rectification rt 244 – lect #2 rev 2012 1. wavelength x-ray is measured in hertz or angstrom...
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RECTIFICATION
RT 244 – Lect #2 Rev 2012
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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
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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
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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 )
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Rectification
• process of changing AC to pulsed DC• rectifier
– device that allows I to flow in only one direction– types valve tube solid state
Ie
I
+ -
Ie
I
+ -
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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
TransformerSection
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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
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8FLOW OF CURRENT
• VALVE TUBE RECTIFIES
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SOLID STATE - DIODES – Semi Conductor
• Allows current to only flow in one direction
• Most common type used in rectifiers
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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-
3) p-n junctionp-n junction– union of the two types of materials
n-type- - - --
--- - --
--
p-type
p-nj
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TYPES OF RECTIFICATIONSINGLE PHASE
½ RECTIFIED + FULL RECTIFIED
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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
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AC – single phase
• Only ½ of cycle usable
• By converting other ½ more useful
• Draw back –• 100% ripple • (0-100-0) output
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Single Phase – Full rectifiedVoltage
• 1Ø AC has a 100% voltage ripple
+ V
- V
+ V
- V
0 0
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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 & Vmin
1/60s +
-
V
1/60s +
-
V
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3 PHASE – MORE EFFICIENT
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3Ø comparison to 1Ø
• 3Ø more efficient than 1Ø• 3Ø requires more complex circuitry• 3Ø more expensive to install
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(3 Φ ) 3 PHASE
• LESS RIPPLE • MORE EFFIECIENT OUTPUT
• HIGH FREQUENCY – 1% RIPPLE• MOST EFFIEICENT – VERY EXPENSIVE
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VOLTAGE RIPPLE
Ripple effect differences
• 100 % RIPPLE• 4 – 10 % RIPPLE• 1% RIPPLE
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Voltage Waveforms (Bushong)Voltage Waveforms (Bushong)
Single phase
Rectification Type
Ripple
Half wave 100%
Full wave 100%
Rectified 3-phase
Rectification Type
Ripple
Three phase, 6 pulse
13%
Three phase, 12 pulse
4%
Three phase, high frequency
<1%
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Generator SummaryType p/c p/s # % Wave Form (2 cycles)
rectifiers Ripple
+
-
+
-
+
-
+
-
+
-
1ØSelf 1 60 0 100%
1Ø1p 1 60 1 or 2 100%
1Ø2p 2 120 4 100%
3Ø6p 6 360 6 or 12 12-15%
3Ø12p 12 720 12 3-5%
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Comparison of X-ray Machines1Ø 3Ø High freq.
V ripple 100% < 15% <1%p/s 60 or 120 720 13,000X-ray Quantity “X” more highestX-ray Quality “X” higher min time 1P (~8 ms) 1 ms machine $ “X” higher highestoperation $ “X” lower ??generator size moderate largest smallest
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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
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High Frequency Generators
• x-ray tube nearly constant– ripple <1%
• small in size• Expensive – most efficient
rectifiers smooth choppedHVT rectifiers smooth tube
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Full-Wave Rectification
Positive HV pulse Negative HV pulse
+
-
I
+
- I
+
-
+
-
V
+
-
I
Ie
x rays
+
-
I+
-
+
-
V
+
-
I
Ie
x rays
FYI - Don’t need to know the direction of the current
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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)
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1Ø vs. 3Ø HV Transformers
Each phase must have step-up transformer
1Ø
primary secondary
3Ø
primary secondary
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3 Transformer Configurations
simple 3 6 pulse: delta/wye () primary secondary
15% voltage ripple
Star Wye or delta
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3 Trans. Configurations (cont.)
complex 3 6 pulse: delta/wye/wye () primary secondary
12% voltage ripple
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3Ø Transformer Coils
1/60s +
-
V
Wye -- delayed output
Delta -- either side
Wye -- secondary only
Delta Coils Wye Coils
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3 Rectification• each secondary coil requires 2 rectifiers• 3Ø6 with coils = 6 rectifiers
positive pulse negative pulse
-+
-+
+-
-+
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3 Rectification (cont.)
• 3Ø6 with coils = 12 rectifierspositive pulse negative pulse
-+
-+
-+
-+
-+
-+
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3 Rectification (cont.)
• 3Ø12 with coils = 12 rectifiers
positive pulse negative pulse
-+ -
+
+ -
-+ -
+
+ -
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A star or delta connection is used to ?
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3 Transformer Configurations
simple 3 6 pulse: delta/wye () primary secondary
15% voltage ripple
A star or delta connection is used to combine three-phase current
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Rectification• AC to DC• Keeps e- flowing from
cathode to anode• Uses rectifying bridge
between HVT & tube
HVT
tube
HVT
tube
AC DCbridge
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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
HVT rectifiers capacitor tube
PLUG IN PORTABLE MACINES
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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