ELECTRICAL
TECHNOLOGY
GRADE 12
Trevor Adams
Steve Mitchell
CAPS
Electrical Technology
Grade 12 Learner’s Guide
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ISBN 978-1-77581-012-4 First published 2013
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Chapter 1: Occupational health and safety
Dangerous practices ...2
Risk analysis ...3
Human rights in the workplace ...4
Work ethics ...5
Bleeding ...8
Chapter 2: Three-phase AC generation Advantages and disadvantages of single-vs. three-phase systems ...12
Single phase generation ...15
Three-phase generation ...16
Three-phase systems: star-vs.-delta (delta-vs.-star) ...18
Power in three-phase systems and calculations ...22
Losses ...26
Concept of power factor correction ...27
kWh meter ...30
Measurement of power in three-phase systems ...35
Chapter 3: Three-phase transformers Three-phase transformers ...42
How to connect a three-phase transformer in star or delta ...45
Type of transformers ...46
Transformers (star/star, delta/delta, star/delta, delta/star ...51
Calculations (efficiency at 100%) ...54
Application of transformers ...57
Safety and transformers ...58
Chapter 4: Three-phase motors and starters Principle of operation of the three-phase squirrel cage induction motor ...66
Connections of motor in star or delta ...71
Calculations on synchronous speed, slip, power and efficiency ...72
Electrical and mechanical inspections/fault finding ...75
DOL starter with overload ...81
Forward/reverse starter with overload ...86
Automatic star-delta starter with overload ...89
Sequence motor control starter with overload (without timer)...92
Sequence motor control starter with overload (with timer) ...94
Chapter 5: RCL circuits Inductive reactance ...106
Power ...108
How to make sense of series and parallel circuits ...111
Calculations of series RCL circuits ...115
Parallel RCL circuits ...118
Basic operation of PLC ...143
Basic ladder logic instructions ...145
Ladder logic diagrams ...146
Logic gates and its ladder equivalents ...149
Converting Boolean expressions to ladder diagrams ...151
Karnaugh maps ...154
Combination logic networks ...162
Motor starter control: a simple approach ...166
Chapter 7: Amplifiers Characteristics of the ideal operational amplifier ...180
Principle of operation of negative/positive feedback of operational amplifiers ...183
OP-amp as comparator ...184
OP-amp as inverting amplifier ...188
OP-amp as a non-inverting amplifier ...190
OP-amp used as a summing amplifier ...193
OP-amp used as an integrator ...196
OP-amp used as differentiator ...198
Bi-stable multivibrator ...201
Mono-stable multivibrator...203
Astable multivibrator ...205
The Schmitt trigger ...208
Operational amplifier oscillators ...211
Icon Description
Key word
Did you know?
Take note
Activity
Chapter 1
Occupational health and safety
Electrical shock
Medical
emergencies
A
B
Human rights
OHS Act
A
B
Introduction
Safety and first aid are the two most important things that must always be high on the priority list of any person working in any workshop, big or small and on any construction site. Being aware of safety and knowing some basic first aid can be very helpful and can even save a life. We must remember, safety is the responsibility of every single person, not just of the employer or factory owner. Most accidents are caused by the careless acts of humans. Because electricity is very dangerous, care must be taken to be safety conscious all the time when busy in a workshop. In South Africa, we have the Occupational Health and Safety Act (OHS Act) that regulates workplace safety in general in our country. It is the aim of the Act to eliminate or reduce work-related accidents. It also tries to ensure that workers have a safe environment in which to work. According to the Act, an accident is an unplanned, uncontrolled event caused by unsafe acts and conditions.
The Act compels the employer to ensure the workplace meets safety standards by appointing people and committees whose task it is to monitor their safety in the workplace. The employers can be punished and fined if machinery or working conditions are found to be unsafe.
Hundreds of accidents occur annually on our roads, in mines, in the workplace and on construction sites, not only because of unsafe working conditions but also because people become complacent, act negligently, lose concentration or are too tired or too hasty. To ensure safety in the workplace means that every single person has to be aware of potential dangers at all times. Learners have to acquire the necessary knowledge and skills to enable them to prevent accidents from happening.
In general, safety is understood to be the absence of danger or risk. However, accidents occur when workers are too complacent and then take risks because they think nothing will happen to them.
Unsafe actions
As mentioned earlier, most accidents are caused by human carelessness. However, accidents also occur when workers are too complacent and then take risks because they think nothing will happen to them.
Below are some unsafe actions responsible for most accidents in workshops. • Failure to wear protective clothing and eye wear – when grinding, drilling or
working with acid and chemicals. • The unsafe placement of tools.
• Horseplay in workshop – running around and playing the fool. • The unsafe use of equipment or incorrect use of equipment. • Trying to make adjustments or working on moving equipment. • Taking up unsafe positions.
• Working too quickly.
Dangerous practices
Dangerous practices most commonly refer to processes or activities that have some form of risk or hazard when performed. In an electrical workshop, many activities or tasks done can be referred to as dangerous practices. If the user of the workshop is not cautious of such situations or practices, they could lead to serious injury or
The etching of printed circuit boards (PCBs) by making use of chemicals and acids. The chemicals can be harmful as they could damage your clothes or even be dangerous to your skin. It is therefore important for anybody involved in the etching of PCBs to observe the safety rules, i.e. wear protective clothing, for example, rubber aprons, rubber gloves, respiratory masks and eye protection. Another dangerous practice is the use of power tools (electric hand drill, bench and hand-held grinding machines). Learners are sometime very reluctant to observe all the safety rules regarding the use of power tools, which can lead to serious injury. Drill bits that are not securely tightened, using your hand to hold the piece of material to be drilled, not wearing protective eye wear and protective clothing, not tucking away all loose-hanging clothes and long hair, not inspecting the electrical cords of the tools, can all lead to unnecessary accidents.
Incorrect use of hand tools is also a very dangerous practice in a workshop. Tools must be fully functional and must be used for the purpose for which they were made. Under no circumstances should anyone use a file without a handle or with a broken handle. Never use a screwdriver as a lever or a chisel. It is important that you select the correct size screwdriver for a job, failing to use the correct size will result in the tip being damaged, making it less effective. When you use any pliers or cutters, please ensure the handle is insulated to prevent electrical shocks. Always ensure the blade of a blade knife is retracted when not in use to prevent anyone from being cut.
Unsafe conditions
Unsafe conditions are also a big contributor to many accidental mishaps in the work-place. Unsafe conditions refer to a hazardous, risky or dangerous environment or surroundings that can lead to accidents. Let us look at a few unsafe conditions. • Inadequate guarding of machines.
