Electrical Machines-II

Electrical Machines-II Important Questions for JNTUH examinations

Previous year Questions
Subject :Electrical Machines –II for
Branch : B.Tech EEE –II –II SEM

List any two differences between cage rotor and wound rotor. [2]
 Explain the concept of RMF. [3]
 Draw torque – slip characteristic of three phase IM.
 [2] What is cogging? Explain? [3]
e) Why two reactances are required for the analysis of Salient pole alternator? [2]
f) List the differences between distributed and concentrated windings. [3]
g) What is synchronizing torque? [2]
h) What is Synchronous induction motor? Explain? [3]
i) Why single phase IM is not self starting? [2]
j) List Applications of shaded pole motor.
1.a) What is rotating magnetic field of a three phase induction motor. [2]
b) Define slip of an induction motor. Write the equation for the same. [3]
c) What is cogging of an induction motor? [2]
d) List three different speed control methods of a three phase induction motor. [3]
e) List any two advantages of rotating magnetic field in Synchronous Generators. [2]
f) List any three differences between smooth Cylindrical and Salient pole types of Alternator. [3]
g) Why a 3-phase synchronous motor always runs at synchronous speed? [2]
h) List any three conditions to be satisfied for parallel operation of alternators. [3]
i) Classify different types of single phase induction motors. [2]
j) Explain Double revolving field theory of Single phase Induction motor. [3]
1.a) What is slip and slip speed of an Induction motor? [2]
b) Give the applications of 3-phase induction motor. [3]
c) What is crawling in Three-phase Induction motor? [2]
d) What is the principle of V/f speed control in Induction motor? [3]
e) What is pitch factor? [2]
f) Draw the load characteristics of an alternator. [3]
g) What is Synchronous reactance of an alternator? [2]
h) Write short note on Synchronous induction motor. [3]
i) State the disadvantages of single phase Induction motor. [2]
j) Give the applications of shaded pole induction motor. [3]
Write the merits and demerits of slip-ring induction motor. [2]
h) Write the effects of crawling and cogging on the performance of induction motor. [3]
i) What is the procedure to conduct the blocked rotor test on 3-ϕ induction motor? [2]
j) Why starting methods are needed for 3-ϕ induction motor
What is the effect of skewing the rotor slots of an induction motor? [2]
h) Draw the speed-torque characteristics of a deep-bar double cage induction motor. [3]
i) What are the limitations of speed control by conventional methods? [2]
j) What will be the effect on torque developed by an induction motor if applied voltage is reduced to
half with frequency unchanged?
Define slip. And explain necessity of slip in an induction motor. [2]
h) Define frequency of rotor current and its condition during starting and synchronous speed. [3]
i) What information can be obtained from circle diagram of 3-ϕ Induction Motor? [2]
j) What is the necessity of speed control of a three phase induction motor?
Derive the condition for maximum torque at the time of starting in a 3-ϕ I.M. [2]
h) Derive the condition for maximum torque under running conditions in a 3-ϕ I.M. [3]
i) Explain the working principle of Induction generator. [2]
j) Explain the speed control of 3-ɸ IM using cascade connection
10 marks qtns
2.a) Discuss the points of similarities and dissimilarities between a transformer and an induction
motor. Hence, explain why an induction machine is called a generalized transformer?
b) Explain why three phase induction motor, at no load operates at very low power factor?
[5+5]
OR
3.a) Describe the principle of operation of three phase induction motor. Explain why the rotor is
forced to rotate in the direction of rotating magnetic field?
b) Show that the voltage generated in the rotor circuit of a three phase induction motor at any slip s is
equal to s times the voltage generated at standstill? [5+5]
4.a) Explain the principle of speed control of a 3-phase induction motor by V/f method and draw the
corresponding torque-speed characteristics and discuss the applications and limitations of these
methods.
b) A three phase, 400 V, 50 Hz induction motor takes a power input of 34 kW at its full load speed of
980 rpm. The total stator losses are 1.2 kW and the friction and windage losses are 1.5 kW. Calculate
(i) slip (ii) rotor ohmic losses (iii) shaft power (iv) shaft torque and (v) efficiency. [5+5]
OR
5.a) Draw and explain the equivalent circuit of three phases IM.
