Example 37.30. A 3.5-MVA, Y-connected alternator rated at 4160 volts at 50-Hz has the open- ircuit characteristic given by the following data : Field Current (Amps) 50 100 150 200 250 300 350 400 450 E.M.F. (Volts) A field current of 200 A is found necessary to circulate full-load current on short-circuit of the alternator. Calculate by (i) synchronous impedance method and (ii) ampere-turn method the full- oad voltage regulation at 0.8 p.f. lagging. Neglect resistance. Comment on the results obtained. 1620 3150 4160 4750 5130 5370 5550 5650 5750 (Electrical Machines-II, Indore Univ. 1984)

Example 37.30. A 3.5-MVA, Y-connected alternator rated at 4160 volts at 50-Hz has the open- ircuit characteristic given by the following data : Field Current (Amps) 50 100 150 200 250 300 350 400 450 E.M.F. (Volts) A field current of 200 A is found necessary to circulate full-load current on short-circuit of the alternator. Calculate by (i) synchronous impedance method and (ii) ampere-turn method the full- oad voltage regulation at 0.8 p.f. lagging. Neglect resistance. Comment on the results obtained. 1620 3150 4160 4750 5130 5370 5550 5650 5750 (Electrical Machines-II, Indore Univ. 1984)

Introductory Circuit Analysis (13th Edition)

13th Edition

ISBN:9780133923605

Author:Robert L. Boylestad

Publisher:Robert L. Boylestad

Chapter1: Introduction

Section: Chapter Questions

Problem 1P: Visit your local library (at school or home) and describe the extent to which it provides literature...

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b) Explain the conditions of parallel connection of alternators and how these conditions are fulfilled.
A three-phase 3300V. 50 Hz star-connected alternator has an effective resistance of 0.5 ohm/phase. A field current to of 30A produces full-load current of 180A on short- circit and a line to line emf of 1000V on open circit Determine(i) the power angle of the alternator when it delivers full-load at 0.8p.f(lag) (ii)the SCR of the alternator D
Eo(LL) (V) a) 3000 b) 2500 2000 1500 1000 500 0 No-load excitation curve @3600rpm 0 1 2 3 4 5 6 DC Excitation current (A) 7 The figure above shows the no-load excitation (LL) curve of a 3-ph, 2-pole, 60 Hz, 1000 kVA, 2.3 kV synchronous alternator at rated speed. The alternator has negligible resistance and Synchronous Reactance Xs = 1.2 n/phase. If the alternator is synchronized to the Power Grid at 2.3 kV (Line to Line), calculate the following: 8 9 10 The value of excitation current required for full load current at 0.7 pf lagging (over-excited). |EO(LL)I IF The alternator current and reactive power when it is delivering 80% rated power (0.8 MW) and its DC excitation current is 4 A. IL Q3ph
4. A 2 MVA, 2.3kV, three-phase Wye connected alternator operates at rated kVA at a power factor of 80 %. The dc armature winding resistance between terminals is 0.08-ohm. The field takes 70-A at 125-V from the exciter equipment. Friction and windage loss is 20-kW, iron loss 36-kW and stray power loss are 2.0-kW. Calculate the efficiency of the alternator. Assume the effective armature winding resistance is 120% of the dc resistance.
A 2MVA, 2.3kV three – phase Y – connected alternator operates at rated kVA at a power factor of 80%. The dc armature winding resistance between terminals is 0.08 ohm. The field takes 70 A at 125 V from an exciter equipment. Friction and windage loss is 20kW, iron losses 36kW and stray load losses are 2.0 kW. Calculate the efficiency of the alternator. Assume the effective armature winding resistance is 120% of the DC resistance. a. 94.0 % b. 92.1 % c. 93.2 % d. 89.5 %
A three phase star connected alternator gives 575 V on open circuit when the field current is 10 A. For same field current, a full load current of 75 A is observed on short circuit. The armature resistance between terminals is 2.16 ohms. On 0.8 leading power factor of full load condition, induced emf per phase is 6000 V. Find its rated terminal voltage between the lines. Also calculate kVA rating of the alternator.
A three phase 6000 V alternator has the following O.C.C. at normal speed: Field current : 14 18 23 30 43 Terminal voltage: 4000 5000 6000 7000 8000 With armature short circuit and fall load current flowing the field current is 17 A And when the machine is suppling fall-load of 2000KVA at zero-power factor the field current is 42.5 A, and the terminal voltage is 6000 V. Determine the field current that required when the machine is supplying the fall-load current at 0.7 p.f. lagging.
Two identical 2000 kVA alternators operate in parallel. The governor of the first machine is such that the frequency drops uniformly from 50 Hz at no-load to 48 Hz at full-load. The corresponding uniform speed drop of the second machine is 50 Hz to 47.5 Hz. What is the maximum unity pf load that can be delivered without overloading either machine
A three-phase, 4-pole, 1,500 rpm alternating having a double layer-windings with 18 turns per coil. The slots per pole per phase are 10 and the coil span is 22 slots. The spread phase is 60 degrees and the flux per pole is 0.0625 Weber. If the armature windings are wye connected. a) Determine the generated voltage per phase and the line to line voltages. b) If the alternator is delivering a three - phase load of 1,200KW at 0.8 lagging power factor what is the line current of the alternator
Q2: A 5-MVA, 6.6 kV, 3-p, star- connected alternator has a resistance of 0.075 Q per phase. Estimate the regulation for a load of 500 A at p.f. (a) unity and (b) 0.9 leading (c) 0.71 lagging from the following open-circuit and full-load zero power factor curve. Open-circuit terminal voltage (V) Field current (A) Saturation curve zero p.f. 32 3100 50 4900 1850 75 6600 4250 100 7500 5800 140 8300 7000
b) Two alternators are connected in parallel to feed a load together. The no-load frequency of the first alternator is 51.5 Hz and its slope is 1MW / Hz. The no-load frequency of the second alternator is 51.0 Hz and its slope is 1MW / Hz. The total power of the load fed by these two alternators is 2.5MW and the power factor is 0.8. Accordingly, at what frequency does this system work and what are the power values transferred to the load by each alternator.
Determine the efficiency of a 1500 kVA 2300V, three phase, Y-connected alternator which operate at rated output with a power factor of 80%. The dc armature resistance at 70 degrees Celsius between terminals is 0.08 ohms. The field takes 70A at 120V from the exciter equipment. Friction & Windage losses are 15kW, iron loss is 35kW and stray load losses is 1.5kW. Assume the effective armature resistance is 1.5 times the DC value.