Electromagnetic Induction and Voltage Transformation • The principal function of the x-ray generator is to provide current at a high voltage to the x-ray tube. • Electrical power available to a hospital or clinic provides up to about 480 V. • Much lower than the 20,000 to 150,000 V needed for x-ray production.
Transformers are principal components of x-ray generators; • They convert low voltage into high voltage through a process called electromagnetic induction.
Electromagnetic induction is an effect that occurs with changing magnetic fields and alternating (AC) electrical current. • Note that for constant-potential direct current (DC),like that produced by a battery, magnetic induction does not occur.
Electromagnetic induction is reciprocal between electric and magnetic fields. • An electrical current (e.g., electrons flowing through a wire) produces a magnetic field, whose magnitude (strength) and polarity (direction) are proportional to the magnitude and direction of the current . • With an alternating current, such as the standard 60 cycles per second (Hz) AC in North America and 50 Hz AC in most other areas of the world, the induced magnetic field increases and decreases with the current.
Transformers • Transformers perform the task of “transforming” an alternating input voltage into an alternating output voltage, using the principles of electromagnetic induction. • The generic transformer has two distinct, electrically insulated wires wrapped about a common iron core. • Input AC power (voltage and current) produces oscillating electrical and magnetic fields.
One insulated wire wrapping (the “primary winding”) carries the input load (primary voltage and current). • The other insulated wire wrapping (“secondary winding”) carries the induced (output) load (secondary voltage and current).
The primary and secondary windings are electrically (but not magnetically) isolated by insulated wires.
The induced magnetic field strength changes amplitude and direction with the primary AC voltage waveform and freely passes through electrical insulation to permeate the transformer iron core, which serves as a conduit and containment for the oscillacing magnetic field. • The secondary windings are bathed inthe oscillating magnetic field, and an alternating voltage is induced in the secondary windings as a result.
The Law of Transformers states that the ratio of the number of coil turns in the primary winding to the number of coil turns in the secondary winding is equal to the ratio of the primary voltage to the secondary voltage: • where Np is the number of rums of the primary coil, Ns is the number of turns of the secondary coil, Vp is the amplitude of the alternating input voltage on the primary side of the transformer, and Vs is the amplitude of the alternating output voltage on the secondary side.
A transformer can increase, decrease, or isolate voltage, depending on the ratio of the numbers of turns in the two coils. • For Ns > Np, a “step-up” transformer increases the secondary voltage;
For Ns < Np, a “step-down” transformer decreases the secondary voltage;
And for Ns = Np, an Isolation” transformer produces a secondary voltage equal to the primary voltage (see later discussion).
A key point to remember is that an alternating current is needed for a transformer to function.
Power is the rate of energy production or expenditure per unit time. • The SI unit of power is the watt (W), which is defined as 1 joule (J) of energy per second. • For electrical devices, power is equal to the product of voltage and current:
Because the power output is equal to the power input (for an ideal transformer), the product of voltage and current in the primary circuit is equal to that in the secondary circuit:
Therefore, a decrease in current must accompany an increase in voltage, and vice versa. • Power losses due to inefficient coupling cause both the voltage and current on the secondary side of the transformer to be less than predicted by these equations.
The high-voltage section of an x-ray generator contains a step-up transformer, typically with a primary-to-secondary turns ratio of 1:500 to 1:1,000. • Within this range, a cube voltage of 100 kVp requires an input peak voltage of 200 V to 100 V, respectively.
The center of the secondary winding is usually connected to ground potential (“center tapped to ground”). • Ground potential is the electrical potential of the earth. • Center tapping to ground does not affect the maximum potential difference applied between the anode and cathode of the x-ray tube, but it limits the maximum voltage at any point in the circuit relative to ground to one half of the peak voltage applied to the tube.
Therefore, the maximum voltage at any point in the circuit for a center-tapped transformer of 150 kVp is -75 kVp or +75 kVp, relative to ground. • This reduces electrical insulation requirements and improves electrical safety. • In some x-ray tube designs (e.g., mammography), the anode is maintained at the same potential as the body of the insert, which is maintained at ground potential. • Even though this places the cathode at peak negative voltage with respect to ground, the low kVp (less than 50 kVp) used in mammography does not present a big electrical insulation problem.