• Bad ventilation.
• Rough, wet or slippery floors. • No personal protective equipment. • A disorganised workshop.
• Overcrowding in a workshop. • Badly planned workshop.
• Loose-hanging clothing and long hair. • Insufficient light in workplace.
Risk analysis
Risk analysis is the process of defining and analysing the dangers to individuals and businesses posed by potential natural and human-caused adverse events. A risk analysis report can be either quantitative or qualitative.
With a quantitative risk analysis, an attempt is made to determine numerically the probabilities of various adverse events and the likely extent of the losses if a particular event were to take place.
On the other hand, a qualitative risk analysis does not involve numerical
probabilities or predictions of loss. Instead, the qualitative method involves defining the various threats, determining the extent of vulnerabilities and generating
It is therefore important for any employer to do a risk assessment in the workplace to make sure that no one gets hurt or becomes ill – that a person returns home safely after work. Risk management is nothing more than a careful examination of what could cause harm to people in your workplace (a qualitative risk analysis). In many instances, straightforward measures can readily control risks, for example, ensuring spillages are cleaned up promptly so people do not slip, or cupboard drawers kept closed to ensure people do not trip. For most, that means simple, cheap and effective measures to ensure that your most valuable asset – your workforce – is protected.
The following things must be kept in mind when doing risk management: • involve workers in the process
• don’t use it to justify a decision that has already been made • consider good practice in your industry
• keep records of any risk management activities undertaken.
Risk management is a five step process for controlling exposure to health and safety risks associated with hazards in the workplace.
• Identify the hazard.
• Decide who might be harmed and how. • Evaluate the risk and decide on precautions .
• Record your findings and implement them .
• Review your assessment and update if necessary. When thinking about your risk assessment, remember:
• a hazard is anything that may cause harm, such as chemicals, electricity, working from ladders, an open drawer, etc; and
• the risk is the chance, high or low, that somebody could be harmed by these and other hazards, together with an indication of how serious the harm could be.
Human rights in the workplace
Chapter 2 of the South African Constitution contains the Bill of Rights, which contains the human rights which protect all South Africans.
Human rights are also called natural rights. It is argued that they belong to people just because they are human beings. People are entitled to them regardless of where they live in the world or of their position in society. It doesn’t matter what a person’s race, sex, age, class, language, beliefs, culture or religion are, or how much money or education a person has, we all have the same human rights. Most people would support human rights that are based on basic values, such as respect for human life and human dignity.
Many of the principles of the Human Rights Act are designed to protect you as a worker within the workplace. It is about protecting the human dignity of the workers. Human rights are about ensuring that your human dignity is not infringed and that you, as the worker, are treated with dignity, respect and are not exploited.
For example, you have the right to a private and family life. So an employer who discriminates against a gay worker, for example, may be violating that worker’s right to a private life.
Your employer has the right to monitor communications within the workplace as long as you are aware of the monitoring before it takes place. Monitoring can cover: • e-mails
• Internet access • telephone calls • data
• images.
You have the right to see any information held about you (for example, e-mails or CCTV footage). Your right to a private life means you have the right to some privacy in the workplace. You can’t be monitored everywhere. If your employer doesn’t respect this, he/she will be breaching human rights law.
Human dignity at work is also about making sure that working conditions for workers are good, i.e. good ventilation, enough light, enough space to work, an environment that is not harmful to their health or well-being. Other issues that are also covered by human rights are:
• Your right to not to be discriminated against because of your race, sex, religion, language, disability, etc.
• Your right to earn a living wage. • Your right to work reasonable hours. • Your right to belong to a trade union.
• No one may be subjected to slavery or forced labour. • Everyone has the right to fair labour practices.
Work ethics
Work ethics include not only how one feels about your job, career or occupation, but also how one performs your job or responsibilities. This involves attitude, behaviour, respect, communication, and interaction; how one gets along with others. Work ethics demonstrate many things about the person and who he/she is and how he/she behaves.
Work ethics involve such characteristics as honesty and accountability. Essentially, work ethics break down to what one does or would do in a particular situation. Work ethics are intrinsic; they come from within.
Steps towards better work ethics 1. Attendance
Attendance and punctuality often have a large impact on individual and team success. Lateness or absenteeism can also greatly impact job performance and retention.
How you can maintain good attendance: • Make work a high priority.
• Know your schedule. • Make use of an alarm clock. • Get enough sleep.
• Arrange your transportation.
• Inform your supervisor of an absence. 2. Character
An employer expects employees to work together toward achieving the
objectives of the company. The wise employee who is interested in having a good relationship with an employer will try to help the employer achieve success. The
employer expects employees to develop certain desirable traits that will help them to perform their jobs well so that the company can succeed. Some of these traits include: • loyalty • honesty • trustworthiness • dependability • reliability • initiative • self-discipline • self-responsibility. 3. Teamwork
It is vital that employees work as a team. It is important not only to their personal success and advancement, but also to that of their co-workers and to the company. Teamwork involves the following aspects:
• Respecting the rights of others. • Being a team worker.
• Being co-operative. • Being assertive.
• Displaying a positive customer service attitude. • Seeking opportunities for continuous learning. • Demonstrating polite behaviour.
• Respecting confidentiality. 4. Appearance
The first impression of a person is generally created within three seconds. If you appear untidy and your clothes are creased, you may give the impression that your work is sloppy. If you dress as a professional, the first impression you give will be excellent.
5. Attitude
It is very important to demonstrate a positive attitude, appear self-confident, and have realistic expectations for yourself. Developing and maintaining a positive attitude involves setting realistic expectations for ourselves at work. These goals should be challenging, but obtainable.
6. Productivity
In order to be a productive employee, a person must follow safety procedures, conserve materials, keep the work area neat and clean and follow directions properly.
7. Organisational skills
Employers consider effective time management and organisational skills as good work habits.
Medical emergencies
Medical emergencies are injuries, or an illness that is severe and poses a serious risk to a person’s life or long-term health. The treatment of medical emergencies would require the services of a doctor, nurse or any other person with the necessary medical/first aid qualifications. We must also note that not all medical emergencies are life-threatening but only require medical attention to prevent any future medical problems.
Medical emergencies in the workplace are seldom expected and very rarely properly planned for. Should there be a medical emergency where you work, your level of preparedness could mean the difference between life and death. It is therefore very important that any workshop has a well-trained first aider with a well-stocked first aid box. It is also essential for each workshop to have the telephone numbers of all emergency services close at hand.
