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9.3 transformers



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  • 1. Transformers & Electrical Distribution Systems HSC Module 9.3 Motors & Generators Copyright Jeff Piggott, 2003. All rights reserved.
  • 2. Objectives
    • Discuss why some electrical appliances in the home that are connected to the mains supply use a transformer.
    • Identify some of the energy transfers and transformations involving the conversion of electrical energy into more useful forms in the home and industry
    • Analyse the impact of the development of the transformer on society.
  • 3. Transformer: Basic Structure
    • A transformer consists of two or more coils coupled magnetically by way of a “core”.
    • Side (coil) of transformer where source
    • (or input) voltage/current is applied = “primary coil” .
    • Side (coil) of transformer where induced
    • (or output) voltage/current is produced = “secondary coil” .
  • 4. Transformer: Principle of Operation
    • A transformer operates on the principle of mutual inductance ie.
    • a changing current in one coil (primary) induces an emf in another (secondary) coil.
  • 5. Purpose and Principle of the Transformer
    • 1. The changing current
    • in the primary coil, is
    • usually achieved by applying
    • an alternating voltage,
    • resulting in an alternating
    • current ( AC )
    AC input AC output
    • As the alternating current changes
    • magnitude and direction, a magnetic
    • field is produced, which changes in a
    • corresponding manner
    • The field from the primary coil is
    • intensified and concentrated (also
    • referred to as increasing the “flux
    • linkage”) through the secondary coil by an iron core
    4.The changing flux through the secondary coil, induces a potential difference across the secondary coil
  • 6. Step-Up Transformer Flux.  AC Input Primary Coil Secondary Coil Core AC Input Flux.  AC Output ( increased! ) Primary Coil Secondary Coil Core # turns on secondary > # of turn on primary n s > n p
  • 7. The Induction Coil
    • Induction coil = step-up transformer with a much greater number of turns on the secondary
    • (~5 000) than on the primary (typically < 100).
    • Input voltage = 6V; Output voltage =~30 000V
  • 8. Operation of an Induction Coil
    • NOTE : Pulsed DC is used because the rate of change of flux
    • is much greater than that produced by 6 V AC.
    Electrical contact broken as coil becomes magnetised - magnetic field starts to collapse. Field builds when current flows in coil. + – I iron cored coil reed switch DC supply I + –
  • 9. Step-Down Transformer Flux.  AC Output decreased Primary Coil Secondary Coil Core AC Input # turns on secondary < # of turn on primary n s < n p
  • 10.
    • Provides a channel for magnetic fields (enables redirection and strengthening of magnetic field)
    •    = total magnetic field lines (in Wb)
    • B = flux density = # of field lines/ unit area (in teslas, T)
    Transformer Core B =  / A
  • 11. Core Material
    • Amount of flux produced in the core depends on a property of the core material - “ permeability ” ,  , – a constant for different types of material.
    • Materials that cause lines of flux to move further apart ie. decrease flux density are called “diamagnetic” ; those that concentrate flux by 1 – 10 times are called are called “paramagnetic” ; and those that concentrate flux by >10 times are called “ferromagnetic” .
    • Certain ferromagnetic materials, especially powdered or laminated iron, steel, or nickel alloys, have µ that can range up to about 1,000,000.
  • 12. Transformer Equation
    • In ideal transformers, there is no power loss and power input to primary coil equals power output from secondary coil.
    • The rate of change of flux in both coils is the same, =  /  t .
    • From Faraday’s Law (  =-  /  t) to:
    • (i) the secondary coil: V S = n S  /  t……..(1)
    • (ii) the primary coil: V P = n P  /  t………….(2)
    • Dividing equation (1) by equation (2):
    V P /V S = n P /n S
  • 13. Transformers and Conservation of Energy
    • The Principle of Conservation of Energy states that: “Energy cannot be created or destroyed, merely changed from one form to another.”