b) A 3-phase, 400 V, 50 Hz induction motor has a rotor resistance of 0.1 Ω and standstill reactance
of 0.9 Ω per phase. The full load slip is 4%, Calculate (i) Full load torque as a percentage of
maximum torque and the value of extra resistance to be added in the rotor circuit to have 70% of
maximum torque at start
6.a) What is armature reaction? Explain the effect of armature reaction on the terminal voltage of an
alternator at ZPF lag and ZPF lead with the help of necessary phasor diagram?
b) A 500 kVA, 1000 V, 50 Hz star connected 3-phase alternator has armature resistance per phase of
0.1 Ω and synchronous reactance per phase of 1.3 Ω. Find its voltage for 0.8 pf lag. Also find the
voltage regulation. [6+4]
OR
7.a) Briefly explain the procedure to conduct slip test on three phase alternator? From test results
how to calculate Xd and Xq?
b) Explain how the potier triangle can be drawn with the help of OCC and any two points on the ZPF
characteristic? What are the observations we can derive from the potier triangle? [5+5]
8.a) Two alternators A and B operate in parallel and supply a load of 10 MW at 0.8 p.f lagging By
adjusting steam supply of A, its power output is adjusted to 6000 kW and by changing its excitation,
its p.f is adjusted to 0.92 lag. Find the p.f of alternator B.
b) What is synchronizing power of an alternator? Derive an expression for the synchronizing power
between the two alternators connected in parallel. [5+5]
OR
9.a) What is synchronous condenser? What are the advantages of installing a synchronous condenser
in an electrical system? Illustrate your answer with an example.
b) A 400V, 50 Hz, 33kW, 3 phase star connected synchronous motor has a full load efficiency of
85%. The synchronous impedance of the motor is (0.2+j1.4) Ω/ph. If the excitation of the motor is
adjusted to give a leading p.f of 0.8, Calculate induced emf developed for full load? [5+5]
10. Explain the constructional details and principle of operation of a split phase induction motor. List
out its industrial applications. [10]
OR
11. What is double field revolving theory? Explain the equivalent circuit of single phase induction
motor based on double filed revolving theory? [10]
2.a) Define i) Synchronous speed, ii) Actual speed of an induction motor.
b) A three phase, 400V, 100hp, 50Hz, 4-pole induction machine delivers rated output power at a slip
of 5%. Determine the i) synchronous speed ii) motor speed (actual speed) iii) frequency of the rotor
induced voltage.
c) 12 pole, 3 phase alternator driven at a speed of 500rpm supplies power to an 8pole, 3phase
induction motor. If the slip of the motor at full load is 0.03, calculate the full load speed of the motor.
[10]
OR
3.a) The active power input to a 415V, 50Hz, 6 pole 3-phase Induction motor running at 970rpm is
41kW. The input power factor is 0.9. Calculate line current and slip.
b) With the help of neat diagram explain the constructional features of wound rotor type of
induction motor. Write any one advantage and dis- advantage of wound rotor type of induction
motor. [5+5]
The following test results are obtained from a three phase, 100 hp, 460 V, eight pole, star-connected
squirrel-cage induction machine.
No-load Test: 460 V, 60 Hz, 40 A, 4.2 kW
Blocked rotor test: 100 V, 60 Hz, 140 A, 8.0 kW.
Average dc resistance between two stator terminals is 0.152 Ω.
(a) Determine the parameters of the equivalent circuit and (b) the motor is connected to a three phase,
460 V, 60 Hz supply and runs at 873 rpm. Determine the input current, input power, air gap power,
rotor copper loss, mechanical power developed, output power, and efficiency of the motor. [10]
OR
5.a) The following data refers to a 12 pole , 420 V, 50 Hz, 3-phase mesh connected induction motor:
R1=2.95Ω, X1=6.82Ω, R21=2.08Ω, X21=4.11Ω per phase. On no load, the line value of magnetizing
current is 6.7 A and the total core loss is 269 W, Determine the p.f., input current, equivalent rotor
current and torque developed by the motor at a slip of 3% using exact equivalent circuit.
b) Why starters are necessary for starting an induction motor? What are the various types of starters
used for Induction motor starting? [5+5]
6.a) Explain the direct axis synchronous reactance and quadrature axis synchronous reactance of
salient pole alternator. On which factors these values depend.
b) A 3 phase 8 pole star connected alternator has the armature coils short chorded by one slot. The
coil span is 1650 electrical. The alternator is driven at the speed of 750rpm.if there are 12 conductors
per slot and flux per pole is 50 mWb. Calculate the value of induced emf across the terminals. [5+5]
OR
7. The following test results are obtained on a 6600V alternator:
Open circuit voltage 3100 5000 6600 7500 8300
Field current(amps) 16 25 37.5 50 70
A field current of 20A is found necessary to circulate full load current on short circuit of the
armature. Using ampere-turn method, find the full load regulation at 0.8 pf lagging.
