Vocabulary.
Connected directly across the armature – соединено напрямую через якорь
Steady-state condition – установившийся режим
The effect is negligible – эффектом можно пренебречь
Wide range – широкий диапазон
Field resistance – сопротивление обмотки возбуждения
Coincident – совпадающий
Permissible value – допустимое значение
Buildup – возрастание
Versus – против, в оппозиции к
self-inductance – самоиндукция
direct-axis – продольная ось
Answer the questions.
1. What are three characteristics that specify the steady-state performance of a dc generator?
2. Describe the open-circuit characteristic.
3. Describe the open characteristic.
4. Describe the load characteristic.
5. What do the other characteristics depend on?
6. What is the terminal voltage related to?
7. What is voltage buildup?
8. What may be reasons for failure of voltage buildup?
9. Give the definition of cumulatively compound generators.
10. Give the definition of flat-compounded generators.
11. Give the definition of under-compounded generators.
12. Give the definition of differentially compounded generators.
What do we call:
1. The shunt-field circuit is connected directly across the armature, without including the series field.
2. The no-load magnetization curve, which gives the relationship between the generated emf and field current at constant speed.
3. It gives the relationship between the terminal voltage and load current at constant speed.
4. It gives the relationship between the terminal voltage and field current, with constant armature current and speed.
5. Vt = Ea – IaRa
6. It is also used sometimes as a tachogenerator when a signal proportional to motor speed is required for control or display purposes.
7. In this type of generator the series and field winding mmfs are adding.
8. In this type of generator the series winding mmf opposes that of the shunt –field winding.
Read and the following text.
In a compound generator, both series and shunt excitation are combined. The shunt winding can be connected either across the armature only (short-shunt connection) or across armature plus series field (long-shunt connection). The compound generator can be cumulatively compounded or differentially compounded generator. The latter is rarely used in practice. Therefore, we shall discuss the characteristics of cumulatively compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical.
External characteristic
The series excitation aids the shunt excitation. The degree of compounding depends upon the increase in series excitation with the increase in load current.
(i) If series winding turns are so adjusted that with the increase in load current the terminal voltage increases, it is called over-compounded generator. In such a case, as the load current increases, the series field m.m.f. increases and tends to increase the flux and hence the generated voltage. The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases.
(ii) If series winding turns are so adjusted that with the increase in load current, the terminal voltage substantially remains constant, it is called flat-compounded generator. The series winding of such a machine has lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage.
(iii) If series field winding has lesser number of turns than for a flat compounded machine, the terminal voltage falls with increase in load current. Such a machine is called under-compounded generator.
