X-ray generator is a device which supplies electric power to the X-ray tube and permits selection of X-ray energy (kVp), X-ray quantity (mA), and exposure time. The generator has three main interrelated electric circuits to serve three main functions: 1) The filament circuit supplies the power needed to heat the cathode filament and boil off electrons, 2) the high-voltage circuit supplies the high-voltage needed to accelerate these electrons from the cathode to the anode for production of X-rays, and 3) the timer circuit (exposure timer) controls the length of the medical x-ray production. A generator may be considered to have two main components, the console or control panel, and the transformer assembly. The control panel allows the operator to select the exposure parameters kVp, mA and exposure time; the actual mA and kVp during exposure are shown by meters on the control panel. There is usually an exposure button with a standby function that starts rotation of the anode and heats the cathode filament prior the actual exposure. The transformer assembly has a low voltage filament transformer which is a step-down transformer, and a high-voltage step-up transformer. The transformer assembly also includes rectifiers for the high-voltage circuit. Due to the high potential differences between the high-voltage circuit and filament circuit (up to 150 kV), the transformers and rectifiers are usually immersed in oil which serves as an insulator and prevents sparking.
There are numerous generator designs, some of which are listed under the entry word generator of high voltage. In most traditional generators (not the constant potential generator or the modern medium-frequency generator), the line voltage is connected to the generator through an autotransformer. This consist of a single winding and functions by self-induction. The line alternating current (AC) is connected to the winding at two points thus encompassing a certain number of turns. The AC induces a magnetic flux around the core of the winding, which in turn induces a voltage into each turn of the winding, e.g. two volts per turn. By selecting a certain number of turns on the output side of the autotransformer, a voltage may be selected that is either less than or larger than the input line voltage. Within a certain small range, the autotransformer thus acts as either a step-down or step-up transformer. From the autotransformer, appropriate voltages are selected for the primary windings of both the filament transformer (e.g. 100-220 V) and the high-voltage transformer (e.g. 320 V). A simple single phase X-ray generator with full-wave rectification is shown schematically in Fig. 1. The X-ray tube current follows the pulsed high voltage in a non-linear way at low kV due to the space charge effect. The cycling high-voltage and tube current produce poor quality radiation with a large amount of low-energy quanta.
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).

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