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