X ray generators
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
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
The mechanism of an x ray generator is continued
in two separate compartments:
A control panel or console
A transformer assembly
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
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
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
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
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)
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
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
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
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
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
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
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
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
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
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
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
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 losses3. Hysteresies4. Flux leakage losses
LAWS OF TRANSFORMERS
The laws govern the behaviour of a transformers1. The voltage in the two circuits is proportional to the
number of turns in the two coils = = number of turns in the primary coil = number of turns in the secondary coil = voltage in the primary circuit = Voltage in the secondary circuit
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
=
= 30000 V
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
One with fewer turns in the secondary coil decreases the voltage and is called a step down transformer
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
= = voltage in the primary coil = current in the primary coil = voltage in the secondary coil = 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 = 0.1 A(100 m A )
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
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
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
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
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
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
X ray circuit
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Halfwave Rectifier Circuit
+
-
X Second Half Cycle:Diodes openNo voltage applied to tubeNo tube current (mA)
+
-
First Half Cycle:Diodes closedVoltage applied to tubeTube current (mA) results
-
-
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)
Fullwave Rectifier
+ B
- AX
X
First Half Cycle Second Half Cycle
Voltage applied to tube(also mA waveform)
X
X
+A
-B
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
Three-Phase Generators
Commercial power generally delivered as 3 phase phases 120o apart
Single Phase Power Three Phase Power
Three-Phase Generators Rectifier circuit
Inverts negative voltage sends highest of 3 phases to x-ray tube
To X-Ray Tube
Input 3 Phase VoltageRectified
Three-Phase Generators Produces nearly constant potential much higher tube ratings than single phase more efficient than single phase
shorter exposures High repetition rates
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
Medium FrequencyGenerator Operation
incoming AC converted to pulsed DC
AC DC
Medium FrequencyGenerator Operation
Pulsating DC smooth to constant voltage
Constant DCPulsating DC
Medium/High Frequency Transformer efficiency: V ~ fnA By increasing frequency(f), cross sectional area (A) reduced for
same power Frequency of invertor ranges from 5-100 kHz! Feedback loop controlled – during exposure if kV drops off,
increase invertor frequency & kV increases
Medium Frequency Generators
Advantages immune to power line fluctuation Timer accuracy Shorter exposures (<10 ms) low ripple small size of electronics & transformer
Today’s trend in generators
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