Week 7: Generators and Charts Flashcards
Single-phase power: Produced by a generator, single-phase 60 Hz AC changes directions ____ times per second
120
Single-phase power: With 60 Hz AC, there are two voltage half-cycles (2 pulses) in _____ of a second
1/60th
With 60 Hz AC, there are ___ positive half-cycles (60 pulses) and ___negative half-cycles (60 pulses) in 1 second.
60, 60
Single phase image
Direct Current (DC)
• Electrons flow in only one direction
• The voltage does NOT fluctuate and
maintains a constant potential until the circuit switch is opened and voltage goes back to 0
• DC is the most efficient power type, but as electrons travel long distances, they lose energy and the voltage drop increases the further current flows in one direction
Three-phase power
• With complex electrical engineering, three-phase power can be created where three single-phase voltage waveforms can be superimposed over each other
• Creates a voltage waveform that
maintains a nearly constant potential (minimal voltage drop from the peak amplitude)
• There are six voltage half-cycles (6 pulses) per 1/60th of a second. There are 360 pulses in 1 second
Three-phase Power: There are __ voltage half-cycles ( __ pulses) per 1/60th of a second. There are ____ pulses in 1 second
6, 6, 360
Single-phase, half-wave rectification (single-phase, one-pulse) (1Φ 1P)
• Single phase 60 Hz AC is rectified so that only the positive half-cycles (positive pulses) of voltage are transmitted through the x-ray tube during an exposure
• This can be created through self-rectification, or by adding 1-2 rectifiers to the high-voltage circuit with the x-ray tube
• HWR wastes half of the supply of power and requires twice the exposure time
• The pulsed x-ray output of a HWR machine occurs 60 times per second (60 x-ray pulses per second)
The pulsed x-ray output of a HWR machine occurs ___ times per second (___ x-ray pulses per second)
60, 60
Single-phase, full-wave rectification (single-phase, two-pulse) (1Φ 2P)
• Single phase 60 Hz AC is rectified so that both the positive and negative half cycles (pulses) of AC can be transmitted through the x-ray tube, which allows for half of the exposure time when compared to HWR
• This can be created by adding at least 4 rectifiers to the high-voltage circuit with the x-ray tube
• The double efficiency of FWR permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used with FWR.
• The pulsed x-ray output of a FWR machine occurs 120 times per second (120 x-ray pulses per second)
The pulsed x-ray output of a FWR machine occurs ____ times per second (____ x-ray pulses per second)
120, 120
Three-phase, six-pulse (3f 6P) (3Φ 6P)
• When full-wave rectification is applied to three-phase power, it produces 6 pulses in 1/60th of a second, or 360 pulses per second through the x-ray tube
• Notice how voltage supplied to the x-ray tube does not drop to 0 like it does with the single-phase generators that are HWR or FWR
• The efficiency of 3Φ 6P permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to single-phase generators
Three-phase, six-pulse (3f 6P) (3Φ 6P): When full-wave rectification is applied to three-phase power, it produces __ pulses in 1/60th of a second, or ____ pulses per second through the x-ray tube
6, 360
Three-phase, twelve-pulse (3f 12P) (3Φ 12P)
• With further improvements in engineering, 3Φ 12P generators can produce 12 pulses in 1/60th of a second, or 720 pulses per second through the x-ray tube
• Notice how voltage supplied to the x-ray tube does not drop to 0 like it does with the single-phase generators that are HWR and FWR, or as much from peak amplitude as the 3Φ 6P generator
• The efficiency of 3Φ 12P permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to single-phase generators and 3Φ 6P generators
Three-phase, twelve-pulse (3f 12P) (3Φ 12P): With further improvements in engineering, 3Φ 12P generators can produce ___ pulses in 1/60th of a second, or ____ pulses per second through the x-ray tube
12, 720
High-frequency (HF)
• HF generators produce a nearly constant potential voltage waveform that most closely resembles DC
• The efficiency of HF permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to all the other radiology generators
• HF offers the best image quality with the least amount of patient dose because voltage barely fluctuates during an exposure, resulting in fewer lower energy x-rays that leave the tube
• HF is common with fixed radiography, mammography, and CT
Capacitor discharge generator
• Commonly found on mobile units, a capacitor can be charged by batteries prior to an x-ray exposure. During the exposure, the charge is released (discharged) from the capacitor to form the x-ray tube current needed to produce x-rays
• After an exposure, the capacitor can continue to discharge after the usable exposure creating leakage radiation
• Grid biased x-ray tubes, automatic lead beam stoppers, or both can be used to stop the leakage radiation
• From the start of the exposure to the end of the exposure is termed wavetail cutoff
Battery operated mobile units
• Mobile units can operate on battery supplied AC current
• This AC can be converted to 3Φ 12P or greater frequencies
• These generators do not exhibit leakage radiation and offer the benefits of three-phase or greater x-ray exposures
• These have become popular in mobile radiography
Falling load generator
• Common in interventional radiology when short exposure times are necessary
• During an exposure, the initial tube loading is higher and drops during the exposure
• Notice that 500 mAs was achieved starting with close to 2000 mA that drops during the exposure to help create the 500 mAs needed. Because of this, a 300 ms exposure time was used instead of a 500 ms exposure time
Voltage ripple
• Voltage ripple refers to the percentage drop of voltage from peak amplitude
• The most efficient method of producing x-rays involves the lowest ripple
• Less voltage ripple results in greater radiation quantity and quality
• Quantity is higher because for any projectile electron emitted by the x-ray tube filament, a greater number of x-rays are produced when electron energy is high than when it is low
• Quality is higher because fewer low-energy projectile electrons pass from cathode to anode to produce low energy x-rays. The average x-ray energy of the primary beam is higher as voltage ripple decreases
• As voltage ripple decreases, the exposure to the patient will be less as more photons will have higher energy with less likelihood of photoelectric absorption
• As voltage ripple decreases, lower kVp/mAs is needed for proper IR exposure because of the higher average energy of the primary beam
Phase, Pulse, & Frequency
• Phase refers to the number of distinct wave cycles. Examples- single-phase & three-phase
• Pulse refers to the quantity of voltage pulses per cycle
• Frequency refers to the number of pulses per cycle
Generators contribution to the exposure
• As generator phase increases, this increases the pulses, which increases the frequency of the voltage supplied to the x-ray tube
• These 3 increases will decrease voltage ripple, which will increase the average primary beam energy measured in keV that leaves the tube
• These 3 increases will also increase x-ray intensity, meaning the quantity of photons leaving the tube will increase
Power Ratings
• Generator power ratings are determined by the greatest load the generator is capable of sending to the x-ray tube. Power is expressed in kilowatts because x-ray generators operate in kilovoltage and milliamperage
-P=V x A for 3Φ generators
-P=V x A x .7 for 1Φ generators
-Because 1Φ generators have a lower average photon emission energy, the .7 constant is used
3Φ generators and 1Φ generators Formulas
-P=V x A for 3Φ generators
-P=V x A x .7 for 1Φ generators
-Because 1Φ generators have a lower average photon emission energy, the .7 constant is used
