Sri Eshwar College of Engineering
Coimbatore – 641202
Department of Electrical and Electronics Engineering
Question Bank
EE 6401 – Electrical Machines I
Unit I - Magnetic Circuits and Magnetic Materials
Part A
Sl. No. / Question / BT / CO / Month / YearDefine magnetic flux along with its unit. / R / CO1 / Nov 2012
A coil of 1500 turns carrying a current of 5 A produces a flux of 2.5 mWb. Find the self-inductance of the coil. / Ap / CO3 / Nov 2012
Name the main magnetic quantities with their symbols having the following units. Webers, Tesla, AT/wb & H/m / R / CO1 / Nov 2013
Nov 2015
How will you minimize eddy current & hysteresis losses? / An / CO3 / Nov 2011
Nov 2013
Nov 2015
What are quasi – static fields? / R / CO1 / May 2014
May 2015
Nov 2015
Define magnetic reluctance. / R / CO1 / May 2010
May 2014
May 2015
What is magnetic leakage flux? / R / CO1 / May 2015
May 2016
Define stacking factor. / R / CO1 / Nov 2015
May 2010
State Ampere’s law. / R / CO1 / May 2016
What are the basic types of rotating electric machines? / R / CO1 / May 2013
Draw the typical magnetization curve of ferromagnetic material. / R / CO2 / May 2013
Clearly define EMF & MMF / R / CO1
CO3 / May 2012
What are core losses and how can this loss be minimized? / R / CO6 / May 2012
Write the Lorentz force equation. / R / CO1 / May2011
Define fringing in magnetic circuit. / R / CO3 / May 2010
May 2011
Define relative permeability. / R / CO1
CO3 / May 2011
What is leakage flux and leakage inductance? / R / CO1
CO3 / May 2011
State Lenz’s law. / R / CO1 / May 2010
What is meant by statically induced emf? / R / CO1 / May 2015
Mention the materials suitable for fabrication of permanent magnets. / R / CO2 / May 2015
How is emf induced dynamically? / U / CO1 / May 2010
Give the analogy between electric and magnetic circuit. / R / CO1 / Nov 2010
Distinguish between statically and dynamically induced emf. / An / CO3 / Nov 2010
Define statically and dynamically induced emf. / R / CO1 / Nov 2011
What is hysteresis loss? / R / CO6 / Nov 2016
Define flux linkage. / R / CO1 / Nov 2016
Define self and mutual inductances of a coil. / R / CO1
Part B
Sl. No. / Question / M / BT / CO / Month / Yeari)Define inductance of a coil.
ii)For the magnetic circuit shown in Fig. 1.1, determine the current required to establish a flux density of 0.5 T in the air-gap.
Fig. 1.1 / 4
12 / R
Ap / CO1
CO3 / Nov 2010
i)Define permeability of a magnetic material and the factors on which it depends.
ii)Explain the operation of a magnetic circuit when A.C. current is applied to the coil wound on iron core. Draw the B – H curve and obtain an expression for hysteresis loss. / 4
12 / R
U / CO1
CO1 / Nov 2010
Compare electric and magnetic circuit by their similarities and dissimilarities. / 16 / An / CO2 / Nov 2011
A ring is composed of three sections. The cross section area is 0.001 m2 for each section. The mean arc lengths are la = 0.3 m, lb = 0.2 m and lc = 0.1 m. An air gap length of 0.1 mm is cut in the ring. µr for sections a, b and c are 5000, 1000 and 10000 respectively. Flux in the air gap is 7.5 X 10-4Wb. Find (i) mmf, (ii) exciting current if the coil has 100 turns, (iii) reluctance of the sections. / 16 / Ap / CO3 / Nov 2011
i)Explain the losses in magnetic materials.
ii)The field winding of DC electromagnet is wound with 800 turns and has a resistance of 40Ω when exciting voltage is 230 V and the magnetic flux around the coil is 0.004 Wb. Calculate self-inductance and energy stored in magnetic field. / 10
6 / U
Ap / CO6
CO3 / Nov 2012
i)Derive the expression for self and mutual inductance of the coil.
