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Direct Current Electric Machines
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D.C. machines converting mechanical energy into electrical one are referred to as d.c. generators. Those, on the other hand, which convert electrical energy into mechanical one, are spoken of as d.c. motors. D.c. machines are reversible and, therefore, identical in construction. They are composed of two main parts – the stationary part which is called the frame provided with the main poles and the commutating poles, and the rotating part which is said to be the armature, its windings and commutator being positioned on it.
The frame is made of steel in the form of a closed magnetic conductor. The main polesfor the purposes of decreasing the losses are assembled of sheet steel laminations. Mounted on them, the field windings are built of copper isolated conductors. The main poles are intended to produce the main magnetic flux.
The commutating polesare also made of steel and arranged midway between the main poles. Their windings as well as those of the main poles are built of copper conductors. The commutating poles with the windings are designed to ensure non-sparking operation of an electric machine.
The armature makes up a cylindrical core made of sheet steel laminations, a two-layer winding being fixed in their slots. The former is built of sections made of isolated copper conductors.
The commutatoris constructed of separate copper bars isolated from each other and from the frame. The section leads of the armature winding are connected to the bars. The commutator is designed for the conversion of an alternating e.m.f. induced in the armature winding into a direct one.
The brush gearis an element designed for collecting the current from the armature winding and leading it to the latter. It is composed of brushes, brush-holders, brush-studs, brush-rockers and current collecting bars.
The commutator and the brush gear are the most essential parts of a d.c. machine, its reliable operation being dependent on their condition.
The operative principleof a d.c. machine is built upon the laws of electromagnetic induction and electromagnetic force. As the armature is rotated in the magnetic flux of the main poles by a drive motor, the e.m.f. is induced in the armature winding. In the loaded machine, the e.m.f. brings into existence the current with which it coincides in direction. This current interacting with the magnetic flux produces the electromagnetic torque directed in opposition to that produced by a prime mover. As this takes place, the machine operates as a generator. The mechanical power consumed from the drive motor is converted into the electrical one and is given up to the mains.
When a d.c. machine is connected to the electrical source of supply, the current is generated in the armature winding. Interacting with the magnetic flux of the poles, it produces the electromagnetic torque which brings into rotation the armature. The e.m.f. directed in opposition to the current direction is induced in the armature winding. In this case the machine works as a motor. The electrical power consumed from the mains is converted into the mechanical one.
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Exercises
I. a) Find the Russian equivalents of the following English word combinations:
1. stationary part; 2. field winding; 3. to ensure non-sparking operation; 4. made of sheet steel laminations; 5. induced in the armature winding; 6. current-collecting bar; 7. coincide in direction; 8. interacting with the magnetic flux; 9. bring into rotation; 10. consumed from the mains;
1. обеспечивать безыскровую работу; 2. совпадать по направлению; 3. сделанный из листов электротехнической стали; 4. обмотка возбуждения; 5. индуктируемая в якорной обмотке; 6. неподвижная часть; 7. потребляемый от сети; 8. взаимодействуя с магнитным потоком; 9. токособирающая шина; 10. приводить к вращению.
b) Translate these Russian words and phrases from the text into English:
одинаковый, вращающийся, станина, якорь, обмотка возбуждения, добавочные полюса, устанавливать, сердечник, щёточный аппарат, приводной двигатель, совпадать, электромагнитный момент, сеть, обеспечивать.
II. Make sentences out of two given parts:
| 1. D.c. generators… 2. The main poles… | is a stationary part of d.c. machines provided with the main poles and commutating poles. |
| 3. The commutator… 4. The brush gear… | converts an alternating e.m.f. induced in the armature winding into a direct one. |
| 5. The frame… | are machines which convert mechanical energy into electrical one. |
| are intended to produce the main magnetic flux. | |
| is an element designed for collecting the current from the armature winding and leading it to the latter. |
III. Answer the questions according to the text context:
What type of a d.c. machine is referred to as a d.c. motor?
What are the two main parts of a d.c. machine?
What is the frame composed of?
Is the construction of the commutating poles identical to the one of the main poles?
What does the armature make up?
What is the commutator designed for?
What are the most essentials parts of a d.c. machine?
What elements is the brush gear constructed of?
What laws is the operative principle of a d.c. machine based upon?
In what case does a d.c. machine operate as a d.c. generator?
Under what condition does a d.c. machine work as a d.c. motor?
IV. Put the verbs in brackets into the correct form – Present Simple Active or Present Simple Passive:
1. The frame (to make) of steel in the form of a closed magnetic conductor. 2. The commutating poles (to design) to ensure non-sparking operation of an electric machine. 3. The main poles (to assemble) of sheet steel laminations and (to decrease) the losses. 4. D.c. motor (to convert) electrical energy into mechanical one. 5. The operative principle of a d.c. machine (to build) upon the laws of electromagnetic induction and electromagnetic force. 6. When a d.c. machine (to connect) to the electrical source of supply, the current (to generate) in the armature winding. 7. Interacting with the magnetic flux of the poles, this current (to produce) the electromagnetic torque which (to bring) into rotation the armature.
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V. Speak on:
the construction of d.c. machines: the frame, toe poles, the armature, the commutator, the brush gear;
the principle of operation: d.c. generators, d.c. motors.
Text 2B
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