-
Define Generator:
Converts mechanical energy into electrical energy
-
Define motor:
convert electrical energy into mechanical enery
-
Slip rings
complete the alternating circuit in simple electric generator (attached to carbon "brushes")
-
How long does it take for one full cycle to complete in an AC generator
1/60 of a second
-
Sine curve
graph representing the sine of the angle between the plane of the armature and a plane perpendicular to the magnetic field
-
Which pole influences the movement of the armature?
North
-
A generator must have....
- a powerful electromagnet
- an armature
-
a motor must have...
2 interacting magnetic fields (one created by a current carrying wire, and an existing magnetic field)
-
Values used to define resistance in an AC circuit...
- Effective resistance
- root mean square (rms) value
-
Properties of resistance in a DC circuit?
- Material
- Length
- Cross sectional area
- Temperature
-
Properties of impedence in an AC circuit..
- Resistance (Material, Length, area, temp.)
- Inductance reactance (Lenz's Law (back emf))
- Capacitive reactance
-
DC generators use _____ to complete the circuit?
split-ring commutator
-
Advantages of AC?
- 1. Can be transformed
- 2. Less power loss (through heat loss)
-
Formula for power
P (watts)= V (volts) x I (amps)
-
Formula for power loss?
PL = I2R
-
Left hand thumb rule:
- Thumb- direction of the electron flow
- fingers- wrap in the direction of the magnetic field
-
Right hand thumb rule:
- Thumb- direction of the current flow
- fingers- wrap in the direction the magnetic field
-
Left hand generator rule
- thumb- direction of the armature movement
- index finger- direction of the magnetic field (S)
- middle finger- direction of the electron flow
-
Right hand generator rule
- thumb- direction of the armature movement
- index finger- direction of the magnetic field (S)
- middle finger- direction of the current flow
-
Left hand motor rule
- thumb- direction of the armature movement
- index finger- direction of the magnetic field (S)
- middle finger- direction of the current flow
-
Right hand motor rule
- thumb- direction of the armature movement
- index finger- direction of the magnetic field (S)
- middle finger- direction of the electron flow
-
Three types of Electic Motors
- DC motor
- synchronous AC
- induction AC
-
-
Synchronous AC motor
turns at the same speed as the armature of the AC generator supplying the current (or a multiple of the armatures speed) (used in clocks or timing devices)
-
Induction AC motor
- uses the stationary magnets of a stator, energized in sequence by a multiphase current, to interact through the motor principle with the copper bars (wrapped around an iron core) of a rotor
- -uses no direct contact
- -used for a rotating anode
-
Typical speed of an anode?
- 3500 rpm
- -anode always spins at the same speed
-
Transformer
- -Changes the voltage of AC with no electrical contact or moving parts (electomagnetic induction principle)
- -no appreciable loss of electrical enery (less than 5%)
-
Types of transformers
- Air core
- open core
- closed core
- shell
-
Air core transformer
two insulated coils laying side by side
-
Transformer Law
- VS/VP =NS/NP
- Voltage (emf) in the secondary coil is to the emf in the primary coil, as the # of turns in the secondary is to the # of turns in the primary
-
Open core transformer
- -iron core inserted in coils of wire
- -experiences leakage flux (loss of magnetic flux at the ends of the iron cores)
-
Closed core transformer
- -heavily insulated coils called 'windings' wrapped around a square or circular iron core
- -experiences transformer loss in the form of eddy currents
- -lamination hinders the formation of eddy currents
- -this is the type of transformer used for x-ray generating equipment
-
Shell type transformer
- -used as commercial or power transformer
- -laminated silicon steel with 2 rectangular holes
- -windings around central portion
-
Types of Electrical power loss
- -copper loss (I2R)- resistance
- -eddy loss-heat generated by changing electical current (reduced by lamination w/silicon steel plates)
- -hysteresis- heat generated by constant reorientation of magnetic dipoles (coercivity -requires energy to reorient poles [causes hysteresis] (reduced by silicone steel lamination))
-
Autotransformer
- -Varies input voltage to a transformer to control output voltage by selection of # of turns tapped on the secondary side
- -sets your kV
- -between source of AC and the primary side of the transformer
- -single coil serves as primary and secondary
-
Autotransformer Law
seconday voltage/primary voltage= #tapped turns /#primary turns
-
2 circuits of the X-ray tube?
