Module 21. Unit How is an AC alternator different in construction from a DC generator? Slip rings

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1 Module 21 Unit 5 1. How is an AC alternator different in construction from a DC generator? Slip rings instead of commutator 2. Why is a revolving armature type generator not typically used? Limited to lowcurrent, low-voltage applications 3. What is the purpose of the brushes and slip rings in a revolving field AC alternator? Supply excitation power from an external source 4. Describe the stator in a revolving field AC alternator? Thin steel punchings or laminations securely clamped and held in place in the steel frame of the generator 5. What are the advantages of the revolving field AC alternator over the revolving armature AC generator? High voltage (13,800 V) can be generated because the stationary windings are not subject to vibration and centrifugal forces. High currents can be produced as they are taken from the windings directly (No brushes and slip rings) 6. What are the two types of construction for the revolving field AC generator? Salient pole and cylindrical rotor 7. Where is the salient-pole AC alternator used? Low speed applications (below 1800 RPM) such as diesel and water turbine 8. Why are cylindrical rotors used for high-speed applications? What characteristics make it suitable? Quiet and strong. Cylindrical and small in diameter 9. What are the most common methods for exciting the DC field in an AC alternator? DC generator on the same shaft

2 10. What is the purpose of the field discharge circuit in an AC alternator? Eliminate danger to personal and equipment 11. What are two advantages of brushless exciters on AC alternators? Can be used in explosive or extremely wet conditions and where low maintenance is desirable 12. How is the size of an AC alternator specified on the nameplate? kva 13. What is the formula for frequency of an AC generator? F=PN/ What is the only practical way to control an AC alternator? Field control (rheostat) 15. What is the terminal voltage of an AC alternator with no load connected to the circuit? Same as the generated voltage 16. What two factors affect terminal voltage in an AC alternator? Resistance of the armature conductors and inductive reactance of the armature coils 17. What is percent voltage regulation for an AC alternator? E no load E full losf /E full load x100% 18. How much armature reaction occurs in a motor at unity power factor? Minimal 19. How does a load with a leading power factor affect terminal voltage in an AC alternator? Increases 20. How does a load with a lagging power factor affect terminal voltage in an AC alternator? Decreases 21. How do automatic voltage regulators work to keep terminal voltage constant in an AC alternator? Adjust the alternator field current

3 22. What are the advantages of the series wye connection for an AC alternator? Two different voltages (phase and line), Reduces insulation requirements to phase levels, provides a neutral point for grounding 23. What are the advantages of the parallel wye connection for an AC alternator? Current advantage while maintaining the features of the series wye connection 24. What are the advantages of the series delta connection for an AC alternator? Gains a current advantage. Line current is 1.73 times greater than phase current so smaller coil wire sizes may be used for alternator construction 25. What are the advantages of the parallel delta connection for an AC alternator? Gains another current advantage double over the series delta 26. How do the different connection combinations affect the kva rating of an AC alternator? No effect 27. Why are some AC alternators not designed for both wye and delta connection? Impedance differences in the coils may set up circulating currents in the stator 28. Why are AC alternators often paralleled? Load requirements change at different times of day, power requirements for an area may increase, one machine can be shut down for maintenance without an interruption 29. Why is it necessary to synchronize AC alternators before connecting them to a common line? Severe damage can result if they are not 30. What are two methods for making sure alternators are in step before connecting them to a common line? Three dark method and two bright, one dark method

4 31. What is the main problem with the three dark method of synchronizing alternators? Lamps can remain dark even with a substantial voltage across the terminals 32. Using the two bright, one dark method of synchronizing alternators, what is the indication to close the switch? Lamps 2 & 3 will be equally bright 33. What is indicated when the pointer of a synchroscope stops moving? What is not indicated? Frequency is the same. Voltage may not be the same 34. What is indicated when the pointer of a synchroscope stops in the vertical upright position? Frequencies are equal and voltages are in phase 35. Briefly, what are the steps involved in paralleling two alternators? 1. Energize the prime mover to drive Alternator 1 and adjust the speed for the rated frequency. 2. Use the rheostat to match the voltages for Alternators 1 & Check phase rotation. 4. Check frequency and voltage and make any adjustments to Alternator 1 5. Using the synchroscope adjust the speed of Alt 1 prime mover and close the switch just before reaching the index mark 6. Alternators are now paralleled and Alternator 1 is floating online 36. What does the term floating online mean in relation to AC alternators? Carrying little or no load 37. How is the load adjusted between two AC alternators connected to a common bus? What precaution must be taken? Gradually increase the mechanical power to the one with no load and decrease the mechanical power of the other. Governors must be changed at the same time to keep frequency consistent

5 38. What is the result of increasing the field current on paralleled AC alternators? Circulating currents will start to flow between the two machines 39. What is drooping speed-load characteristics and how does it affect paralleled AC alternators? As the torque load increases the speed decreases 40. What is hunting in relation to paralleled AC alternators? How is it minimized? Pulsating or oscillating effect that causes current to surge back and forth between alternators in parallel. A heavy flywheel or a damping squirrel cage winding will minimize hunting The End