Week 6: Transformers and Recitification Flashcards
Transformers are composed of
two coils placed near one another, and there is no physical contact between the two coils
The coil that is supplied with current is the _________ coil, and the coil in which current is induced is the _________ coil
Primary, secondary
Transformers operate using
mutual induction
Transformers change the intensity of
voltage and current
Transformers operate only with
AC
*DC applied to the primary coil will induce no current in the secondary coil because there is no relative motion between the secondary coil and the lines of flux with DC
T or F: The number of turns of wire in the primary coil is designed to be different from the number of turns in the secondary. This causes the induced voltage and current to be different in the secondary coil
True
In a hypothetical transformer, the lines of force induced by 5 turns, or loops of the primary coil may cut ___ turns, or loops of the secondary coil, resulting in ___ times the voltage in the secondary coil than in the primary coil. This is an example of a ___________ transformer because voltage has been increased.
10, 2, step-up
When the voltage is increased from the primary to the secondary it is called a
step-up transformer
In a hypothetical transformer, the lines of force induced by 10 turns, or loops of the primary coil may cut __ turns, or loops of the secondary coil, resulting in __ of the voltage in the secondary coil than in the primary coil. This is an example of a __________ transformer because voltage has been decreased.
5, 1/2, step-down
When the voltage is decreased from the primary to secondary it is called a
step-down transformer
A step-up transformer __________ voltage from primary to secondary, while ___________ amperage
increases, decreasing
- For example, if the voltage is doubled in the secondary, the current is halved. The primary voltage is measured in volts, and the secondary voltage is measure in kilovolts (kV). The primary current is measured in amps, and the secondary current is measured in milliamps (mA).
A step-down transformer ___________ voltage from primary to secondary, while ___________ amperage
decreases, increasing
- For example, if the voltage is halved in the secondary, the current is doubled.
the terms “step-up” and “step-down” refer to the ___________ change, not the ___________ change
voltage, amperage
Step-up and Step-down transformers turns and loops
Transformer law equation
Transformer law equation problem:
Induced current equation
Induced current equation problem:
Transformer efficiency is usually above ___%, but some power is lost. Lost power is given off as ______
95, heat
T or F: In a perfect world, the power (P=VI) on the primary coil will equal the power on the secondary coil, but some energy is lost
True
Transformer efficiency is affected by
• I²R Loss (Copper loss)- all conductors inherently have resistance to current flow. Ideally, a conductor will have a large diameter, using a high voltage and low amperage.
• Hysteresis loss (lagging loss)- occurs on the ferromagnetic core as AC magnetizes, demagnetizes, and remagnetizes the core continuously. Dipoles can lag in this process and give off heat.
• Eddy current loss- occurs in the ferromagnetic core as a result of currents opposing the cause that produced them. Laminating the core reduces eddy current loss by dividing the core into thin layers, reducing the strength of eddy currents.
Transformer types: Air-core, Open-core, Closed-core, Shell-type
• Air-core: two coils of wire placed in close proximity; 2 solenoids
• Open-core: two coils of wire, both filled with a ferromagnetic iron core, placed in close proximity; 2 electromagnets
• Closed-core: closing the core will direct the lines of flux from primary to secondary cores toward each other, resulting in a net increase in field strength and efficiency (less power loss).
• Shell-type: both the inside and outside lines of force are directed through the core, resulting in greater field strength and efficiency
In modern x-ray generators, the use of ____________ core, shell-type transformers is the norm
laminated
*A laminated core is a core made of several layers instead of a single piece of ferromagnetic iron.
Autotransformer
•Consists of an iron core with only one winding of wire above it. The single winding acts as both the primary and secondary windings. The autotransformer operate using self-induction instead of mutual induction because there is only 1 coil.
• The power (P=VI) supplied to the x-ray imaging system is delivered first to the autotransformer, and the autotransformer is able to vary this voltage to the step-up transformer (high-voltage transformer) and the step-down transformer (filament transformer).
• The voltage the autotransformer receives (primary voltage) and the voltage it provides (secondary voltage) can vary depending upon the kVp the technologist sets at the operating console.
• Some older operating consoles have separate adjustments for major kVp and minor kVp, which allow the technologist the ability to make precise kVp settings. The minor kVp adjustment “fine tunes” the technique.