- filament circuit- carries current to heat the filament
- tube circuit-carries the current that passes from cathode to anode (electrodes) to produce x-rays
-
Types of rectification (3)
- Self-rectified
- vacuum tube
- solid state
-
Self-rectification
- -uses no diodes
- -no protection for the filament
- -self-half wave
- -single pulse
- -100% rippple with a pause
- -forward bias (useful half of the voltage cycle)
- -low heat loading capacity
- -limited to low exposure factors
- -100% suppression
- -single phase
-
One diode rectification
- -half-wave
- -one-pulse
- -100% ripple w/ a pause
- -100% suppression
- -diode protects tube filament
- -single phase
-
2 diode rectification
- -single phase
- -half-wave
- -one- pulse
- -100% ripple w/pause
- -100% suppression
- -increased load capacity
- -increased effeciency
- -increased filament protection
-
4 diode rectification
- -full-wave
- -2 pulse
- -single phase
- -100% ripple
- -100% inversion
- -high % of low energy x-rays produced
-
Electron flow in diodes....
from n to p
-
Conventional electrical current flow in diodes.....
from p to n
-
What does a rectifier do?
Converts AC to useable DC
-
3 energy bands in materials
- -conduction
- -forbidden
- -valence
-
commom semi-conductor and # of valence electrons
silicon; 4 e-
-
arsenic
- -5 valence e-
- -
added to Si - -creates donor or n-type atom
- -makes Si a conductor
-
gallium
- -3 valence e-
- -
added to Si - -creates acceptor or p-type atom
- -makes Si an insulator
-
barrier voltage or potential hill
the potential difference that exists between a n-type ion and a p-type ion in a diode
-
thyristor or silicon-controlled rectifier
- -more complex
- -2 p-type and 2 n-type layer (makes 3 n-p junctions)
- -can hold or release large amounts of current
-
Advantages of silicone rectifiers
- -compact size
- -no filament (means no filament transformer)
- -low forward voltage drop
- -low reverse current
- -long life
-
Main switch
-primary side
-
Fuses
- -thin strip of metal prevents overload
- -primary side
-
Autotransformer
- -changes voltage to set kV
- -primary side
-
Pre-reading kilovoltmeter
- -Primary side
- -metering device that indicates the desired kVp
-
Timer /Exposure switch
- -primary side
- -controls the current to the primary coil serving to complete the x-ray exposure
-
Remote control switch
- -primary side
- -completes the circuit
- -protects the person making the exposure from electical shock
-
Timer
- -primary side
- -built into the exposure switch
- -5 types (mechanical, synchronous, electronic, mAs, and AEC)
-
mechanical timer
- -Primary
- -operates by clockwork
- -exposure time must be greater than 1/4 of a second
-
Synchronous timer
- -Primary
- -powered by synchronous motor
- -minimum exposure time is 1/60 sec.
- -timer counts by waveform
-
Electronic timer
- -primary
- -we use
- -most sophisticated
- -operate on variable resistance
- -can be use for rapid serial exposure
- -exposure time as small as 1 ms
-
mAs timer/meter
- -secondary (b/c it monitors actual tube current)
- -factors time and mA to shut off at correct mAs
- -not really a timer
-
AEC timer
- -primary
- -measure quantity of radiation reaching the IR to provide the required optical density
-
Back-up timer and Minimum exposure timer
- -primary
- -terminates the exposure in the event of AEC failure
- -minimum exposure response time is 1 ms
-
Circuit breaker
- -Primary
- -additional protection from overloading
-
Filament Circuit
- -primary
- -supplies the heating current for the filament
- -rheostat (variable resistor) reduces current to 3-5 amp and 6-12 Volts
-
Filament Ammeter
- -primary
- -connected in series with the filament circuit to measure current (for proper heat production w/corresponding mA)
- -space charge compensator automatically corrects to maitain constant mA over wide range of kV
-
Primary coil of Transformer
- -primary
- -primary winding of the transformer
-
-
Secondary Coil of transformer
- -secondary
- -smaller wires that primary b/c of smaller amps
-
Miliammeter
- -secondary
- -connected in series with the high voltage/low amp secondary circuit of the transformer
-
Rectifier
- -secondary
- -changes AC to DC
- -increases heating capacity permitting larger exposures
-
Cables
- -secondary
- -hazard reduced by insulation, grounding, and grounding of the secondary winding of the step-up transformer at its mid-point
-
-
Anode
- -secondary
- -considered part of the x-ray tube
- -contains stator induction motor
-
3 Phase Generator
- -6 pulse-13% ripple
- -12 pulse- 5% ripple
- -Coils are wound in delta or star pattern
- -primary is always delta
- -secondary can be delta or star
- -rated up to 1000 mA (kV max. 80)
- -exposure time down to 1 ms
- -nearly constant potential (small ripple)
-
Hi-Frequency Generator
- -1% ripple
- -uses high-frequency circuit
- -60 Hz converted to 500-25,000 Hz
- -inverter circuits convert DC to square pulses
- -Uses an inverter module
|
|