Generators

(Siemens)

Diagram showing typical waveforms from x-ray generator output circuits

Ripple
The deviation of the voltage waveform across the x-ray tube from constant voltage is named ripple
.the variation in the high-voltage expressed as the percentage of the maximum high-voltage across the X ray tube during X-ray production:

Ripple factor (%) = 100 x (Vmax - Vmin)/Vmax

The ripple causes corresponding but relatively higher variations in the X-ray output. It is an unwanted phenomenon in the X-ray production due to the lengthening of the exposure time and the reduction in the average kV. The ripple is theoretically 100% for the old-fashioned single phase X ray generator (in practice, it is less, however, due to the smoothening effect of the high-voltage cable capacitance). The three-phase X-ray generator have ripple factors in the range of about 3–25% (3-phase 6-pulse generator: 13–25%, 3-phase 12-pulse generator: 3–10%). In the medium frequency generator, the ripple factor decreases with increasing kVp. In this type of generator (also named high frequency or inverter generator), the kV is controlled by adjusting the frequency of the current prior to high-voltage transformation. Ripple is usually in the range of 4–15%. There is practically no ripple in the constant potential X ray generator

Constant potential x-ray generator,
An X ray generator providing a nearly flat high voltage waveform for the X ray tube.
The term may refer to
1) any generator providing high voltage with a ripple factor less than a certain limit, e.g. 5%, or
2)* a special generator type briefly mentioned below.

1) A voltage ripple limit of 5% would include the 3-phase 12-pulse generator and the medium frequency generator.

2)*The so-called constant potential X-ray generator is a very large and expensive generator that provides the highest average X-ray energy of any X-ray generator type. It is now used only for the most demanding applications. This generator uses a three-phase line voltage coupled directly to the primary windings of the high-voltage transformer, i.e. without an intermediate autotransformer. Regulation of the kilovolt peak kVp and exposure time is done on the secondary (high voltage) side of the transformer by means of high voltage electron tubes (triodes or tetrodes). The high voltage supplied to the X-ray tube has a nearly flat waveform with a ripple less than 2%.

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--------------------------------------------------------------------------------------------------------1) One pulse self rectified
In the simplest case the tube acts as a rectifier and the - a self rectified circuit

(Stylised graph of voltage v Time)

--------------------------------------------------------------------------------------------------------2) One pulse half wave rectified
In order to prevent the anode producing electrons as it becomes hotter and the electrons flowing backwards and striking the filament a single rectifier can be placed in series with the x-ray tube to ensure current flows only from filament to anode. -

(Stylised graph of voltage v Time)

Advantages and Disadvantages
Inefficient use of power no x-rays produced in negative half cycle
Possibility of reverse conduction - low power output - unless a rectifier is used
Minimum exposure time 0.02 S to include one whole AC cycle
However the unit can be made relatively small and cheaply for situations requiring limited output.

--------------------------------------------------------------------------------------------------------3) Two pulse full wave rectified

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A bridge rectifier circuit (see rectification) inverts the negative half cycles and double the number of positive cycles are produced per unit time compared with a single rectifier.
- the ripple is said to be 100%.

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Advantages and Disadvantages
The principal disadvantages are the inefficiency of radiation production due to the pulsating waveform no providing enough voltage to produce x-rays for a portion of the time, and the inability to select short exposure times.

--------------------------------------------------------------------------------------------------------4) Constant potential x-ray generator
The constant potential generator circuit has two capacitors across the output from the rectifiers to smooth the pulsating waveform.

To further condition the waveform, there is a triode valve in series with each lead and  these control the output via a grid connection which has control signals fed to it from a high resistance across the output of the triodes. Once the tube voltage has been monitored via the high resistance potential divider it is constantly corrected by the control unit attached to the triode valve control grids.
The triode valves also from part of the timer switching and can operate at microsecond intervals with good accuracy.  The high voltage supplied to the X-ray tube has a nearly flat waveform with a ripple less than 2%.
 

Block Diagram of Constant potential x-ray generator circuit

             
 

Output waveform from the rectifiers and the smoothed output across the x-ray tube

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Advantages and Disadvantages
High x-ray output per mAS
Smaller range of x-ray energies
Very small exposure times possible

However these generators are very expensive and tend to be large and have the possibility of more to go wrong.
--------------------------------------------------------------------------------------------------------5) Three phase 6 Pulse Generator
Commercial electric power, the line voltage, is usually produced and delivered as three phase alternating current. The period of each single phase may be 50 or 60 Hz. The period of a 50 Hz AC has a duration of 1/50 s, or 20 ms. The three phase X ray generator transforms and rectifies this AC into a high-voltage direct current (DC) with either six or twelve forward pulses per 20 ms period. As compared to the 100% ripple factor of single-phase generators, three-phase generators dramatically reduces voltage ripple (13–25% for 3-phase 6-pulse, 3–10% for 3-phase 12-pulse).

