Xray Generators,Transformers,Rectification

Producing Xray Isn't no joke , A lot of Equipment and science is behind this task. Read in to learn more

1. HIGH TENSION GENERATORS AND RECTIFICATION PRESENTER Akshay singh Bsc. 2 nd year MODERATOR Dr. S.C Bansal Lecturer Department Of Radio-diagnosis And Imaging PGIMER , CHANDIGARH-160012 15 Jan,2014

2. INTRODUCTION An X-ray generator is the device that supplies the electric power to the X-ray tube . It provides electrical energy to > Boil electrons from the filament. > Accelerate these electrons from cathode to anode. It regulates the length of exposure. 2

3. ELECTRICITY & CURRENT Electricity is the flow of current. Flow of charge is called electric current Its SI unit is Amperes. i.e ., I = q/t where, q= charge t=time I= electric current 3

4. ELECTRIC CURRENT ALTERNATING CURRENT An electric current that reverses its direction many times a second at regular intervals. DIRECT CURRENT Electric current which flows in one direction only through a circuit or equipment. 4

5. 5

6. SINGLE PHASE A.C Only one coil rotates between two magnetic poles and give rise to one sine wave . 6

7. Poly phase A.C In poly-phase A.C or 3 phase A.C , three coils are made to rotate simultaneously by placing them in front of magnet equidistantly which produces three separate supplies of Alternating current. 7

8. N S A B C 360 o 240 o 120 o A B C 8

9. Distribution of Electricity Since electricity is generally generated in form of poly-phase. Windings of circuits can be arranged in the following two ways : star type or wye type Delta type 9

10. Star or Wye type In this type one end of the three windings is connected to a common centre point and four terminals are used. The three conductors 1, 2, 3 are the three lines of supply & 4 th is the neutral. Coil A Coil B Coil C 2 1 3 4(Neutral) 10

11. 1 2 3 4(Neutral) P1 P2 L1 L2 P1,P2 got phase voltage i.e., 230-240V L1,L2 got line voltage i.e.,400-415V 11

12. Delta type The windings in this type can be arranged as a delta, i.e., ∆. It may also be described as mesh connection 12

13. Advantages of wye connection over delta Machinery is cheaper. Less stress and liability to break down in regard to insulation. Can supply two different voltage as per need of different types of user, e.g. low voltage for domestic user and high for industrial user or large installations 13

14. HIGH TENSION CIRCUITS An X-ray equipment is just not directly connected to the source where electricity is generated. There are many other components required to complete the circuit. Components of H.T. Circuit: 1.Transformer Step up transformer Step down transformer Autotransformer 2. Rectifiers Self rectifier Half wave Full wave 14

15. TRANSFORMERS

16. Definition Device that converts an alternating current of a certain voltage to an alternating current of different voltage without change of frequency. It is based on the principle of electromagnetic induction. When current flows through the primary coil , it creates a magnetic field with in the core , and this magnetic field induces a current in the secondary coil . Principle 16

17. 17

18. Types of transformers on the basis of construction 18

19. 1 st law: The voltage in the primary or secondary coil is directly proportional to the number of turn in the coil. i.e., where Vs , Vp =voltage in secondary, primary coils and Ns , Np = number of turns in secondary, primary coils respectively Hence, In step up transformer Ns > Np In step down transformer Np > Ns LAWS OF TRANSFORMERS 19 Ns = Vs Np Vp

20. 2nd law : It is based on law of conservation of energy & states that Power in primary coil = power in secondary coil Since, Power = current × voltage input power = output power Vp × Ip =Vs × Is 20

21. It is the ratio of output power in watts to the input power in watt expressed as % age. Efficiency = power output in watts In Ideal conditions , it’s value should be one. However in real world, it’s value is always less than one. For good transformer, it’s efficiency is nearly 95% 21 power input in watts X 100 Transformer efficiency

22. Why a transformer is not 100% efficient The reason behind reduced efficiency are the certain amount of energy losses which occur in transformer , explained as under : Loss in winding includes : Copper losses due to resistance Loss in iron core includes : > Hysteresis loss > Eddy current loss ? 22

23. Copper loss due to resistance Some power is lost to over come the resistance i.e., opposition to flow of current and is called copper loss. i.e. greater the resistance , greater the loss. R =  L A Where R is resistance ,  is the resistivity of conductor L is the length of the conductor A is the cross-sectional area Hence, for resistance to be minimum we need wire of shortest length, greatest area of cross section and material of low resistance, such as copper 23