ii)Two coils A and B are wound on same iron core. There are 600 turns on A and 3600 turns on B. A current of 4 amps through the coil A produces a flux of 500 X 10-6Wb in the core. If this current is reversed in 0.02 seconds, calculate average emf induced in coils A and B. / 8
8 / Ap
Ap / CO1
CO3 / Nov 2012
i)Distinguish between statically and dynamically induced e.m.f.
ii)The core loss (hysteresis + eddy current loss) for a given specimen of magnetic material is found to be 2000 W at 50 Hz. Keeping the flux density constant, the frequency of the supply is raised to 75 Hz resulting in a core loss of 3200 W. Compute separately hysteresis and eddy current losses at both the frequencies. / 4
12 / An
Ap / CO3
CO3 / Nov 2013
Nov 2013
May 2015
Nov 2015
Draw and explain the typical magnetic circuit with air-gap and its equivalent circuit. Hence derive the expression for air-gap flux. / 16 / Ap,U / CO1 / May 2013
The magnetic circuit has dimensions: AC = 4 X 4 cm2, lg = 0.06 cm, lc = 40 cm and N = 600 turns. Assume the value of µr = 6000 for iron. Find the exciting current for BC = 1.2 T and the corresponding flux and flux linkages. / 16 / Ap / CO3 / May 2013
A steel ring has a mean diameter of 20 cm, a cross section of 25 cm2 and a radial air-gap of 0.8 mm cut across it. When excited by a current of 1 A through a coil of 1000 turns wound on the ring core, it produces an air-gap flux of 1 mWb. Neglect leakage and fringing. Calculate
i)Relative permeability of steel and
ii)Total reluctance of the magnetic circuit / 16 / Ap / CO3 / Nov 2013
For the magnetic circuit shown in Fig. 1.2, find the self and mutual inductance between the two coils. Assume core permeability = 1600.
Fig. 1.2 / 16 / Ap / CO3 / May 2014
i)Explain the AC operation of magnetic circuits.
ii)Explain about a typical magnetic circuit with air-gap. Also prove that the core reluctance may be neglected in practice. / 8
8 / U
U / CO1
CO1 / May 2014
Nov 2015
Nov 2016
May 2014
Explain the methods of energy conversion via electric field, with examples of electrical machines. / 16 / U / CO1 / May 2014
i)Specify the causes for hysteresis and eddy current losses in electrical machines. Also suggest the methods in construction to minimize the above losses.
ii)State properties of magnetic material suitable for fabrication of permanent magnet and electro magnet. / 8
8 / U
R / CO6
CO2 / May 2015
i)Discuss the magnetic circuits and the electrical analogy of magnetic circuits.
ii)Explain the eddy current and the eddy current loss. / 10
6 / U
U / CO1
CO6 / May 2015
Explain the power losses that occur in a magnetic material when it undergoes cyclic magnetisation. / 10 / U / CO6 / May 2015
Explain clearly the statically and dynamically induced emf. / 16 / U / CO1 / Nov 2015
i)Derive an expression for an energy density in a magnetic circuit.
