Exam 3 Concepts

Transformers

electromagnetic device that changes an AC current from low voltage ←→ high voltage with minimal energy loss

• uses electromagnetic induction to transfer electricity between circuits without any electrical contact or moving mechanical parts

Mutual Induction

primary side

• voltage on primary side creates fluctuating magnetic fields due to the incoming AC supplied

secondary side

• fluctuating magnetic fields cut the coils, inducing a voltage on the secondary coil

Principle of Operation

the changing magnetic field produced by primary coil induces an EMF in the secondary coil

• induced EMF determined by # of coils on each side

Transformer Types

air core - 2 overlapping coils (least efficient w/ no core)

open core - core in each coil (lots of flux not used… leakage flux losing efficiency)

closed core - square donut w/ coils on opposite sides

shell type - closed + air cores combined w/ pillar down middle (most efficient)

Efficientmaxxing Cores

• increasing core sophistication increases transformer efficiency

• laminating the core steel reduces heat in the core & improve transformer efficiency

Step Up Transformers

• increases voltage in secondary coil

• decreases amperage (current)

• secondary side has more coils than primary

• Turn’s Ratio is >1

Step Down Transformers

• decreases voltage in primary coil

• increses amperage (current)

• secondary side has fewer coilers than primary

• Turn’s Ratio is <1

Transformer Law

the EMF induced in the secondary coil is to the EMF in the primary coil, as the number of turns in the secondary is to the number of turns in the primary coil

• voltage change is directly proportional to the Turn’s Ratio

Turn’s Ratio

the number of turns in the secondary coil is to the number of turns in the primary coil: N_s/N_p

• fraction, whole number or ratio

Transformers and the Law of Conservation of Energy

Power In = Power Out

• if voltage increases, current must decrease (to keep power constant, so compensation of either one)

Transformer Law: Voltage

voltage is DIRECTLY proportional to Turn’s Ratio: V_s/V_{p =} N_s/N_p

Transformer Law: Current

current is INVERSELY proportional to Turn’s Ratio: I_p/I_{s =} N_s/N_p

Transitive Property

current change is inversely proportional to voltage change

1. V_s/V_{p =} N_s/N_{p =} I_p/I_s

2. V_s/V_{p =} I_p/I_s

Autotransformer

operate on the principle of electromagnetic self-induction

• single coil acts as primary and secondary

• number of turns are adjustable

• variable transformer either step up or step down

• sends precise voltage to the primary side of the step-up transformer

Autotransformer Law

if the number of tapped turns is decreased, it will reduce the voltage on the secondary side, and vice versa

Power Loss (+ Efficiency)

• transformers are 95+% efficient

• Power Output/Power Input x 100

  • (2ndary V x I)/(primary V x I) x 100

Power Loss Causes + Prevention

power loss in a transformer is due to heat

1. copper loss: loss of power due to inherent resistance of copper (silver better)

   1. to prevent, use larger diameter

2. eddy current loss: swirling currents that build up in the core

   1. to prevent, spin until iron heats up, laminate & put in cooling oil bath

3. hysteresis loss: constantly changing magnetic domains in the core

Prevention (generally)

1. submerge transformer in an oil bath

2. laminate the transformer core

X-Ray Tube (Contains + Function)

Contains: thermionic vacuum diode

• cathode

• anode

Function: converts electrical energy into electromagnetic energy

Conditions to Produce X-Ray Photons

1. source of electrons (large amount)

2. high PD

3. vacuum

kVp

Kilovoltage Peak

• beam: energy level (penetrability) of the X-ray beam

• sets PD (voltage) between anode & cathode

mAs

Milliamperage per Second

• beam: # of photons produced & length of exposure

• sets the amount of current (or e^-) per second during exposure

• mA x exposure time (s)

X-Ray Circuits

1. Filament Circuit - determines filament current (mA setting)

   1. current that heats filament

   2. measured in Amps (low voltage & high amperage)

      1. 6-12 V & 3-5 A

2. Tube Circuit - determines the tube current during the exposure

   1. passes between 2 electrodes & produces x-rays

   2. measured in mA (high voltage & low amperage)

      1. 50-120 kV & 0.1.0.5 A

• e^- produced by filament are driven across the gap by the voltage applied to the cathode & anode

• a small change in filament current produces a large change in tube current

Generator Divisions

Sections

1. low voltage (primary side)

2. high voltage (secondary side)

3. tube

Circuits

1. tube circuit (high voltage to tube)

2. filament circuit (high amperage, low voltage to filament)

Primary Side (Low Voltage)

• adjusts incoming voltage to the technical factors we choose

  • tube circuit - adjust kVp

  • filament circuit - adjust mAs

• Components

  • autotransformer

  • rheostat

  • kVp meter

  • timing circuit

Main Switch

2-Stage Switch

1. current to induction motor & filament

2. closes tube circuit & begins exposure

Autotransformer & kVp Selectors

variable transformer (single coil); adjusts incoming supplied voltage to the desired voltage for the X-ray tube (also operates using self-induction)