24. It is the power used to maintain the alteration of the magnetic flux of the core. In their windings transformers have AC which is constantly changing in direction. Core is magnetized first in one direction then in the opposite. Energy is used in establishing the mag. Field in each new direction & power is lost as the heat in the core. Hysteresis losses 24

25. Eddy current loss Changing magnetic fields associated with the windings induces current in the core called eddy currents . This is the lost energy which is dissipated as heat on the core. Eddy currents are reduced by laminating the core. 25

26. Transformers used in X-ray circuit Autotransformer Step-up transformer Step-down transformer 26

27. It is a transformer used to obtain different values of high kV required to operate an X-ray tube. It has a single winding wound on a laminated closed core having number of connections or taps. Works on the principle of self induction. Function: Provides a convenient location for the kVp meter. Provides voltage for the a) X-ray tube filament circuit b) primary of the high-voltage transformer c) subsidiary circuits Autotransformer 27

28. 28 110 volts 55 turns 230 volts 115 turns 320 volts 160 turns 230 VOLTS 115 TURNS INCOMING POWER SUPPLY WORKING OF AUTOTRANSFORMER A B O X P Y

29. Definition : Transformers that convert a high voltage into a lower voltage are called step-down transformers . Purpose : To heat filament in order to emit the electrons (thermal electrons ). Primary connection from autotransformer. Secondary connected to filament. Step-down transformers 29

30. Definition: Transformers that convert a low voltage into a higher voltage are called step-up transformers. Purpose : To provide very high voltage(of the order of 40kV - 150 kV) to produce x– rays. Primary connection from autotransformer. Secondary connection to rectifier circuit. Provide attachment for mA meter. Step-up transformer 30

31. Placements in circuit 31

32. RECTIFICATION

33. Process by which pulsating alternating current is changed to unidirectional direct current is known as rectification & device use for this purpose is called rectifier . A rectifier can be valve type or vacuum type or it can be solid state/semiconductor/metallic type rectifier. X-ray tube itself is a sort of valve type rectifier or diode rectifier as it has two electrodes and allows the current to flow in one direction only Rectification 33

34. Rectification is required because if an AC voltage is directly applied then after every half cycle there will be reversal of polarity of cathode to anode . Now electron will flow from target to filament. Such electrons would not produce a useful X-ray They will further heat the filament and reduce its lifetime. Why we need rectification ? 34

35. A diode consists of an evacuated glass tube into which are sealed into two separate electrodes . The cathode or filament is constructed so that as current flows through it, a space charge of electrons developed around it as result of thermal effect of the current The anode or the plate is the other electrode When positive it attracts electrons across the valve THERMIONIC DIODE VALVES 35

36. anode cathode Electron flow No electron flow Electrons can pass only from cathode to anode as there is no space charge around the cold anode Thermionic valve allows electrons to flow in one direction, but not in the reverse direction 36

37. As the name solid state implies, conduction takes place by electron travel through solid materials . The material used are semiconductors. Their characteristics come in between metals and nonmetals . Si and Ge are semiconductors generally used in these . SOLID STATE RECTIFIERS P N 37

38. Semiconductors are materials whose electrical properties lie between Conductors and Insulators. Ex : Silicon and Germanium Pure semiconductor have little conduction towards electricity, so to enhance the conductivity impurities are added by a process called doping. SEMICONDUCTORS According to addition of impurities it Can be classified into two types. P-type semiconductor N-type semiconductor 38

39. P type semiconductor A P-type material is one in which holes are majority carriers i.e. they are positively charged materials . the impurities added are called acceptors. E.g. Indium, Gallium, Aluminium added to Si 39

40. Semiconductor which are obtained by adding a impurities in the silicon atom to increase the number of electron. The impurities added are known as Donor So, valence-five elements is added to a valence-four elements. E.g., arsenic, antimony added to Silicon. N-type semiconductor 40

41. It is formed by joining P- type and N- type semiconductors together called PN junction . Thus electron easily flow from the N type layer(electron rich) towards P type layer(hole rich) i.e. from d onor towards Acceptor but not in opposite direction from P towards N type. Hence unidirectional flow of current is obtained and rectification is done . The block to the current in reverse direction occurs at the junctions between the two materials N type and P type i.e. the region where the barrier exists is very thin, Construction Of Solid State Rectifier 41

42. h h h Electrons flow across junction h - - - - - - - - h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h h - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - h h h h h h h h - P side N side 42 Junction barrier

43. Symbol of solid state rectifier Direction of current flow Direction of electron flow 43

44. Forward biased Reversed biased Connection of diode to potential source is called biasing. When higher potential of sources is connected to p-side of diode then it is forward biased. When higher potential of sources is connected to n-side of diode then it is reverse biased. Working of solid state rectifier 44

