+ - HIGH TENSION GENERATOR & RECTIFICATION SYSTEM 1 MODERATOR MR. S.C. BANSAL (LECTURER) PRESENTED BY BIVEK RAJ B. SC. MED. TECH. 3RD YEAR DEPARTMENT OF RADIO-DIAGNOSIS AND IMAGING PGIMER, CHANDIGARH-160012
An electric current in which amplitude and polarity of the current vary with time.
Its a sinusoidal wave which varies amplitude and periodic reversal of polarity.
Battery DC A Switch A Time Direct Current Electric current which flows in one direction only through a circuit or equipment. The associated direct voltages, in contrast to alternating voltages, are of unchanging polarity. 5
Tube current is nonlinear and drops off below 40 kV
Capacitors hold and release charge in a time delayed manner
In polyphase AC or three phase AC, 3 coils are made to rotate simultaneously by placing at equidistance from each other in front of the magnets which will produce three separate supplies of alternating current. The voltage provided by these separate sources of electricity are out of phase e.g. 415Volt 10
Distribution Of Electricity The generated electricity is distributed as follows: 2 methods in which the windings of the circuit containing a polyphase generator can be connected are as below: Star type 3 phase circuit or Wye type Delta type 11
Definition: An electrical device that transforms the voltage of alternating current electricity from one parameter to another .
Principle : It is based on the principle of electromagnetic induction.
Types Of Transformer On The Basis Of Construction: 17
Construction Working: P. coil supplied with A.C. As the current flows through it , a changing magnetic field is set up around it in the soft iron core As the principle of e.m. induction and current is also induced in the S. coil Current flows when magnetic field is increased or decreased and not when it is stable 18
Hysteresis losses: It is a heat loss caused by the magnetic properties of the core. When an core is in a magnetic field, the magnetic particles of the core tend to line up with the magnetic field. When the armature core is rotating, its magnetic field keeps changing direction. The continuous movement of the magnetic particles, as they try to align themselves with the magnetic field, produces molecular friction. This, in turn, produces heat. This heat is transmitted to the armature windings. The heat causes armature resistances to increase. Eddy current loss: These currents are also set up due to changing magnetic field and loss appears as the heat in the core.
When current flows through a wire , the material of the wire offer resistance to its flow .some power is lost to over come the resistance and is called copper loss, i.e. greater the resistance and current greater the loss.
Copper loss ά length of wire and ά Solution : material as low resistance as possible is selected e.g. pure Cu 1 area of the cross section 23
Types Of Transformer Used In X-ray Circuit: In x-ray circuit , three type of transformer are used: 24
The autotransformer is designed to supply voltage of varying magnitude to several different circuits of the x-ray machine including both the filament circuit and high voltage circuits. The autotransformer has only one winding and one core. The single winding has a number of connection or electric taps.
Because there is only one winding, the primary and secondary circuits are in metallic (conductive) connections with each other. This fact makes an autotransformer unsuitable for transforming high voltages from one value to another.
In comparison with two-winding transformers, autotransformers are smaller in size, are economical of copper wire and cost less.
Diagram Of Autotransformer: 27 c A d 300V 450V 300V 2:2 TURN RATIO 2:2 TURN RATIO 2:3 TURN RATIO e f g B
Step Up Transformer Or High Tension Transformer:
Definition: Transformers that convert a low voltage into a higher voltage are called step-up transformers.
A step down transformer has less turns of wire on the secondary coil,which makes a smaller induced voltage in the secondary coil.
Placement : It is connected directly to the filament of x-ray tube.
Purpose: the filament of the x-ray tube needs to be heated up in order to emit the electrons (thermal electrons).
The Tube Current is controlled through a separate circuit called the filament circuit which is connected to filament transformer.
Diagram of x-ray circuit showing filament transformer: + High Voltage Transformer Rectifier Circuit Timer Circuit Line voltage compensator Auto- trans-former TO FILAMENT TRANSFORMER PRIMARY mA selector 31
Two electrodes within the glass envelope, one of which is a heated filament.
The filament of a valve is heated by a step down transformer and emits electrons which are drawn across to the anode when a potential difference is applied across both the electrodes and the valve passes current.