[10]
8. A three phase, 250 hp, 2300 V, 60 Hz, Y connected non-salient rotor synchronous motor has a
synchronous reactance of 11 Ω per phase. When it draws 165.8 kW, the power angle is 15 electrical
degrees. Neglect ohmic losses. Determine (a) The excitation voltage per phase EF (b) The supply line
current Ia and (c) The supply power factor. [10]
OR
9. Two identical 2000 KVA alternators operate in parallel. The governor of first machine is such that
the frequency drops uniformly from 50 Hz on no load to 48 Hz on full load. The corresponding speed
drop in second machine is 50 to 47.5 Hz.
a) how will the two machines share a load of 3000kw? b) what is the maximum load a unity power
factor that can be delivered without overloading either machine? [10]
10.a) Develop equivalent circuit of a single phase induction motor in terms of its circuit parameters.
b) Find the input current, power factor and efficiency of a ½ h.p., 110V, 50Hz, single phase induction
motor based on double revolving field theory with the following data at a slip of 5%.
Stator impedance = 2+j3Ω
Equivalent rotor impedance = 2+j3Ω
Magnetizing impedance = 50Ω
Friction and windage loss = 25 W. [5+5]
OR
11.a) Explain with neat sketches the construction and working principle of a Shaded-pole induction
motor.
b) A single-phase, 120 V, 60 Hz, four-pole, split-phase induction motor has the following standstill
impedances.
Main winding: Zm = 5 + j6.25
Auxiliary winding: Za = 8 + j6
(i) Determine the value of capacitance to be added in series with the auxiliary winding to obtain
maximum starting torque.
(ii) Compare the starting torques and starting current with and without the added capacitance in the
auxiliary winding circuit when operated from a 120V, 60 Hz supply. [5+5]
---ooOoo---
2.a) Explain the R.M.F production in an 3-phase Induction motor.
b) A 3-phase induction motor is wound for 4 poles and is supplied from 50 Hz system. Calculate (i)
the speed at which magnetic field of the stator is rotating. (ii) the rotor speed when slip is 4% and (iii)
the frequency of the rotor currents when the slip is 0.03 and (iv) the frequency of the rotor currents
when at stand still. [5+5]
OR
3. Explain the working of 3-phase Induction motor. Compare the constructional features of both
squirrel cage induction motor and slip ring induction motor. Discuss the merits of one over the other.
[10]
4.a) Sketch and explain torque-slip characteristic of an induction motor working at rated Voltage and
frequency.
b) No load test determines which parameters of the circuit model of induction motor? Explain in
detail. [5+5]
OR
5.a) Explain the speed control of 3-phase Induction motor by cascade connection.
b) A 6 pole, 50 Hz, 3 phase induction motor running on full load with 3% slip develops a torque of
160 N-m at its pulley rim. The friction and windage losses are 210W and the stator copper and iron
losses equal 1640W. Calculate (i) Rotor output (ii) Rotor copper loss (iii) Efficiency at full load. [5+5]
6.a) Derive an expression for e.m.f induced in an alternator.
b) Calculate the r.m.s value of the induced e.m.f per phase of a 8 pole, 3 phase, 50 Hz alternator with
3 slots per pole per phase and 5 conductors per slot in two layers. The coil span is 1400. The flux per
pole has a fundamental component of 0.1 Wb and 15% third harmonic component. [5+5]
OR
7.a) Explain the procedure to find the voltage regulation of an alternator by A.S.A method.
b) Explain the suppression methods of harmonics generated in synchronous generator. [5+5]
8.a) Explain the variation of power factor and current of 3-phase synchronous motor with excitation.
b) A 3300V, star connected synchronous motor has synchronous impedance of (0.4+j5) Ω per phase.
