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Code No: R22024

II B. Tech II Semester Regular Examinations April/May 2013

ELECTRICAL MACHINES - II(Electrical and Electronics Engineering)

Time: 3 hours Max. Marks: 75

Answer any FIVEQuestionsAll Questions carry EqualMarks

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1. a) Develop the phasor diagram of a single-phase transformer under load condition. Assumelagging power factor load.

b) A single-phase transformer has 550 primary turns and 40 secondary turns. The primary is

connected to a 3300 V, 50 Hz supply. Neglecting losses, calculate i) the secondary voltage

and ii) the primary current when the secondary current is 200 A.

2. a) Define voltage regulation of a transformer. Derive the expression of voltage regulationusing the equivalent circuit.

b) A transformer has no-load loss of 55 W with a primary voltage of 250 V at 50 Hz and 41 W

with primary voltage of 200 V at 40Hz. Compute the hysteresis and eddy current losses at a

primary voltage of 300V at 60Hz of the above transformer. Neglect the small amount of

copper at no-load.

3. a) What is Sumpners test? Draw a circuit diagram to conduct this test and explain itsprocedure.

b) Two single-phase transformers rated 600 kVA and 500 kVA are connected in parallel to

supply a load of 1000 kVA at 0.8 lagging power factor. The resistance and reactance of the

first transformer are 3% and 6.5% respectively, and of the second transformer are 1.5% and

8% respectively. Calculate the kVA loading and the power factor at which each transformer

operates.

4. a) What are the distinguishing features of Y-Y, Y-, -Y and -3-phase connections?b) Two 100 V, single-phase furnaces take loads of 600 kW and 900 kW respectively at a power

factor of 0.707 lagging and are supplied from 6600 V, three-phase supply through a Scott

connected transformer. Calculate the currents in the three-phase lines. Neglect transformer

losses.

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5. a) Describe the principal of operation of a three-phase induction motor. Explain why the rotoris forced to rotate in the direction of rotating magnetic field.

b) A 10kW, 400V, 3-phase, 4-pole, 50Hz delta connected induction motor is running at no-load

with a line current of 8A and an input power of 700 W. At full load, the line current is 18A

and the input power is 11 kW. Stator effective resistance per phase is 1.2 and friction,

windage losses are 400 W. For negligible rotor ohmic losses at no-load, calculate

i) Stator core loss;

ii) Total rotor losses at full load;

iii) full-load speed

iv) Internal torque, shaft torque and motor efficiency.

6. a) Develop the equivalent circuit for a three-phase induction motor and explain how themechanical power developed is taken care in the equivalent circuit.

b) A three-phase, 400 V, 4-pole, 50 Hz induction motor has a star-connected stator and rotor.

The rotor resistance and standstill reactance/phase are 0.25 and 1.2 respectively. The

ratio of stator to rotor turns is 1.3. The full-load slip is 4 percent. Calculate:

i) Power and torque developed at full-load

ii) Maximum torque and the speed at which it occurs.

7. a) Explain the procedure of drawing circle diagram of an induction motor. What informationcan be drawn from circle diagram.

b) Find the ratio of starting to full load current for a 10 kW, 400 V three-phase induction motor

with star-delta starter, given that the full-load efficiency is 0.86, the full-load power factor is

0.8 and the short-circuit current is 30 A at 100 V.

8. a) Explain the method of speed control using stator voltage control. Draw the speed-torquecharacteristics.

b) Explain the principle of operation of an induction generator.

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1. a) Describe the operation of a single-phase transformer, explaining clearly the functions of thedifferent parts.

b) A 3300/230 V, 50 Hz, single-phase transformer is to be worked at a maximum flux density

of 1.2 T in the core. The effective cross-sectional area of the core is 150 cm2. Calculate the

suitable values of primary and secondary windings.

2.

a) Describe various losses in a transformer. Explain how each loss varies with the load current,supply voltage and frequency.

b) A transformer has maximum efficiency of 0.98 at 20 kVA at upf. During the day it is loaded

as follow:

12 hours : 2 kW at p.f 0.6



Determine the all-day efficiency of the transformer.