Functioning of a Diode Valve: If the valve is connected in a complete circuit such that cathode is –ve with respect to anode electrons are drawn towards the anode and valve passes current. If the cathode is positive with respect to anode, no electrons will be drawn across the valve and it blocks the current thus the supply of current to X-ray tube is unidirectional only. But these diode valves which were used earlier are replaced with solid state rectifiers. Hence, its function is to pass current in one direction only and to block any reverse flow. 35
Solid State Rectifier As the name solid state implies, conduction takes place by electron travel through solid materials as opposed to electron flow through a vacuum in a valve Solid materials used are semi-conductors whose characteristics place them midway between metals, which are conductors of electricity and non-metals, which mostly are non-conductor of electricity and are insulators. Semiconductors can be made either to conduct or insulate. Selenium, Silicon or Germanium are some commonly used semiconductors to rectify the high tension for an X-ray tube in place of vacuum tube. 36
P-Type semiconductor: P-type semiconductor is obtained by adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charge carriers. 39
N-type Semiconductor An N-type semiconductor is obtained by by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers. 40
It is formed by joining p type and n- type semiconductors together called PN Junction. Thus electrons flow easily from the n type layer towards p type layer i.e. from Donor towards Acceptor but not in opposite direction from p towards n type. Hence uni-directional flow of current is obtained and rectification is done.
The block to the current in reverse direction occurs at the junctions between the two materials N type and P type.
i.e. the region where the barrier exists is very thin, hence it is also called junction diode or barrier layer rectifier. A P-N JUNCTION DIODE 42
Advantage of Solid State Rectifier over diode valve:
HIGH TENSION GENERATORS The high tension transformer together with other components such as rectifiers is called a High Tension Generator. Various types of HT generators varying from simple to complex ones are as below:
The self rectified HT Circuits (Single pulse generators)
X-ray tube connected to secondary of high voltage transformer through diode rectifiers
Alternating voltage applied to secondary of high voltage transformer
Voltage applied to tube 53
Half wave Rectifier Circuit: + First Half Cycle: Diodes closed Voltage applied to tube Tube current (mA) results - - X Second Half Cycle: Diodes open No voltage applied to tube No tube current (mA) + R1 - R2 R1 - 54 R2
Applied to X-ray Tube Output of High Tension Transformer Applied to x-ray tube Blocked (not used) Limitations:
60 pulses per second
only positive half cycle of high tension transformer used
Single Phase Full Wave Rectified H T Generator: In this circuit both half cycles of AC are used to produce X-Rays by employing a bridge of four rectifiers Secondary of High Voltage Transformer Voltage applied to tube 56
Voltage applied to tube (also mA waveform) + - X X X X - + Actually what happens? First Half Cycle Second Half Cycle R1 R1 R4 R4 R2 R3 R2 R3 57
Ripple factor is 100% as it is pulsating X-Ray beam with voltage variation between zero to peak and again to zero.
THREE PHASE, SIX PULSE,SIX RECTIFIER GENERATOR
This design employs a delta wound 1* transformer with a star wound 2* transformer . The out put of the 2* winding is rectified with six solid state rectifier.
Winding A and B works as a system with R2, R4, R1, R5
Winding B and C works as a system with R1, R6, R3, R4
Winding A and C work as a system with R2, R6, R3, R5.
61 A B C R1 R4 R5 R2 R3 R6
Rectified Input 3 Phase Voltage To X-Ray Tube
When rectified there will be six positive maximum voltage per cycle thus the term six pulse.