45. Longer life. No filament heating. More robust. Smaller in size. More compact, i.e. occupy less space, better for mobile units. More reliable Advantages of solid state rectifier over diode valve 45

46. Lower forward voltage drop of the order 200V compared to 18-20 kV. Very high resistance to reverse current, hence lesser no. of barrier layer are required. Ability to work at higher temperature, approx. 392ºC compared to 266ºC. Smaller in size at the same rating. Advantages of silicon rectifier over selenium rectifier 46

47. HIGH TENSION GENERATORS

48. The high tension transformer together with other components such as rectifiers is called High Tension Generator. Various types of HT generators varying from simple to complex ones are as below: The self rectified HT Circuit (Single pulse generators) Half wave rectified circuit (Single Pulse). Single phase full wave rectified circuits (Two Pulse). Three phase full wave rectified circuits(Six Pulse). Three phase 12 pulse generators . 48

49. High frequency generators. Battery powered generators . Capacitor discharged generators . Falling load generators . Anatomical programmed generators . 49

50. It is a X-ray generator in which : X-ray tube acts as rectifier Current only flows from cathode to anode Cathode is source of free electrons 50 Secondary of High Voltage Transformer mA waveform Voltage applied to tube Self Rectified HT Generator A B

51. Applications Small in size. Simple design. Light in weight. Less cost. Simple to operate. 51 Advantages Portable and low power mobile and dental unit.

52. The peak value which the tube current reaches during the cycle is 3 times of the average value. E.G. Peak current = 3* average current. The rating of a given x-ray tube is more limited when the tube is placed in a self rectified circuit than when it is used in any other type of ht generator. Greater strain on cables 52 X-ray Tube Current -ve +ve Time C B A 1/50 sec 1/00 Sec Average Tube Current Primary of HT Transformer -ve +ve Time C B A 1 Cycle 1/2 Cycle LIMITATIONS

53. Used wasted Voltage applied to the X-ray tube mA waveform X-ray produced Hot anode can emit electrons. Accelerate & can destroy filament. Half of electrical cycle wasted. 53

54. + - X-ray tube connected to secondary of high voltage transformer through diode rectifiers Alternating voltage applied from the secondary of high voltage transformer Voltage applied to tube HALF WAVE RECTIFIED GENERATORS 54

55. + - 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 - - R1 R2 R1 R2 Half wave rectifier circuit 55

56. Single Phase Full Wave Rectified H T Generator In this circuit both half cycles of AC are used to produce X-rays by employing a bridge of four rectifiers Secondary of High Voltage Transformer Voltage applied to tube 56

57. + - First Half Cycle Second Half Cycle Voltage applied to tube (also mA waveform) + - R1 R2 R3 R4 R1 R2 R3 R4 57

58. less difference in Average and peak current. Peak current is 1 ½ times of the average current. both the halves of AC cycle and voltage during both halves is alike. both + & - half cycle of high tension transformer used. Advantages 58

59. Costly. More complex. Heavier, not easy to transport. Larger in size. Ripple factor is 100% as it is pulsating X-ray beam with voltage variation between zero to peak and again to zero. LIMITATIONS 59

60. Even after rectification the voltage across the tube still fluctuates from zero to its maximum level and X-ray are generated in120 short bursts each second. The disadvantage of pulsed radiation is loss of exposure time when the voltage is in the valleys..thus bombarding the target with low energy electrons  and low energy X-rays and raised patient dose. Thus a need for a three phase generator….. Why we need a three phase generator ? 60

61. Phase 1 at 0˚ Phase 2 lags 120 behind phase 1˚ Phase 3 lags 120 behind phase 2˚ Advantages. Produces an almost constant voltage , because there are no deep valleys between pulse. Higher tube rating(2000mA)for extremely short exposure. THREE PHASE GENERATORS 61

62. This design employs a delta wound primary transformer with a star wound secondary transformer . The out put of the secondary winding is rectified with six solid state rectifier. Winding A and B works as a system with R2, R4, R1, R5 Winding B and C works as a system with R1, R6, R3, R4 Winding A and C work as a system with R2, R6, R3, R5 . 62 R1 R2 R3 R5 R4 R6 A B C THREE PHASE,SIX PULSE,SIX RECTIFIER GENERATOR

63. Input three phase voltage Rectified To X-ray tube After rectification there will be six maximum voltages per cycle thus the term six pulse is used. Since the voltage supplied never falls to zero, the ripple factor is significantly reduced to 13.5 %. 63

