X-RAY GENERATOR COMPONENTS Electromagnetic Induction and Voltage Transformation  The principal function of the x-ray generator is to provide current at a high voltage to the.

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Transcript X-RAY GENERATOR COMPONENTS Electromagnetic Induction and Voltage Transformation  The principal function of the x-ray generator is to provide current at a high voltage to the.

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
Electromagnetic induction is an effect
that occurs with changing magnetic
fields and alternating (AC) electrical
• 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 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 in the 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:
P(watts)  V (volts)  I (amps)
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:
VP  I P  VS  I S
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-tosecondary turns ratio of 1:500 to
• 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
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.
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
• 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,
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
The switched autotransformer is used ro
adjust the kVp produced by an x-ray
• 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 highvoltage 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
• 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 xray beam quality (penetrability), which plays a
role in the subject contrast.
The x-ray tube current (mA) determines the xray 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 milliampereseconds (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
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,
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
Several x-ray generator circuit designs
are in common use, including singlephase, three-phase, constant potential,
and medium/high-frequency inverter
• 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
• 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 xray 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 highvoltage circuit and from the cathode to the anode in the xray 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
The polarity across the x-ray tube is thus maintained with
the cathode negative and anode positive throughout the
X-rays are produced in two pulses per