X-ray production is therefore much more efficient. The so-called constant potential X ray generator produces a voltage ripple less than 2% (hence the name), and it produces the highest average X-ray energy of any X-ray generator type, with exposure times less than 1 ms. This kind of generator is, however, very bulky, with high costs and inefficient power consumption. The preferred modern generator today, is therefore the almost equally efficient, much smaller and less costly medium-frequency generator (also known as high frequency and inverter generator).

An X ray generator using a 3-phase alternating current (AC) line source, i.e. three wires, each with a single phase AC that is one third cycle (120°) out of phase with the other two (Fig.1). The three-phase transformer used in this generator has three sets of primary windings and three sets of secondary windings, i.e. in effect three separate high-voltage interconnected transformers. The three primary and secondary windings are connected either in a wye¹ configuration or a delta² configuration. In the three-phase six-pulse generator, rectifiers in the high-voltage circuit produce two pulses for each line, resulting in a total of six pulses.

Waveforms from the various circuit in a 3 phase unit parts

Star and delta windings                                       
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Wye or Star configuration, ¹
a star-shaped configuration or interconnection of the three windings in the primary or secondary of a transformer in a three phase X ray generator.

Delta configuration, ²
one possible configuration of the windings in the primary or secondary side of a three-phase transformer. The windings in this transformer can be arranged as a “D“

Combinations of these configurations in the primary and secondary windings of a transformer will give rise to a phase shift of 30. Using one delta and one wye configuration as secondary windings and (usually) a delta configuration as primary winding will therefore give twelve pulses per period of mains AC voltage.

Advantages and Disadvantages
High x-ray output per mAS
Smaller range of x-ray energies

However these generators are expensive and tend to be large and have the possibility of more to go wrong.

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Three-phase twelve-pulse generator
In the three-phase twelve-pulse generator, a different configuration of transformers, one of each one star and  delta wound secondaries and rectifiers resulting in a total of twelve pulses per cycle. These generators have very low ripple factor.

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Falling Load Generator

--------------------------------------------------------------------------------------------------------See the Mobile Generator notes in the Tutorials section for details of generators used in Mobile equipment

--------------------------------------------------------------------------------------------------------Medium-frequency generator,
A state of the art generator design, also named high-frequency generator and inverter generator, which uses a high-frequency current to produce nearly constant potential voltage to the X ray tube with a transformer of much smaller size than found in ordinary X-ray generators.

The incoming power supply to a medium frequency generator may be an ordinary 50 Hz (230 V) single phase current (Fig.1). This current is rectified and smoothed and then fed to a chopper and inverter circuit which transforms the smooth, direct current (DC) into a high-frequency (5 - 100 kHz) alternating current (AC). (The chopper "chops" the continuous DC into high-frequency DC pulses and the inverter transforms this into AC.) A transformer converts this high-frequency low-voltage AC into high-voltage AC, which then is rectified by half wave rectification and smoothed to provide a nearly constant potential high voltage to the X-ray tube. The voltage is controlled by varying the frequency of the chopper/inverter circuit, which determines the frequency of the current delivered to the transformer. Fast exposure switching, in the order of 1 ms, is easily obtained with the medium frequency generator.

Outline of a medium frequency generator

(Siemens)

Waveforms from the various circuit parts

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Advantages and Disadvantages
One of the great benefits of this generator design, is the reduced weight and size. The main components of the generator may be placed within the same enclosure as the X-ray tube, or in e.g. the C-arm of the equipment. This generator principle was previously used only in small mobile and/or battery-powered generators with low power rating, but today it is applied to all modern high-voltage generators up to the highest needed power ratings above 100 kW.

Typical Generator Diagrams (Siemens)

Typical Fluoroscopy Unit Block Diagram (Siemens)

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Advantages and Disadvantages of different generator types
The design of a generator needs to optimise the following points, and be matched as closely as possible to the clinical requirements of the generator usage, a point to note is that the late 1990 saw the pinnacle of development of film screen radiography with exposure values required for typical examinations being much less than even ten years earlier, thus a typical chest x-ray in 1979 before rare earth screens may have been around 25 mAS @70Kv whilst today this may have dropped to as little as 2mAS@90 Kv, interestingly some of the new Digital radiography systems require more exposure than the film screen combinations they replace.

Efficiency of conversion of electrical energy to useful x-ray energy
Maximum dose rate per mAS
Power output
Low Ripple
Cost
Size / weight
Minimum exposure time
Reliability

Advantages and Disadvantages of  Constant Potential and multiphase generators compared with a basic single phase generator. (Stockley)

Advantages
More efficient conversion of electrical power to x-ray energy
More x-rays generated per mAS
Shorter exposure times possible
Sleeplessly variable range of exposure times

Disadvantages
More expensive to purchase
X-ray tube to cope with higher loading required
Lower image contrast
Shorter tube life
Equipment larger and heavier
Possibly more prone to failure due to greater complexity

--------------------------------------------------------------------------------------------------------Indicative Reading
Stockley, S  (1986) A Manual of Radiographic Equipment. London, Churchill Livingstone

Mobile X-Ray Generators: a review By Evans, Harris, Lawinski & Hendra
Radiography March/April 1985 Vol. 51 No: 506

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