1. RECTIFICATION RT 244 – Lect #2 Rev 2012 1

2. Wavelength X-ray is measured In HERTZ OR Angstrom Elsevier items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc. 2 XRAY IS MEASURED IN HERTX OR ANGT

3. 3 Alternating Current - AC AC is produced when a coil of wire turns in a magnetic field ½ turn current is moving in one direction Then ½ turn it moves in the other direction IF 3 coils of wire are wrapped around a core – they rotate 1/3 (120º apart) – then 3 different AC currents are generated

4. 4 Rectification Converts AC to DC Located after Step up Transformer On secondary side of Circuit Can be single phase (1Φ) Or 3 phase (3 Φ )

5. 5 Rectification process of changing AC to pulsed DC rectifier – device that allows I to flow in only one direction – typesvalve tube solid state IeIe II +- IeIe II +-

6. 6 Rectification purpose – convert HV AC to pulsed DC for x-ray tube location – between HV secondary coil & x-ray tube types – full wave -- most common – half wave -- limited use – self -- rarely Transformer Section

7. 7 Valve tube (diode) glass vacuum tube w/ 2 electrodes 1) negative electrode -- cathode – thermionic emission of e - 2) positive electrode -- anode – cold metallic plate cold hot anode cathode

8. 8 FLOW OF CURRENT VALVE TUBE RECTIFIES

9. 9 SOLID STATE - DIODES – Semi Conductor Allows current to only flow in one direction Most common type used in rectifiers

10. 10 Solid State Rectifier 1)"n-type" material (donor) [similar to cathode] – contains loosely bound e - 2)"p-type" material (acceptor) [similar to anode] – spaces in molecular structure to accept e - p-n junction 3) p-n junction – union of the two types of materials n-type - - - - - - - - - - - - - p-type p-nj

11. 11 TYPES OF RECTIFICATION SINGLE PHASE ½ RECTIFIED + FULL RECTIFIED

12. 12 Rectification Process rectifiers placed in circuit to convert AC current to pulsed DC methods – suppress unwanted part of the AC (half wave rectification) – redirect unwanted part of AC (full wave rectification) + - + - + - + - V V I I

13. 13 AC – single phase Only ½ of cycle usable By converting other ½ more useful Draw back – 100% ripple (0-100-0) output

14. 14 Single Phase – Full rectifiedVoltage 1Ø AC has a 100% voltage ripple + V - V + V - V 00

15. 15 Single vs. Three Phase Power (cont.) Three phase (3Ø ) power supplied/used w/ a new cycle beginning each 120 ° – results in three overlapping sine waves – 3 + pulses & 3 - pulses occur in the length of time of one complete cycle (1/60s) – each wave can be rectified (3Ø6p) – pulses overlap reducing voltage ripple & V min  1/60s  + - V + - V

16. 16 3 PHASE – MORE EFFICIENT

17. 17 3Ø comparison to 1Ø 3Ø more efficient than 1Ø 3Ø requires more complex circuitry 3Ø more expensive to install

18. 18 (3 Φ ) 3 PHASE LESS RIPPLE MORE EFFIECIENT OUTPUT HIGH FREQUENCY – 1% RIPPLE MOST EFFIEICENT – VERY EXPENSIVE

19. 19 VOLTAGE RIPPLE Ripple effect differences 100 % RIPPLE 4 – 10 % RIPPLE 1% RIPPLE

20. 20 Voltage Waveforms (Bushong) Single phase Rectification Type Ripple Half wave100% Full wave100% Rectified 3-phase Rectification Type Ripple Three phase, 6 pulse 13% Three phase, 12 pulse 4% Three phase, high frequency <1%

21. 21 Generator Summary Type p/cp/s# % Wave Form (2 cycles) rectifiers Ripple + - + - + - + - + - 1 Ø Self1600100% 1 Ø 1p1601 or 2100% 1 Ø 2p21204100% 3 Ø 6p63606 or 1212-15% 3 Ø 12p12720123-5%

22. 22 Comparison of X-ray Machines 1Ø3ØHigh freq. V ripple100%< 15%<1% p/s60 or 120 72013,000 X-ray Quantity“X”morehighest X-ray Quality“X”higher  min time1P (~8 ms)1 ms  machine $“X”higherhighest operation $“X”lower?? generator sizemoderatelargestsmallest

23. 23 Ripple Factor (%) variation in tube V during an exposure What is the ripple of the following waveform?  1/60s  + - V  80 kV  68 kV = look in book for comparisons

24. 24 High Frequency Generators x-ray tube nearly constant – ripple <1% small in size Expensive – most efficient rectifiers smoothchopped HV T rectifierssmoothtube

25. 25 Full-Wave Rectification Positive HV pulse Negative HV pulse + - I + - II + - + - V + - I IeIe x rays + - II + - + - V + - I IeIe FYI - Don’t need to know the direction of the current

26. 26 High Frequency Power machine designed to have a low ripple (<1%) & use 1Ø power 1Ø AC rectified then smoothed & chopped into high frequency output (kHz)

27. 27 1Ø vs. 3Ø HV Transformers Each phase must have step-up transformer 1Ø1Ø primary secondary 3Ø3Ø primarysecondary

28. 28 3  Transformer Configurations simple 3  6 pulse: delta/wye (  ) primarysecondary  15% voltage ripple Star Wye or delta

29. 29 3  Trans. Configurations (cont.) complex 3  6 pulse: delta/wye/wye (  ) primarysecondary  12% voltage ripple

30. 30 3 Ø Transformer Coils  1/60s  + - V Wye -- delayed output Delta -- either side Wye -- secondary only Delta CoilsWye Coils

31. 31 3  Rectification each secondary coil requires 2 rectifiers 3Ø6 with  coils = 6 rectifiers positive pulse negative pulse - + - + + - - +

32. 32 3  Rectification (cont.) 3Ø6 with  coils = 12 rectifiers positive pulse negative pulse - + - + - + - + - + - +

33. 33 3  Rectification (cont.) 3Ø12 with  coils = 12 rectifiers positive pulse negative pulse - + - + + - - + - + + -

34. 34 A star or delta connection is used to ?

35. 35 3  Transformer Configurations simple 3  6 pulse: delta/wye (  ) primarysecondary  15% voltage ripple A star or delta connection is used to combine three-phase current

36. 36 Rectification  AC to DC Keeps e - flowing from cathode to anode Uses rectifying bridge between HVT & tube HVT tube HVT tube ACDC bridge

37. 37 Capacitor Discharge Generator capacitor bank charged by rectified high voltage during exposure capacitors provide kV across tube problems – as capacitors drain kV drops (~1 kV/mAs) – any residual charge may shock operator HV T rectifiers capacitor tube PLUG IN PORTABLE MACINES

38. 38 TECHNIQUE CHANGES SINGLE PHASE (1Φ) TO 3 Φ 12 PULSE CUT MAS BY ½ SINGLE PHASE (1Φ) TO 3 Φ 6 PULSE CUT MAS BY 1/3 MORE EFFICIENT = LESS MAS NEEDED