В
There are two circuits, one through the shunt field and one through the armature. On small motors the connections are made internally and only two wires are brought out. Therefore, to test such a motor, it must be disassembled in order to reach the field and armature wires.
If the wires are accessible, test each circuit separately. The lamp should light brightlv when the armature circuit is tested. The field should produce a dim light. If it is not known which of the four wires' are the shunt field and which the armature, they can be determined by this test. If the armature circuit shows an open, the trouble may be the brushes, the connections to the brushes, or the armature windings; if the field tests open, then the trouble is either the field coil or its connections.
C
For testing purpose the motor is considered as having three circuits. Six leads are brought out of a motor.
If the armature leads are tested with the test lamp, the lamp should light, indicating a complete circuit. If the open is in the armature circuit, the trouble is in the brushes or connections to the brushes or the
interpole. If the trouble is in the field, test for a complete circuit from one coil to another.
23 15
Practice pronunciation. Open your books and repeat in pauses 9 paragraphs of the recorded text “Duties of Electricians” after the speaker. Then close your books and do the same with paragraphs 10—14.
24 6
Listen to the tape-recorded dialogue and be prepared to dwell upon its subject.
Long Distance Cables
Bob: The old man I shan’t be able to. . .
Norah: What old man?
В.: I mean H. B., that’s to say Henry Burton, the managing director of the works.. .
N.: Oh, I see! And what does he say?
B.: He says I shan’t be able to go away for the holidays unless I trace the fault!
N.: Are you serious?
B.: Of course, I am! The cable, I mean to say the long distance cable, has broken down and I’ll have to stay here until. . .
N.: Oh, Bob! That’s really dreadful: But will you be able to find out what’s wrong?
B.: Of course I will!
N.: But you won’t be allowed to leave London until you’ve put everything right!
B.: But I tell you I will, and when I do I shall get a huge reward...
N.: But what’s happened?
B.: Er... well, you see, a cable consists of a core dielectric and a metal sheath. Now, the life of a cable varies according to the material from which it has been made, and the current it carries. This time it was a high-tension current, but to come back to the subject when a cable has been used for some time, voids are formed in the dielectric, and are filled with gases. Gradually the voids get larger and larger until the day when they touch the core and the sheath. Then the current begins to flow...