²ʲв
:
³
ʳ
'
˳
˳
ϳ
'
㳿
Գ
Գ
Գ
Գ


How Electrical Energy is Produced.

 

There are several methods of producing electricity for practical purposes. The battery of a pocket torch may be contrasted with the source of enormous energy represented by a larger power station. Both are examples of the application of electrical energy to a particular purpose, and in general the purpose determines the nature of the method used to produce the energy. Practical methods of producing electricity may be enumerated as follows:

1. Chemical, as represented by the various types of batteries or primary cells in which the electricity is produced by purely chemical actions.

2. Electromagnetic, forming the basis of operation of rotating generators in which the electricity is produced by conductors moving through a magnetic field. This is the method employed in practice for generators of various sizes.

3. Thermo-electric, in which the heating of the Junction between two different metals produces a very small voltage, which may be used for purposes of temperature measurement and as a source of power.

4. Pieso-electric, in which a very small voltage is produced across certain faces of a crystal by application of mechanical pressure. This effect is used, for example, as a means of frequency control in radio oscillators or for gramophone pick-ups, but it is suitable for power supply.

5. Electronic, characterized by the flow of electrons through evacuated or gas-fi1 led tubes, and having the following forms:

a) Thermionic emission. In which electrons are produced by the heating of special materials.

b) Photo-electric emission, in which electrons are liberated at the surface of certain substances by the action of light.

c) Secondary emission, in which electrons are driven from a material by the impact of electrons or other particles on its surface.

d) Field emission, in which electrons are drawn from the surface of a metal by the application of very powerful electric fields.

 

Electrochemistry, Batteries and Other Sources of e.m.f.

 

Pure liquids are good insulators but liquids containing salts conduct electricity.

An ion is an atom, which has either lost an electron (a positive ion) or has gained an electron (a negative ion).

Electrolysis is the process of decomposing an electrolyte by the passage of electric current through it; this results in chemical action at the electrodes, that is, the anode and the cathode. Electrolysis is the basis not only of many forms of chemical extraction and refining but also of the electroplating industry. Faradays laws describe the laws, which govern electrolysis. An electrical cell consists of two sets of plates immersed in an electrolyte. Cell can be either dry or wet. A primary cell cannot be recharged but a secondary cell can be recharged. A battery is an interconnected group of cells. All cells have an internal resistance whose value is reduced by the use of a depolarizes.

Electricity can be produced by a number of different methods including chemical action, thermoelectricity, the Hall effect, the piezoelectric effect and the photovoltaic effect.

 

Resistors and Electrical Circuits

 

A resistor may either be fixed or variable. Variable resistors may either have a sliding contact or may be tapped at various paints along their length; they may be connected as potentiometers to provide a variable output voltage. The resistance of a resistor depends on several factors including the resistively the length, the cross-sectional area and the temperature of the material. The conductance of a conductor is equal to the reciprocal of the resistance. In the case of a conductor an increase in temperature causes an increase in resistance and vice versa. In an insulator and a semiconductor, an increase in temperature causes a decrease in resistance. When resistors are connected in series the resistance of the circuit is greater than the highest individual value of resistance in the circuit and the resistance of the circuit is less than the lowest individual value of resistance the circuit.

 

Electromagnetism

 

A magnetic field in a ferromagnetic material is produced by magnetic domains. Lines of magnetic flux are said to leave a N-pole and enter a S-pole. Like magnetic poles repel one another and unlike magnetic poles attract one another.

The magneto motive force (m.m.f.) produced by an electromagnet causes a magnetic flux to be established in the magnetic circuit. The effective resistance of a magnetic circuit to magnetic flu is known as its reluctance (S) The relationship between the flux (F) the reluctance and the m.m.f. (F) is (Ohms law for the magnetic circuit) =F8. Equipment can be screened from a strong magnetic field by surrounding it with a material of low reluctance. An e.m.f. may be induced in a circuit either by self-induction or induction by motion in a magnetic field or by mutual induction. The magnitude and direction of the induced e.m.f. can be predicted using Faradays laws and Lenss law.

Electrical Generators and Power Distribution.

 

Motors action is caused by the force acting on a current-carrying conductor in a magnetic field. The direction of the force can be predicted by Flemings left hand rule.

A d.c. motor consists of a rotating part (the armature) and a fixed part (the frame). Electrical connection to the armature is made via carbon brushes and the commutator. When the armature rotates a back e.m.f. is induced in the armature conductors (this is by generator action) which opposes the applied voltage.

The four main types of dc motor are the separately excited, the shunt wound, the series wound and compound wound -machines.

 

The Transformer

 

The transformer depends for its operation on the principle of mutual induction. The primary winding of the transformer is connected to the power source (which must be a AC) and the load is connected to the secondary winding.

The transformer may have either a single winding (when it is known as an autotransformer) or more than one winding (two winding transformers are the most common single phase transformers). The iron circuit of the transformer is laminated to reduce the eddy-current power-loss. Important rules relating to transformer design are:

1. Each winding supports the same number of volts per turn.

2. Ampere-turn balance is maintained between the windings. The efficiency of a transformer is the ratio of the power it delivers to the load to the power absorbed by the primary winding.

