

phenomena with the help of graph.

Q.12 Prove that current in pure inductive circuit lags its voltage by 90º.

Q.13 Define the following terms with respect to AC waveforms (1) phase (2) Time period

Q.14 A certain waveform has a form factor of 1.2 and a peak factor of 1.5.If the maximum

value is 100,find the r.m.s value and average value.

Q.15 Explain series resonance circuit. Draw resonance curve.

Q.16 Prove that in a purely capacitive circuit power consumed is zero when a.c. voltage is

applied. Draw relavent phasor diagram and waveforms.

Q.17 An inductive coil of resistance R and inductance L is connected in parallel with a

capacitor of C. Derive an expression for resonant frequency and Q factor.




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Assignment-2_ Three Phase A.C. Circuits:

Q.1 Explain the variations of watt-meter readings for 3-phase power measurement by

two watt-meter method as power factor takes the values of unity, 0.5, between 0.5 &

0 and 0.

Q.2 A 3-phase load consists of three similar inductive coils of resistances of 50 Ω and

inductance 0.3 H. The supply is 415 V 50 Hz. Calculate:(i) the line current (ii)the

power factor and the total power when the load is star connected.

Q.3 Derive the relation between phase and line values of voltages and currents in case of

3-phase (i) star (ii) delta connection.

Q.4 Three 100 non-inductive resistances are connected in (a) star (b) delta, across a 400

V, 50 Hz, 3-phase supply mains. Calculate the power taken from the supply system

in each case.

Q.5 Prove that the sum of readings of two wattmeters connected to measure power in 3-

phase a.c. circuit, gives total power consumed by the circuit.

Q.6 Three 100 Ω non-inductive resistances are connected in (a) star (b) delta, across a

400 V, 50 Hz, 3-phase supply mains. Calculate the power taken from the supply

system in each case.

Q.7 Write down the line value and phase value relationship of voltages and currents in 3

phase star and delta connected systems.

Q.8 The input power to a 3 phase load is measured by two wattmeter method. The ratio

of the readings of the two wattmeters connected for 3 phase balanced load is 41׃. The

load is inductive. Find the load power factor.




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Assignment-1_ DC Circuits :

Q.1 State and explain Kirchhoff’s voltage and current laws.

Q.2 The Thevenin’s equivalent voltage source for a network is 2 Volt and Thevenin’s

equivalent resistance is 2 Ohm. Find the Norton’s equivalent current source for the

same network.

Q.3 Super position theorem is applicable to ___________ network.

(A) Linear (B) Bilateral (C) Linear and Bilateral (D) None of these

Q.4 State and Explain in brief: (i) Thevenin’s theorem (ii) Norton’s theorem (iii) Super

position theorem.

Q.5 Find the value of all currents and voltage across 5Ω resister for the network

shown in figure 2 using mesh analysis.

Q.6

Calculate the value of branch currents for the network shown below, using

nodal analysis. Values of resistors are in ohm.



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Q.7

Find the current passing through 3 Ohm resistor in the following circuit using nodal

analysis.

Q.8 Calculate the node voltages for all the nodes shown in figure: 1 using nodal analysis.

Q.9 For the circuit shown in figure: 4 find the voltages and currents for all circuit

elements using mesh analysis.

Q.10 State and prove Thevenins Theorem, find Rth and Vth. for the network shown in Fig.



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Q.11 Draw the Thevenin’s equivalent of the circuit shown in figure and find current

through load resistance (between terminal bb).

Q.12

Obtain the value of Norton’s equivalent current and Norton’s equivalent resistance

for the network shown in the following figure.

Q.13 In Fig. if 2V source is replaced by an open circuit then find Thevenin’s and Norton’s

equivalent circuits across V2 and V3.

Q.14 State and explain Superposition Theorem. Hence Find current in 20 ohm resistance

in the circuit shown in figure using superposition theorem.



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Q.15 Find voltage VX in the network shown in figure 4 using superposition theorem.

Q.16 Find the current in the 4 ohm resistor in Fig- using Thevenin’s Theorem and

Superposition theorem.

Q.17

Find the current in the 5 ohm resistor for the circuit shown in figure using Norton’s

theorem.

Q.18 For the network of the figure, determine the Thevenin equivalent network for the

load 𝑅𝐿.



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Q.19 State Thevenin’s theorem. Calculate current passing through 60Ω resistance in the

circuit shown in figure using thevenin’s theorem.


assignment-3 singal-phase transformers

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