ii)Explain in detail “Eddy current loss”. / 6
4 / Ap
U / CO1
CO6 / Nov 2015
A single phase, 50 Hz, 100 kVA transformer for 12000 / 240 V ratio has a maximum flux density of 1.2 Wb/m2 and an effective core section of 300 cm2, the magnetising current (RMS) is 0.2 A. Estimate the inductance of each wire on open circuit. / 6 / U / CO3 / Nov 2015
Summarize the properties of magnetic materials. / 16 / U / CO2 / Nov 2015
Explain the hysteresis and eddy current losses and obtain its expression. / 16 / U / CO6 / Nov 2015
Obtain the expression for dynamically induced EMF and force. / 16 / Ap / CO3 / Nov 2016
Unit II–Transformers
Part A
Sl. No. / Question / BT / CO / Month / YearWhat is meant by all day efficiency in transformer? / R / CO3 / Nov 2012
May 2016
State the advantages and applications of auto transformer / R / CO5 / Nov 2012
Nov 2013
May 2015
Define voltage regulation of a transformer. / R / CO3 / Nov 2013
May 2015
Nov 2016
Differentiate between a core and shell type transformer. / U / CO1 / May 2014
What is the basic purpose of tertiary winding? / R / CO1 / May 2014
May2015
Why is the efficiency of a three phase induction motor less than that of a three phase transformer? / An / CO3 / May 2015
What are the no load losses that occur in a two winding transformer and state the reasons for such losses. / R / CO6 / Nov 2010
Nov 2015
Mention the conditions to be satisfied for parallel operation of two winding transformers? / R / CO1 / May 2010
Nov 2010
Nov 2015
Why transformer rating in kVA? / An / CO1 / Nov 2011
May 2015
What happens when a DC supply is applied to a transformer? / An / CO1 / Nov 2015
What is Inrush current in a transformer? / R / CO1 / May 2016
What are the losses that occur in a transformer? / R / CO6 / May 2013
What is polarity test in transformers? / R / CO3 / May 2011
Draw the phasor diagram of an ideal transformer. / R / CO3 / May 2011
Draw the no load phasor diagram of a transformer. / R / CO3 / May 2010
Write the condition for maximum efficiency of a transformer. / R / CO1 / May 2010
Give the principle of transformers. / R / CO1 / May 2010
Compare two winding transformer and auto transformer. / U / CO1 / Nov 2011
What material is used for making the magnetic core of transformers? / R / CO1 / Nov 2003
Draw scott connection of transformer. / R / CO1 / Nov 2016
A sinusoidal flux 0.02 wb links with 55 turns of a transformer secondary coil. Calculate the rms value of the induced emf in the secondary. The supply frequency is 50 Hz. / Ap / CO3
Why does the OC test on a transformer performed on the low voltage side? / An / CO1
Why does the SC test on a transformer performed on the high voltage side? / An / CO1
What are the possible three phase transformer connections? / R / CO1
Why does the Auto-Transformer not used as Distribution Transformer? / An / CO1
List some of the applications of transformers. / R / CO5
What will happen if a power transformer designed for operation on 50 Hz is connected to a 500 Hz source of same voltage? / An / CO3
What are the advantages and disadvantages of 3 phase transformers over 3 single phase bank of transformers? / R / CO1
What is the principle of operation of transformer? / R / CO1
Why do we represent leakage flux in a transformer by inductive reactance? / U / CO4
Part B
Sl. No. / Question / M / BT / CO / Month / Yeari)Define “voltage regulation” of a two winding transformer and explain its significance.
ii)A 100 kVA, 6600 V/330 V, 50 Hz single phase transformer took 10 A and 436 W at 100 V in a short circuit test, the figures referring to the high voltage side. Calculate the voltage to be applied to the high voltage side on full load at 0.8 power factor lagging when the secondary terminal voltage is 330 V. / 4
12 / R
Ap / CO1
CO3 / Nov 2010
i)Explain the reasons for “tap changing” in transformers. State on which winding the taps are provided and why?
ii)A transformer has its maximum efficiency of 0.98 at 15 kVA at UPF. During the day it is loaded as follows:
12 hours / 2 kW / at 0.5 p.f
6 hours / 12 kW / at 0.8 p.f
4 hours / 18 kW / at 0.9 p.f
2 hours / No load
Find the “All Day Efficiency”. / 4
12 / U
Ap / CO1
CO3 / Nov 2010
i)Explain clearly the causes of voltage drop in a power transformer on load and develop the equivalent circuit for a single phase transformer.
ii)Derive an expression for saving of copper when an auto transformer is used. / 10
6 / U
Ap / CO4
CO3 / Nov 2011
Nov 2011
Nov 2015
A 3 phase step down transformer is connected to 6.6 kV mains and takes 10 A. Calculate the secondary line voltage and line current for the (i) Δ/Δ (ii) Y/Y (iii) Δ/Y (iv) Y/Δ connections. The ratio of turns per phase is 12 and neglect no load losses. / 16 / Ap / CO3 / Nov 2011
i)Derive the emf equation of a transformer.
