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X ray generators

  1. 1. X ray generators
  2. 2.  An x ray generator is the device that supplies electric power to the x ray tube  It begins with a source of electrical energy  The x ray generator modify this energy to meets the needs of x ray tube
  3. 3.  The tube require electric energy for two purposes: 1.To boil electrons from the filament 2.To accelerate these electrons from the cathode to anode  X ray generator has a circuit for each of these functions refer them as 1. filament 2. high voltage circuits 3. timer mechanism, which regulates the length of x ray exposure
  4. 4. The mechanism of an x ray generator is continued in two separate compartments: A control panel or console A transformer assembly
  5. 5.  Control panels may be simple or quite complex  The console allows the operator to select the appropriate kVp , mA and exposure time for a particular radiographic examination.  Meters measure the actual mA and kVp during the exposure
  6. 6.  One exposure button readies the x ray tube for exposure by heating the filament and rotating the anode  The other button starts the exposure  The timing mechanism terminates the exposure
  7. 7.  The transformer assembly is a ground metal box filled with oil 1. It contain low voltage transformer for filament circuit 2 High voltage transformer and a group of rectifiers for the high voltage circuit - the potential difference in these circuits may be as high as 1,50,000 V, so the transformer and rectifier are immersed in oil - the oil serve as an insulator and prevents sparkling between the various compartments
  8. 8. Transformers  A transformer is a device that either increase or decrease the voltage in a circuit  The x ray generator receives 115 or 230 V,60-Hz (cycles per second) alternative current.  Filament heating requires a potential difference of approx.10V  Electron acceleration requires a PD that can varied between 40,000 V and 1,50,000 V  Transformer are used to change the potential difference of incoming electric energy to appropriate level
  9. 9.  A transformer consists of two wire coils wrapped around a closed core  The core may be a simple rectangle with the windings wound around opposite sides of the rectangle  The circuit containing the first coil (which is connected to the available electric energy) is called the primary circuit  The circuit containing the second circuit (from which comes the modify electric energy)
  10. 10.  The core of a transformer is laminated  It is made up of thin sheets of special iron alloy separated from each other by thin insulating layers  This layers clamped tightly together  The purpose of lamination is to reduce eddy currents which is waste power and appear as heat in the transformer core
  11. 11.  When current flows through the primary coil, it creates a magnetic field within the core ,and this magnetic field induces a current in the secondary coil  Current only flows through the primary coil , It creates a magnetic field induces a current in the secondary coil i.e either increasing or decreasing
  12. 12.  In the primary circuit is connected to a battery and the secondary circuit to a voltmeter  No secondary current flows while the magnetic field in the core is in a steady state
  13. 13.  When the switch in the primary circuit is closed,the battery drives current through the primary coil ,which creates magnetic in the iron core  As magnetic field increases,it induces a current through the secondary coil  Thus current builds up a potential difference between the two ends of the coil, the voltmeter needles swings to one side
  14. 14.  As soon as the magnetic field stabilizes ,the potential across the secondary coil drops to zero and remains there until the switch in primary coil is opened  When the switch is opened ,the magnetic field induces a potential difference across the secondary coil  The polarity of the potential is reversed ,and the voltmeter needle moves in the opposite direction
  15. 15.  The important fact to remembers that a current only flows in the secondary circuit when the magnetic field is increasing or decreasing  Alternating current is used for a transformer because it is produced by a potential difference (voltage) that changes continuously in magnitude and periodically in polarity  Current flows in one direction while the voltage is positive and in opposite direction while the voltage is negative
  16. 16.  The most important characteristic of alternating current is that its voltage changes continuously so it produces a continuously changing magnetic field  Therefore an alternating current in the primary coil of a transformer produces an alternating current in the secondary coil
  17. 17. CORE  The transformer cores are always designed so that they form a closed circuit  A core with a closed magnetic circuit has a high permeability and is very efficient  there are 3 types of core  Core type  Shell type  Cross type or H type
  18. 18. Core type  In this the primary winding is on one leg and secondary winding is on other leg  This is easily assembled and has a good cooling surface  Alternatively both primary and secondary windings are made as two halves  This is most preferred
  19. 19. Shell type  In this the primary and secondary are wound around the central limb  The magnetic circuit is shorter  Most efficient design in terms of energy conversion and efficiency (98 %)  So it is used most commonly
  20. 20. Cross or H type  It is called as modified shell type since it is combination of two shell cores set at right angles to each other  In this the coils are surrounded by four legs  The windings are located over the central core which is four times the area of each of the outside legs  This type of core is cooled easily  So it is used in large power transformers