Autotransformers • A simple autotransformer consists of a single coil of wire wrapped around an iron core. • It has a fixed number of turns, two lines on the input side and two lines on the output side.
When an alternating voltage is applied to the pair of input lines, an alternating voltage is produced across the pair of output lines. • The Law of Transformers applies to the autotransformer, just as it does to the standard transformer. • The output voltage from the autotransformer is equal to the input voltage multiplied by the ratio of secondary to primary turns. • The primary and secondary turns are the number of coil turns between the caps of the input and output lines, respectively.
The autotransformer operates on the principle of self-induction, whereas the standard transformer operates on the principle of mutual induction. • The standard transformer permits much larger increases or decreases in voltage, and it electrically isolates the primary from the secondary circuit, unlike the autotransformer.
A switching autotransformer has a number of taps on the input and output sides, to permit small incremental increases or decreases in the output voltage.
The switched autotransformer is used ro adjust the kVp produced by an x-ray generator. • Standard alternating current is provided to the input side of the autotransformer, and the output voltage of the autotransformer is provided to the primary side of the high-voltage transformer. • Although variable resistor circuits can be used to modulate voltage, autotransformers are more efficient in terms of power consumption and therefore preferred.
Operator Console • At the operator console, the operator selects • The kVp, • The mA (proportional to the number of x-rays in the beam at a given kVp), • The exposure time, and • The focal spot size.
The peak kilo-voltage (kVp) determines the x-ray beam quality (penetrability), which plays a role in the subject contrast. • The x-ray tube current (mA) determines the x-ray flux (photons per square centimeter) emitted by the x-ray rube at a given kVp. • The product of tube current (mA) and exposure time (seconds) is expressed as milliampere-seconds (mAs).
Some generators used in radiography allow the selection of “three-knob” technique (individual selection of kVp, mA, and exposure time), whereas others only allow “two-knob” technique (individual selection of kVp and mAs). • The selection of focal spot size (i.e., large or small) is usually determined by the mA setting: • low mA selections allow the small focus to be used, and • higher mA settings require the use of rhe large focus due ro anode heating concerns.
On some x-ray generators, preprogrammed techniques can be selected for various examinations (i.e., chest; kidneys, ureter, and bladder; cervical spine). • All console circuits have relatively low voltage and current levels that minimize electrical hazards.
Several x-ray generator circuit designs are in common use, including single-phase, three-phase, constant potential, and medium/high-frequency inverter generators. • All use step-up transformers to generate high voltage, step-down transformers to energize the filament, and rectifier circuits to ensure proper electrical polarity at the x-ray tube.
Rectifier Circuit • A rectifier is an electrical apparatus that changes alternating current into direct current. • It is composed of one or more diodes.
In the x-ray generator, rectifier circuits divert the flow of electrons in the high-voltage circuit so that a direct current is established from the cathode to the anode in the x-ray tube, despite the alternating current and voltage produced by the transformer. • Conversion to direct current is important. • If an alternating voltage were applied directly to the x-ray tube, electron back-propagation could occur during the portion of he cycle when he cathode is positive with respect to the anode. • If the anode is very hot, electrons can be released by thermionic emission, and such electron bombardment could rapidly destroy the filament of the x-ray rube.
To avoid back-propagation, the placement of a diode of correct orientation in the high-voltage circuit allows electron flow during only one half of the AC cycle (when the anode polarity is positive and cathode polarity is negative) and halts the current when the polarity is reversed.
As a result, a “single-pulse” waveform is produced from the full AC cycle, and this is called a half-wave rectified system.
Full-wave rectified systems use several diodes (a minimum of four in a bridge rectifier) arranged in a specific orientation to allow the flow of electrons from the cathode to the anode of the x-ray tube throughout the AC cycle (see Fig. 5-26B).
During the first half-cycle, electrons are routed by two conducting diodes through the bridge rectifier in the high-voltage circuit and from the cathode to the anode in the x-ray tube. • During the second half-cycle, the voltage polarity of the circuit is reversed; electrons flow in the opposite direction and are routed by the other two diodes in the bridge rectifier, again from the cathode to the anode in the x-ray tube. • The polarity across the x-ray tube is thus maintained with the cathode negative and anode positive throughout the cycle.