ii)The voltage per turn of a single phase transformer is 1.1 V. When the primary winding is connected to a 220 V, 50 Hz AC supply, the secondary voltage is found to be 550 V. Find the primary and secondary turns and the core area if the maximum flux density is 1.1 Tesla. / 8
8 / Ap
Ap / CO3
CO3 / Nov 2012
May 2015
Nov 2012
i)Explain in detail, the step by step procedure to draw the equivalent circuit of transformer.
ii)In a 25 kVA, 2000 V / 200 V transformer, the constant and variable losses are 350 W and 400 W respectively. Calculate the efficiency on unity power factor at full load and half the full load. / 8
8 / U
Ap / CO4
CO3 / Nov 2012
Obtain the equivalent circuit of a 200 / 400 V, 50 Hz, 1 phase transformer from the following test data:
O.C. test: 200 V, 0.7 A, 70 W – on L.V side.
S.C. test: 15 V, 10 A, 85 W – on H.V side.
Calculate the secondary voltage when delivering 5 kWat 0.8 p.f lagging, the primary voltage being 200 V. / 16 / Ap / CO4 / May 2013
i)Describe the construction and principle of operation of single phase transformer.
ii)Derive an expression for maximum efficiency of a transformer. / 8
8 / U
Ap / CO1
CO3 / Nov 2013
Nov 2015
A 500 kVA transformer has 95% efficiency at full load and also at 60% of full load both at UPF.
i)Separate out the transformer losses.
ii)Determine the transformer efficiency at 75% full load, UPF. / 16 / Ap / CO6
CO3 / Nov 2013
Nov 2015
i)Draw the phasor diagram of transformer when it is operating under load and explain.
ii)The parameters of approximate equivalent circuit of a 4 kVA, 200/400 V, 50 Hz, single phase transformer are: RP’=0.15Ω; XP’=0.37Ω; R0=600Ω; Xm = 300Ω. When a rated voltage of 200V is applied to the primary, a current of 10 A at lagging power factor of 0.8 flows in the secondary winding. Calculate,
i)The primary current.
ii)The secondary terminal voltage. / 8
8 / U
Ap / CO3
CO3 / May 2014
i)What is meant by inrush current in transformer? Specify the nature of inrush currents and its problem during transformer charging.
ii)A 500 kVA transformer has a core loss of 2200 W and a full load copper loss of 7500 W. If the power factor of the load is 0.9 lagging, calculate the full load efficiency and the kVA load at which maximum efficiency occurs. / 6
10 / R
Ap / CO1
CO3 / May 2015
i)Specify the condition for parallel operation of transformer. Also explain the effect of load sharing due to impedance variation between transformers during parallel operation.
