r05310204 electrical machines iii

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Code No: R05310204 Set No. 1

III B.Tech I Semester Regular Examinations, November 2007ELECTRICAL MACHINES-III

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

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1. (a) What is short pitch winding & distributed winding? Why the armature wind-ing is distributed & short pitch type?

(b) Calculate the EMF of a 4 pole, 3-Φ, star connected alternator running at 1500

r.p.m from the following data: Flux per pole = 0.1 wb, Total number of slots= 48, Conductors per slot (in two layers) = 4, coil span = 150o. [8+8]

2. (a) Explain the load characteristics of an alternator.

(b) The phase EMF of a 3-Φ alternator consist of fundamental, 20 % 3rd harmonic& 10 % fifth harmonic. The amplitude of fundamental is 1000 V. Calculatethe RMS value of line & phase voltage, when the alternator is connected in

i. Star

ii. Delta. [8+8]

3. (a) Explain the voltage regulation method of an alternator by which the armaturereaction & leakage reactance can be separated.

(b) A 3-Φ, star connected salient pole synchronous generator is driven at a speednear synchronous with the field circuit open and the stator is supplied froma balanced 3-Φ supply. Voltmeter connected across the line gave minimumand maximum readings of 1196 V & 1217 Volts. The line current fluctuatedbetween 120 & 225 Amp. Find the direct and quadrature axis reactances perphase. Neglect armature resistances. [8+8]

4. (a) Explain the ‘two bright one dark’ & ‘all dark’ method of synchronization of alternators.

(b) The EMFs of two alternators are 3000200 &2900 00 V. Their synchronousimpedances are 2 + j20 Ω/ph & 2.5 + j30 Ω/ph. The load impedance is 10 + j4 Ω/ph. Find the circulating current. [8+8]

5. (a) Explain the construction & working principle of Synchronous motor.

(b) A 3- Φ, 400 V star connected SM has effective per phase armature resistance& synchronous reactance of 0.2 Ω & 2 Ω respectively. It takes 20 A to delivera certain load. Calculate the excitation EMF induced in the motor if it works

withi. 0.8 pf lagging

ii. 0.8 pf leading

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iii. UPF. [8+8]

6. (a) Explain the construction of ‘excitationcircle’ for a synchronous motor.

(b) Calculate the synchronous coefficient (in kW & Nm per mechanical degree) atfull load for a 1 MVA, 0.8 pf lagging, 6.6 kV, 8 pole, star connected generatorof negligible resistance & synchronous reactance of 0.8 pu. [8+8]

7. (a) Explain the operation of ‘split phase single phase Induction motor’ withvector diagram.

(b) Explain how the direction of a single phase Induction motor cab be reversed.[8+8]

8. (a) Compare AC series motor & Universal motor.(b) Compare variable reluctance stepper motor & permanent magnet stepper mo-

tor. [8+8]

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1. (a) Explain the various winding factors? Explain the effects of each of them.

(b) Determine the slot distribution and the pole phase group sequence for a 45slot, 6 pole, 3-Φ winding. [6+10]

2. (a) Explain the effect of armature reaction on the EMF induced. Is it possible toobtain load voltage more than EMF induced? if yes, how?

(b) A 200 kVA, 480 V, 50 Hz, star connected synchronous generator with a ratedfield current of 5A was tested and the following data were obtained:

OC test: 540 V between lines on open circuit.SC test: 300 A.

When a DC voltage of 10 V was applied to two of its terminals, a current of 25 A was measured, find the value of synchronous impedance, synchronousreactance voltage regulation of 0.6 p.f leading. [8+8]

3. (a) How the MMF method is different from EMF method in finding voltage reg-ulation of an alternator? Explain the drawbacks of each method.

(b) A 1 MVA, 11 kV, 3-Φ, star connected synchronous machine has following OCCtest data: [6+10]

I f 50 110 140 180E OL-kV 7 12.5 13.75 15

(where EOL is the line voltage at no load)The short circuit test yielded full load current at a field current of 60 A. TheZPF yielded a full load current at terminal voltage for a field current of 150

A. The armature resistance is negligible. Calculate the voltage regulation atfull load 0.866 pf lagging by Potier triangle method.



5. A 20 pole, 30 kW, 660 V, 50 Hz, star connected synchronous motor is operatingwith it’s per phase generated voltage exactly equal to the phase voltage applied

to armature. At loaded condition the motor is retarded by 5o mechanical from itssynchronous position. Per phase synchronous reactance & the effective armatureresistance are 10 Ω & 1 Ω respectively. Calculate:

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(a) Armature current

(b) The total power drawn by the motor from bus.(c) The developed power. [16]

6. (a) Explain the construction of damper winding. Clearly show the location of damper winding.

(b) With neat diagram & explanation, show how damper winding prevents oscil-lations. [8+8]

7. (a) Why single phase motors are not self starting?