A number of medical emergencies which could happen in an electrical workshop are listed below.
Burns
Burns in a workshop can be caused by steam from hot liquids, contact with flames, contact with hot surfaces (tips of soldering irons), electrical burns caused by someone touching a bare electrical conductor, or chemical burns (caused by chemical spillage on the skin). Managing burn injuries properly is important because they are common, painful and can result in disfiguring and disabling scarring of affected parts or even death in severe cases.
For minor burns
• Remove the person from the heat source and remove any burned clothing, except clothing imbedded in the skin.
• Run cool – not cold – water over the burn or hold a clean, cold compress cloth on it until the pain subsides. Do not use ice. Do not use butter or other types of grease.
• Remove jewellery or tight clothing from around burned areas and apply a clean bandage. You can also apply antibiotic cream.
• Never break blisters resulting from a burn.
• Remember not to remove clothing stuck to burned skin.
• If you are helping someone with a serious burn, keep the burned areas elevated to reduce swelling.
First aid for electrical burn victims
Electrical burns vary in severity, depending on the strength of the current, the duration of the electrical shock as well as the direction of the current through the body. Often these burns are deep. Electrical burn wounds may look small from the outside but could be severe on the inside. The following must be done in case of an electrical burn.
• Check that the victim is not in a state of shock (cold, clammy, pale and having a rapid pulse). If the victim is in shock, lay the victim down with feet slightly higher than the head.
• Do not apply grease or oil to the burn. • Cover the burn with a dry, sterile dressing. • Do not try and remove clothing stuck to the burn. • Cover the victim with a blanket to maintain body heat. • Call for medical attention.
What to do for chemical burns
• Dry chemicals should be brushed off the skin by a person wearing rubber gloves. • Remove the person’s clothing and jewellery and rinse chemicals off the skin by
placing the person in a shower for 15 to 20 minutes. (Be careful to protect your eyes and the eyes of the injured person.)
• Wet chemicals should be flushed off affected areas with cool running water for 20 minutes or longer or until emergency help arrives.
• If you or someone else has swallowed a chemical substance or an object that could be harmful (e.g. watch battery), call poison control first and then the medical emergency numbers. It is helpful to know what chemical product has been swallowed. Take it with you to the hospital.
Bleeding
Bleeding is mainly caused by accidents where blood escapes the body when a vein is cut. Excessive bleeding can lead to shock or even death. Any bleeding wound should be treated using medical gloves. Try to use latex gloves when treating someone who is bleeding. Latex gloves should be in every first aid kit. People allergic to latex can use a non-latex, synthetic glove. You can be infected by HIV/ AIDS if infected blood gets into an open wound – even a small one.
The following should be done to control bleeding. • Apply direct pressure or use a pressure
bandage.
• Keep the victim calm.
• Keep the bleeding point above the heart level if possible.
• When there is severe bleeding, where a major artery has been severed, pressure may be insufficient and a tourniquet may be used. Pressure from tourniquets must be relieved periodically to prevent damage to the tissue from lack of oxygen.
The following should not be done when treating bleeding.
• DO NOT apply a tourniquet to control bleeding except as a last resort.
• DO NOT peek at a wound to see if the bleeding has stopped. The less a wound is disturbed, the better the control of bleeding. • DO NOT probe a wound or pull out any
object that might still be inside a wound. This will usually cause more bleeding and harm.
• DO NOT remove a dressing if it becomes soaked with blood; just add a new one on top.
• DO NOT try to clean a large wound. This may cause heavier bleeding. • DO NOT try to clean a wound after you get the bleeding under control. Get
medical help.
There are four types of bleeding that can result from wounds: • Arterial bleeding
Full of oxygen and bright red in colour, the blood has just come from the heart so it is under pressure and often spurts from a wound in time with the heartbeat. This type of bleeding is the most dangerous because the patient can lose a lot of blood in a short time.
• Venous bleeding
Containing less oxygen and a darker red, venous blood flows at a lower pressure than arterial blood and will not spurt. It may gush freely if a major vein is torn. • Capillary bleeding
Usually minor wounds where blood oozes from the wound. Blood loss is limited but the risk of infection is very high.
Figure 1.1: Apply direct pressure
on external wounds with a sterile cloth or your hand, maintaining pressure until bleeding stops
Figure 1.2: Use a tourniquet only
as a last resort, if bleeding cannot be stopped and the situation is life-threatening
• Internal bleeding:
Bleeding may not be visible due to internal injuries. This can be very dangerous and may develop following an injury to the abdomen or the chest.
Wounds at the workplace
A worker on a construction site may suffer any of the wounds listed below. The treatment for all wounds is more or less the same, except that bleeding from deep cuts and puncture wounds may be more difficult to stop.
Cut
Caused by a knife, razor or even the sharp edge of paper. The wound may bleed profusely because cleanly cut blood vessels do not contract easily.
Tear wound (laceration)
Barbed wire, machinery or the claws of an animal may tear the skin in a ragged way. These wounds tend to bleed less severely because torn blood vessels contract more quickly than those that have been cut cleanly.
Puncture wound
Nails, needles, garden forks and even teeth may result in serious internal injury. If the wound is deep, the risk of infection is high because dirt and germs may have been carried into it.
Graze
A graze normally results from a sliding fall. Superficial layers of skin are scraped off, leaving a tender, raw area. These wounds often contain dirt or grit and may easily become infected.
Gunshot wound
These wounds can result in serious internal injuries. The exit wound is often much larger than the entry wound. Internal organs, tissues and blood vessels may be damaged by the passage of the bullet through the body. In addition to external bleeding, there may also be internal bleeding.
Bruise
A fall or a blow to the body by a blunt object. The skin is split and the surrounding tissues are bruised. With a bruise, damaged blood vessels leak blood into the tissues although the skin remains unbroken.
Listed below are a few simple steps that can be taken in the event of any medical emergency.
• Stay calm: The worst thing you can do in any emergency medical situation is to panic. For the sake of the victim and your colleagues, try to remain calm, cool and collected. You will be more effective and efficient this way.
• Assess the situation: Quickly assess the scope of the injuries and collect information. If the injured person is conscious, ask him/her to tell you if
anything hurts and observe where on the body he/she may be physically injured. Do not move an injured person, especially if he/she is reporting pain, unless there is imminent danger.
• Call the emergency services: If a person is badly injured, call the emergency services immediately. If there is any doubt as to whether emergency services are needed, it is better to be safe than sorry. Stay calm and provide your address, location in the building, phone number, name and any information you have gathered about the injuries.
