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ELECTRICAL ENGINEERING DEPARTMENT

B.E. 6th Sem. (Electrical Machines-III_160901)

Assignment-[ Chapter: 02 ]

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Synchronous Machines: Introduction, generated emf, Harmonics in voltage waveforms, MMF of distributed windings, rotating magnetic field, Torque, Operations, Machine efficiency, Armature reaction and it’s compensation, Short circuit ratio, Effect of increase in excitation, Effect of change in torque and speed, Determination of Synchronous reactance, regulation by ZPF method, AIEE methods, Synchronizing & load shadding between two machines Operating characteristics, Load angle and Power flow equations, Capability curves, Two reaction model of Salient pole machines, Parallel operations, Load sharing between generators, Effect of unequal voltages & percentage impedance, Governor characteristics, Hunting, Short circuit transients, single phase generators, Slip test for measurement of Xd and Xq, Sudden short circuit of Synchronous machine. Methods of starting of synchronous motors, Different torques in Synchronous motor, Stability, Synchronous condenser, Synchronous phase modifiers, V-curves and O-curves of Synchronous motors., hunting of sy. machines and its prevention. …………………………………………………………………………………………………..

(01) Derive emf equation of alternator. Explain distribution factor with effect of

harmonics.

(02) What are the advantages of connecting alternators in parallel? Explain with diagram

three dark lamp method of synchronizing two three phase alternators.

(03) Explain coil span factor and distribution factor of an alternator.

(04) Briefly explain the ZPF method of finding the voltage regulation of Alternator.

(05) What is armature reaction? Explain the effect of armature reaction on the terminal

voltage of an alternator.

(06) What are the causes of harmonics in the voltage waveform of an alternator? How can

these be minimized?

(07) Explain an experimental method of determining ‘V’ curves for a synchronous motor.

(08) Explain the two reaction theory of salient pole synchronous machine.

(09) Discuss different torques in synchronous motor.

(10) Draw the combined Space and Time Phasor Diagram of Cylindrical rotor alternators

for the following conditions (i) lagging p.f. (ii) leading p.f. and (iii) unity p.f.

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(11) Derive the expression for the input and output power developed by synchronous

motor. Also derive the maximum input and output power developed by synchronous

motor.

(12) Explain V and inverted V curve of synchronous motor.

(13) List different methods for finding voltage regulation of an alternator and explain

Potier method.

(14) Explain the two reaction theory of salient pole machine in detail with phasor diagram.

(15) Why synchronous motor is not self starting? Explain the methods of starting of

synchronous motor.

(16) With necessary diagram explain the effect of leading power factor on alternator.

(17) Explain the slip test for measurement of Xd and Xq of synchronous machines.

(18) Explain hunting of synchronous machines and methods of its prevention.

(19) Explain the effect of varying excitation at constant load on synchronous motor.

(20) Explain an experimental method of determining ‘V’ curves for a synchronous motor.

(21) Explain concept of pitch factor and distribution factor in case of an alternator. How

these values change with harmonics. Also explain how e.m.f. in alternator can be

evaluated when it contains non sinusoidal variation of fluxes.

(22) A 3-phase, 8-pole, 750 rpm star-connected alternator has 72 slots on the armature.

Each slot has 12 conductors and winding is short chorded by 2 slots. Find the induced

emf between lines, given the flux per pole is 0.06Wb.

(23) A 3.5 MVA, 3-phase synchronous generator rated at 6.6 KV has 32 poles. Its direct

and quadrature axis synchronous reactances as measured by the slip test are 9.6 Ω and

6 Ω respectively. Neglecting armature resistance, determine the regulation and

excitation emf needed to maintain 6.6 KV at the terminals when supplying a load of

2.5 MW at 0.8 p.f. lagging. What maximum power can generator supply at the rated

terminal voltage, if the field becomes open circuited?

(24) A star connected, 11 KV alternator, with synchronous impedance of 1+j10 per phase

is connected to infinite bus at rated voltage. The alternator delivers an armature

current of 100 A at unity power factor to the bus bar. With the alternator output

remaining constant, the alternator excitation is increased by 15%. Find the new values

of armature current, load angle and p.f.

(25) A 2,000 KVA, 3-phase, 8-pole alternator runs at 750 rpm in parallel with other

machines on 6,000 V bus-bars. Find synchronising power on full load 0.8 p.f. lagging

per mechanical degree of displacement and the corresponding synchronising torque.

The synchronous reactance is 6 ohm per phase.

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(26) A 4 KVA, 3 phase, 110V, 50Hz, star connected alternator has Xd = 3 ohm and Xq = 2

ohm. The machine is delivering full load current of 0.8 p.f lagging at rated voltage.

Find the induced emf, load angle and maximum power output of the alternator.

(27) A 3 phase star connected 1000KVA, 11000V alternator has rated current of 52.5 A.

The ac resistance of the winding per phase is 0.45 ohm. The test results are given

below:

O.C. Test: field current = 12.5 A, voltage between lines = 422 V

S.C. Test : field current = 12.5A, line current = 52.5A

Determine the full load voltage regulation of the alternator for (i) 0.8 p.f lagging and

(ii) 0.8 p.f leading loads with synchronous impedance method.

(28) A 3 phase, 6 pole, 2.3 KV, 200KVA, star connected synchronous motor has

synchronous reactance of 12 ohm per phase and negligible resistance. The motor is

initially operating at a load of 120KW with the field current adjusted such that the

armature current is minimum. The field current is now increased such that the

armature current is increased by 50%. With this field current, the load is reduced to

60KW. Calculate the new values of armature current and power factor.

(29) A 60- KVA, 220 V, 50 Hz, single phase alternator has effective armature resistance of

0.016 Ω and an armature leakage reactance of 0.07 Ω. Compute the voltage induced in

the armature when the alternator is delivering rated current at a load p.f. of (a) unity

(b) 0.7 lagging (c) 0.7 leading.

(30) A 4 KVA, 3 phase, 110V, 50Hz, star connected alternator has Xd = 3 ohm and Xq = 2

ohm. The machine is delivering full load current of 0.8 p.f lagging at rated voltage.

Find the induced emf, load angle and maximum power output of the alternator.

(31) In a 50 KVA , star connected, 440 V, 3 phase, 50 Hz alternator, the effective armature

resistance is 0.25 ohm per phase. The synchronous reactance is 3.2 ohm per phase and

leakage reactance is 0.5 ohm per phase. Determine at rated load and unity power

factor. (a) Internal e.m.f. Ea, (b) no-load e.m.f. Eo . (c ) percentage regulation on full

– load (d) value of synchronous reactance which replaces armature reaction.

(32) A 3 phase star connected 1000KVA, 11000V alternator has rated current of 52.5 A.

The ac resistance of the winding per phase is 0.45 ohm. The test results are given

below:

O.C. Test: field current = 12.5 A, voltage between lines = 422 V

S.C. Test : field current = 12.5A, line current = 52.5A

Determine the full load voltage regulation of the alternator for (i) 0.8 p.f lagging and

(ii) 0.8 p.f leading loads with synchronous impedance method.