ii)A 100 kVA, 3300 V/240 V, 50 Hz, single phase transformer has 990 turns on the primary. Calculate the number of turns on secondary and approximate value of primary and secondary full load currents. / 6
10 / U
Ap / CO1
CO3 / May 2015
Describe the method of calculating the regulation and efficiency of a single phase transformer by OC and SC tests. / 16 / U / CO3 / May 2015
May 2016
Calculate the efficiency at half, full load of a 100 kVA transformer for unity and 0.8 p.f. the copper loss is 1000 W at full load and iron loss is 1000 W. / 10 / Ap / CO3 / May 2015
Nov 2015
The primary of the transformer is rated at 10 A and 1000 V. The open circuit readings are V1 = 1000 V, V2 = 500 V, I = 0.42 A, Pac = 100 W. The short circuit readings are I1 = 10A, V1 = 125 V and Pac = 400 W. Draw the equivalent circuit of the transformer. Predict the output voltage for the load impedance of ZL = 19 + j12 ohms and draw the phasor diagram. / 16 / Ap / CO3 / Nov 2015
Explain the various three phase transformer connections and parallel operation of three phase transformer. / 16 / U / CO1 / May 2016
The following data were obtained on a 20 kVA, 50 Hz, 2000 / 200 V distribution transformer:
Voltage / Current / Power
OC test with HV open / 200 / 4 / 120
SC test with LV short / 60 / 10 / 300
Draw the approximate equivalent circuit of the transformer referred to the HV and LV sides respectively. / 16 / Ap / CO4 / Nov 2016
With circuit, explain Sumpner’s test and how to obtain efficiency of a transformer? / 16 / U / CO3 / Nov 2016
Unit III - Electromechanical Energy Conversion and Concepts in Rotating Machines
Part A
Sl. No. / Question / BT / CO / Month / YearDraw the diagram indicating the flow of energy in electromechanical energy conversion via coupling medium. / R / CO1 / Nov 2012
Give the expression for energy stored in the magnetic field. / R / CO3 / Nov 2012
Define pitch factor. / R / CO1 / Nov 2012
What is meant by mechanical angle? Explain. / R / CO1 / Nov 2012
Draw a schematic indicating flow of energy in the conversion of mechanical to electrical energy / R / CO1 / Nov 2013
Why do all practical energy conversion devices make use of magnetic field as a coupling medium rather than an electric field? / U / CO1 / Nov 2013
May 2014
May 2015
What is meant by SPP? What is its significance? / R / CO1 / Nov 2013
Nov 2015
Enumerate the advantages of using short – pitched winding in a synchronous machine. / R / CO1 / Nov 2013
Define co-energy. / R / CO1 / May 2013
May 2014
May 2015
May 2016
Why synchronous machine does not produce torque at any other speed? / U / CO1 / May 2014
Draw the power flow diagram for motor and generator operation. / R / CO1 / Nov 2010
May 2015
In a magnetic circuit with a small air gap, in which part the maximum energy is stored and why? / U / CO1 / Nov 2010
May 2015
What are the requirements of the excitation systems? / R / CO1 / Nov 2015
Why fractional pitched winding is required than full pitched winding. / U / CO1 / Nov 2015
May 2013
May 2012
Define winding factor. / R / CO1 / Nov 2015
What is meant by winding inductance? / R / CO1 / May 2016
Give examples of multiple excitation systems. / R / CO1 / May 2013
Write the expression for mechanical force for singly excited system. / R / CO1 / May 2011
Draw the i – λ characteristics of a non-linear magnetic circuit when the armature is moved from X1 to X2. / R / CO1 / May 2011
Write the relationship between mechanical and electrical angle. / R / CO1 / Nov 2010
May 2011
What is rotating magnetic field? / R / CO1 / May 2011
Draw the mmf space wave of a single coil. / R / CO1 / May 2011
Write the equation which relates rotor speed in electrical and mechanical radian/second. / R / CO1 / May 2015
In a linear system, prove that the field energy and co-energy are equal. / Ev / CO3 / May 2010
What are the assumptions to be made to determine the distribution of coil mmf? / R / CO1 / May 2010
Write an expression for stored energy in a magnetic field. / R / CO3 / May 2010
What is magnetic saturation? / R / CO1 / Nov 2016
What is meant by distributed winding? / R / CO1 / Nov 2016
Part B
Sl. No. / Question / M / BT / CO / Month / Yeari)Derive an expression for the magnetic energy stored in a singly excited electromagnetic relay.
ii)The relay shown in Fig. 3.1 is made from infinitely permeable magnetic material with a movable plunger also of infinitely permeable material. The height of the plunger is much greater than the air-gap length (h>g). calculate the magnetic energy stored as a function of plunger position (0<X<d) for N = 1000 turns, g = 2.0 mm, d = 0.5m, l = 0.1 m and I = 10A.