3. a) Discuss the procedure for conducting OC and SC tests on a single phase transformer anddevelop the equivalent circuit from results obtained by these tests.

b) A 200/ 400 V, 20 kVA and 50 Hz transformer is connected as an auto transformer to

transform 600 V to 200 V.

i) Determine the auto transformer ratio k

ii) Determine the kVA rating of the auto transformer

iii) With a load of 20 kVA, 0.8 pf lagging connected to 200 V terminals, determine the

currents in the load and the two transformer windings.

4. a) Why are tap changing transformers required? Explain the operation of a on-load tap- changing transformer.

b) A 100 kVA, 3-, 50Hz, 3300/400V transformer is delta-connected on the HV side and star

connected on the LV side. The resistance of the HV winding is 3.5 per phase and that of

the LV winding 0.02 per phase. Calculate the iron losses of the transformer at normal

voltage and frequency if its full load efficiency be 95.8% at 0.8 p.f. (lag).

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5. a) Discuss the constructional features of both squirrel-cage induction motor and slip-ringinduction motor.

b) A 6-pole, 50 Hz, 3-phase induction motor running on full load develops a useful torque of

150 N-m at a rotor frequency of 1.5 Hz. Calculate the shaft power output. If the

mechanical torque lost due to friction and windage be 10 N-m, determine i) rotor copper

loss, ii) the input to the motor and iii) the efficiency.

6. a) Derive the relationship for torque developed by a three-phase induction motor. Draw thetypical torque-slip characteristic.

b) A 7.5 kW, 440 V, 3-phase, star-connected, 50 Hz, 4-pole squirrel cage induction motor has

its full-load internal torque at a slip of 0.05 when operated at rated voltage and frequency.

Rotational losses are to be neglected. The parameters of the motor are:

R1= 1.32 /phase; X1= = 1.46 /phase; Xm= 22.7 /phase. Determine the maximum

motor torque at rated voltage and the slip at which it will occur. Also calculate the starting

torque.

7. a) Explain the procedure of no-load and blocked rotor tests on a three-phase induction motor.How are the parameters of equivalent circuit determined from test results?

b) A three-phase delta connected cage type induction motor when connected directly to 400 V,

50 Hz supply takes a starting current of 100 A in each stator phase. Calculate

i) The line current for direct-on-line starting

ii) Line and phase currents for a 70 % tapping on auto-transformer starting

8. a) Explain the method of speed control of three-phase induction motor by varying the supplyfrequency.

b) Two 50 Hz, three-phase induction motor having six and four poles respectively are

cumulatively cascaded, the six-pole motor being connected to the main supply. Determine

the frequencies of the rotor currents and the slips referred to each stator field if the set has a

slip of three percent.

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1. a) Explain briefly the action of a transformer and show that the voltage ratio of the primary andsecondary windings is the same as their turns ratio.

b) A single-phase transformer has 400 primary and 1000 secondary turns. The net cross-

sectional area of the core is 60 cm2. If the primary winding is connected to a 50 Hz supply

at 500 V, calculate

i) The peak value of the flux density in the core and

ii) The voltage induced in the secondary winding.

2. a) Define power efficiency and all day efficiency of a transformer. Obtain the condition formaximum efficiency of a single-phase transformer.

b) A single-phase transformer working at unity power factor has an efficiency of 90% at both

half load and at the full-load of 500 W. Determine the efficiency at 75% full load and the

maximum efficiency.

3. a) State the reasons for parallel operation of transformers. Assuming that two transformershaving equal ratios of transformation are connected in parallel, indicate how the total load

divides between them.

b) Two similar 40-kVA, 1-phase transformers gave the following results when tested by the

back to back method:

W1in the supply line: 850 W

W2in the secondarys series circuit at rated current: 1050 W.

Calculate the efficiency of each transformer at full load unity p.f and half full load 0.8 p.f

lagging.

4. a) Explain with the help of connection and phasor diagrams, how Scott connections are used toobtain two-phase supply from 3-phase supply mains.

b) What are three winding transformers? Give the equivalent circuit and mention its

applications.

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5. a) Show that a rotating magnetic field is can be produced when a balanced supply is given toinduction motor stator.

b) A 12-pole, 3-phase alternator is coupled to an engine running at 500 rpm. It supplies a

three-phase induction motor having a full-load speed of 1440 rpm. Find the percentage slip,

number of poles of motor, frequency of rotor currents and rotor mmf speed with respect to

stator.

