Download SMU-DDE-Assignments-Scheme of Evaluation PROGRAM Bachelor

SMU-DDE-Assignments-Scheme of Evaluation
PROGRAM
SEMESTER
SUBJECT CODE &
NAME
BK ID
SESSION
MARKS
Q.No
1.
A
Bachelor/Diploma in Medical Imaging Technology
II
BMI 203– Medical & Radiation Physics – I
B1949
WINTER 2015
30
Criteria
Marks
Total
Marks
Discuss single-phase and three-phase rectifiers.
(Unit 2;Section 2.3 & 2.4;Pg 29-34)
There are two types of single-phase rectifiers: half-wave rectifiers and full
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wave rectifiers.
Half-wave rectifier:
 A half-wave rectifier is the one that changes only one half of a cycle
of AC into DC.
 For a single-phase supply of half-wave rectifiers, the positive half or
the negative half of the AC wave is applied, whereas the other half is
obstructed or blocked.
 The mean voltage is low in this case because only a half of the input
waveform reaches the output. This rectification requires a single
diode in a single-phase supply.
 Rectifiers provide a unidirectional, but a pulsating DC.
 However, as compared to full-wave rectifiers, half-wave rectifiers
create extreme ripple and much more filtering is required to remove
harmonics of the AC frequency from the output.
 The diode is forward biased as the anode is positive with respect to
the cathode during each positive half cycle of the AC wave, as a
result, the current flows through the diode.
 As the DC load is resistive (resistor, R), the current flowing in the
load resistor is thus proportional to the voltage (according to the
Ohm’s law), and the voltage across the load resistor will thus be equal
to the supply voltage, Vs. Hence, the DC voltage across the load is
sinusoidal for the first half cycle only, such that: V out = Vs, where Vout
is the voltage output.
 During each negative half cycle of the AC wave, the diode is reverse
biased as the anode is negative with respect to the cathode; therefore,
no current flows through the circuit or diode.
 In the negative half cycle of the supply, current does not flow in the
load resistor as voltage does not appear across it, such that: Vout = 0
 The circuit becomes unidirectional in this case because the current on
the DC side of the circuit flows in one direction and the value of the
DC voltage, VDC, across the load resistor is calculated as follows:
VDC = Vmax/π
= 0.318 Vmax
= 0.45Vmax
Here, Vmax is the maximum voltage value of the AC supply, and VS is the
SMU-DDE-Assignments-Scheme of Evaluation
2.
A
r.m.s. value of the supply.
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 A full-wave rectifier is the one that converts both halves of a cycle of
AC into DC.
 The whole input waveform gets transformed to one of the constant
polarity (positive or negative) at its output by a full-wave rectifier.
 Both polarities of the input waveform are converted to DC by fullwave rectification and give a higher mean output voltage.
 For this rectification, two diodes and a centre-tapped transformer or
four diodes in a bridge configuration and any AC source (including a
transformer without centre tap) are required.
 Similar to the half-wave circuit, a full-wave rectifier circuit produces
an output voltage or current, which is purely DC or has some specific
DC component.
 Full-wave rectifiers have some basic advantages over halfwave
rectifiers, as the average (DC) output voltage of full-wave rectifiers is
more than that of half-wave rectifiers.
 The output of the full-wave rectifier has lesser ripple than output of
the half-wave rectifier, which produces a smoother output waveform.
There are two types of three-phase rectifiers: three-phase half-wave
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rectifiers and three-phase full-wave rectifiers.
Three-phase half-wave rectifier
 In an uncontrolled three-phase half-wave circuit, three diodes are
required, one connected to each phase.
 It is the simplest three-phase rectifier, but it experiences relatively
high harmonic distortion on both AC and DC connections.
 This rectifier has a pulse-number of three because the output voltage
on the DC side contains three distinct pulses per cycle of the grid
frequency.
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Three-phase full-wave rectifier
 A rectifier circuit with improved harmonic performance can be
attained if the AC supply is fed by means of a transformer on which
the secondary windings contain a centre tap.
 Six diodes are required in a three-phase fullwave rectifier, one
connected to each end of each transformer secondary winding.
 This circuit has a pulse-number of six and hence can be thought of
as a six-phase, half-wave circuit.
 Traditionally, with mercury-arc valves, the half-wave and full-wave
circuits using a centre-tapped transformer were commonly used in
industrial rectifiers.
 These were used before the availability of solid-state devices for
various applications. However, the three-phase bridge circuit has
become the most widely used circuit with the introduction of diodes
and thyristors.
 For a three-phase full-wave diode rectifier, the ideal, no-load average
output voltage (VAV) is given as follows: VDC = VAV = 3√3 Vmax /π
Explain rotating anode x-ray tube.
(Unit 3;Section 3.3;Pg 66-73)
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 The rotating anode X-ray tube is an assembly and a multi-part piece
of electro-mechanical production including about 350 parts based on
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SMU-DDE-Assignments-Scheme of Evaluation
150 assembly operations.
The rotation of the anode target disc is actually based on a highly
specialised ball bearing system.
 The cathode radiates a stream of electrons focused at the target that
are accelerated due to an increased potential difference between the
cathode and the target disc.
 When the electron beam strikes the anode an X-ray beam is produced.
Design
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 The rotating anode X-ray tube unit is designed for special
requirements with particular anode diameters with the help of highly
energised radiographic procedures.
 Continuous power dissipation is ensured by highly proficient air
cooling for a self-contained unit.
 The body of the rotating anode X-ray tube is made of aluminium that
is lined with lead coating.
 The tube is filled with insulating oil under vacuum.
 The tube is fitted with an internal oil circulation system for
standardisation of temperature.
 A bimetallic thermal switch is connected in series with stator common
cable for thermal safety purposes. This cable is assembled internally.
 Temperature range from −10°C to +80°C is required for its storage
and shipment.
 There is no requirement of user maintenance.
Speed of anode rotation
2
Angle of anode inclination
2
Power rating and use of rating charts
3
3.
Discuss various radiation detectors and their principles.
(Unit 5;Section 5.2-5.6;Pg 104-114)
A
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 Gas-filled radiation detectors
 Scintillation Radiation Detectors
 Dosimeter
Gas-filled radiation detectors
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1. Ionisation chambers
2. Proportional counters
3. Geiger-Muller tubes
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 A detector in which a scintillator is excited by the ionising radiation
that displays scintillation (a property of luminescence) is called a
scintillation radiation detector.
 Explanation
3
 Dosimeter is an instrument that is used to measure the degree of
exposure to radiation that an individual or object is exposed to while
working in a potentially hazardous environment.
 Explanation of thermoluminescent dosimeter
*A-Answer

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