 

8. Measuring Devices. Ammeters and Voltmeters.

Ammeters measure the current flowing in a circuit and normally have scales, which are graduated or calibrated in amperes, milliamperes or microamperes.

Voltmeters are used to measure the potential difference between two points in a circuit. The calibration of voltmeters is usually in volts, millivolts and microvolts.

The main difference between the two instruments of the same type or design is in the resistance of the operating coil identical moving units may be used for either meter. An ammeter is connected in the positive or negative lead in series with a circuit and, therefore, must have a low resistance coil otherwise the readings would be incorrect as the coil would absorb an appreciable amount of power.

A voltmeter is connected in parallel across the points of a circuit where the difference of potential is to be measured. The resistance of the operating coil must, in this instance, be as high as possible, to limit the amount of current consumed by it, or else a drop in potential due to the meter would occur and the pointer indication would not represent the true potential difference across the circuit.

Wattmeters. - The measurement of the power in a D.C. circuit at any instant can be achieved by means of an ammeter and voltmeter as the power in watts is the product of the current and the voltage. With A.C. circuits however, the instantaneous values are always changing. To measure A.C. power correctly, therefore, it is necessary to use the third instrument to measure the phase difference. The normal practice, however, is to combine these three instruments in one which will give a direct reading of power in watts.

 

9. Care of the Electrical equipment.

As a rule electrical equipment operates reliably. Still it does not mean that it deserves no attention. It is necessary to give the equipment frequent inspections, keep it well cleaned, lubricated and repaired. Undue heating, vibration, sparking should be immediately removed.

Heating may be due to overload or to a short circuit between turns, lack of oil in bearings, vibration may be due to improper foundation, unbalance in the moving parts of the machine.

Conductors may get heated because of overload or by reason of damage of the insulation of the conductor.

An electrical machine of any kind requires certain conditions under which it may operate reliably: temperature and freedom of access of surrounding air, need for protection against dirt, dust, type and duration of load, etc. Rotating machines should be placed on solid foundations. Conductors should be protected against mechanical damage. All measures or safety precaution must be undertaken.

 

LESSON 8.

TECHNICAL BOOKS AND REPORTS

 

1 Dont use defective electrical equipments.

2 Report faulty electrical equipment immediately.

3 Never touch electrical equipment with wet hands.

4 Report all frayed cables.

5 Dont make faulty electrical connections.

6 Use the advice or an authorized electricians.

7 Check the cart hiding on all power tools.

 

Electrical Danger-Points:

1. Wet hands;

2. Frayed cables;

3. Unearthed plugs;

4. Home-made connections.

 

Step 1 Are these statements true or false? Correct the false ones:

1. Defective electrical equipment is safe.

2. Frayed cables are well insulated.

3. Power tools should be earthen.

4. Wet hands conduct electricity easily.

5. An electrician should check faulty electrical devices.

 

Step 2.

Now change warnings into instructions. Use must. Look at the example.

Dont use defective electrical equipment.

Defective electrical equipment must not be used.

Then make true statements. You may use these phrases:

Faulty electrical equipment, called an authorized electrician reported, Unearthed plugs,immediately, electrical equipment, replaced, faulty brakes, used, faulty electrical connections, tightened, frayed cables, recharged, damaged tools, touched with wet hands.

A flat battery must not be checked regularly.

A loose nut never measured accurately.

A broken fuse repaired.

A defective machine adjusted.

A broken light bulb made. Voltages worn correct clothing.

A faulty switch worn tyros.

 

Ask and answer questions about the table above.

 

Look at the examples: why were the cables replaced? Because they were faulty.

 

Step 3

PROTECTIVE DEVICES

 

The cables that carry electric current to the different appliances in the factory are called conductors. Their resistance to the flow of electric current causes heat to be generated. If the flow of electricity in a circuit suddenly increases, the heat in the conducting wires increases and can cause the insulation to burn, resulting in damage to the equipment and possibly causing a fire. All electric circuits must therefore be protected by manual trip switches, fuses or automatic trip switches, which cut the supply where there is a fault.

 

Manual Trip Switches

Manual trip switches can be inserted in the circuit and are used to isolate equipment from the supply if a fault occurs.

 

Fuses

Every circuit should have a fuse in the supply line to the equipment. The fuse can be of the cartridge type which is held in place by two spring clips or can be a piece of wire joining two points in the circuit. Fuses are rated in amperes and the rating must be correct so that if there is an overload the fuse will melt and break the circuit before any damage to the circuit wiring or the equipment occurs. If a fuse blows the correct manual switch must be switched off and the fault rectified before replacing the fuse. When a fuse is changed the correct rating must be used. It is dangerous and can be expensive to fit a replacement fuse with a higher rating.

 

Automatic Trip Switches

Automatic trip switches are often fitted to electrical devices to protect them from overloads. Usually, they are terminal switches that break the circuit when an overload causes a bi-metal strip in the switch to bend. These switches have a re-set button, which resets the trip switch and completes the circuit. If the circuit is broken because of an increase in current, the switch cannot be re-set immediately. The bi-metal strip needs time to cool (approx. 30 seconds) before the switch can be re-set.