Three Phase , Six Pulse , Twelve Rectifier The circuit is twelve pulse but this circuit has fixed potential to ground. This allows a 150 KVp generator to have a transformer that provide a voltage of -75KV to +75KV and hence, further reduces ripple. 63
Voltage In Respective Coils And X-ray Tubes. At any instant of time, the voltage in all coils is not the same i.e. they are out of step with each other, so two pairs which supply the tube are those pairs which have highest voltage in comparison to 3rd pair. Thus voltage across x-ray tube never falls to zero and takes the form of ripples. This is 20%. 64
Features Of A Three Phase High Tension Generator. Voltage wave forms: on the circuits the alternating voltage from the three phases of mains supply are fed into a triple high tension transformer. Here the voltage exists on the secondary winding as three phases of alternating voltage stepped up to the peak values required by the x-ray tube in operation. During the period of time occupied by one full cycle of voltage changes, the x-ray tube has six half waves of useful voltage applied to it. 65
So the voltage across the x-ray tube takes the rippling form and varies from the maximum in the cycle to some value less than the maximum but it does not fall all the way to zero. i.e. does not fall below 80% of the peak value. Hence there is 20% ripple theoretically its value is 13.5%.
This ripple can be reduced further by 6-pulse 12 rectifier i.e., 3%.
Average And Peak Current In 3 phase circuits, there is a difference b/w avg. mA during the cycle and the peak value is reached but is narrow as compared to single phase half wave rectified circuit where peak is about 3 times and in single phase full wave rectified circuit And this difference between peak and average value of current is further reduced in 12 rectifier circuit than the six rectifier circuit as average tube current is close to the peak value of the current. 67
Application: Useful in procedures requiring shortest or repetitive exposures e.g. angiography or serial radiography. Advantages: As load is distributed equally overall three phases of the supply when the x-ray exposure is made it can draw larger amounts of power. Hence three phase HT generators can supply higher mA e.g. 1000-1200mA for the x-ray tube than single phase generators. Disadvantages:
High Frequency Generator The newest development in high-voltage generator design uses high frequency circuits. Full wave rectified power at 60 Hz is converted to a high frequency usually 500-50000 Hz. H.F. voltage generation uses inverter circuit. The DC power supply produces a constant voltage from either a single phase or three phase input line source. An inverter circuit creates the high frequency AC waveform. This AC current supplies the high voltage transformer and creates a waveform of fixed high voltage and corresponding low current. 69
contd. After rectification and smoothing two high voltage capacitor on the secondary circuits accumulates electron charges. These capacitors produce a voltage across the x-ray tube, that depends upon the accumulated charges according to the relation: V=Q/C where, V=voltage Q=charge(coulomb) C=capacitance (farad) 70
It’s small size can be placed within the tube housing, produce a nearly constant voltage waveform, resulting into improved quality at lower pt. dose. Portable x-ray imaging system were the first to use this technology, but now a days, all stationary imaging systems use H.F. voltage generation.
50KW to 70KW: There are six pulse generators with max mAupto 700 and maximum KV upto 150Kv. The minimum exposure time is usually 0.01 sec and fastest repetition rate upto 8 exp/sec. useful for general radiography and for some angiography – peripheral and abdominal.
70KW to 100KW:There are 12 pulse generators with mAupto 1000-1250 and KV 150. Shortest exposure time is 0.003 sec. rate of repetition not less than 8 exp/sec. useful in angiography studies undertaken with serial film changers. Also useful in busy trauma centers and ortho departments.
150KW-200KW:these are 12 pulse with mA 1200 at KV 150 as voltage. Shortest exposure are down to 0.001 sec even upto 0.0001 second and repetition rate not less than 80 films/sec. useful in cine radiography.
Single Phase Output Constant Potential or High Frequency Output 78
Generator Kilowatt (kW) Rating The essential function of HT generators in x-ray equipment is to provide such power as is needed by the x ray tube to which they are connected. So power output of generator is very important specification and it is expressed in Watts- unit of power or Kilowatts. But rating of HT generator is evaluated when the unit is under load and formula involve multiplying Kilovolts and the milliamperes which constitutes the load as below: 79
1000 mA @ 70 kVp 800 mA @ 80 kVp 600 mA @ 100 kVp 300 mA @ 120 kVp three phase
Battery Powered Generators: Some portable x-ray machines employ a series of batteries to generate the high voltage and filament currents and are useful to operate in areas where the electric supply is inadequate for the conventional generators. Each cell in the battery pack supplies potential difference of 1.5 volts, so thousands of cells are required to provide high voltages used in diagnostic radiology. 82
The KV selector in H V Circuit regulate the potential difference across the tube by adding or subtracting batteries from the series. The filament current is adjusted by variable resistor in the filament circuit.