• Report the injury to the appropriate authority: Depending on the size of your workplace, you may need to notify management about the situation.
• Administer first aid and CPR: If required, CPR or first aid should be performed by a person who is trained. If there is no skilled person, wait for emergency personnel to arrive. Do not administer medical treatment or medications. Be careful not to come in contact with blood, vomit or other bodily fluids.
Activity 1
1. In your own words, what is the general aim of the OHS Act? 2. What is your definition of an accident?
3. Unsafe actions and conditions are the cause of many accidents in workshops. Give THREE examples of each.
4. Why is it important to work in a well-ventilated workshop?
5. Write a short definition for what you understand by ‘dangerous practices’. 6. Why is the incorrect use of tools regarded as a dangerous practice?
7. Name at least four important things that must be taken into consideration when doing risk management.
8. Briefly distinguish the difference between a hazard and a risk.
9. In your own words, briefly explain your understanding of ‘Human rights in the workplace’.
10. Name at least FOUR issues that are also covered by human rights in the workplace.
11. Teamwork can be seen as one aspect that can lead to improvement in work ethics. How is this possible?
12. Name at least THREE other aspects which play a role in work ethics. 13. Define ‘medical emergencies’
14. Burns caused by steam, flames or even hot liquids can be very unpleasant and must therefore be treated correctly. Explain the steps to be taken when someone experiences a burn of some kind.
15. When a person experiences bleeding, name at least FOUR things you must not do when you try to help the person who is bleeding.
16. Name the FOUR types of bleeding and briefly explain each one. 17. List and explain the FIVE steps which can be taken in the event of any
Chapter 2
Three-phase AC generation
Instruments
Power
A
B
Phasor diagrams
Waveforms
A
B
Introduction
In this chapter, comparisons will be made between single-phase and three-phase generation. Both the waveforms as well as the phasor diagrams will be referred to. However, the focus will be on three-phase systems. Star and delta connections will be shown in diagrammatic as well as schematic format. For calculations, the line and phase relationship between voltage and current will clearly be shown, as well as all aspects of power calculations.
Further insights will also be given to matters like losses and efficiency. The measurements required in three-phase systems will be discussed and will include the wattmeter, kWh-meter, power factor meter as well as calculations with respect to these meters.
Principles of three-phase AC generation
When mankind first started generating electricity, it was in the form of direct current (DC). While supply and demand circuits were close together, this system worked well, but the moment this electricity had to be fed to circuits further removed, this became problematic as there was no way that DC could be increased or decreased at these faraway points.
This led to the development and introduction of single-phase AC systems, which also worked well for a certain period. But with the introduction of single-phase motors, this single-phase AC showed some shortcomings. For example, these motors were not self-starting, meaning something else had to be added to them to ensure they could start rotating with the required torque.
With the introduction of phase generation came many advantages. A three-phase generator means that instead of a single coil rotating though a magnetic field (two wires), three coils now rotate (meaning 6 wires). In the following section, comparisons will be made between the single and the three-phase systems.
Advantages and disadvantages of single-phase compared
to three-phase systems
Electrical power is generated at the power plant. This means the spinning of a coil in a magnetic field, or spinning a magnetic field in a coil. Something must spin the generator, and in South Africa this is usually done by means of the following:
Power plants Substation Commerical and industrial users Residential users System distribution loop 115 00 volts Local distribution 13 800 volts Overhead service line Substation Underground service line Underground service line
Type of generation Name of plant
Hydroelectricity Vanderkloof and Gariep Nuclear Koeberg in Cape Town
Pumped storage Palmiet in the Western Cape, Drakensberg in KwaZulu- Natal
Gas turbine Arcacia, Port Rex, Ankerlig, Gourikwa
Coal (Those in excess of 3 500 MW) Duhva, Lethabo, Matmba, Tutka, Majuba, Kendal, Matla Wind Klipheuwel, Darling
Single-phase is what is used in your house. Generally this is a 240 V 50 Hz supply available at every plug point in the house. Whether it is switching on a kettle, toaster, TV, microwave, radio or lamps, it is all connected to single phase. But that still leaves the problem of single-phase motors. Although additional circuitry can be added to make it self-starting, it is expensive and cheaper alternatives needed to be considered.
However, in industry the use of three-phase motors far outweighs that of single-phase motors, as they are self-starting, and have a better efficiency and power factor. And so the ‘three-phase generation’ was born. Three-phase generation is not a magical system. It is simply three coils positioned at 120° with respect to each other.
The supplier (Eskom) will generate the power. From there it goes via what we call transmission lines. These are the power lines that carry the power from the supplier to the consumer. The next time you drive down the road and look at the power lines, that is exactly what this section of the work is about.
There are numerous advantages to three-phase generation over single-phase generation, not only for the consumer (the people), but also for the provider (Eskom). A lot of these advantages may not make sense immediately, but all these aspects will be addressed in the relevant chapters and they will unfold as we work through each part.
The advantages of three phase over single-phase will be split into three categories. 1. The generation process.
2. The transmission and distribution process. 3. The load.
1. The generation process
• For three-phase and single-phase alternators of similar physical sizes, three phase will generate more power.
• Three-phase can supply power to single and three-phase loads.
Take note
Electricity can be created by means of a generator or an alternator.
Rotation Single-phase loads 230V Three-phase loads 400V L1 L2 L3 N Generation
• Three phase is cheaper to generate. • Three phase requires less maintenance.
• Three phase has two connection options, namely star and delta, each with their own advantages (single-phase only has one option)
• In three phase, losses are limited (phase voltages are 57,7% of the line voltages).
• The star point can be used as a neutral.
• A three-phase supply has three times the power of a single phase. 2. The distribution and transmission process
Power distribution happens over long distances and the idea is to make that happen at higher voltages and lower currents.
• Lower currents means less heat. • Less heat means less losses.
• Lower current means thinner cables can be used and associated cost savings. • Pylons required to support thinner cables will require less metal during construction and associated cost savings.
• A neutral point is available when connected in star. • Load distribution and phase balancing becomes possible. 3. The 3-phase load
Figure 2.1 (a): A three-phase motor Figure 2.1 (b): How three-phase and single-
phase loads are connected to a three-phase supply
• Three-phase motors are more efficient.
• Three-phase motors have a higher power factor.
• Three-phase motors have a much higher starting torque. • Three-phase motors do not need additional starting circuitry.
• For three- and single-phase motors of similar physical sizes, three-phase will produce more power.
• Three phase can be connected in star or delta.
• Three-phase motors are easier and cheaper to manufacture.