    • This means that energy obtained from secondary coil, at most (without heat losses), can only equal energy supplied to primary coil. Also, since power = rate of supply of energy :
    • P primary = P secondary
    • But P=VI, therefore:
    • V P I P = V S I S
    • Combining this equation with the transformer equation gives:
    I S /I P = n P /n S
  • 14. Eddy Currents
    • Eddy currents are induced currents that result when there is a B field acting on part of a metal object and there is relative movement between the object and the field, such that the conductor cuts across magnetic flux lines.
    • Eddy currents are
    • circular currents.
    • They are an
    • application of
    • Lenz’s Law .
    Eddy current motion X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X
  • 15. Eddy Currents Reduce Transformer Efficiency
    • Energy output of a real transformer is always less than the energy input.
    • Energy losses occur because eddy currents induced in the transformer core by the alternating current, result in resistive heat losses (the transformer core heats up).
    • The ratio of the energy output to the energy input, expressed as a percentage is called the efficiency of the transformer.
    Energy input Energy output energy losses Input 240 V Output 12 V transformer
  • 16. Core Laminations Splitting the core into laminations – thin sheets – reduces effects of eddy currents by restricting them to shorter pathways. Laminated iron core Insulating layers
  • 17. Effect of Core Lamination Thickness 0.50 0.0127 0.75 0.0254 0.85 0.0508 0.90 0.10 to 0.25 0.95 0.27 to 0.36 Eddy Current Losses Lamination Thickness (mm)
  • 18. Transformers & Electrical Distribution In Australia, 23,000V AC generated, 330,000V or 500 kV AC HV transmission line, 240VAC 50 Hz end use single phase, 415VAC 50 Hz 2 and 3-phase.
  • 19. Electric Power Distribution System - Structure
    • The typical delivery system for the supply of electrical power is based on central-station service.
    • The power generating station produces AC electricity
    • Step-up transformers increase the voltage level of the electricity for bulk transmission
    • Transmission lines carry large amounts of electricity across the nation.
    • Substation transformers lower voltage so that electricity can be delivered to local homes and businesses.
    • The electricity reaches the customer over a system of distribution wires.
  • 20. Commercial Power Generators
    • Commercial power stations use AC generators to produce their electrical energy.
    • AC generators are preferred because:
    • (i) Easy to step up AC emfs to higher voltages for transmission.
    • (ii) AC electricity transmitted with low energy losses.
  • 21. Step-up Transformers at Power Generation Plants
    • Electricity generated at a power station is usually produced at a voltage ranging from a few hundred volts to tens of kilovolts. (Eraring power station at Lake Macquarie has four 660 MW generators with an output of 23 kV).
    • It is transformed to 330 kV or 500 kV for transmission over the distribution grid .
  • 22. Transmission Grid Conductors
    • The transmission grid consists of high voltage overhead lines and underground cable made of either copper or aluminium.
    • Copper is much heavier than aluminium so it is used primarily in insulated wires and cables.
    • Aluminium is suitable for transmission and distribution and allows the use of much lighter and more economical support structures. The tensile strength of pure aluminium is not high enough for most applications so aluminium alloys or steel reinforced aluminium alloys are used.
  • 23. Electrical Transmission Lines – Insulation of Wires In dry air, electrical sparks can jump the following distances for the given potential differences: 10 000 V --------- 1 cm 20 000 V --------- 2 cm 100 000 V ------- 10 cm 330 kV -------- 33 cm *Distances smaller in very humid air
  • 24. High-Voltage Insulator
    • Prevents electrical sparks
    • jumping from high voltage
    • lines to support poles or towers.
    • Insulators made of individual
    • sections:
    • (i) Shape prevents build up
    • of dust or grime (which
    • conducts when it absorbs water)
    • (ii) Increases distance current
    • must flow over insulator surfaces,
    • so decreases chance of sparking.
    Static Dischargers Transmission cable Disk-shaped ceramic/glass insulators Suspension insulator for 330 kV transmission line
  • 25. Why Ceramic or Glass Insulators?
    • Glass and ceramics lack a crystal structure - called amorphous materials .
