Radiology Physics CH 3 : Generation And Control Of X-Ray
TRANSFORMER
The transformer is an electrical device, which can convert electrical energy from one coil to another coil. The transformer is working on the principle mutual induction. The transformer basically consists of two coils, namely, primary and secondary (Fig. 4.1). These coils are wound on a iron core. The alternating voltage, which is to be transferred, is applied in the primary coil as input. This produces a changing magnetic flux in the iron core, which produces an alternating emf in the secondary coil. The induced emf’s in the coils are directly proportional to the respective number of turns of the coil.
Basically, there are three types of transformers, namely, step up, stepdown and isolation transformers. If a transformer, transfers power of low voltage and high current into power of high voltage and low current, it is called step-up transformer. In this type, the secondary coil will have more number of turns than the primary, i.e. Ns > Np. If a transformer, transfers power of high voltage and low current into power of low voltage and high current, it is called step-down transformer. In this type, the primary will have large number of turns than the secondary, i.e. Ns < Np. If Ns = Np, this results in an isolation transformer.
Efficiency
The efficiency of the transformer is the ratio between the output power and input power.
In actual transformer, the output power is always lesser than the input power due to some energy losses. Hence, the efficiency is always less than <100%
TRANSFORMER LOSSES
In practice, the output power is always lesser than the input power and hence, the efficiency of the transformer is always less than 100%. This implies that some amount of energy is lost in the form of heat. This energy loss can be considered as copper losses, eddy current losses, hysteresis and flux leakage losses.
Copper Losses
Whenever a current I flows through a resistance R, an amount of power equal to I2× R × watt is converted into heat. This can arise in both copper coils and iron core. The copper coil has resistance. If current flows through this coil, electrical energy equal to I2Rt is converted into heat. To reduce this loss, the current cannot be reduced because the normal operation of the transformer will be affected. Instead, the resistance of the coil must be minimized by using wire of low resistivity. Therefore, thicker wire should be used as transformer coil. The optimum thickness will be decided by comparing the cost,
space and saving of power. Copper is the best coil material available to day and hence, it is commonly used.
Eddy Current Losses
The iron core consists of concentric layers of iron, each acts as a circuited single turn coil. Whenever the magnetic field changes, an emf will be induced in the core. The current produced by the induced emf in the core is called eddy current, which will give rise to I2Rt heat losses. These eddy currents can be eliminated by making the iron core in the form of thin sheet of metal, and each sheet is insulated from its neighbor by a thin layer of paper. This type of core is known as laminated core. The core is usually made up of stelloy, an alloy of steel. Some design employ high resistance ceramics as core material.
Hysteresis Losses
The transformer core is a magnetic material. The core is magnetized twice in each cycle of the alternating voltage. When the direction of AC changes, the magnetization is also gets reversed. During this reversal, some energy is lost due to the molecular friction and the energy appears as heat. The loss of energy by molecular friction is called hysteresis loss. This can be reduced in practice by choosing a suitable magnetic material, such as mu-metal, which has low hysteresis loss. Mu-metal is a ferromagnetic alloy containing 78% nickel, 17% iron, and 5% copper. It has high permeability.
Flux Leakage
All the magnetic flux linked with the primary is not linked with the secondary coil. This is said to be flux leakage, which results in loss of energy. This can be minimized by using good core design like shell type of core.
TRANSFORMER CONSTRUCTION
A practical transformer differs considerably from the ideal transformer. The following points should be considered during the construction of the transformer.
Winding
The transformer is usually made with single primary winding, whereas the secondary will have more than one winding. For example, the
primary of a transformer used in control equipment, may be designed for 200 V input. The secondary may be designed with three windings for three out put such as 500 V low current, 50 V low current and 6 V high current. The primary will have a wire of medium thick, whereas the secondary have thin and thick wires. Thicker wire offer lesser resistance and allow flow of high current in the primary. Whereas secondary handle only low current, hence, thin wire with relatively higher resistance is preferred to save cost and power loss.
Core
The transformer cores are always designed so that they form a closed circuit. A core with a closed magnetic circuit has high permeability and is very efficient. At the same time, the core is laminated to eliminate eddy current loses. There are three types of core, namely, (i) core type, (ii) shell type, and (iii) cross type or H type (Fig. 4.2).
In a core type transformer, the primary winding is on one leg and the secondary winding is on other leg. This is easily assembled and has a good cooling surface. Alternatively, both primary and secondary windings are made as two halves. The secondary is wound over the
primary winding on each leg. This is the most preferred transformer core type, used in X-ray generators. In a shell type transformer, the primary and secondary are wound around the central limb, and the magnetic circuit is shorter. Shell type is the most efficient design in terms of energy conservation and efficiency (98%). Hence, it is used most commonly.
The cross or H type core is called modified shell type, since it is a combination of two shell cores set at right angles to each other. In this, the coils are surrounded by four legs. The windings are located over the center core, which is four times the area of the each of
the outside legs. This type of core is cooled easily and hence used in large power transformers, where the voltage drop and cost is kept minimum.
