x ray generators, transformers,

1. X ray generators

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

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)

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

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

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

38. X ray circuit

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

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

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

59. Three-Phase Generators
 Commercial power generally delivered as 3 phase
 phases 120o apart
Single Phase Power Three Phase Power

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

63. Medium Frequency
Generator Operation
 incoming AC converted to pulsed DC
AC DC

64. Medium Frequency
Generator Operation
 Pulsating DC smooth to constant voltage
Constant DCPulsating DC

66. Medium/High Frequency
 Transformerefficiency: V~ fnA
 Byincreasing frequency(f),cross sectionalarea(A) reducedforsamepower
 Frequencyof invertorrangesfrom5-100kHz!
 Feedbackloop controlled–duringexposureif kVdropsoff,increaseinvertorfrequency& kVincreases

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