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. 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. The mechanism of an x ray generator is continued
in two separate compartments:
A control panel or console
A transformer assembly
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. 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.
8. 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
9. 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
10. 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)
11.
12. 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
13. 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
14. 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
15. 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
16. 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
17.
18. 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
19. 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
20. 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
21. 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
22. 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
23.
24. 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
25. 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
26. 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
27. 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
28. 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
29. One with fewer turns in the secondary coil
decreases the voltage and is called a step down
transformer
30. 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
31. 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 )
32. 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
33. 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
34. 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
35. 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
36. 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
37. 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
39. 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
40. 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
41. 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
42. 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
43. 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
44. 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
45. 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
46.
47. 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
48. 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
49. 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
50. 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
51. 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
52. 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
53.
54. 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
55. 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
-
-
56. 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)
57. Fullwave Rectifier
+ B
- A
X
X
First Half Cycle Second Half Cycle
Voltage applied to tube
(also mA waveform)
X
X
+A
-B
58. 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
60. 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
61. Three-Phase Generators
Produces nearly constant potential
much higher tube ratings than single phase
more efficient than single phase
shorter exposures
High repetition rates
62. 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
67. Medium Frequency Generators
Advantages
immune to power line fluctuation
Timeraccuracy
Shorterexposures (<10ms)
low ripple
small size of electronics & transformer
Today’s trend in generators