 

Connecting Plugs

A lamp is connected with the switch open. The lamp holder can only become live when a fault occurs in the insulation, which is not isolated by the switch.

When plugs for operating electrically powered hand tools are connected, the minimum rating for the connector must not be exceeded.

 

Step 4

Are these statements true or false? Correct the false ones.

1. Electric circuits must be protected against current overloads.

2. An increase in current causes heat to be generated in conductors.

3. Burning insulation can be caused by fuses.

4. Defective circuits must be isolated from the supply.

5. Fuses can be replaced immediately after melting. Use a higherrating if necessary.

6. A bi-metal strip consists of one piece of metal.

7. A bi-metal strip must cool before the circuit is complete again.

8. The current manual switch must not be switched off before replacing the fuse.

 

Step 5.

Put the correct words. Choose from this list.

rectified damage joining supply

piece should clips rated

must melts breaks of

blows manual occurs there

held cartridge

 

Every circuit... have a fuse in the ... to the equipment. The fuse can be of the ... type, which is ... in position by two springs ... or it can be a ... of ... two points in the circuit. Fuses are ... in amperes and the rating ... be correct so that if ... is an overload ... current, the fuse wire ... and ... the circuit before any ... to the circuit wiring ... . If a fuse ... the correct ... switch (be switched off and the fault ... before replacing the fuse.

Step 6.

Explain the difference between automatic trip switches, manual switches and fuses and why all electrical circuits must be protected against current overloads.

Examine the electrical system and appliances in your University, draw diagrams of the circuit and supply. Describe all the protective devices, which you find.

 

LESSON 9.

SAFETY AT WORK

Tuning-in

Step 1

What do these warning labels on chemicals mean? Match each label to the correct warning.

a) highly flammable

b) harmful

) explosive

d) corrosive

e) oxidizing

f) toxic

 

Step 2.

List some of the potential dangers in your laboratory, workshop, or place of work. How is the risk of these hazards reduced?

 

Step 3.

Study the safety instructions from a workshop below, and then answer these questions.

a) Who are the instructions for?

b) Who wrote them?

c) What was the writers purpose?

1. Wear protective clothing at all times.

2. Always wear eye protection when operating lathes, cutters, and grinders and ensure the guard is in place.

3. Keep your workplace tidy.

4. The areas between benches and around machines must be kept clear.

5. Tools should be put away when not in use and any breakage s and losses reported.

6. Machines should be cleaned after use.

 

Reading.

Understanding the writers purpose.

Knowing what the writers purpose is, who the writer is and who the intended readers are can help us to understand a text. The safety instructions in Step 3 are clearly intended to encourage employees to be safety conscious and reduce the risk of accidents. The writer is perhaps a supervisor or the company safety officer, and the intended readers are machine operatives. Knowing these things can help us to work out the meaning of any part of the text we may not understand.

 

Step 4.

Study the company document on safety on the net page, and then answer these questions.

Who is this document for?

a) machine operatives

b) managers

) all employees

d) injured employees

Who wrote this document?

a) trade union representative

b) technician

) manager

d) medical staff

3. What is the writers intention?

a) to prevent accidents;

b) to ensure speedy help for injured employees;

) to protect the company;

d) to warm about dangers.

Accident investigation.

 

Whenever an accident occurs that results in an injury (medical case), damage of equipment and material, or both, prompt accident investigation by the immediate manager is required. A written preliminary investigation will be completed by the end of the particular shift or business day on which the accident occurred.

In no event should there be a delay of more than 24 hours. Failure to comply with this requirement may subject the immediate manager to disciplinary action up to and including discharge. Without adequate accident investigation data the Company may be subjected to costs, claims, and legal action for which it has no deference. As a minimum, the preliminary accident investigation report will include the following:

1. Name, occupation, and set of injured worker.

2. Place, and date/ time of accident.

3. Description of how the accident happened.

4. Immediate causes of the accident - unsafe acts and unsafe conditions.

5. Contributing causes - manager safety performance, level of worker training, inadequate job procedure, poor protective maintenance, etc.

6. Witness(es) - name and department.

7. Corrective action taken - when.

The employee who was injured and any employee(s) who witnessed the incident should be separately interviewed as soon possible. A copy of the report must be submitted to the Manager of Human Resources for review. An other copy of the report is to be retained for a period of not less than the injured employees length of employment plus five (5) years.

 

 

Step 5.

Study this brief report of an accident. In which points does it not meet company policy on reporting accidents?

 

To:

Name

Department and Location

Date

Manager

Human Resources

17 May

From:

Name

Department and Location

Tel.

D. Taylor Mech. Eng. Workshop

Subject

Preliminary Report, Accident

12 May

While turning a brass component on Tuesday, last week, Kenneth Oliver, machinist, received an injury to his eye. He was taken to the Eye Hospital where I understand he was operated on. I believe the accident was due to carelessness.

 

Language study. Making safety rules.

© 2013 wikipage.com.ua - wikipage.com.ua |