The batteries must be recharged periodically for numerous x-ray exposures.
Low voltage on grid controls flow of electrons to anode
Exposure start & stop controlled by voltage on grid
HV from transformer charges capacitor slowly just before exposure
Negative voltage on grid blocks tube current from cathode to anode
Capacitor discharged through tube for exposure when grid turned off
In a 1μf design the kV drop is around 1 kV per mAS used and the effective voltage is 1/3 of the voltage drop lower than the starting voltage. Hence the equivalent kV=Starting kV-1/3 x mAS used. Example for an exposure of 87Kv and 20mAS 87-(1/3 x 20)= 87-6.6 = 80Kv Effective. 85
The starting exposure must be such that the end exposure contributes to the image, in practice any end exposure of less than 50 kV will contribute little to image formation but will cause excessive skin dose.
When the exposure has been made there is still a residual charge in the capacitor.
To remove this a special discharge circuit is employed, the capacitor is discharged through the tube but a special lead shutter closes the exposure aperture to protect the operator and patient. This operation has to be performed if the kV selected needs to be lowered the charge cycle has to be reduced to zero and then reset.
Anatomically Programmed Generator Radiographers have been traditionally selecting & settings of kV, mA & sec. But this problem is overcome by using modern circuitry in anatomically programmed generator. In this generator there is a no. Of push – button setting. Each pushbutton is designated to a particular anatomical part or region. When the chosen push- button is pressed , the circuitry associated with it automatically sets the appropriate factor s of kV , mA & selects the appropriate focal spot on the x-ray tube which is to be used. 87
Technologists operating unfamiliar equipment may work with greater certainty no need of use of technique chart.
In fluoroscopic room , if anatomical programming is used for spot filming, there is obvious benefit when exam are changed
e.g. a pt. being fluoroscoped to investigate a possible gastric ulcers succeeded by one undergoing an oral cholecystogram.
Technologist may find the range of selection not wide enough to embrace all the examinations.
Technologist must manipulate the exposure factor like in case of pt. physical build and certain known pathologies (osteoporosis, pleural effusion).
Well trained and experienced radiographers must be needed for evaluating the good quality of radiograph.
Falling Load Generators: These are usually specially designed three phased or high frequency generators. They take full advantage of the current loading capacity of the X-ray tube by beginning the exposure with a high milliamperage and then allows it to fall during the exposure. This can be achieved with a constant potential circuit. It requires that both mA and KV be regulated independently. These must be used with automatic exposure controls or rely on mAs timer instead of independent mA and time controls. 89
Can shorten x-ray tube life considerably as they use higher mA settings, thus causing the filament to wear out more quickly.
Function with mA unknown to the operator so it is impossible for the operator to set the correct time to achieve the desired mAs, therefore, required to be used with automatic exposure controls or rely on mAs timer instead of independent mA and timer controls.
mA TIME IN AN EXPOSURE 90
Q.C. For Diagnostic X-ray Generators: 1.Accuracy of tube voltage 2. Linearity of mA 3.Exposure time accuracy 91
Safety Rules for Radiological Technologist The radiological technologists operate two main categories of X-Ray equipment: 1.Permanent Installation 2.Movable Equipment Electrical hazards are most communally to be greater with movable equipment. 92
There are certain rules to be observed by radiological technologist:
All movable x-ray equipment should be checked regularly and often by electrician.
The R. Technologist using the equipment should at once any damage or defect.
R. Technologist should not put plugs into or out of sockets which are live.
Cables and plugs should be treated as kindly as if they are patient!
The essential function of high tension generators in x-ray equipments is to provide such power as is needed by the x-ray tube. No doubt, x-ray tube is the heart of the x-ray equipment, like human body, heart has no importance without blood, similarly, x-ray generators provide power which acts as blood in the x-ray tube. The development in high tension generator has always enhanced the quality of production in x-ray beamby reducing the ripple factor.
Radiographic Equipments By Chesney.
Christensen’s Physics For Diagnostic Radiology.