  21. 21. Transformer losses  The output power is always lesser than the input power  So the efficiency of a transformer is always less than 100%  This implies that some amount of energy is lost in the form of heat  EFFICIENCY =power output / power input  Energy loss can be considered as 1. Copper losses 2. Eddy current losses 3. Hysteresies 4. Flux leakage losses
  22. 22. LAWS OF TRANSFORMERS The laws govern the behaviour of a transformers 1. The voltage in the two circuits is proportional to the number of turns in the two coils 𝑁 𝑃 𝑁 𝑆 = 𝑉 𝑃 𝑉 𝑆 𝑁 𝑃 = number of turns in the primary coil 𝑁𝑆 = number of turns in the secondary coil 𝑉𝑃 = voltage in the primary circuit 𝑉𝑆 = Voltage in the secondary circuit
  23. 23.  Example:  The primary coil has 100 turns and the secondary coil has 30000 turns. If the potential difference across the primary coil is 100 V, the potential difference across the secondary coil will be  100 30000 = 100 𝑉 𝑆  𝑉𝑆= 30000 V
  24. 24.  A transformer with more turns in the secondary coil than in the primary coil increases the voltage of the secondary circuit is called a step up transformer
  25. 25.  One with fewer turns in the secondary coil decreases the voltage and is called a step down transformer
  26. 26.  The second law of transformer is simply a restatement of law of the conversion of energy  A transformer can not create energy  An increase in the voltage must be accompanied by a corresponding decrease in current  The product of voltage and current in the two circuits must be equal
  27. 27.  𝑉𝑃 𝐼 𝑃= 𝑉𝑆 𝐼𝑆  𝑉𝑃 = voltage in the primary coil  𝐼 𝑃 = current in the primary coil  𝑉𝑆 = voltage in the secondary coil  𝐼𝑆 = current in the secondary coil  Example:  The voltage across primary coil was 100 V, that across secondary coil was 30000 V. if the current in the primary coil is 30 A, then the current in secondary coil will be  100 × 30 = 30000 𝐼𝑆  𝐼𝑆 = 0.1 A(100 m A )
  28. 28.  The product of voltage and current is power  If the potential difference in volts and current is in amperes, then power will be in watts  W = V × I  In the last example the power in transformer is 3000 watts  It is the same on both high voltage and low voltage sides of the transformers
  29. 29.  The wire in the transformer must be large enough to carry the current without over heating  As a result, high voltage transformers are both large and heavy which also make them very expensive
  30. 30.  There are two basic circuits in a diagnostic x-ray unit  One circuit contains the step up transformer and supplies the high voltage to the x-ray tube  The other circuit contain step down transformer and supplies the power that heats the filament of x-ray tube  autotransformer supplies the primary voltage for both these circuit
  31. 31. THE AUTOTRANSFORMERS  The voltage supplied to the x-ray room connects to the x- ray generator through an autotransformer in most cases  Functions :  Provides voltage for x-ray tube filament circuit  Provides voltage for the primary coil of the high voltage transformer  Provides a convenient location for kVp meter that indicates the voltage to be applied across the x-ray tube
  32. 32.  An autotransformer consists of a single winding wound on a laminated closed core  The autotransformer works on the principle of self induction  An alternating current applied between the input points will induce a flow of magnetic flux around the core  This magnetic flux will link with all the turns forming the coil , inducing voltage into each turns of winding
  33. 33.  Example  If 230V are applied between points A and B connect to 115 turns of the autotransformer winding the volts per turn will be 2  By suitable selection if taps one may select the number of turns to supply the necessary voltage to the other components of the x-ray generator
  34. 34. X ray circuit
  35. 35. FILAMENT CIRCUIT  The filament circuit regulates current flow through the filament of the x-ray tube  The filament is a coiled tungsten wire that emits electrons when it is heated by this current flow  Not much power is needed to heat this filament to the necessary high temperature
  36. 36.  A current flow of 3 to 5 A with an applied voltage of about 10 V are typical values  This current merely heats the filament does not represent the current across the x-ray tubes  The power to heat the x-ray filament is provided by small stepdown transformer called filament transformer  The filament is connected directly to the second winding of this transformer
  37. 37.  The primary winding of filament transformer obtain its voltage by tapping of an appropriate number of turns from the autotransformer  This voltage will be around 100 to 220 V across the primary winding  To reduce this to the desired 10 V range , the primary coil in the stepdown transformer in the filaments circuit has appropriately 10 to 20 times as many turns of wire secondary coil
  38. 38.  The secondary winding of filament transformer has only a very small voltage across it and is connected to the filament of x-ray tube  The x-ray tube of course has very high voltage across it  This makes it necessary to provide high voltage insulation between the secondary and primary windings of the filament transformer  The filament transformer is usually placed in the same oil field grounded metal tank as the high voltage transformer
  39. 39.  Precise control of filament heating is critical, because of a small variation in the filament current resulting in large variation in x-ray tube current  The x-ray tube current is produced by the flow of electrons from their point of origin(filament) to anode(target) of x-ray tube