6. a) What is the purpose of using deep-bar cage rotors? Explain the construction and working ofa deep-bar cage motor.

b) The rotor resistance and reactance of a 4-pole, 50 Hz, three-phase sip ring induction motor

are 0.4 and 4 per phase respectively. Calculate the speed at maximum torque and the

maximum torque to starting torque ratio. What value should the resistance per phase have so

that the starting torque is half of maximum torque?

7. a) Why a starter is necessary to start an induction motor? Explain auto-transformer starter indetail.

b) A 400 V, three-phase, star-connected induction motor gave the following test results:

No-load 400 V 8.5 A 1,100 W

Blocked rotor 180 V 45 A 5,700 W

Determine the ohmic values of the components in the circuit model and calculate the line

current and power factor when the motor is operating at 5% slip. The stator resistance per

phase is 1.5 and the standstill leakage reactance of the rotor winding referred to the stator

is equal to that of the stator winding.

8. Write a short notes on the following speed control methods of IMa) Cascade connection

b) Pole changing method

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1. a) Describe the two general types of transformers. Why is the low-voltage winding placed nearthe core?

b) A 100 kVA, 2400/240 V, 50 Hz single phase transformer has no load current of 0.64A and a

core loss of 700 W, when its high voltage side is energized at rated voltage and frequency.

Calculate the two components of no-load current. If this transformer supplies a load current

of 40 A at 0.8 lagging power factor on its l.v. side, determine the primary current and itspower factor. Ignore leakage impedance drop.

2. a) Discuss the effect of variations of supply voltage and frequency on core losses.b) The maximum efficiency of a 100 kVA, single-phase transformer is 98 % and occurs at

80% of full load at 0.8 power factor lagging. If the leakage impedance of the transformer is

5%, find the voltage regulation at full load.

3. a) Explain the principle of operation of an auto-transformer. Derive an expression for saving ofcopper when an auto-transformer is used.

b) Open circuit and short circuit tests on 5 kVA, 220/400 V, 50 Hz, single-phase transformer

gave the following results:

O.C. test 220 V, 2 A, 100 W (l.v.side)

S.C. test 40 V, 11.4 A, 200 W (h.v.side)

Determine the efficiency and approximate regulation of the transformer at full load 0.9

power factor lagging.

4. a) Explain inrush current phenomena in a transformer?b) Explain how the parameters of a three winding transformer can be determined

experimentally.

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5. a) Discuss the production of starting torque, through the concept of interaction of flux and mmfwaves in a 3-phase slip-ring induction motor. Hence show that the rotor is forced to rotate in

the direction of rotating flux wave.

b) In a 6-pole, three-phase, 50 Hz induction motor with star-connected rotor, the rotor

resistance is 0.25 /phase, the reactance at standstill is 1.4/phase, and an emf between the

slip-rings on open-circuit is 160 V. Calculate i) slip at a speed of 940 rpm ii) rotor emf

per phase and iii) rotor frequency and reactance at a speed of 900 rpm.

6. a) Derive the relationship between the torques developed by outer and inner cages of a doublecage induction motor.

b) A three-phase induction motor has a 4-pole, star connected stator winding and runs on 50 Hz

with 400 V between lines. The rotor resistance and standstill reactance per phase are 0.4

and 3.6 respectively. The effective ratio of rotor to stator turns is 0.67. Calculate i) the

gross torque at 4% slip, ii) the gross mechanical power at 4% slip, iii) maximum torque,

iv) speed at maximum torque and v) maximum mechanical power (gross). Neglect stator

impedance.

7. a) Explain the operation of star-delta starter used for squirrel cage induction motor. Whichmethod is preferred for starting wound rotor induction motor?

b) The following test results were obtained on a 7.5 kW, 400 V, 4-pole, 50 Hz, delta-connected

induction motor with a stator resistance of 2.1 /phase:

No-load 400 V 5.5 A 410 W

Blocked rotor 140 V 20 A 1,550 W

Obtain the approximate equivalent circuit model. Estimate the braking torque developedwhen the motor, running with a slip of 0.05 has two of its supply terminals suddenly

interchanged.

8. Write a short notes ona) Slip energy recovery

b) Induction generator

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