    • To conduct electricity, a material must have &quot;free&quot; electrons ( not the same as excess electrons ).
    • In glass and ceramics all of the electrons are localised ie. bound to a nucleus, whereas in metals, some electrons (“free” electrons) are not bound to nuclei  conduction.
    Early glass electrical insulator
  • 26. Electrical Transmission Lines – Protection from Lightning Strike
    • Lightning usually strikes highest point.
    • Electrical transmission systems usually use a single cable – continuous earth line - running between poles & sitting above the 4 transmission lines (3 phase lines and return ground line)
    • Continuous earth line normally carries no current - conducts charge from lightning strike to earth .
  • 27. Powerline Energy Losses
    • Low resistance transmission cables used so that resistive heating and energy loss are minimised.
    • Power is transmitted at high voltages [500 kV typical] , thus reducing the magnitude of the current, I, flowing in the lines.
    • P= VI
    • * Resistive heat losses:
    • P lost = I 2 R where I is small
  • 28. WARNING: Two Types of Voltage to Consider
    • There are two voltages to consider in most electrical transmission problems:
    • “ Floating voltage” = voltage of transmission (energy per coulomb given to charges at the switching yard).
    • This CANNOT be used in:
    • P=VI to find power loss in wires!!
    • “ Voltage loss” = difference in voltage at either end of transmission line (energy per coulomb lost by charges during transmission). This is most easily found from V=IR
      • where I = current transmitted
      • R =total resistance of transmission wire.
  • 29. Sample Problem – Power Loss
    • A generator produces 20 kW of power at 200 V. The 1.0
    • km long transmission lines over which the power is
    • transmitted have a total resistance of 0.50 W. Determine
    • the power lost in the lines and the voltage available at the
    • end of the lines.
    • Solution:
    • (i) P = VI  I = P/V
    • I = 20000 / 200 = 100A.
    • Power lost in wires:
    • P = I 2 R = (100) 2 X 0.5 = 5 kW .
    • (ii) Voltage loss during transmission:
    • V = IR
    • V = 100 x 0.5 = 50 V
        • Therefore, voltage available = 200 V – 50 V = 150 V
  • 30. Superconducting Transmission Lines
    • Superconducting transmission cable is a technology intended to increase transmission capability. High temperature superconductivity (HTS) cable has no resistance.
    • HTS has the potential to deliver twice the power capacity with the same power loss and smaller diameter as conventional cable.
    • One potential design which is well-suited for retrofitting in networks has an HTS conductor enclosed in a cryogenic environment which is covered by conventional room-temperature dielectric. Prototype cable systems have been developed in the US and actual systems are expected there over the next few years.
  • 31. Sub-Stations & Local Transformers
    • Step-down transformers are required at local
    • substations to step down the very high voltages
    • from transmission lines to lower voltages (11 kV) for
    • suburban distribution. Finally, local transformers
    • step the voltages down further for domestic use
    • (240 V).
  • 32. Household Uses of Transformers
    • Step-down transformers are
    • found in all electronic devices that
    • can be run from the domestic 240 V
    • AC supply, since most electronic
    • devices require low voltages to
    • operate the semiconductor
    • components that they depend for
    • their operation, for example, a
    • computer will include components
    • that run on 12V, 5V or 1.5 V .
    If not AC, otherwise would have to be provided by batteries = high cost . TV’s need high voltages to function .
  • 33. Transformers & Electrical Appliances in the Home Electronically operated domestic appliances require both a step-down transformer to change 240 volts to about 5 - 20 volts & a rectifier to change the low voltage AC to DC. TV, stereo, computer, CD player, clock radio, fluorescent lights, home security systems, microwave oven, answering machines, air conditioner, fax machines, washing machines, microwave oven kettle, hot water heater, toaster, older room heaters, hair dryers, incandescent lights, old model refrigerators, some clothes dryers Appliances with a transformer Appliances without a transformer
  • 34. Energy Transfers in the Home (1)
    • Much of the energy transferred in homes is
    • electrical energy . This is because electrical
    • energy is readily transferred as:
    • a) heat (thermal energy)
    • b) light
    • c) sound
    • d) kinetic energy (movement).