• kV selector - taps on secondary side of the autotransformer

  • kV major - adjusts in units of 10

  • kV minor - adjusts in units of 1

kVp Meter

voltmeter that prereads the kVp going to step-up transformer (to be applied to the tube) & ensures correct voltage reaches it

• wired in parallel

• if there are voltage fluctuations, autotransmitter is adjusted (V not kV here)

• lets you know if something is wrong

Timing Circuit

responsible for terminating the exposure

Rheostat

variable resistor (mA selector); adjusts incoming current to the desired amperage to be sent to the step-down transformer (filament)

• 3-5 amps & 6-12 volts

Focal Spot Selector

determines which filament receives current

• dual focus tube

  

Step-Up (High Tension) Transformer

increases voltage (from autotransformer) to tube as it leaves from primary to secondary coil

Secondary Side (high voltage) (distinction)

• primary vs. secondary distinction made at step-up, high tension transformer

• primary side - incoming volts adjusted by autotransformer

• secondary side - high voltage to be sent to the tube

Step Down (filament) Transformer

sends increased amperage to filament for thermionic emission

mA Meter

measures tube current

• wired in series on secondary side of high tension (step-up) transformer

mAs Meter (not in diagram) - needed for short exposure times less than 100 ms because ammeter will not measure it accurately

Rectification Circuit

converts AC → DC

• enhances heating capacity of X-ray tube, permitting larger exposures

X-Ray Tube (+ Components)

utilizes high voltage to accelerate electrons toward the target and produce x-rays

1. filament - thermionic emission (boiling off of electrons when metal is heated)

2. cathode - negative diode to create high PD

3. anode - positive diode to create high PD

X-Ray Tube Function

Function: converts electrical energy into electromagnetic energy

1. large source of e^-

2. high PD

3. vacuum

X-Ray Tube Envelope

Material: pyrex glass (older) or aluminum (easier to ground w/ less buildup)

Function: create vacuum

• also tube port (window) which allows primary beam to exit

Cathode (parts + functions)

the negative electrode/diode

1. focusing cup - tungsten (or NICKEL?)

   1. focuses e^- cloud

   2. prevents stem or off-focus radiation

2. filament(s) - thoriated tungsten

   1. thermionic emission

   2. dual focus: large & small focal spot size (FSS)

      1. control for spatial resolution in images

Anode (parts + functions)

the positve electrode/diode

1. stationary - copper cylinder

2. rotating - molybenum disk (attached to rotor of induction motor)

   1. high thermal capacity

   2. high melting point

   3. dissipates heat quickly

   4. remains stable when heated at high RPM

Functions

1. supports target

2. dissipates heat

3. conduct current

Heat Units

kVp x mA x time x generator constant (1.45)

Factors Determining Heat During Beam Production

1. kVp

2. mA

3. exposure time

4. type of x-ray generator

Excessive Heat Build Up & Reduction

1. creates resistance in the tube circuit (lowers photon production)

2. causes damage to the anode & possibly the induction motor

Reduction: use lowest kV, mA & time combinations possible

Target (parts + function)

tungsten-rhenium alloy embedded in anode face

Function: to convert the kinetic energy of e^- to electromagnetic energy (x-ray photons)

Tungsten Because…

• high melting point: 6,192 °F

• produces photon energies ideal for imaging

Rhenium Because…

• stabilizes tungsten while it spins

Induction Motor

spins anode during exposure (3,000-10,000 RPM)

• increases target area, spreading heat load surface area

• rotor inside vacuum tube & held in place by a series of bearings

Tube Failure

most tube failure is due to heat build up within the tube

1. anode pitting and melting

   1. wrong RPM or improper rotation

2. anode cracking

   1. high heat exposure on cold anode/temp differences

3. induction motor damage (bearings)

   1. molybdenum overheating moves onto rotor bearings w/o cooling time

4. tungsten vaporization

   1. thin then breaks

5. filament breakage

   1. can cause arcing/lightning

Life Span Determined By… (+ Extending Tube Life)

how it’s used

1. use lowest kVp, mA & time combinations

2. use appropriate warm up procedures

Label the Boxes

Left to Right (Top then Bottom)

• induction motor rotor

• molybdenum disk

• tube envelope

• anode

• tube port (window)

• tungsten-rhenium target

• cathode

• focusing cup

• filament

Rectification

the process of changing AC → DC

(x-ray tubes operate most efficiently when supplied by high voltage DC)

AC Sine Wave

above: positive/forward bias

• cathode to anode (- → +)

below: negative/reverse bias

• anode to cathode (+ → -)

Valve Tube Rectifier (+ disadvantages)

• simplest type of rectification

• e^- only flow from cathode to anode during positive pulses of AC current (no current during negative pulses)

Disadvantages

1. low heat capabilities

2. burnt out often

3. no longer used (replaced by solid state conductors)

What is this?