64. SIX PULSE TWELVE RECTIFIER GENERATOR This circuit has a 13.5 % theoretical ripple factor. It has a fixed potential to ground This allows a 150 kv generator to have a transformer that provides a voltage of -75 kv to +75 kv to the X-ray tube, Simplifying insulating requirements . 64

65. M Fixed potential to ground Two wye Secondary windings Delta primary winding 65

66. TWELVE PULSE GENERATOR Secondary winding is not a double wye but a delta and a wye connection. When delta and wye are connected together, output of the delta will lag wye by 30º Thus the output of one winding fill the ripple of the other, resulting in a 12 pulse output. 66

67. RIPPLE FACTOR The ripple factor is the variation in the voltage across the X-ray tube expressed as a percentage of the maximum value. When three phase generator are operated under the load , ripple factor is accentuated ,known as the load ripple factor & is always greater than the theoretical ripple. eg . the load ripple factor of a 12 pulse system is about 5% 2-PULSE 100% 1/60 SEC 6-PULSE 13.5% 1/60 SEC 1/60 SEC 12-PULSE 3.5 % _________ Ripple factor = X 100 (V max –V min ) V max 67

68. 6 pulse generators Maximum ma up to 700 and Maximum kv upto 150kv. The minimum exposure time is usually 0.01 second Fastest repetition rate up to 8exp/sec. Useful for general radiography and for angiography . 50 kW to 70 kW 12 pulse generators ma up to 1000-1250 and KV 150. Shortest exposure time is 0.003 second Rate of repetition not less than 8 exp/sec. It is useful in angiography studies undertaken with serial film changers. Also useful in busy trauma centers and ortho departments. 70 kW to 100 kW Choice of a three phase generator 68

69. These are 12 pulse ma upto 1200 & kV upto 150. Shortest exposure are down to 0.001 second even upto 0.0001 second Repetition rate not less than 80 exp/sec. Useful in cine radiography. 150 kW to 200 kW 69

70. HIGH FREQUENCY GENERATOR Also known as “medium frequency generator”. It uses a high frequency current to produce an almost constant potential voltage to the x-ray tube. The basic principle involved : in a transformer the voltage induced in the secondary coil is proportional to the rate of change of current in the primary coil. A high frequency generator converts the 50 hz power line frequency to up to 5000 hz before it is fed to the primary coil of the transformer 70

71. How high frequency generator works ? Incoming power supply of 50 Hz current is rectified and smoothed This DC then fed to a DC chopper which converts the smooth DC into a chopped DC with frequency of about 5000Hz. Then the inverter converts the DC into AC This power supplies the primary of step up transformer which step up the voltage 71

72. The high voltage 5000 Hz output of the transformer is rectified to produce 10,000 V pulses per sec. Then the supply is smoothened by filters. Then applied to the X-ray tube. 72

73. 50 HZ POWER SUPPLY RECTIFIER 100 HZ PULSED DC FILTER DC SMOOTHING SMOOTH DC CONVERT DC TO PULSED DC DC CHOPPER + INVERTER 5000 HZ PULSED DC HIGH VOLTAGE TRANSFORMER HIGH VOLTAGE 5000 HZ AC HIGH VOLTAGE RECTIFIER HIGH VOLTAGE 10000HZ PULSED DC HIGH VOLTAGE SMOOTHING HIGH VOLTAGE FILTER SMOOTH HIGH VOLTAGE X-RAY TUBE 73

74. WAVEFORMS FROM THE VARIOUS CIRCUIT PARTS 74

75. Why the size of high frequency generator is small ? Output of the transformer is determined by the rate of change of flux, and is proportional to the frequency , the number of windings in the secondary and the cross-sectional area of core. V ~ fnA V= output voltage f= frequency n= number of windings A= core cross-sectional area For a given transformer , a constant output voltage may be maintained by increasing the frequency and decreasing the number of turns or the cross sectional area . 75

76. Advantages Supplies constant voltage to the X-ray tube regardless of the input power. No special power supply or voltage regulator are required. Very small size. Present high-frequency systems are more cost effective than conventional generators. A high-frequency generator waveform has less ripple, in many cases less than 2%. 76

77. Power storage generators It supplies power for the X-ray independent of external power supply as needed with mobile radiographic equipment. Remote locations Inadequate power from power line Outlet inaccessible APPLICATIONS : 77