    • Amount of electrical energy transferred depends
    • on:
    • a) time appliance is switched on;
    • b) appliance power rating
    • W [ work] = P [power] x t [time]
  • 35. Energy Transfers in the Home (2) Copy and complete the table below.  Washing machine  VCR  Hair dryer  Electric drill  Air conditioner  Blender  Radio  Television Energy Transformation Household Appliance
  • 36. Example: Domestic Transformer
    • For the transformer shown here:
    • a) What is the ratio of the number of turns on the primary to the number of turns on the secondary coil?
    • b) Suggest a possible use for this transformer.
  • 37. Transformers Problem #1
    • A transformer has input voltage and current of 12.0 V and 3.0 A. It has an output current of 0.75 A.
    • a) If there are 1200 turns on the secondary coil, how many turns are on the primary?
    • b) What is the output voltage?
  • 38. Transformers Problem #2
    • An ideal transformer has 100 turns on the primary coil and 2 000 turns on the secondary coil. The primary voltage is 20 V. The current in the secondary coil is 0.5 A.
    • a) What is the secondary voltage?
    • b) What is the output power?
    • c) What is the input power?
    • d) What is the current flowing through the primary coil?
  • 39. Impact of Transformers on Society (1)
    • The first practical transformer, using AC, was developed in 1883.
    • Prior to this, direct current was seen as being the logical way to distribute energy using electricity.
    • AC triumphed, and by the early 1900s, its future impact on society was inevitable.
    • Transformers permitted the long-distance transfer of electrical energy with low resistive energy losses.
    • Without the high voltages possible through the use of transformers, the electrical wires required to transmit large amounts of electrical energy would have to have been too large to be practical.
  • 40. Impact of Transformers on Society (2)
    • Transformers were a key to establishing electrical
    • energy as the driving force behind technological and
    • industrial development in the 20th century.
    • Electrical energy rapidly became the means of
    • lighting homes and cities, with its distribution
    • facilitated by the use of transformers .
    • Electrically operated machines thus replaced less efficient machines, resulting in the rapid growth of industry and commerce.
    • Communication networks grew rapidly as a result of electrical energy and its intimate association with radio, then television and ultimately the computer revolution of the late 20th century.
    • Every home has dozens of appliances that make use of transformers, permitting a host of electronic devices to be operated from the mains.
  • 41. Effect of High Voltage Power Lines on Humans Alternating E field induces an alternating current to flow in body Sign changes 100 times/ s. + - - - - - - - - - + + + + + + - - + + + + + + + + - - - - - -- +
  • 42. What are the Health Implications?
    • Studies still in progress
    • At least one study has shown that exposure to strong electric and magnetic fields increases likelihood of developing cancers and leukemia.
  • 43. Phasing and em Radiation Exposure Levels Phasing assists to reduce the E fields when multiple power lines are present. “ Code” related health effects refer to wiring codes where the conductors are far apart. The closer the supply and return wires are together, the lower the fields due to phase cancellation - - - - - - - - - + + + + - + + + + + + + + - +
  • 44. Phasing and em Radiation Exposure Levels (cont)
    • Phasing assists to reduce the B fields when multiple power lines are present as with E-field.
    • Dynamic magnetic field causes currents to flow in a circular fashion within the body.
    • They will reverse 100 times / second
    Current direction B Magnetic field arising from current Induced current in body
  • 45. em Field Exposure
    • Typical values:
      • Under power line 10 microT and 10,000 V/m
      • 10m from 12kV line 0.2-1 microT and 2-20 V/m
      • Within home 150-0.02 microT depending on proximity to electrical appliances
        • >0.20 microT at 1m distance only for
        • washing machines,dishwashers, can openers, microwave ovens
      • Electric train ~ 60microT at seat