Valve Tube Rectifier

Solid State Rectifiers

• semi-conductor used in diodes is silicon

• impurities added to silicone to create two types of semiconducting material

  • P-Type: positive, electron holes (anode)

  • N-Type: negative, free/excess electrons (cathode)

• currrent will only flow if e^- pass from N → P

• positive bias creates a PD b/t the two materials & negative bias removes the PD

What is this?

Solid State Rectifiers

Rectification Options (2 Waves)

1. half wave: suppressing the half of AC that lies below the line

2. full wave: change the negative half cycle to a positive one

Half Wave Rectification

• 60 pulses/sec

• 1/60 sec periods with no current/beam

• leads to longer exposure times, high amount of low energy photons & higher patient dose

Full Wave Rectification

• 120 pulses/sec

• no periods with no current

• leads to a decrease of exposure time

Methods of Rectification (2)

1. self-rectification - half wave

2. diode rectification (valve tube or solid state of 1-2 diodes) - full wave

Self Rectification (+ Disadvantages)

• simplest type of rectification

• x-ray tube acts as its own rectifier

Disadvantages

1. low heat load capabilities

2. half wave rectification (aka single pulse)

   1. no longer used

Diode Rectification Half Wave

• diodes allow for better heat loading than self-rectification

• 1-2 diodes can provide half wave rectification

  • 2 diodes are more efficient & increase heat loading more than one diode

Diode Rectifcation Full Wave

• 4 diodes can be used together to provide full wave rectification (aka 2-pulse rectification)

• only one correct way to connect rectifier diodes to each other + x-ray tube + transformer

Half Wave vs. Full Wave Rectification

Half Wave

• self-rectification

• 1-2 diodes

• voltage drops to zero (1/60 sec)

• lower efficiency (lower energy, more tube heat load & longer exposure time)

Full Wave

• diode rectification only

• no less than 4 diodes

• more constant PD

• higher efficiency (higher energy beam, less tube heat load & shorter exposure time)

Disadvantage of 4 Diode Rectification (+ corrections)

a large number of low energy x-rays are produced

• correct with filtration

• corrected with 3-phase or high frequency generators

Label Parts of the Diode

vertical line: cathode

bottom of triangle: anode

e^- flow from right to left on this one (think megaphone)

3-Phase Current Production (+ 2 types)

generator with 3 armatures producing currents 120° out of phase (3 overlapping AC currents)

1. 3-phase 6 pulse

2. 3-phase 12 pulse (utilizes specialized high-tension transformer to double 3-phase current to tube)

Voltage Drop (ripple)

the percentage voltage decreases per cycle of current

• any decrease in voltage decreases average photon energy and reduced the number of photons produced → heterogenous beam

Heterogenous Beam

creating a more heterogenous beam improves image quality, shortens exposure time & reduces patient dose

3-Phase Generators

x-ray generators that utilize incoming 60 Hz 3-phase AC to produce a more consistent primary beam

3-Phase Current at the X-Ray Tube

1. single phase half wave = 60 pulses/sec - 100% voltage drop

2. single phase full wave = 120 pulses/sec - 100% voltage drop

3. three phase 6 pulse = 360 pulses/sec - 13% voltage drop

4. three phase 12 pulse = 720 pulses/sec - 3% voltage drop

5. high frequency - continuous DC - < 1% voltage drop

3-Phase X-Ray Transformers

both types of 3-phase generators require

• 3 autotransformers for kV selection

step up (high voltage) transformers require

• 3 primary coils in a delta pattern

3-phase 6 pulse - 3 secondary coils in star (Wye) pattern

3-phase 12 pulse - 6 secondary coils one in each pattern

Compared to Single Phase: 1 secondary coil

3-Phase X-Ray Rectifiers

3-phase 6 pulse requires 6 rectifiers

3-phase 12 pulse requires 12 rectifiers

Compared To Single Phase: 4 rectifiers for full phase

Rectifiers (+ Devices to Test Rectification & Timers)

if a rectifier fails, radiographs will be under-exposed

• most modern equipment will show an error or shut down if there is rectifier failure

3-phase generators & high frequency: oscilloscope

single-phase generators: spinning top

3-Phase Beam Production Pros

more constant PD

1. more homogenous beam

2. shorter exposure times

3. lower patient dose

Capacitor

device that stores a static electrical charge

• measured in capacitance; Farad unit

• discharge DC

What is this?

parallel plate capacitor - two charged plates separated by a dielectric

High-Frequency X-Ray Generators

generator utilizes a series of capacitors & transformers to convert pulsed AC into continuous voltage DC

• nearly constant PD < 1% voltage

• smaller (used for portable & CT)

• more reliable

• shorter & more accurate exposure times

High Frequency Generator Steps

1. AC enters rectifier → full wave pulsed DC

2. low voltage capacitor smooths voltage → low voltage DC

3. DC enters the inverter circuit → squared pulses of AC

4. square pulsed AC enters step up transformer → high voltage AC

5. high volt AC enters volt rectifier → pulsed DC

6. pulsed DC to high volt capacitor → smooth high voltage DC

7. smooth DC delivered to tube → nearly homogenous x-ray beam

High Frequency = …

constant beam