Laboratory Exercises

7. Listen and read the following word combinations:

principle of electric motor, trolley buses, escalators and lifts, a vacuum cleaner, fundamental parts, soft-iron core, spins round, permanent magnet, is capable of turning, the face of the coil, a south pole, repulsion occurs, two insulated halves, through the coil, from brush to brush.

8. Listen and repeat in pauses:

Electric motors are used to produce work. They convert electrical energy into mechanical energy. They contain two fundamental parts: the armature and the field magnet. The armature is a coil of wire wound on a soft-iron core, which spins round. A coil of wire is capable of turning about axis. A commutator is a device which automatically changes the direction of the current in the coil. It rotates with the coil.

9. Listen to these questions on the above text and answer them:

1. What energy do electric motors convert? 2. What fun­damental parts of electric motors do you know? 3. What is the armature? 4. What is a commutator? 5. What does a commutator rotate with?

10. Ask questions using the model.

Model: A coil of wire is wound on a soft-iron core. Where is a coil of wire wound?

1. A coil of wire is situated between the poles of a mag­net. 2. The coil is spinning round the core. 3. An armature is capable of turning about an axis. 4. A current from a bat­tery flows round the coil.

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11. Listen to this short dialogue and learn it by heart:

— Under what conditions are the electric motors de­signed to perate?

— They are designed to operate when connected to a single-phase source. Do you know what kind of work they perform?

— Certainly, I do. They are known to perform a great variety of useful services in the home, the office, the factory, on the farm, and many other purposes where electricity is available.

— What characteristic features of such motors do you know? Such motors are generally constructed in small size, operate at high speed and include special design features, so that commutation and armature-reactance difficulties are minimized. They may be connected to any of the commonly available supply sources.

12. Suggested topics for oral narration:

1. Describe the construction of an electric motor.

2. What is meant by the terms "commutator", "arma­ture", "the field magnet"?

Additional Material

Read the text and get its central idea. Describe the con­struction and operation of an electric bell.

Electric Bell Circuit

T he electric bell con­tains an electromagnet consisting of two sole­noids wound in opposite directions on two paral­lel soft iron cores joined by a soft iron yoke (Fig. 9). Adjoining the cores is a soft iron arma­ture mounted on a short flat spring. Attached to the armature by a light spring is a silver contact which bears against a silver-tipped contact ad­justing screw. The va­rious components of the bell are connected by

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insulated wire as shown in the diagram, and the circuit is completed through a cell and push switch.

When the push button is pressed, current flows from the! cell, through the two solenoids, down the armature, across the contacts and thence back to the cell. This causes the cores to become magnetized with opposite polarity and the armature is attracted towards them. As a result the striker hits the gong and, at the same time, the circuit is broken] at the contact. Magnetism disappears from the cores and the armature is pulled back by the spring so as to restore contact. The action just described begins all over again, with the result that the armature moves to-and-fro, and the gong is struck repeatedly. If a bell circuit does not work we may suspect, among other causes, that a) the contact breaker needs adjustment, or b) a new battery is needed, or c) the bell-push is faulty, or d) the coil is broken.

UNIT 18