THE DIRECT - CURRENT MACHINES.

A D-C generator is an electric machine which converts the mechanical energy of its prime mover into electric energy which is delivered to the consumer. For its operation, a D-C generator depends on electromagnetic induction. It consists essentially of a rotating armature carrying a winding, and a stationary field structure with electromagnets which produce the magnetic field.

The armature core is a cylinder assembled of separate 0.5.-mm-thick silicon-steel punchings, which are insulated from one another by a coating of varnish or thin paper. When the core is assembled, the notches on the punchings form slots which receive the armature winding.

The armature shaft carries a commutator consisting of separate cooper segments each soldered to a definite portion of the armature winding.

The poles of the electromagnets are of steel and are bolted to the yoke of the machine.

The yoke is usually cast of steel. In small machines, the yoke and poles are cast integral. In the

larger machines, the poles are assembled of silicon-steel punchings. The poles carry field coils wound with insulated copper wire. The current traversing the field coils sets up a magnetic field around the poles.

The armature circuit is connected to the load circuit by means of brushes. The brushes are carried in brush-holders which are mounted on brush-holder studs or brackets. When the armature rotates, its winding cuts across the magnetic field and an alternating current is produced in it. So that the terminal voltage can always act upon the external load in the same direction, some device must be inserted between the armature winding and the terminals. This device must

reverse the connection of the winding conductors to the external circuit at the instant when the voltage of the conductors is zero and changing in direction. Such a device is called

a commutator. There are some types of direct-current generators. The MMF necessary to establish the flux in the magnetic circuit of a DC generator can be obtained by means of (1)

a permanent magnet; (2) field coils excited by the generator itself; (3) field coils excited by some external source.

A permanent-magnet generator consists of an armature and one or several permanent magnets encircling the armature. In the case of separately excited generators, the field coils may be excited by any D-C constant -potential source as a storage battery, a D-C generator (called an exciter), a rectifier, etc. Where the field coils are excited by the generator itself, it is said to be self-excited. Self-excitation consists in the following. In the absence of current in the field coils, the armature rotates in the weak magnetic field produced by the residual magnetism of the pole cores. The weak EMF induced in the armature winding sends a slight current through the field coils. The magnetic field of the field coils builds up, and so does the EMF in the armature winding. This raises the exciting current. This continues until the current in the field coils attains a value corresponding to the resistance of the field circuit.

Therefore, self-excitation will take place only if and when (a) the current in the field coils produces a field increasing the field produced by residual magnetism and (b) the resistance of the field circuit does not exceed a definite ( critical) value.

Self-excited generators are further subdivided into: (1) shunt-wound units with the field coils connected in parallel with the armature winding; (2) series-wound units; in which the field coils are connected in series with the armature winding; (3) compound-wound machines, in which there are two windings on each pole, one connected in parallel and the other in series with the armature winding. The type of connection of the two windings determines the construction and performance of the generator.

P. Translate the sentences into Russian in writing. Cover the English variants and

translate the Russian ones back into English.

1. Materials having very high resistance are called insulators.
2. The transmission lines are demagnetized.
3. The generators constructed at the plant have no commutators.
4. A radio locating apparatus is called radar.
5. The energy lost in the capacitor appears in the form of heat being generated in the dielectric.
6. The device is used to detect charges moving through the circuit.
7. Various kinds of windings are used in dependence on the type of building and location.
8. The device being faulty, one cannot rely on its readings.
9. What is an electric arc? It is a discharge accompanied by a temperature of over 3.000*C, produced when an electric current flows through a gap between two electrodes.
10. We know of Kondakov’s having made the first synthetic rubber.
11. We know of silver, copper, and some other metals being widely used as conductors of electricity.
12. A fuse is a device for preventing an excessive current from passing through the circuit.

It is known to include a wire made of metal with low melting point.

1. Magnetism is referred to in the old writings of a man.
2. Starting resistors have a number of advantages; they are unbreakable, light, rigid, they can oppose, without variation, vibrations and shock.

Q. Group work. Make up a number of questions which help you to discuss and revise the

main ideas of the Lesson 9.

Date: 2014-12-28; view: 974