• Three-phase motors have fewer moving parts, therefore less maintenance. • Three-phase motors do not use centrifugal switches, less arcing, less of a fire
To summarise, most of the above simply means smaller machines are required, less materials are required to manufacture them, they’re cheaper to produce and maintain, shorter manufacturing times, easier to install and longer lasting. Those are more than enough reasons to go with the three-phase systems.
Waveform of single- and three-phase systems
Single-phase generation
Very basically, a single phase can be generated by rotating a conducting loop of wire through a magnetic field.
Figure 2.2: A generator is really a wire moving within a magnetic field
The only problem could be that the wires from the rotor could twist themselves together as the rotor turns, and a split ring commutator is used to prevent this from happening. However, in this section no attention will be paid to that process, as the objective here is three-phase generation.
A better way to see that objective is shown by the diagram below.
Figure 2.3: The generation of a single-phase AC supply by rotating a conductor through
a magnetic field
Field pole Rotation
Voltage and current produced
Field pole
However, it is a lot easier to explain this generation by using the basic sketches below.
Figure 2.4: Simplified version of the generated single phase
• At position A, the conductor moves parallel (//) with respect to (wrt) the magnetic field (MF) and zero current is induced.
• At position B, it moves at 90° wrt MF – maximum current induced. • At position C, it moves // wrt MF – zero current induced.
• At position D, it moves at 90° wrt MF but in the opposite direction – max. current induced but flowing in the opposite direction.
• At position A, it moves // wrt the MF and zero current is induced. This completes one cycle of the generated AC wave.
Three-phase generation
As mentioned before, there is no magic involved in three-phase generation. It is simply three coils placed at 120° with respect to each other. They are usually referred as to the RED, YELLOW and BLUE phases, but occasionally in our descriptions they will simply be called phase 1, 2 and 3.
As one can imagine, when the three rotating conducting loops turn in the magnetic field, six wires will twist together. However, this is very easily eliminated, by rotating the magnetic field instead, and keeping the conducting loops stationary.
Figure 2.5: Generation of a three-phase wave by rotating three coils through a
magnetic field
It is easier to describe how the red coil’s wave is generated for one rotation, and then fit the yellow and the blue coils, keeping their phase relationship with respect to the red in mind.
N
A B D C AS
C A B DN
Rotating magnetS
Red phase Yellow phase Blue phase Possible 4th wire (Natural) 120˚ 120˚ 120˚ B Y RExplanation of how three-phase is generated
• At the start, the red phase moves parallel (//) with respect to (wrt) the magnetic field (MF) and zero current is induced.
• After rotating 90°, it moves perpendicular with respect to the magnetic field – maximum current induced.
• After 180°, it moves parallel with respect to the magnetic field – zero current induced.
• After 270°, it moves perpendicular with respect to the magnetic field, but in the opposite direction – max. current induced but flowing in the opposite direction. • After one revolution (cycle), it is moving parallel to the magnetic field again – zero current is induced.
This completes one complete cycle for the red phase.
The yellow phase follows the red phase exactly, except it lags 120° with respect to the red phase.
The blue phase follows the red phase exactly, except it lags 240° with respect to the red phase.
Figure 2.6: The generated three waves with the 120 degrees phase shift indicated
between phases
Phasor diagram of single- and three-phase systems
Whether talking about single- or three-phase generation, it is always inductive. The reason for this is that conducting loops of wire are used, and this brings a certain amount of inductance into the equation.
Phasor diagram for a single-phase system
As the inductive part causes a phase shift between current and voltage, the same principle is used as in RCL circuit, to determine where they will be with respect to each other.
Take note
When drawing the waves, it is important to indicate exactly at which degrees the phases start.
R Y B
Time
Figure 2.7: The phase relationship between current and voltage through a coil (if
voltage is the reference)
Phasor diagram for a three-phase system
In the three-phase system (if connected in star), the EMFs (voltages) generated in each phase will be 120° out of phase with each other. This means that the voltage in the red, yellow and blue phases would be represented as follows.
Figure 2.8: A phasor diagram showing the three voltages (one per phase)
3-phase systems; Star vs. Delta (Delta vs. Star)
There are three voltages generated, one in each phase. The idea is not to use them as three individual single-phase voltages, but to combine them in specific ways to form a three-phase system.
There are two ways of connecting the end point of the three coils (or phases) together. They are known as the star and the delta connections.
This can be represented by means of a schematic or diagrammatic representation. Schematic (sketch without indication of components) is making use of lines, while diagrammatic (sketch with components) is almost the way it would be drawn in a circuit diagram.
C I V I L
COIL
Voltage (V) leads current (I)
Take note
Examiners like to ask for either schematic or diagrammatic representations. It is important for all learners to take note of this. 90˚ V I 120˚ 120˚ R 120˚ B Y
L1 N L2 L3 a1 c2 a2 b2 c1 b1 a1 a2 c2 b2 c1 b1 L1 N L2 L3
•
In a star-connected system, all the end points of each phase are joined and the beginning points will go to Live 1, Live 2 and Live 3. The beginning points are labeled as a1, b1, c1 and the end points as a2, b2, and c2. The common end points can be used as a neutral.
Figure 2.9: Diagrammatic versus schematic representations for a star connection
In a delta connection, the end of phase 1 will go to the beginning of phase 2, and the end of phase 2 to the beginning phase 3, and the end of phase 3 to the beginning of phase 1. All the beginning points will go to Live 1, Live 2 and Live 3.
Figure 2.10: Diagrammatic versus schematic representations for a delta connection
Only balanced loads
Since all alternators, generators and motors use windings of copper wire, each phase would have the resistance of that copper wire, as well as a certain inductance. The reason for the inductance is that the wires of each phase are turned around a core to form a coil. L1 L2 L3 a1 c2 c1 a2 b2 b1 L1 L2 L3 a1 a2 c2 b2 b1 c1
Therefore each phase would have a certain resistance and inductive reactance, and the total ohmic value for each phase is called the impedance of that phase.
It is of utmost importance that these rotating coils generate the same voltages and currents in each of the three phases and that is only possible if the impedance of all three-phases is matched. This is referred to as a balanced load.
Figure 2.11: A balanced three-phase load showing the impedance of each phase
Introduction to star and delta calculations with reference to basic line/phase values
In three-phase systems, reference is made to phase values and line values. A phase value is the voltage across one of the phases and the current flowing through that phase only. Line values refer to the voltage that is measured across any two of the lines and the current flowing between those lines. (e.g. between L1 and L2). Calculations for STAR connections
When looking at the star connection, it is clear that the voltage between L1 and L2 is actually across phases 1 and 3. However, this does not mean that one can just add the voltages of phase 1 and phase 3 together to get the line voltage, as the two phases are 120° out of phase with each other.