(b) Explain the necessary arrangements made to make single phase Induction

motor self starting & with neat diagram explain the operations of same. [8+8]

8. (a) Give the applications of various types of permanent magnet motors.

(b) Explain the working principle of permanent magnet motors. [8+8]

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1. (a) Obtain the expression for the RMS value of EMF induced in an alternator.

(b) An alternator is operating at no load has an induced EMF of 346.4 V/ph anda frequency of 60 Hz. If the pole flux is deceased by 15 % & the speed is

increased by 6.8 %; determine [8+8]i. The induced EMF

ii. Frequency.

2. (a) A 16 pole, 3-Φ star connected alternator has 144 slots. The coils are shortpitched by one slot. The flux per pole is Φ = 100 sin θ + 30 sin 3θ + 20 sin5θ. Find the harmonics as percentage of phase voltage & line voltage.

(b) Define

i. synchronous reactance

ii. Synchronous impedance and

iii. leakage reactance in an alternator. [10+6]

3. (a) With proper explanation & diagram, Justify the statement ‘M MF method

for finding voltage regulation is optimistic and EMF method for finding

voltage regulation is pessimistic’.

(b) The no load excitation of an alternator required to give rated voltage is 160 A.In a short circuit test with full current flowing in the armature, the field exci-tation was 135 A. Determine the approximate excitation that will be requiredto give full load current at 0.8 PF lagging at the rated terminal voltage. [8+8]



5. (a) Explain, why synchronous motor is not self starting?

(b) A 3-Φ, 600 V, star connected SM has effective per phase armature resistance& synchronous reactance of 0.4 Ω & 3.6 Ω respectively. Calculate the inducedEMF per phase if the motor works on full load delivering 326 kW. The full

load efficiency is 87 % having power factor of 0.8 leading. Also calculate theload angle. [6+10]

6. (a) Explain the various starting methods of synchronous motor.

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(b) Explain the characteristics of synchronous induction motor. [8+8]

7. (a) Explain, the speed of single phase Induction motor can be controlled bysupply voltage where as them is not possible with 3− Φ IM , why?

(b) The name plate of single phase IM, 4 pole Induction motor gives the followingdata:

Output = 410 W, Supply voltage = 230 V, Frequency = 50 Hz,Input current = 3.2 A, Power factor = 0.7, speed = 1410 RPM.

Calculate:

i. The efficiency of the motor

ii. The slip of the motor when delivering rated output. [8+8]

8. (a) Compare AC series motor & Universal motor.

(b) Compare variable reluctance stepper motor & permanent magnet stepper mo-tor. [8+8]

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1. (a) Explain the various winding factors? Explain the effects of each of them.

(b) Determine the slot distribution and the pole phase group sequence for a 45slot, 6 pole, 3-Φ winding. [6+10]

2. (a) Justify the statement ‘The terminal voltage of an alternator is not only de-pends on the load current, but also on the nature of load’.

(b) A 3-Φ, star connected alternator is rated at 1600 kVA, 13500 V. The arma-ture effective resistance & reactance is 1.5 Ω/ph. & 30 Ω/ph. respectively.Calculate percentage regulation for a load of 1280 kW at power factors of

i. 0.8 lagging

ii. 0.8 leading. [6+10]

3. A 3-Φ, 200 kVA, 1.1 kV, 50 Hz star connected alternator having an effective per

phase resistance of 0.62 Ω gave the following results: [16]

Field Current (A) 20 35 50 80 100 120OC Voltage V L 692.82 1120 1450 1750 1953 2180SC Current (A) 0 22 44 66 88 110

Using MMF method, find the voltage regulation at 100 A

(a) 0.8 pf lagging

(b) 0.8 pf leading.

4. (a) Explain all the necessary conditions for successful parallel operation of alter-nators.

(b) A 2 MVA, 3-Φ, star connected, 4 pole, 750 RPM alternator is operating on6000 V bus bars, ×s is 6 Ω/ph. Find synchronizing power and torque for fullload 0.8 power factor lagging. [8+8]

5. (a) A sub-station operating at full load of 1200 kVA supplies a load at 0.7 powerfactor lagging. Calculate the permissible additional load at this power factorand the rating of synchronous condenser to raise the substation power to 0.9lagging.

(b) Derive the expression for the maximum power developed by a synchronousmotor. [8+8]

6. (a) Explain the various starting methods of synchronous motor.

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(b) Explain the characteristics of synchronous induction motor. [8+8]

7. (a) Why single phase motors are not self starting?

(b) Explain the necessary arrangements made to make single phase Inductionmotor self starting & with neat diagram explain the operations of same. [8+8]

8. (a) Explain the static characteristics of stepper motor.

(b) Explain the role of compound winding in the operation of AC series motor.[8+8]

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r05310204 electrical machines iii

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