Direct-current (dc) generators Theory of Operation

A generator is a machine that converts mechanical energy into electrical energy. This is done by rotating an armature, which contains conductors, through a magnetic field. The movement of the conductors through a magnetic field produces an induced emf in the moving conductors. In any generator, a relative motion between the conductors and the magnetic field will always exist when the shaft is rotated.

Parts of a dc Generator

The principal parts of a DC generator are an armature, commu­tator, field poles, brushes and brush rigging, yoke or frame, and end bel1s or end frames.

Armature

The armature is the structure upon which the coils are mounted. These coils cut the magnetic lines of force. The armature is attached to a shaft. The shaft is suspended at each end of the machine by bearings set in the end bel1s. The armature core, which is circular in cross section, consists of many sheets of soft iron. The edge of the laminated core is slotted.

­The brushes col1ect the current generated by the armature coils. The brush holders transfer the current to the main terminals. The commutator bars are insulated from each other. Thus, each set of brushes, as it contacts the conunutator bars, col1ects current of the same polarity. This results in a continuous flow of direct current. The finer the division of the commutator bars, the less the ripple that appears in the current, and therefore, the smoother the flow of the dc output.

Commutator - The commutator is that part of the generator that rectifies the generated alternating current to provide direct current output. It also connects the stationary output terminals to the rotating armature. A typical commutator consists of commutator bars. The bars are wedge-shaped segments of hard-drawn copper. These segments are insulated from each other by thin strips of mica. Commutator bars are held in place by steel V -rings or clamping flanges. These are bolted to the commutator sleeve by hexagonal cap screws. The com­mutator sleeve is keyed to the shaft that rotates the armature. A mica collar or ring insulates the commutator bars from the com­mutator sleeve. The commutator bars usually have risers or flanges to which the leads from the associated armature coils are soldered. These risers serve as a shield for the soldered connec­tions wben the commutator bars become worn. When risers are not provided, it is necessary to solder the leads from the armature coils to short slits in the ends of the commutator bars.

S HAFT COMMUNATOR COILS SHAFT

COIL ENDS

2. Make up a plan in the form of questions.

3. Give the summary of the text according to your plan in a written form.

IV Comprehensive skills

1. Read and remember.

perfect – завершений, точний

bestow – дарувати

The great inventor Thomas Edison

Edison was now so famous as an inventor that people thought there was nothin he could not do. They began to call him the "Wizard" as if he could produce an invention like magic. Few people realized how hard Edison worked – often 20 hours a day – and that most of his inventions were the results of hundreds of experiments.

No one but Edison knew how hard and long he had worked to achieve his great victory with the light bulb. Nor did he stop there. He not only developed an improved dynamo to provide the power for electric lighting, but he also perfected other electric power equipment such as the generator, conductors, and underground power cables. The carbon filament lamp and the system of electrical distribution which he devised were key developments in the modern electronics revolution.

For more than 50 years Thomas Alva Edison was the world's leading inventor. He patented over 1,000 inventions which changed our way of living. He designed the central power station which became the model for the first public electric plant in New York City, providing electric power for thousands of homes and businesses. Edison was one of the earliest inventors of the motion-picture machines. His invention of the phonograph was joined with photography to produce talking pictures. He also perfected the electric motor which made streetcars and electric trains possible.

Therefore, it is no wonder that Edison received many honors during his life for his contributions to the progress of mankind. America bestowed on him its highest award, a special Congressional Medal of Honor.

2. Listen to the text “The great inventor Thomas Edison” and try to understand it.

3. True or false statements.

1. Thomas Edison as a famous American writer.

2. Most of his inventions were the results of hundreds of experiments.

3. He invented only the bulb.

4. He developed everything else needed to invent a movie camera.

4. Listen to the text once again and answer the following questions:

1. Edison was a famous inventor wasn’t he?

2. Did he work hard? Why do you think so?

3. What were his great inventions?

4. What was his contribution to the modern electronics?

V Communicative skills

Topic for discussion: How can the motors and generators improve the future?