  40. 40.  The x-ray filament current may be controlled by altering the voltage to the primary of the stepdown transformer by addition of resistors connected in a series in the circuit leading from the autotransformer  If the resistance is increased more voltage must be used to push current through the resistance, making less voltage available to the filament transformer primary
  41. 41. High voltage circuit  The circuit has 2 transformers , an autotransformer and a step up transformer  The auto transformer is actually kVp selector and is located in control panel  The voltage across the primary coil of stepup transformer can be varied by selecting the appropriate number of turns in the autotransformer  The kVp can be adjusted in steps from approximately 40 to 150 kVp
  42. 42.  The stepup transformer is sometimes called high voltage transformer  It has many more turns in the secondary coil than the primary coil and it increases the voltage by a factor of approximately 600  The potential difference across the secondary coil may be as high as 1,50,000 V  So it is immersed in oil in the transformer assembly for maximum insulation
  43. 43.  Two meters are incorporated in to the high voltage circuit, one to measure kVp and the other to measure mA  The meters themselves are located under control panel  They indicate potential across the x-ray tube and the actual current flowing through the tube during x-ray exposure  The voltmeter measures the difference in electrical potential between two points  Electrons moving through the difference in potential constitute an electric current
  44. 44.  In a closed circuit the same number of electrons flows through all points  An ammeter counts the number of electrons flowing past a point per unit time and can be placed in the circuit wherever it is most convenient  The ratio of voltage across the primary and secondary coils in a transformer is propotional to the number of turns in two coils
  45. 45.  kVp meter can be placed in the circuit between the autotransformer and step up transformer  The voltage which energizes the kVp meter is the voltage from autotransformer that will be applied to the primary windings of high voltage transformer when exposure begins  Because the kVp meter records the selected kVp before the actual exposure begins is usually term the prereading peak kilovolt meter
  46. 46.  The voltage in this circuit is relatively small and the meter can be located on control panel with minimum of insulation and without serious risk of electrical shock  The connections for the mA meter must be in the secondary coil of the high voltage transformer to record current flow accurately  The mA meter is in a circuit with a potential difference of up to 150 kilo kVp to minimize the risk of electric shock
  47. 47.  The connections are made at the point at which the transformer is grounded, which is the center of coil with a voltage across the coil of 150kVp, the potential on one side is +75 kVp and on other side -75 kVp  The center of coil is at zero potential  If the meter is connected at this point, it may be placed on a control panel without risk of shock to the operator
  48. 48. Rectification  Changes alternating current(AC) output of high voltage transformer to direct current(DC)  allows current flow in one direction only  x-ray tube is a rectifier because current will not flow from anode to cathode
  49. 49. Halfwave Rectifier Circuit + - X Second Half Cycle: Diodes open No voltage applied to tube No tube current (mA) + - First Half Cycle: Diodes closed Voltage applied to tube Tube current (mA) results - -
  50. 50. Fullwave Rectifier  Four diodes  120 pulses/second  exposure times half of halfwave circuit Secondary of High Voltage Transformer Voltage applied to tube (also mA waveform)
  51. 51. Fullwave Rectifier + B - A X X First Half Cycle Second Half Cycle Voltage applied to tube (also mA waveform) X X +A -B
  52. 52. Full-Wave Rectification  Rectifiers  Four diode “bridge” configuration used with single phase  both + & - half cycle of high tension transformer used  efficient  circuit reverses negative half cycle & applies to x-ray tube Applied to X-ray TubeOutput of High Tension Transformer Tube
  53. 53. Three-Phase Generators  Commercial power generally delivered as 3 phase  phases 120o apart Single Phase Power Three Phase Power
  54. 54. Three-Phase Generators  Rectifier circuit  Inverts negative voltage  sends highest of 3 phases to x-ray tube To X-Ray Tube Input 3 Phase Voltage Rectified
  55. 55. Three-Phase Generators  Produces nearly constant potential  much higher tube ratings than single phase  more efficient than single phase  shorter exposures  High repetition rates
  56. 56. Medium (or high) Frequency Generators  higher frequency square wave voltage sent to primary of high voltage transformer  very efficient  transformer & generator very small  some transformers integral with x-ray tube head
  57. 57. Medium Frequency Generator Operation  incoming AC converted to pulsed DC AC DC
  58. 58. Medium Frequency Generator Operation  Pulsating DC smooth to constant voltage Constant DCPulsating DC
  59. 59. Medium/High Frequency  Transformerefficiency: V~ fnA  Byincreasing frequency(f),cross sectionalarea(A) reducedforsamepower  Frequencyof invertorrangesfrom5-100kHz!  Feedbackloop controlled–duringexposureif kVdropsoff,increaseinvertorfrequency& kVincreases
  60. 60. Medium Frequency Generators  Advantages  immune to power line fluctuation  Timeraccuracy  Shorterexposures (<10ms)  low ripple  small size of electronics & transformer  Today’s trend in generators