Alternating Current Generators.

Q1. Magnetic induction occurs when there is relative motion between what two elements?

A1. A conductor and a magnetic field.

Q10. In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?

A10. In series.

Q11. What determines the phase relationship between the voltages in a two-phase AC generator?

A11. Placement of armature coils.

Q12. How many voltage outputs are available from a two-phase three-wire alternator?

A12. Three.

Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?

A13. C is 1.414 times greater than A or B.

Q14. In a three-phase alternator, what is the phase relationship between the individual output voltages?

A14. Each phase is displaced 120º from the other two.

Q15. What are the two methods of connecting the outputs from a three-phase alternator to the load?

A15. Wye and Delta.

Q16. Ships' generators produce 450-volt, three-phase, AC power; however, most equipment uses 117- volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?

A16. Three single-phase, delta-delta, step-down transformers.

Q17. What two factors determine the frequency of the output voltage of an alternator?

A17. Speed of rotation and number of poles.

Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

A18. 120 Hz.

Q19. The variation in output voltage as the load changes is referred to as what? How is it expressed?

A19. Voltage regulation. As a percentage.

Q2. What is the part of an alternator in which the output voltage is generated?

A2. Armature.

Q20. How is output voltage controlled in practical alternators?

A20. By varying the voltage applied to the field windings.

Q21. What generator characteristics must be considered when alternators are synchronized for parallel operation?

A21. Output voltage, frequency, and phase relationships.

Q3. What are the two basic types of alternators?

A3. Rotating armature and rotating field.

Q4. What is the main advantage of the rotating field alternator?

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

Q5. Most large alternators have a small dc generator built into them. What is its purpose?

A5. To provide dc current for the rotating field.

Q6. How are alternators usually rated?

A6. Kilovolt-amperes (volt-amperes).

Q7. What type of prime mover requires a specially designed high-speed alternator?

A7. Steam turbine.

Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?

A8. Internal combustion engines, water force and electric motors.

Q9. What does the term single phase indicate?

A9. One voltage (one output).