Transformer designed for higher output voltages, such as 100 kV, needs special care. The secondary winding must be designed very carefully to avoid electrical break down due to ionization of the surrounding air. Transformers are cooled by oil or forced air, to avoid
overheating. Transformers never be immersed in water for cooling. During accidental flooding, if the transformer is immersed in water, immediately the water should be pumped out.
Oil
High voltage transformers are usually enclosed in a metal tank filled with oil. This oil penetrates into the inner spaces of the windings and increases the effectiveness of the insulation. The oil prevents the windings from dust and moisture and also acts as a cooling medium. The oil is a good insulator than air, it avoids electrical short circuiting. Oil also provides effective cooling to the transformer.
Autotransformer
The autotransformer consists of a single winding wound on a laminated iron core and it is working on the principle of self induction (Fig. 4.3). The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits, therefore, have a number of windings turns in common.
The alternating current applied between the input points will induce a flow of magnetic flux around the core. This magnetic flux will link with all the turns forming the coil, inducing a voltage into each turn of the winding. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer, part of the current flows directly from the input to the output, and the other part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.
These transformers are widely used where electrical isolation between primary and secondary is not necessary. Autotransformer occupy very important place in X-ray generator circuits. In the X-ray generator, the autotransformer is used to adjust the voltage applied to the primary of the high voltage transformer with high efficiency and convenience.
An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does. A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output. If there is a break in the part of the winding, then the transformer acts as an inductor in series with the load.
RECTIFIER CIRCUIT
RECTIFICATION
Rectification is the process of changing alternating current into direct current. The device that produces the change is called a rectifier.A rectifier allows an electrical current to flow in one direction but does not allow current to flow in the other direction. Rectifiers are connected into the X-ray circuit in series. They are mainly divided into half wave and full wave rectifiers.
If alternating voltage is applied directly to the X-ray tube, the anode will emit electrons, whenever it is negative with respect to cathode. These electrons will travel towards the cathode and bombard the filament and destroy the filament. This is called back projection which is avoided by the supply of rectified DC voltage. Thus, rectifiers play an important role in X-ray production .
HALF WAVE RECTIFIER
Vacuum tube diodes or solid state (semiconductor) diodes can be used for rectification. In a half wave rectifier, a single diode is used, as shown in the Figure 4.6. An alternating voltage is applied to the diode as input. The output is obtained across the resistance R. When the plate is positive, the diode will allow the current to flow. When the plate is negative, the diode will not allow the current. Therefore, the diode will allow the current only during those half cycles when the plate is positive. Hence, the output current is always in one direction. This circuit is known as half wave rectifier and it is mainly used in mobile and dental X-ray units. A single solid state diode cannot prevent reverse current at higher voltages. Hence, many diodes are placed in series in a stick to do rectification.
FULL WAVE RECTIFIER
In the half wave rectifier, the input voltage is used only in one half of the cycle. The other half of the cycle is not used. Therefore, there is a need for a rectifier, which will use the full cycle of the input. This is possible by having two or more number of diodes, as shown
in the Figure 4.7. The alternating voltage is applied between A and B. The output is obtained across the resistance R.
THYRISTOR
A thyristor is a silicon-controlled rectifier which has four layer semiconductors (n-p-n-p). It is used to switch larger currents, which the transistor cannot handle. It has two large terminals, namely, anode and cathode, which connects the main circuit (Fig. 4.8). The third terminal is the gate, which is smaller in size. Initially, the junction J1 and J3 are forward biased and junction J2 is reverse biased. Hence, only small current flows through the circuit and the thyristor is said to be in OFF state.
If a positive voltage is applied in the gate terminal, holes flows through J3 and the barrier across J2 breaks down. This will facilitates movement of electrons across J2 junction and makes the thyristor to ON condition. The conduction continues in the circuit, even after the gate voltage is removed. The conduction ceases only when the potential difference
across the anode and cathode falls to zero. Thyristor conduct current only in one direction and can be used to switch the alternating current.
HALF WAVE RECTIFICATION X-RAY CIRCUIT
The half wave rectification is the most commonly used circuit. The circuit uses two rectifier stacks connected in series with the X-ray tube, as shown in Figure 4.9. The center point of the secondary winding of the step up transformer is grounded. The electrons flow through the X-ray tube from the cathode to anode in the first half cycle. When the voltage reverses, in the second half cycle, the rectifier stops current flow. Since there are two rectifiers, the circuit is symmetric and each has to withstand only half of the peak voltage (Vp). The anode goes only to +Vp/2 at the peak of the conducting cycle and the cathode to -Vp/2. The tube voltage, tube current and X-ray pulse are shown as a function of time.
The discontinuous nature of X-ray yield reveals that the tube is inoperative at least half the time. This means that the exposures must be twice as long to get the same X-ray flux. This increases the chance of organ motion during the exposure, with a loss of diagnostic
information. The advantage of the half wave rectification is that they protect the X-ray tube from the full potential of the inverse cycle.
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