78. POWER STORAGE GENERATOR CAPACITOR DISCHARGE GENERATOR BATTERY POWERED GENERATOR

79. Capacitor is an electric device, is used for storing charge. Output of the step up transformer is rectified and is used to charge a capacitor. Charged capacitor is discharged during X-ray exposure It provide a very high milliamperage for very short exposure time. As the exposure begins the capacitor begin to discharge, so the kV will fall during the exposure. Capacitor – Discharge Generator 79

80. Small sized. Easy to move. Limitation to its mAs output(30~50 mAs) kV fall during the exposure(say 1kV/1mAs) Capacitor must be charged immediately prior to use. Advantages Disadvantages 80

81. A standard power supply is used to charge large Nickel Cadmium battery. While exposure, the output of battery is fed in to DC chopper which interrupt the current about 500 times per second produced 500Hertz pulse DC current that is supplied to the primary winding of step up transformer. The high voltage output is then rectified and supplied to X-ray tube as a 1000 pulse waveform . This wave has 100% ripple at this frequency ,so this waveform must be smoothed to provide constant potential . BATTERY POWERED GENERATOR 81

82. Make exposures independent of a power supply. Supply a constant output of kV and mA throughout the exposure. Store considerable energy to generate X-ray. Heavy . Requires regular battery maintenance. Advantages Disadvantages 82

83. Used to provide the highest mA settings at the shortest time possible. The operator selects mAs settings. The computer automatically calibrates the time of exposure (allowing consistently shorter exposures) The mA is therefore controlled by the falling-load generator. The mA starts at the highest possible setting and “falls” throughout the exposure. Falling load generator 83

84. 600 mA x .05 sec = 30 mAs 500 mA x 0.15 sec = 75 mAs 400 mA x 0.1 sec = 40 mAs 300 mA x 0.2 sec = 60 mAs Total 0.50 sec = 205 mAs If an exposure of 70 kVp and 200 mAs is desired ,this X-ray tube could be operated at 70 kVp , 200 mA and 1 sec. 84

85. Radiographers have been traditionally selecting & settings values of kV, mA & sec separately. But this problem is overcome by using modern circuitry in anatomically programmed generator In this generator, there is a no. of push – button setting. Each push button is designated to a particular anatomical part or region. When the chosen push- button is pressed , the circuitry associated with it automatically sets the appropriate factors of kV , mA & selects the appropriate focal spot on the X-ray tube which is to be used. Anatomically Programmed Generator 85

86. Examination completed more speedily. Inexperienced technologists may work with greater certainty. In fluoroscopic room , if anatomical programming is used for spot filming, there is obvious benefit when exam are changed. Advantages 86

87. Range of selection is not wide enough to embrace all the examination. Manipulation of the exposure factor like in case of pt. physical build and certain known pathologies (osteoporosis, pleural effusion, kyphosis ) Disadvantages 87

88. It is the power output of generator and is expressed in Watts- unit of power or Kilowatts. R ating of HT generator is evaluated when the unit is under load and formula involve multiplying Kilovolts and the milliamperes which constitutes the load as below : kW = kV x mA kW = 0.7 x kV x mA 1000 1000 88 (for 3 phase) (for single phase) KILOWATT RATING OF GENERATOR

89. Generators may be compared by means of their kilowatt ratings The maximum values of the Kilo voltage and milli amperage cannot be simultaneously obtained. If highest kV is used, then the maximum mA available will be lower than the highest value stated for the generator. Similarly if the highest mA is to be used the available kV is lower than the maximum voltage output given for the generator. e.g. a generator meant for 1000 mA and 150kV might in practice provide as follows: 1000mA at 80kV (80kW) 800mA at 100kV(80kW) 500mA at 150 kV (75kW) 89

90. The radiological technologists operate two main categories of X-ray equipment: 1.Permanent Installation 2.Movable Equipment Electrical hazards are most likely to be greater with movable equipment 90 Safety rule for radiological technologist

91. All movable X-ray equipment should be checked regularly and often by electrician. Radiological technologist should not put plugs into or out of sockets which are live. Cables and plugs should be treated as kindly as if they are patient! Do not stretch the cable. Do not run the equipment on the cable. There are certain rules to be observed by radiological technologist: 91

92. CONCLUSION The essential function of high tension generators in X-ray equipments is to provide power as is needed by the X-ray tube. The development in high tension generator has always enhanced the quality of production in X-ray beam by reducing the size and ripple factor. And if the quality of X-rays is improved then the quality of the images produced is improved consequently, hence ultimately the diagnostic value of images is increased. 92

93. References X-ray equipment for student radiographers by Chesney Christensen's Physics of Diagnostic Radiology www.wikiradiography.com www.e-radiography.net www.radiologyinfo.net 93

94. THANK YOU 94