Lesson 7-8.
1. Find the equivalents of following word combinations.
Field construction с гидроприводом
Salient (definite) pole осевая длина
Design consideration выступающий, явный полюс
Hydraulically driven конструкция обмотки возбуждения
Axial length рассеяние тепла
Compatible с учетом конструкции
Speed consideration взаимозаменяемый
Heat dissipation с учетом скорости
2. Complete the sentences with words given in previous exercise and translate them into Russian.
1. There are two types of _____ used on alternators: salient or round-rotor.
2. The _____construction is required when there are many poles on the rotor.
3. One of the first _____ is to account for the type of prime mover.
4. The _____ alternator must have many poles.
5. The length of the individual conductors lying in the magnetic field can be small, since each one need not generate a high voltage, and hence a slow-speed machine requires a small _____.
6. A design is developed that is _____ with the space limitations, speed considerations, and heat _____ for both electrical machines.
3. Give adequate Russian equivalents of the italicized words.
Subject - общий элемент смысла: подверженность внешнему воздействию. Частотные русские эквиваленты: subject to (Adj.) - подчиненный, зависимый; подверженный; подлежащий и т.п.; subject (N) - предмет, тема; предмет изучения; подданный; подлежащее и т.д.; to subject - подчинять, подвергать и т.д.; subject to (Adv.) - из-за, ввиду, при условии, в зависимости от и т.д.
1. Everything and everybody are subject to the laws of nature. 2.The principles are subject to change or elimination as new scientific facts are added to our knowledge. 3. The process is subject to fluctuations. 4. The plan is sub- subject to governmental approval. 5. This is highly complicated and interesting subject. 6. The subject of my thesis is occupational diseases as a function of industry geography. 7. This decision is not a subject for discussion. 8. The
subject-matter of the book is plasma. 9. The library has an excellent subject catalogue. 10. As a rule the subject opens the sentence. 11. He studied four subjects in his first year at college. 12. The soldiers happened to be subjects of France. 13. The idea was subjected to severe criticism and rejected. 14. To harden the metal it was subjected to intense heat. 15. The construction of the plant was delayed subject to restriction on deliveries of some types of equipment.
4. Read and translate the text.
Field construction. There are two types of field construction used on alternators: salient, or definite-pole, and round-rotor, or cylindrical-pole. Salient, meaning standing out or projecting, is shown in Fig. 6a the cylindrical rotor is shown in Fig.6b.
Fig. 6a Fig. 6b
The salient-pole construction is required when there are many poles on the rotor and is used for medium- and slow-speed machines. The cylindrical rotor is used almost exclusively for two-pole alternators, although it may occasionally be found on a four-pole rotor and therefore is used for the higher speeds. Salient poles are wound very much the same as are the field poles on a dc generator with the coils wound around the center of the pole. The conductors of a cylindrical-pole winding are placed in slots which are parallel to the rotor shaft. It is the placement of the slots which creates the magnetic poles since there actually are no physical poles to be seen as in the salient-pole construction. Indeed, at first glance, the round-rotor field closely resembles a dc armature without the commutator connections.
What determines the type of field construction to be used for a particular ac generator? Although there are certain advantages of one type over the other, these of themselves are of secondary consideration. Basically, it is geography that determines the type of field construction. The major cost of operating a power plant or even an individual alternator lies in the prime mover. This is true because electric machines have efficiencies which are quite high compared with the efficiencies of steam- or gas-driven units. Thus one of the first design considerations is to account for the type of prime mover. This is essentially a slow-speed machine. Since the frequency had already been predetermined by the locality, there is very little choice left in selecting or calculating the number of poles on the rotor. The hydraulically driven alternator must have many poles. In order to accommodate the large number of poles, a large diameter is necessary, and hence the salient-pole construction is used. The prime mover determines the construction in still another manner. Since a large diameter is required for the many poles of a slow-speed alternator, the armature has a large circumference and can hold many conductors to generate the required voltage. The length of the individual conductors lying in the magnetic field can be small, since each one need not generate a high voltage, and hence a slow-speed machine requires a small axial length.
Steam turbines are most efficient at high speeds, and therefore nearly all turbo alternators are two-pole machines. A two-pole machine obviously does not require a large-diameter rotor. Moreover, because of the increased centrifugal force at the high speed, a large diameter is prohibited. The small diameter necessitates fewer armature conductors, and in order to obtain the required generated voltage, a greater length of conductor must lie within the magnetic field. Thus the axial length is increased to permit the use of longer armature conductors.
Summarizing, we can say that cylindrical rotors will most likely be on alternators located where steam power is readily available. Salient-pole rotors will be found where water power is the prime mover source of energy, Diesel engine, gas engine, and gas turbine prime movers are considered medium-speed machines, and their alternators will also have salient poles. Where alternators are driven by other electrical machines, either ac or dc motors, there are no such restrictions on the rotor construction. A design is developed that is compatible with the space limitations, speed considerations, and heat dissipation for both electrical machines. It may be noted here that the terms high-speed and low-speed rotors are sometimes used synonymously with cylindrical- and salient-pole rotors, respectively.