SYNCHRONOUS GENERATORS

The armature. The armature of a revolving-field synchronous generator consists of silicon-

steel punchings assembled into an armature packet. The armature has slots which receive the armature winding. The whole structure is held in a frame which may be of cast iron or welded steel plate. The slot insulation,which must be especially thorough on account of the high voltage involved, uses micanite plate or micanite tape.

2. The field structure. There are two general types of field construction: the salient-pole type and the cylindrical or non-salient-pole type. The salient-pole type is used almost entirely

for slow and moderate-speed generators, since this construction is the least expensive and permits ample space for the field ampere-turns.

It is not practicable to employ salient poles in high-speed generators because of the very high peripheral speed (100 to 170 m/sec) and the difficulty of obtaining sufficient mechanical strength. The salient-pole type consists of a cylindrical steel forging whose rim carries the pole cores. The field coils are placed on the pole pieces and connected in series. The ends of the field winding are taken to two rings mounted on the shaft (or hub) of the field structure. The slip-rings carry brushes which are connected to a d-c generator. The direct current required to excite

the field coils is usually supplies by a d-c generator mounted on a common shaft with a field structure. This d-c generator is called the exciter. The power rating of the exciter is ordinarily 0.25 to 1 per cent of the power rating of the synchronous generator. The rated voltage of the exciter can be between 60 and 350 V.

There are also self-excited synchronous generators. The direct current required to excite the field structure is supplied by selenium rectifiers connected across the armature winding.

Initially, the residual magnetism of the revolving field structure induces a small alternating EMF in the armature. The selenium rectifiers turn it into a direct current, which builds up the field of the rotating unit, the voltage of the generator rises.

Synchronous generators may be cooled either by air or by hydrogen. In the former case,

air is forced through the machine by fans mounted on both ends of the field structure or placed in a pit beneath the machine. In the case of open-circuit ventilation, the incoming air is first passed through filters to remove any dust and is discharged into the atmosphere after it has moved round the machine. Where closed –circuit ventilation is employed; the air is repeatedly circulated through the machine. After the ventilating air has been heated by the various parts of the generator, it is cooled by passing it through a fin-tube cooler in which cooling water is circulated. Hydrogen cooling is a good method of generator ventilation. The thermal conducti-vity of hydrogen is 7.4. times that of air; and so more heat is abstracted. Also, hydrogen has a density 1/14 that of air; consequently, windage losses which account for half the total losses of a synchronous generator, are about one-tenth of their value in air. A further advantage of hydrogen as a cooling medium is that it does not oxidize the insulation.

3. Armature reaction. When the armature circuit of a synchronous generator is completed through a load, an electric current traverses the armature winding, inducing a magnetic flux of its own. The armature MMF interacts with the main field to produce a resultant magnetic flux. This effect is called armature reaction. There is armature leakage reactance.

4. Parallel operation of synchronous generators. For two synchronous generators to be connected in parallel, they must: (a) run at exactly the same frequency; (b) run at exactly the same voltage;

(c) have the same phase position; (d) have the same direction of phase. The difference in phase and frequency between two generators can be indicated by a synchroscope, which is a small induction motor. The deflection of its vane indicates the direction of phase rotation.

J. Pair work. Make up some questions to this text and ask your group-mates to answer

them.

K. Read the Text I again and give the Russian variants of the following words.

armature, core, reactance, mmf, hydrogen, generators, exciter, forging, salient-pole type,

cast, slot, ample, engine, mover, rectifier, ventilation, fan, magnetic flux, synchronous,

induction motor, frequency, oxidize, voltage, density, mount on, total losses, windage

losses, field structure, rim, resistance, self-excited, conductivity, steel forging, discharge.

L. Read and translate this text and give the Russian equivalents of the following words and

word combinations: stator winding, annular core, inner periphery, air gap, squirrel-

cage, armature winding, slip-ring, commutator, resistance, brushes, stationary winding,

try to explain their meanings in English.

Date: 2014-12-28; view: 938