Fig. 3.1 / 8
8 / Ap
Ap / CO3
CO3 / Nov 2010
Two windings one mounted on the stator and the other mounted on a rotor have self and mutual inductances of L11 = 4.5 H, L22 = 2.5 H and L12 = 2.8 cosϴ H, where ϴis the angle between the axes of the windings. The resistance of the windings may be neglected. Winding 2 is short circuited and the current in winding 1as a function of time is i1 = 10 sin ωt A. Derive an expression for the numerical value of the instantaneous torque on the rotor in N-m in terms of angle ϴ. / 16 / Ap / CO3 / Nov 2010
Obtain the expression for mechanical force of field origin in a typical armature attracted relay. / 16 / Ap / CO3 / Nov 2011
May 2014
Nov 2015
May 2016
Nov 2016
Find an expression for the magnetic force developed in a doubly excited magnetic systems. / 16 / Ap / CO3 / Nov 2011
May 2013
Derive the expression for energy and force in a doubly excited magnetic field system. / 16 / Ap / CO3 / Nov 2011
May 2016
Two coupled coils have self and mutual inductance of L11 = 2 + ½x; L22 = 1+½x; L12 = L21 = ½x over a certain range of linear displacement of x. the first coil is excited by a constant current of 20 A and the second by a constant current of –10 A. Find mechanical work done if x changes from 0.5 to 1 m and also the energy supplied by each electrical source. / 16 / Ap / CO3 / Nov 2012
Derive an expression for co-energy density of an electromechanical energy conversion device. / 16 / Ap / CO3 / Nov 2013
May 2015
The doubly excited magnetic field has coil self and mutual inductances of
L11 = L22 = 2
L12 = L21 = cosϴ
Where ϴ is the angle between the axis of the coils. The coils are connected in parallel to a voltage source V = Vm sinωt. Derive an expression for the instantaneous torque as a function of the angular position ϴ. Find the time-average torque. Evaluate for ϴ = 300, gamma = 100 sin 314t. / 16 / Ap / CO3 / Nov 2013
May 2015
i)Show that the torque developed in doubly excited magnetic field system is equal to the rate of increase of field energy with respect to displacement at constant current.
ii)The λ-i characteristics of singly excited electromagnet is given by i = 121λ2X2 for 0<x<10 cm. If the air-gap is 5 cm and a current of 3A is flowing in the coil, calculate
a)Field energy
b)Co-energy
c)Mechanical force on the moving part / 8
8 / Ap
Ap / CO3
CO3 / May 2014
Nov 2015
May 2014
Nov 2015
Two windings, one mounted in stator and other at rotor have self and mutual inductance of L11 = 4.5, L22 = 2.5 and L12 = 2.8 cosϴ H, where ϴ is the angle between axes of winding. Winding 2 is short circuited and current in winding as a function of time is i1 = 10 sin ωt A.
(i)Determine the expression for numerical value in N-m for the instantaneous value of torque in terms of ϴ.
(ii)Compute the time average torque in N-m when ϴ = 450.
(iii)If the rotor is allowed to move, will it continuously rotate or it will come to rest? If later at which value of ϴ0. / 8
4
4 / Ap
Ap
An / CO3
CO3
CO3 / Nov 2015
In an electromagnetic relay, functional relation between the current i in the excitation coil, the position of armature is x and the flux linkage ψ is given by i = 2ψ3+3ψ(1–X+X2), X > 0.5. Find the force on the armature as a function of ψ. / 16 / Ap / CO3 / Nov 2015
Prove that a three phase set of currents, each of equal magnitude and differing in space by 1200 applied to a three phase winding spaced 120 electrical degrees apart around the surface of the machine will produce a rotating magnetic field of constant magnitude. / 16 / Ap / CO1 / Nov 2010
Nov 2015
Explain the concept of rotating MMF waves in AC machine. / 16 / U / CO1 / Nov 2012
Nov 2016
What is meant by current sheet concept? Explain briefly. What is the phase angle difference between a sinusoidally distributed current sheet and its accompanying mmf wave? / 8 / U / CO1 / May 2014
Explain briefly the production of rotating magnetic field. What are the speed and direction of rotation of the field? Is the speed uniform? / 16 / U / CO1 / May 2014
Unit IV - DC Generators