VII. Supplementary reading.

Text 8

A.C. Generators principles

Of operation

Generation of E.M.F.—The fundamental principle on which all a.c. generators depend is that an e.m.f. is induced when a conductor is made to cut across a magnetic flux. In order to obtain an e.m.f. of suitable magnitude, many conductors in series arid a strong magnetic flux are employed, while the rate of cutting is made as high as prac­ticable.

This fundamental principle is illustrated, where a single turn is shown rotating in a constant magnetic field excited from a source of D.C, As the turn is made to rotate at a constant speed, the two active conductors cut the magnetic flux at a rate that gradually increases to a maximum when the turn is horizontal. The vertical portions of the turn in the position shown in the diagram are inactive from the point of view of the inducing of e.m.f., and serve merely as end-connectors for the two active conductors which lie parallel to the axis of rotation.

During the first 90° of rotation the rate of cutting flux is gradually increasing while during the second 90° it is gradually decreasing, owing to the alteration in the angle at which the con­ductors cut the flux. During the second half of the revolution the conductors cut the flux in the reverse direction, and during this period the reverse half of the e.m.f. wave is induced. One complete cycle of e,m.f. is induced in one complete revolution.

If a second turn were rigidly attached to the first, but at right angles to it, an e.m.f. of the same r.m.s. value would be induced in this second turn, but it would be 90° out of phase with the e.m.f. induced in the first turn. The arrangement would constitute a two-phase generator.

In practice three-phase generators are the most common, and a three-phase supply would be obtained by mounting three turns on the same shaft, these three turns being rigidly fixed at 120° to one another.

Speed and Frequency.—A d.c. generator can be designed to run at, any speed within the range of the prime mover driving it, but in a.c. work only certain speeds are permissible, a.c. generators feeding a publiс supply system are required to generate at a definite fixed frequency, and this limits the speed to certain fixed values depending upon the number of poles on the a.c. generator.

The induced e.m.f. goes through one complete cycle every time the rotor is moved through a distance equal to a double-pole pitch; one half-wave of e.m.f. is induced when a conductor passes a north pole, while the reverse half-wave is induced when the conductor passes the south pole which follows. If there are 2p poles, therefore, each con­ductor induces p cycles of e.m.f. in one revolution. If the speed is n.r.p.m., the total number of cycles induced per minute is thus pn and the frequency of the induced e.m.f., is

P n/60 cycles per second or f= pn/60

where f is the frequency in cycles per second.

In order to generate at a definite frequency, therefore, the speed must be equal to

N = 60f/p

Considering the standard frequency of 50 c/s the only possible speeds are given by

N = 60X50/p = 3000/p

For each number of poles there is one speed, and one speed only that will .enable the machine to generate at the desired frequency.

Rotating Field.- In d.c. generators the armature winding is mount­ed on the rotating member which rotates between the poles of a fixed magnet system, but with a.c. generators, or alternators, the stand­ard practice is to place the armature winding upon the stationary element, now called the stator, while the field system is mounted on the rotating element, now called the rotor. This method of construction has many advantages from the designer's point of view, and is always adopted except in the case of a few special purpose machines.

The d. c. exciting current has to be led into the rotor, this being done by means of two insulated slip rings on which press a. number of carbon brushes (на які нажимають вугільні щітки). This d.c. exciting current is usually provided by a special d.c. generator called an exciter, this being directly driven from the shaft of the main a.c. generator. In certain cases where a d.c. supply is available, this exciter is omitted, but in power station work each a.c. generator is provided with its own d.c. exciter.

Engine-driven and Turbo-Generators.— The number of poles and the speed of an a.c. generator are largely determined by the characteristics of the prime mover which is to drive it. If it is to be engine-driven the speed will necessarily be less than 1000 r.p.m. (except for very small sets), and so the a.c. generator must have eight or more poles, assuming the frequency to be 50 c/s. In such cases, speeds of 750, 600, and 500 r.p.m. are available.

If, on the other hand, a steam turbine drive is to be adopted, high­er speeds are essential for the economic design of the turbine, and the only available speeds are 1500 and 3000 r.p.m. These correspond to four and two poles respectively.

Thus it follows that turbo-generators are all either two-or four-pole machines, while engine-driven generators have more poles.

Salient Pole Rotors.—The rotors of engine-driven generators are built in the form of a flywheel, having the various poles mounted on the rim of the flywheel, pointing outwards. This rim acts as the yoke pi the field system. The exciting coils are mounted on the pole& themselves which are provided with shaped pole shoes.

Cylindrical Rotors.—In view of the large centrifugal forces set up at high speeds, the salient pole type of construction is not suitable for turbo-generators, and to combat these forces the cylindrical rotor type of construction has been developed. As its name implies, the rotor is now made in the form of a cylinder, on the outer surface of which a number of slots are cut. These slots receive the exciting wind­ings, the conductors being kept in place by strong wedges inserted in the mouth of each slot. A portion of the cylindrical surface is left unslotted, this portion corresponding to the pole in the salient pole arrangement.