Unit 4. Dc machines constructing parts. Lessons 12-14.

1. Find the equivalents of following word combinations.

laminated sheet steel точка соединения

low reluctance path пружина

eddy currents держатель

junction point пластинчатая листовая сталь (электротехническая сталь)

lines of flux участок низкого магнитного сопротивления

spring вихревые токи

holder линии магнитного потока

2. Complete the sentences with words given in previous exercise and translate them into Russian.

1. The armature core is the material of _____ surrounding the shaft.

2. Each _____ between coils is connected to a commutator segment.

3. _____ are generally produced by an electromagnet.

4. The brushes are held in contact with the commutator by a combination of rectangular brush _____.

5. The tension of _____ may be adjusted.

6. The laminations are required to reduce _____.

7. The armature serves to provide _____for the flux.

3. Translate into Russian the following word chains with the same root and explain the means of their formation.

Generator – generate – generating – generation – generated – generational – generative;

Purpose – purposeful – purposeless – purposely – purposive – purposiveness – purposivism;

Compose your own word chains of words: lamination, rotate, produce.

4. Give adequate Russian equivalents of the italicized words.

Challenge - общий элемент смысла: нечто стимулирующее активное ответное

действие. Частотные русские эквиваленты: challenge (N) - вызов; проблема, требующая решения; трудность и т.д.; to challenge - бросать вызов; подвергать сомнению и требовать действий и т.д.

1. Environmental pollution is a major challenge to mankind in the mid- 20th century. It can be met only by the joint efforts of all advanced countries, for the problem challenges man in all his activities on a global scale. 2. Designers, builders and those who supply materials for teaching have met the challenge of change by providing new and important equipment, audio-visual devices and learning systems. 3. The years to come promise to be at least as

challenging and revolutionary as the years gone by. 4. It is imperative to strengthen the humanistic and intellectually challenging elements of the high school program. 5. The problem of hydrodynamics is a permanent challenge to mathematicians. 6. In his talk Dr. N. challenged the accuracy of the measurements made by his German colleagues.

5. Read and translate the text.

A dc electrical machine (dynamo) converts electrical energy to mechani­cal energy or mechanical energy to electrical energy. A generator, therefore, is dynamo in as much as it converts the mechanical energy imparted to it in the ro­tation of the coils into electrical energy that is supplied to the load.

The armature.

The moving part of the dc generator is called the armature. The armature consists of a shaft upon which all parts are mounted. The material surrounding the shaft is laminated sheet steel called the armature core. This magnetic mate­rial of sheet steel is necessary to provide a path of low reluctance to the lines of flux from the pole. The laminations are required to reduce the eddy currents due to the changes of flux in the core. There are grooves or slots located axially around the periphery of the core, and the sides of the coils are placed in these slots. Each junction point between coils is connected to a commutator segment. The commutator segments form a ring around the shaft of the armature. Thus the armature serves the following three functions: (1) provides low reluctance path for the flux, (2) holds the coils, (3) produces motion so that the coils can cut the flux.

The field poles.

The lines of flux are generally produced by an electromagnet and not by a permanent magnet. There are, however, small special-purpose generators that have permanent magnets. The core of the pole is built up of laminated steel, and the shoe of the pole is curved to produce a more uniform magnetic field. A coil con­sisting of many turns of fine wire is generally wound around the core, the coil is called the shunt field. Also around the core may be found a few turns of heavy wire. This is called the series field. These coils produce the magneto motive force required to yield the necessary flux cut by the rotating conductors.

The magnetic circuit.

The magnetic circuit of the generator may easily be traced by following the lines of flux as they leave the north pole. From the north pole the flux passes through an air gap to the magnetic material of the armature. The air gap is the space between the stationary field pole and the rotating armature. This air gap is made as small as possible, for the larger the air gap the greater the magnetomotive force required to produce the necessary lines of flux. The length of the air gap is in the neighborhood if 1/16 to 1/4 in., with the tendency toward the smaller distance. From the armature the lines of flux pass through a second air gap to the south pole, then to the frame of the generator, called the yoke, and back again to the north pole.

Brushes and brush rigging

The brushes are rectangular in shape and are made of carbon or graphite with a binder to hold the brush together. Some brushes, for low vo1tage dyna­mos may even have copper mixed with the carbon.

The brushes are held in contact with the commutator by a combination of rectangular brush holders .and springs whose tension may be adjusted. The holder is so located that the brush always makes contact with a commutator segment connected to a coil in which no voltage is induced. Except in small dynamos, the current is taken from the brush by means of a flexible copper wire embed­ded in the brush, called the pigtail. In the smaller dynamo, the spring pressing against the brush connects the brush to the external circuit. The brush holder is mounted on an insulating material so that the holder is electrically isolated from the frame of the dynamo. Brushes at equal potential are connected by internal wires.