Current on the other hand is the same in the line and the phase.
Although there is a logical mathematical deduction of how to get this relationship between line and phase values, we will simply use the formula.
Figure 2.12: An indication that the line voltage and phase voltage cannot be the same
in a star system Take note VPHASE 1 +VPHASE 3 ≠ VLINE Z1 Z2 Z3
3-phase transmission line
3-phase generator (supply) 3-phase load
VPHASE VLINE Phase 1 Phase 2 Phase 3 L1 N L2 L3
VPHASE Phase 1 Phase 2 Phase 3 VLINE L1 L2 L3
The relationships between voltages and currents in a star system are as follows. VL = √ 3VPH
IL = IPH Example 1:
A star-connected alternator generates 240 V per phase. Each phase in this balanced system has an impedance of 16 Ω. Calculate the line voltages and currents.
In a star system VL = √ 3VPH = √ 3(240) = 415,69 V IPH = = 240 = 15 A
Z 16 In star IL = IPH = 15 A Example 2:
In a star system, a 11 kV line voltage and a line current of 20 A are generated by an alternator. Calculate the voltage across each phase as well as the impedance per phase. VPH = = 11 000 = 6,35 kV √ 3 √ 3 IL = IPH = 20 A ZPH = = 6350 = 317,54 Ω 20
Calculations for DELTA connections
When looking at the delta connection, it is clear that the voltage between L1 and L3 is actually only across phase 2 and it stands to reason that they would be equal. The current flowing from L1 would split up between phase 1 and 2, but again there is the phase implication to keep in mind.
Again, there is a logical mathematical deduction of how to calculate this relationship between line and phase values but we will simply use the formula.
Figure 2.13: An indication that the phase and line voltages are the same in a delta
system
VL VPH
VPH IPH
The relationships between voltages and currents in a star system are as follows. IL = √ 3IPH
VL = VPH Example 1:
(The same values are deliberately used to show the different outcomes between star and delta calculations.)
A delta-connected alternator generates 240 V per phase. Each phase in this
balanced system has an impedance of 16 Ω. Calculate the line voltages and currents. In a delta system VL = VPH = 240 V
IPH = = 240 = 15 A Z 16
IL = √ 3 IPH = √ 3(15) = 25,98 A Example 2:
In a delta-connected system, a 11 kV line voltage and a line current of 20 A are generated by an alternator. Calculate the voltage across each phase as well as the impedance per phase.
VL = VPH = 11 kV
IPH = = 20 = 11,55 A √ 3 √ 3
ZPH = = 11 000 = 952,63 Ω 11,55
Power in three-phase systems and calculations
Fortunately, the calculations that were done in RCL circuits regarding power stay exactly the same. As mentioned before, any alternator, generator or motor consists of coils of wire which basically makes them an RL load. This means the voltage and current in each coil will not be in phase with each other.
Electric power is transmitted in overhead lines like those shown in the picture on the right, and also in underground high-voltage cables.
Take note
Electric power is the rate of energy consumption or energy transfer in an electrical circuit. Electric power is measured by the capacity to do this power transfer and is expressed in kW or MW.
VPH
IL VPH IPH
Power in 3-phase AC circuits
Before any calculations are done, there are some basics which need to be spelled out clearly.
1. ANY of the calculations done using these formulae always uses the LINE values of voltage and current.
2. Do not confuse the single-phase formula with the three-phase formula.
3. Always make use of a phasor diagram to get clarity with regards to kW and kVA. One also notices a √ 3 in all the three-phase calculations. This simply comes from the fact that P = IV but in a three-phase star system, the voltage generated in the phase is converted to a line value, which is VL = √ 3 VPH and in delta the current generated must be converted to a line value which is IL = √ 3 IPH. This means either the voltage or the current will have a √ 3 attached to it.
Lastly, the phase angle always refers to the phase shift between the voltage and current in the phase and not the line.
Active power (also called real or true power)
The true power is the power that is effectively being used by the load or the circuit, and this would be the voltage and current values that are exactly in phase with each other. The formula used to calculate this true power is:
Pactive (P) =√3 (IL) (VL) cosθ measured in watt (W) or kilowatt (kW) Apparent power
The apparent power is the power that is supplied to the circuit. This is the product of the voltage and the current, ignoring the angle between the two. It is measured in VA or kVA, depending on the size of the values. The formula used to calculate this apparent power is:
Papp (S) = √3 (IL) (VL) measured in VA or kVA Reactive power
Reactive power is the power that is wasted and not used to do work on the load, usually in the form of heat.
Preac (Q) = √3 (IL) (VL) sinθ measured in VAr or kVAr
3-phase calculations (star and delta)
To start three-phase power calculations, the trick is to identify the type of power given. It’s proven that learners who plot the information on a phasor diagram are more successful at calculating the correct answers than those who don’t. Two examples using similar values will be used to try and demonstrate this simple fact.
Take note
Pythagoras can be used to work out a value, but the other two values must be available before the third one can be calculated.
PAPP2 = PTRUE2 + PREAC2
OR PAPP =
Example 1:
An AC star-connected alternator generates 300 kW at a power factor of 0,8 lagging. The phase voltage is 220 V. Calculate the following:
a) Line voltages b) Line current c) Apparent power (S) d) Reactive power (Q) . Answers to example 1:
a) First determine what is given and plot it on a phasor diagram. kW is always the phasor that is in phase with the supply voltage. Once this is established, it is easy to figure out the rest.
a) VL = √ 3 VPH = √ 3 (220) = 381,05 V
b) The power given is in kW, therefore it is the active power. That is the power component in phase with voltage. Now select the correct formula. Make sure you use the calculated line voltage, and remember that cos θ = 0,8 and it is not cos 0,8.
Pact = √ 3 (IL) (VL) (cos θ)
IL = Pact = 300 000 = 568,18A ____________ _______________
√ 3 (VL)(cos θ) √ 3 (381,05)(0,8)
It also helps to fill in values on the phasor diagram as they are calculated. c) Papp = √3 (IL)(VL) = √3 (568,18)(381,05) = 375 kVA
OR Papp = Pact = 300 000 = 375 kVA _____ _______
cosθ 0,8
d) Before using the reactive formula, the angle needs to be calculated. Cosθ = 0,8
θ = cos-1 0,8 = 36,87˚
Preac (Q) = √ 3 (IL)(VL)(sin θ) = √ 3(568,18)(381,05)(sin36,87) = 225kVAr
OR
Preac (Q) = √ Papp2 – Pact2 = √ 375 0002 – 300 0002 = 225kVAr (Pythagoras)
Pact = 300 kW
VT = 220 V PApp
Pr
Example 2:
(The same example, but the given power is now in kVA.)
