Types of Power Supply

The two chief sources of electrical energy are (1) commercial power-supply systems and (2) batteries. Of the six kinds of electric currents, only the continuous, direct, and alternating are used as sources of power supply.

The continuous current, which is obtained from battery cells is used largely for operating portable equipment.

The direct current, usually referred to as d-c, is that type obtained from a rotating machine called the d-c generator. The theory of this machine is also based on the principle of electromagnetic induction.

The generator voltage of the alternator in the large modem power plant is much higher than the voltage supplied to the homes. The extensive use of a-c systems is due largely to the ability to raise or lower the voltage easily by .means of a transformer.

The discovery of the principle of electromagnetic induction is credited to Michael Faraday, who, in 1831, found that, if a conductor were moved through a magnetic field, a current would flow in the conductor.

The relation among the direction of motion of the conductor, the direction of the magnetic field, and the direction of the induced emf is expressed by Fleming's right-band rule.

The principle of electromagnetic induction may be restated as follows. Whenever there is motion between a conductor and a magnetic field, an emf will be induced in the conductor; the motion may be produced by moving the conductor through a stationary magnetic field or by having the magnetic field out by a stationary conductor.

7. Answer the following questions:

1. What are the chief sources of electrical energy?

2. Why are a-c system used so extensively?

3. With what fundamental principle of electricity is Faraday's name associated?

4. Why is Fleming's right-hand rule so important?

5. What is the principle of electromagnetic induction?

8. Ask you fellow-student:

How many kinds of currents there are in electricity; if all the kinds of electric currents are used as sources of power supply;

What kinds of electric currents are used as sources of power supply;

When the continuous current is used;

What principle the theory of operation of the d-c generator is based upon.

9.

1) Give a definition of an alternator.

2) Describe the figure demonstrating Faraday's discovery:

10. Learn the dialogues by heart:

I

A.: This morning I have finished reading the book about Faraday. I liked it very much.

В.: I know the book. What a remarkable man of science Michael Faraday was. Everybody knows something about his discoveries in electricity.

A.: Oh, yes, his discovery of electromagnetic induction is truly fundamental.

В.: The discovery was made as early in 1831. He showed that if a conductor is moved through a magnetic field, a current will flow in the conductor.

II

A.: What are you pointing with your thumb at?

В.: I'm demonstrating Fleming's right-hand rule.

A.: What sort of rule is that?

В.: It expresses the relation among the direction of motion of the conductor, the direction of the magnetic field, and the direction of the induced emf.

Test Exercises

Write the number of the right choice or choose the right letter:

A.

1. The theory of operation of the d-c generator is based upon:

1. the heat of expansion

2. polarity

3. the principle of electromagnetic induction

2. The generated voltage of the alternator in the large modern power plant is:

a. lower than

b. higher than

c. equal to the voltage to the homes

B.

1. a source ... power supply (1. for, 2. of, 3.-)

2. to obtain ... battery cells (1. out of, 2. at, 3. from)

3. to refer ... (1. at, 2. on, 3.to)

4. to based ... (1. at, 2. to, 3. upon)

5. to supply ... (1. by, 2. with, 3. to)

Part III

A. Read the text without dictionary:

Transformers

Electromagnetic induction is the basis of operation of the transformer. The fundamental transformer consists of a core and two windings called the primary and the secondary windings. The core provides a path for the magnetic field. The primary is the winding that receives the energy from the line, and the secondary is the winding that delivers the energy to the load.

If the secondary is connected to a load, a current will flow through the load and also through the secondary winding. There is no electrical connection between the primary and secondary windings. The efficiency of the transformer is very high, often above 95 per cent. As the voltage is stepped up by a transformer the current is stepped down. This is a decided advantage in power-transmission systems.

В.

Test Exercise

Chose the right letter :

1. The core of the transformer

1. provides a path for the magnetic field

2. receives the energy from the line

3. delivers the energy to the load

1. The primary winding of the transformer

   1. delivers the energy to the load

   2. received the energy from the line

2. The secondary winding of the transformer

   1. receives the energy from the line

   2. delivers the energy to the load

C. Retell the text

IV. MOTORS

ambient temperature - температура окружающего

воздуха

armature, n. - якорь

centrifugal force - центробежная сила

coil, n. - катушка

control, n. - управление, контроль

determine, v. - определить (в численных

выражениях)

efficiency, n. - коэффициент полезного

действия

equation, n. - yравнение

friction, n. - трение

gear, n. - шестерня, сцепление

horsepower, n. - лошадиная сила