For an excitation e.m.f of 4000Vand motor input power of 1000kW at rated voltage, calculate the
line current and power factor. [5+5]
OR
9.a) Discuss the conditions to be fulfilled for parallel operation of alternators.
b) A 10MVA, 11 KV, 1500 rpm, 50 Hz alternator runs connected across an 11 KV constant voltage
constant frequency bus bars. The steady S.C. current is twice that full load current and the movement
of inertia of the rotating machine is 20,000 Kg-m2. Determine the synchronizing torque per degree of
mechanical displacement and time for one complete oscillation for full load unity p.f. Neglect
saliency and losses. [5+5]
10.a) Explain the concept of double field revolving theory.
b) What is split phase motor? Give the significance of split phasing. [5+5]
OR
11. Draw the circuit diagram of 1-phase capacitor start and induction motor. Explain its working
with all constructional details. Sketch the performance characteristics of the motor.
8.a) Explain with the aid of diagrams the principal of operation of double cage induction motor.
Sketch the torque –slip curves of such a motor.
b) Obtain the relation between rotor input, rotor copper losses and rotor output in terms of slip(s).
[5+5]
OR
9.a) Explain the principle of production of rotating magnetic field in a 3-phase induction motor.
b) A 10 kW, 400 V, 3-phase, 4 pole, 50 Hz delta connected induction motor is running at no load
with a line current of 8 A and an input power of 660 W. At full load, line current is 18 A and input
power is 11.20 kW. Stator effective resistance per phase is 1.2 Ω and friction, windage loss is 420 W.
For negligible rotor ohmic losses at no load, calculate
(i) stator core loss (ii) total rotor losses at full load (iii) total rotor ohmic losses at full load (iv) full
load speed.
10. A 3-phase, 400V induction motor gave the following test readings:
No-load: 400V, 1250W, 9A
Short-circuit: 150V, 4kW, 38A
Draw the circle diagram. If the normal rating is 20.27 hp (metric), find from the circle diagram, the
full-load values of current, power factor and slip. [10]
OR
11.a) How is the speed of a 3-phase induction motor controlled by its stator voltage control?
b) A 4-pole induction motor and 6-pole induction motor are connected in cumulative cascade at 50
Hz supply. The frequency in the secondary circuit of the 6-pole motor is observed to be 1.0 Hz.
Determine the slip in each machine and combined speed of the set. [6+4]
8.a) What are its advantages and disadvantages of wound rotor type induction motor?
b) Explain the phenomenon of crawling and cogging. [5+5]
OR
9.a) Explain how starting performance of 3-phase squirrel cage type induction motor is improved by
means of double cage rotor windings.
b) Explain the phenomenon of crawling and cogging. [5+5]
10. A three phase, 15 kW, 400 V, 50 Hz, 4-pole, delta connected squirrel cage induction motor has
the following data:
No-load: 400 V, 5.0 A, p.f. 0.2
Blocked rotor: 120 V, 20.0 A p.f. 0.6
The ratio of stator to rotor copper losses on short circuit is assumed to be unity. Draw the circle
diagram and determine (a) the full load current and power factor (b) the maximum power developed
(c) starting torque. [10]
OR
11. Explain the induction motor operation under injection of an e.m.f. into the rotor circuit. [10]
8.a) Derive the expression for the rotor EMF and rotor current of an induction motor.
b) A 3-phase, 50Hz, 6 pole induction motor has a full load speed of 960 pm. Calculate:
i) Synchronous speed
ii) Slip speed
iii) Slip frequency. [5+5]
OR
9.a) Explain why the rotor of a poly phase induction motor can never attain synchronous speed.
b) Explain the concept of crawling of an induction motor. [5+5]
10.a) Explain the pole changing method of speed control in Induction motor.
b) Explain the concept of No load test on a Induction motor. [5+5]
OR
11. Explain the principle of operation of Induction generator with characteristics. [10]
8.a) Explain the torque slip characteristics of 3-ϕ IM.
b) Derive and explain rotating magnetic field in a three phase induction motor. [4+6]
OR
9.a) Briefly explain the working of double Cage Induction motor.
b) The power input to the rotor of 440V, 50 Hz, 6 pole, 3-phase, and induction motor is 80 KW. The
rotor electromotive force is observed to make 100 complete alterations per minute. Calculate (i) the
slip (ii) the rotor speed (iii) rotor copper losses per phase. [4+6]
10.a) Explain the conducting procedure of No load test on three phase induction motor.
b) Explain the working of y- starter used in 3- ϕ induction motor. [5+5]
OR
11.a) Explain the conducting procedure blocked rotor test on three phase induction motor.
b) Explain the speed control of induction motor using Rotor resistance control. [5+5]