An AC star-connected alternator generates 300 kVA at a power factor of 0,8 lagging. The phase voltage is 220 V. Calculate the following:
a) Line voltages b) Line current c) Active power (P) d) Reactive power (Q) Answers to example 2:
First determine what is given and plot it on a phasor diagram. kVA is always the phasor that is at an angle with the supply voltage. Once this is established, it is easy to figure out the rest.
a) VL = √ 3 VPH = √ 3 (220) = 381,05 V
b) The power given is in kVA, therefore it is the apparent power. Now select the correct formula. Make sure you use the calculated line voltage, and remember that cosθ = 0,8 and it is not cos 0,8.
Papp (S) = √ 3 (IL) (VL)
IL = Papp = 300 000 = 454,55 A ____________ _______________
√ 3 (VL) √ 3 (381,05)
It also helps to fill in values on the phasor diagram as they are calculated. c) Pact (P) = √3 (IL)(VL) (cos θ) = √3 (454,55)(381,05) (0,8) = 240 kW OR Pact = Papp (cosθ) = (300 000)(0,8) = 240 kW
d) Before using the reactive formula, the angle needs to be calculated. Cos θ = 0,8
θ = cos-1 0,8 = 36,87°
Preac (Q) = √ 3 (IL)(VL)(sin θ) = √ 3(454,55)(381,05)(sin 36,87) = 180 kVAr
OR
Preac = √ Papp2 – Pact2 = √ 300 0002 – 240 0002 = 180 kVAr (Pythagoras)
Pact
PApp = 300 kVA Pr
Losses
Whenever we convert one form of energy into another there are bound to be losses. No machine is perfect, and therefore not 100% efficient as there are always certain losses. Some are considered electrical losses while others are called mechanical losses. The basic losses can be categorised as follows:
• copper losses
• iron losses or core losses • friction losses
• windage losses
Electrical
Copper losses
These are basically due to the resistance of the copper wire used in the loops of wire that make up the windings of the alternator/generator. Any current that flows will experience a resistance and create heat.
Iron or core losses
The core of a generator/armature is made from soft iron which is a conducting material with desirable magnetic characteristics. Any conductor will have currents induced in it when it is rotated in a magnetic field. The currents that are induced in the generator/armature core are called EDDY CURRENTS. The power dissipated in the form of heat, as a result of the eddy currents, is considered a loss.
Mechanical
Friction losses
The full weight of the rotating parts is carried by bearings. Although these bearings make it easier to rotate and reduce friction to a large extent, they still experience friction, and friction results in unwanted heat.
Windage losses
The alternator/generator has fan blades on the rotor that blow air through and over the alternator to keep it cool. The losses due to forced air movement are called windage losses.
Efficiency: (For calculations: η=100%)
The alternators are usually designed for maximum efficiency to occur at about 80% of full load. To simplify matters, all the calculations we do will be at 100% efficiency.
The symbol used for efficiency is given by η and the formula associated with calculating these losses is:
Take note
Why is the efficiency of a generator greater than that of a motor?
The generator windings are made of thicker windings and hence have less resistance and hence lower copper losses.
In a motor, the armature produces back EMF which tries to oppose the motion of the motor (there are two opposing voltages), but in a generator there are no opposing voltages.
Only concept of power factor correction – no calculations
for exam purposes
When a load (like a three-phase motor) is connected to a three-phase supply, it is always inductive. The reasons for this have been discussed earlier. This motor will draw a certain amount of current from the supply. The user must pay for the current that is drawn, and the supply cables to the motor need to be of sufficient thickness to carry the current safely.
For example, a 20 kVA star-connected motor with a power factor of 0,75 is connected to a supply of 380 V.
Firstly, we plot this on a phasor diagram. It is 20 kVA, so it is the one at an angle to the supply voltage.
The current drawn from the supply will be: Papp = √ 3 (IL) (VL)
IL = Papp = 20 000 = 30,39 A (This is the total current drawn _________ _______
√ 3 (VL) √ 3 (380) from the supply)
This is the line current that is drawn from the supply, and this is what you pay for whether it is used effectively or not. The cables to the motor will also have to be of specified thickness to carry 31 A.
However, the horizontal component of current can be determined by:
IHORIZONTAL = ILINE × cos θ = (30,39)(0,75) = 22,79 A
This means that you pay for 30,39 A, but effectively you are only using 22,79 A. The rest are losses and the major negative implication of this is unwanted heat. The motor runs hotter, the bearings are operating at a higher temperature, maintenance increases and all this time it is completely unnecessary.
PApp = 20 kVA Pact Pr VT = 380 V Cos θ = 0,75 θ Ihor = 22,79 A Pr θ Itotal = 30,39 A VT = 380 V Cosθ = 0,75
A way must be found to reduce the angle between the voltage and the supply current. The 22,79 A will stay exactly the same, but the supply current will become less as the power factor increases. See the diagram below on how supply current decreases with an improved power factor.
Figure 2.14: Supply current decreasing as the power factor is increased
The way to achieve this is to connect a capacitor in parallel with the load. By selecting the correct capacitor, one can get very close to unity power factor. It is not possible to have it exactly correct due to varying load conditions.
A capacitor would result in a vertical current (plotted in the upwards direction), resulting in the top and bottom verticals cancelling each other out (almost like a parallel RCL circuit at resonance).
Figure 2.15: A phasor diagram showing the effect on supply current due to power
factor correction
Figure 2.16: Power factor correction is achieved by connecting a capacitor in parallel
with the load
Take note
The best power factor is 1, and improving it means changing it from 0,75 to as close to 1 as possible.
Take note
A power factor of 1 is also called unity power factor. Cos θ = 1 IT = 22,79 A Cos θ = 0,9 IT = 25,32 A Cos θ = 0,8 IT = 28,49 A Cos θ = 0,75 IT = 30,39 A ICapacitor Ivertical IT Before correction IT After correction Ihor VS
Advantages of power factor correction
For the consumer:
• Less current drawn from the supply. • Cost saving from drawing less current. • Less heat generated.
• Maintenance reduced due to motor running cooler. • Longer life span of load.
For the supplier:
• Thinner supply cables required.
• Reduced cost as a result of thinner cables.