Filament Transformer
• The filament step-down transformer is needed to decrease the incoming line voltage to 5-15 Volts and 3-5 Amps. This voltage and current is used to heat the x-ray tube filament during thermionic emission
• As technologists adjust the mA selector, they are adjusting the heat at the x-ray tube filament. As the current increases to the filament (mA increases), it becomes hotter, and more electrons are released by thermionic emission
• Connections on the autotransformer provide voltage for the filament circuit
Diode (solid state rectifier)
•Most common rectifier type
•Made from silicon, this semiconductor device is classed into two types: n type and p type
• N-type semiconductors have loosely bound electrons that move freely
• P-type semiconductors have mobile spaces called holes, where there are no electrons
• The p-n junction is where the n-type contacts the p-type.
• The direction of electron flow will determine if the diode will allow electrons to pass through it or not
Single-phase AC
-One cycle: one complete turn of the armature through the magnetic field represents 1 cycle
-Half cycle: from** 0° -180° is the positive half cycle**, and from 180° to 360° is the negative half cycle. The only difference between the positive half cycle and the negative half cycle is the direction of current flow in the circuit
*In 1 second, AC will provide 60 positive half-cycles and 60 negative half-cycles of voltage
Half-wave rectification (HWR)
• In early rectification, the anode of the x-ray tube would absorb the negative half cycle, which resulted in an overheated anode. This form of half-wave rectification is termed self-rectification.
• In the self-rectified circuit, you can see how the black arrows demonstrate electrons traveling through the tube to make an exposure during the positive half cycle, but as the AC changes direction, the current, represented by the purple arrows, is absorbed by the anode of the x-ray tube during the negative half cycle
Half-wave rectification: __________ half cycle is transmitted through the x-ray tube, but the _________ half cycle is absorbed by the anode of the x-ray tube
positive, negative
Half-wave rectification is a condition in which the voltage is not allowed to swing ____________ during the negative half of its cycle
negatively
- The gaps seen in the picture is where the negative half cycle of AC should be, but they are absorbed by the anode in self-rectification, or by using 1-2 rectifiers in the non self-rectified circuit
Full-wave Rectification
-During the positive half-cycle, electrons move through rectifier D, through the x-ray tube, and through rectifier A.
-During the negative half-cycle, electrons move through rectifier B, through the x-ray tube, and through rectifier C.
Does the FWR voltage waveform demonstrate gaps?
No
X-ray circuit: Circuit breaker
A device constructed to interrupt a circuit before a dangerous temperature is reached. If there is a power surge in the circuit, it breaks, or opens the circuit, stopping the current flow before damage can occur “down stream” in the circuit and to other devices.
X-ray Circuit: Control console- Autotransformer
Single coil that operates using self-induction; allows for changes in kVp by varying the incoming line voltage to the primary coil of the step-up transformer (high voltage transformer). Connections on the autotransformer also provide voltage for the step-down transformer (filament circuit)
X-ray Circuit: Control console- Voltage meter (kVp meter)
measures the voltage in the circuit in volts but can easily be converted to kilovolts (kV)
X-ray Circuit: Control console- mA selector
Controls a rheostat (variable resistor) that controls the heat at the filament of the x-ray tube. A higher mA selected at the control console’s mA selector increases the current to the filament, which increases the filaments temperature, which increases the number of electrons produced during thermionic emission, which increases the number of photons that exit the tube during an exposure.
X-ray Circuit: Control console- Timing circuit and selector
controls how long the exposure stays on. Electronic timers can create exposures as quick as 1 millisecond (ms) or .001 seconds.
X-ray Circuit: High-voltage section- High-voltage step up transformer
increases the voltage sent from the autotransformer to kilovolts (kV). kV is needed because x-ray exposures are made in kV, not volts.
X-ray Circuit: High-voltage section- Filament step-down transformer
used to decrease the incoming line voltage to 5-15 Volts and 3-5 Amps needed to heat the x-ray tube filament during thermionic emission.
X-ray Circuit: High-voltage section- Rectifiers
this is an example of a full-wave rectified circuit because there are 4 rectifiers(diodes) connected to the x-ray tube. These rectifiers are one-way streets for current flow. With FWR, both the negative and positive half-cycles of voltage are transmitted through the x-ray tube.
X-ray Circuit: High- voltage section- Ground
Circuits can be grounded as a safety feature. The ground is a circuit connected to the ground where excess electricity can be safely directed in emergencies. The ground is said to possess an infinite number of both positive and negative charges and is said to have zero potential.
X-ray Circuit: High-voltage section: Ammeter (mA meter)
measures the current in milliamps (mA)
X-ray Circuit: X-ray tube
converts electrical energy into electromagnetic energy
HWR, FWR, and Single Phase Alternating Current (AC) wave images
