Main Structural Elements of a D. C. Machine

A direct-current machine consists of two main parts, a stationary part, usually called the stator, designed mainly for producing a magnetic flux, and a rotating part, called the armature or the rotor. The stationary and rotating parts should be separated from each other by an air-gap. The stationary part of a d.c. machine consists of main poles, designed to create the main magnetic flux; commutating poles interposed between the main poles; and a frame. It should be noted here that sparkless operation of the machine would be impossible without the commutating poles. Thus, they should ensure sparkless operation of the brushes at the commutator.

The main pole consists of a laminated core the end of which facing the armature carries a pole shoe and a field coil through which direct current passes. The armature is a cylindrical body rotating in the space between the poles and comprising a slotted armature core, a winding inserted in the armature slots, a commutator, and a brush gear.

The frame is the stationary part of the machine to which are fixed the main and commutating poles and by means of which the machine is bolted to its bedplate. The ringshaped portion which serves as the path for the main and commutating pole fluxes is called the yoke. End-shields or frame-heads which carry the bearings are also attached to the frame.

Of these main structural elements of the machine the yoke, the pole cores, the armature core and the air-gap between the armature core and the pole core are known to form the magnetic circuit while the pole coils, the armature windings, the commutator and brushes should form the electric circuit of the machine.

II. Translate the following phrases, using the given variants of translation.

To consist – состоять: to consist of a stationary part and a rotating part; separated – отдельный, изолированный: the stationary and rotating parts should be separated from each other by an air gap; to serve – служить в качестве чего-либо: the ringshaped portion or yoke serves as a path for the main and commutating pole fluxes.

II. Join the beginnings and the ends

Beginnings	Ends
The stationary parts of a d.c. machine are ....	a laminated core the end of which carries a pole shoe and a field coil
The two main parts of a direct current machine are ....	main poles, commutating poles and a frame
The main pole consists of ....	A stationary part or stator and a rotating part, called the armature or the rotor

IV. Arrange synonyms in pairs and memorize them:

a) to consist of; to be separated from; to create; to be interposed between; to pass; to rotate;

в) to be divided with; to produce; to introduce into; to permeate; to roll; to revolve; to comprise.

V. Write out the names of the machine parts and describe their operational characteristics.

UNIT 12

I. Read the text.

The Alternator

The alternator is an electric machine for generating an alternating current by a relative motion of conductors and a magnetic field. The machine usually has a rotating field and a stationary armature. In a synchronous alternator the magnetic field is excited with a direct current. The direction of an induced e.m.f. is reversed each time when a conductor passes from a pole of one polarity to a pole at another polarity. Most machines of this type are used for lighting and power, but there are alternators with a revoking armature and a stationary field. They are used in small sizes mostly for special purposes.

Any electrical machine is reversible. When a machine is driven by a source of mechanical power, it works as a generator and delivers electrical power. If it is connected to a source of electrical power, it produces mechanical energy, and operates as a motor. The alternator may also be operated as a motor.

The a.c. generator, or alternator, does not differ in principle from the d.c. generator. The alternator consists of a field structure and an armature. The field structure is magnetized by a field winding carrying a direct current. An electromotive force is generated in tine winding of the armature. In alternators the field is usually the rotating element and the armature is stationary. This construction has a number of advantages. Only two rings are needed with a rotating field. These rings carry only a relatively light field current, at a voltage generally of 125, and seldom exceeding 250. The insulation of such rings is not difficult. A stationary armature requires no slip rings. The leads from the armature can be continuously insulated from the armature winding to the switchboard, leaving no bare conductor. The alternator with a rotating field may be further divided into the vertical and the horizontal types.

The vertical type is usually applied for large water-wheel generators where it is desirable to mount the water turbine below the generator. The more common horizontal type is used with diesel and steam engine drive. A low-speed alternator of this type is suitable for a diesel engine drive, a high speed alternator is suitable for a steam turbine drive.