• Suppliers’ machines not under so much stress to produce electricity that is wasted.
• Less maintenance. • Longer life span.
It can clearly be seen that it is to the advantage of both the supplier and the consumer to make use of power factor correction.
Purpose of the wattmeter
Power can be measured by connecting a voltmeter (in parallel) and an ammeter (in series) in a circuit. Record the individual readings and then multiply them together. This might work for low power circuits, but with higher values of voltage and current it becomes dangerous, and the use of potential transformers (PT) and current transformers (CT) becomes necessary. This method also does not take into consideration the power factor and therefore the answer would be in VA or kVA [Apparent power (S)]
Figure 2.17: A voltmeter and an ammeter connected to a load
With normal meters, one would simply use P = I × V (measured in VA or kVA) However, this can be time consuming, and there are far easier and more convenient ways of measuring power. The modern wattmeter enables us to measure
power without any difficulty and it measures the active power (P) in W or kW immediately.
There are two main types of wattmeters.
• moving coil (of which there are a few variations) • digital.
Figure 2.18: A variety of old and modern wattmeters
Purpose of wattmeters
• The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit.
• It essentially accumulates or averages readings.
• Electronic wattmeters are used for direct, small power measurements or for power measurements at frequencies beyond the range of electrodynamometer- type instruments.
• It basically samples the voltage and current thousands of times a second. For each sample, the voltage is multiplied by the current at the same instant.
• The average taken over at least one cycle is the real power. Digital type
It is not that easy to describe the operation of the digital meter, as different types use different technologies. What is important is to compare the advantages of using a digital meter compared to the moving coil type.
Advantages:
• The cost to operate the meter.
• Virtually all digital meters can emit a radio signal to forward the power usage for a certain time period to a recording device. This means that a utility provider can drive around the block rather than walking up to each house and recording the power usage every month.
• The best electromechanical wattmeter has a power consumption of 24 W. The leading digital meters have a starting power of less than 5 W.
• The operating power consumption for electromechanical meters is also more than that of digital meters.
• Electromechanical meters have a loss of about 0,7 W compared to the 0,5 W loss of digital meters.
• The display could also be considered an advantage because people who are not trained in reading a scaled meter can read a digital meter easily.
kWh meter
An electricity meter or energy meter is a device that measures the amount of electrical energy consumed by a residence (household) or business, or an electrically powered device.
Take note
All wattmeters are designed to measure the power in a circuit at a specific frequency. If used to measure power in another circuit where the frequency is different, the readings would be inaccurate.
Figure 2.19: A conventional kWh meter and the electronic kWh meter
Electricity meters are typically calibrated in billing units, the most common one being the kilowatt hour [kWh]. Periodic readings of electric meters establish billing cycles and energy used during a certain period of time.
http://www.youtube.com/watch?v=AsQwgagD7IU (disc speeds)
Figure 2.20: Main components of the kWh meter, and a simplified version
Basically, it measures power which is a product of current and voltage. Inside the kWh meter one would find a current coil, a voltage coil and a spinning aluminium disc.
The voltage coil
Figure 2.21: The voltage (potential) coil of thin wires inside the kWh meter
Line Line Stator Potential coil Permanent magnet (braking) Rotor (disc) Current coil Load Load I–coil V–coil AL disc L O A D L N
• A voltage coil is connected in parallel to the supply (because voltmeters are connected in parallel).
• It consists of a coil with many turns of fine wire and a laminated core.
• This coil has a high resistance to prevent as little current as possible flowing through so it does not interfere with the operation of the circuit.
• The voltage coil is designed to produce an air gap magnetic flux, which is proportional to the applied voltage.
Current coil
• The current coil is connected in series with the live wire (ammeters are connected in series).
• It consists of a coil of a few thick turns and a laminated core assembly.
• This coil must have a low resistance (hence thick wire) to not interfere with the operation of the circuit. (The full load current flows through this coil.)
• The current coil assembly must produce an air gap magnetic flux, which is proportional to the current drawn by the load.
Figure 2.22: The current coil of thick wires inside the kWh meter
Aluminium disc
Figure 2.23: The aluminium disc that rotates
• The voltage coil and core assembly and the current coil and core assembly are situated close to each other.
• The aluminium disc is located precisely in the air gap between these two assemblies.
Voltage coil
Gear train to dials
Copper shading ring
Current coils
Magnetic brake Rotating aluminium disc
• The two magnetic fields of the current and potential coils induce an eddy current in the disc creating a torque, causing it to turn.
• The more current that is drawn by the user, the stronger the magnetic field, the faster the disc will rotate.
• The disc is attached to a spindle in which it rotates and in turn it drives a geared mechanism which indicates the units of power used.
Meter register
Figure 2.24: Meter register
• A part of the meter registers the revolutions of the aluminium disc. • This drives a gear train that is attached to the meters (clock mechanism) that are calibrated in kilowatt hours (kWh).
• This in turn tells us how much electricity we have used over a period of time.
Display or measuring face
Prepaid electricity meters
A prepaid electricity meter is a KWh (Kilowatt Hour) meter, measuring electrical consumption. The main difference is that this KWh meter counts backwards as the electricity is consumed and has a relay (an automatic switch) which disconnects the power when the KWh reading on the meter reaches zero. It further incorporates hardware which has the ability to decipher the pin number entered and convert it to KWh. All prepaid meters in South Africa are STS (Standard Transfer Specification) compliant. This means that they all use the same coding system. In SA this is a 20- digit encrypted code, preventing fraudulent vouchers from being generated.
Initiated by Eskom
The development of STS (Standard Transfer Specification) was initiated by Eskom, since they were purchasing numerous prepayment units for various municipalities from different manufacturing companies, which resulted in their purchasing different vending systems for the operation of these units. The introduction of STS was the only means of ensuring that the prepayment tokens issued by the vending system of one manufacturer could also be used by a prepayment unit of a different manufacturer. As a result, all prepaid units, even though different in looks and manufactured by different companies, function using the same STS encryption technology for the production of vouchers. The Standard Transfer Specification (STS) has become recognised as the only globally accepted standard for prepayment systems. It has become established as a worldwide standard for the transfer of electricity prepayment tokens since its initial introduction in South Africa in 1997.
Conclusion
The Standard Transfer Specification has found widespread application. Initially created in South Africa, it has subsequently been distributed to many developed and developing countries. To date, over 10-million STS-compliant meters have been installed at 400 utilities in 30 countries around the world.
http://www.prepaidelectric.co.za
Different types of prepaid meters used in South Africa
