Download rectification - El Camino College

RECTIFICATION
RT 244 – Lect #2
Rev 2012
1
Wavelength
• X-ray is measured
• In HERTZ OR
• Angstrom
XRAY IS MEASURED IN
HERTX OR ANGT
Elsevier items and derived items © 2009 by
Mosby, Inc., an affiliate of Elsevier Inc.
2
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
3
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 )
4
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
+
-
I
 Ie
5
Rectification
• purpose
–
convert HV AC to pulsed DC
for x-ray tube
Transformer
Section
• location
–
between HV secondary coil &
x-ray tube
• types
–
–
–
full wave -- most common
half wave -- limited use
self -- rarely
6
Valve tube (diode)
glass vacuum tube w/ 2 electrodes
anode
cathode
cold
hot
1) negative electrode -- cathode
– thermionic emission of e-
2) positive electrode -- anode
– cold metallic plate
7
• VALVE TUBE RECTIFIES
FLOW OF CURRENT
8
SOLID STATE - DIODES
– Semi Conductor
• Allows current to only
flow in one direction
• Most common type
used in rectifiers
9
Solid State Rectifier
p-type p-nj n-type
- - -- - - - -- -
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 junction
– union of the two types of materials
10
TYPES OF RECTIFICATION
SINGLE PHASE
½ RECTIFIED + FULL RECTIFIED
11
Rectification Process
• rectifiers placed in circuit to convert AC
current to pulsed DC
• methods
– suppress unwanted part of the AC
+
V
-
+
I
-
– redirect unwanted part of AC
+
V
-
(half wave rectification)
(full wave rectification)
+
I
12
AC – single phase
• Only ½ of cycle usable
• By converting other ½
more useful
• Draw back –
• 100% ripple
• (0-100-0) output
13
Single Phase – Full rectifiedVoltage
+V
+V
0
0
-V
-V
• 1Ø AC has a 100% voltage ripple
14
Single vs. Three Phase Power (cont.) Three
+ 1/60s 
•
phase (3Ø )
1/60s 
+
V
V
-
-
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
15
3 PHASE – MORE EFFICIENT
16
3Ø comparison to 1Ø
• 3Ø more efficient than 1Ø
• 3Ø requires more complex circuitry
• 3Ø more expensive to install
17
(3 Φ ) 3 PHASE
• LESS RIPPLE
• MORE EFFIECIENT OUTPUT
• HIGH FREQUENCY – 1% RIPPLE
• MOST EFFIEICENT – VERY EXPENSIVE
18
VOLTAGE RIPPLE
• 100 % RIPPLE
• 4 – 10 % RIPPLE
• 1% RIPPLE
Ripple effect differences
19
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%
20
Generator Summary
Type
1ØSelf
p/c
1
p/s
60
#
rectifiers
0
%
Ripple
100%
Wave Form (2 cycles)
+
+
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%
+
-
21
Comparison of X-ray Machines
1Ø
100%
720
“X”
3Ø
< 15%
13,000
more
High freq.
<1%
X-ray Quality
min time
machine $
“X”
1P (~8 ms)
“X”
higher
1 ms
higher


highest
operation $
generator size
“X”
moderate
lower
largest
??
smallest
V ripple
p/s 60 or 120
X-ray Quantity
highest
22
Ripple Factor (%)
• variation in tube V during an exposure
• What is the ripple of the following waveform?
80 kV
68 kV
+
V
-
1/60s 
= look in book for comparisons
23
High Frequency Generators
rectifiers
smooth
chopped
HV
T
rectifiers
smooth
tube
• x-ray tube nearly constant
– ripple <1%
• small in size
• Expensive – most efficient
24
Full-Wave Rectification
+
I
-
Positive HV pulse
+
+
-
+
V
-
Negative HV pulse
-
-
x rays
+
+
Ie
x rays
Ie
+
I
-
I
+
V
-
FYI - Don’t need to know the direction of the current
I
+
I
-
25
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)
26
1Ø vs. 3Ø HV Transformers
Each phase must have step-up transformer
3Ø
1Ø
primary
secondary
primary
secondary
27
3 Transformer Configurations
simple 3 6 pulse: delta/wye (DU)
primary
secondary
15% voltage ripple
Star Wye or delta
28
3 Trans. Configurations (cont.)
complex 3 6 pulse: delta/wye/wye (DUU)
primary
secondary
12% voltage ripple
29
3Ø Transformer Coils
Delta Coils
Wye Coils
Wye -- delayed output
Delta -- either side
Wye -- secondary only
+ 1/60s 
V
-
30
3 Rectification
• each secondary coil requires 2 rectifiers
• 3Ø6 with DU coils = 6 rectifiers
positive pulse
negative pulse
-
+
+
-
+
+
-
31
3 Rectification (cont.)
• 3Ø6 with DUU coils = 12 rectifiers
positive pulse
+
+
negative pulse
+
+
+
+
-
-
32
3 Rectification (cont.)
• 3Ø12 with DUD coils = 12 rectifiers
positive pulse
+
+
negative pulse
+
+
-
+
-
+
-
33
A star or delta connection is used to ?
34
3 Transformer Configurations
simple 3 6 pulse: delta/wye (DU)
primary
secondary
15% voltage ripple
A star or delta connection is used to
combine three-phase current
35
Rectification
• D AC to DC
• Keeps e- flowing from
cathode to anode
• Uses rectifying bridge
between HVT & tube
tube
HVT
AC
bridge
DC
tube
HVT
36
Capacitor Discharge Generator
HV
T
•
•
•
rectifiers
capacitor
tube
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
PLUG IN PORTABLE MACINES
37
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
38