THREE PHASE POWER
• Sum of three identical AC voltage waveforms 120° out of phase
• Reduces variation (ripple) in supply of DC power
1-phase, 100% ripple
3-phase, 13% ripple
• Results in much steadier x-ray beam intensity
• Created by 3-phase generator (alternator)
1-phase alternator
3-phase alternator
POWER REQUIRED BY A CIRCUIT
DC circuits
P=IxV
I = current, V = power, P = power
Example:
Q. A flashlight runs on 4.5 volts. If a current of 500 milliamps passes through
the flashlight bulb, how much power does the flashlight use?
A.
V = 4.5 V,
I = 500 x 10-3 A
P = (500 x 10-3 A) x (4.5 V)
= 2.25 Watts
[1 Amp x 1 Volt = 1 Watt]
AC circuits
Ppeak = Ipeak x Vpeak
Paverage = IRMS x VRMS x cos(φ)
where φ is the phase difference between the voltage and current waveforms.
If there are no inductors or capacitors in the circuit (i.e. only resistors), then the current
and the voltage are always in phase (i.e. φ = 0).
φ=0
Paverage = IRMS x VRMS
Ppeak = Ipeak x Vpeak
(φ = 0)
(φ = 0)
Example:
Q. A hair dryer operates on a 120 VAC RMS (Root Mean Square) household
current. The RMS current that passes through the hair dryer is 2 amps. If the
the current and the voltage are 30 degrees out of phase, how much power does
the hair dryer use on average?
A.
VRMS = 120 V,
IRMS = 2 A, φ = 30 degrees
Paverage = (2 A) x (120 V) x cos (30)
= 240
3
2
Watts
= 208 Watts
Q. What would be the peak power required by this hair dryer, if the current and
voltage were in phase?
A.
φ=0
Ipeak = IRMS 2
Vpeak = VRMS 2
Ppeak = Ipeak x Vpeak
= IRMS 2 x VRMS 2
= IRMS x VRMS x 2
= (2 A) x (120 V) x 2
= 480 Watts
THE X-RAY CIRCUIT
In Brief
• Main circuit – supplies high voltage to the x-ray tube anode (so that electrons are
accelerated toward it)
• Filament circuit – heats the x-ray tube cathode filament (so that electrons are emitted)
The heat-induced emission of electrons from a substance is called
thermionic emission.
In Detail
Main circuit components
#1
Main breaker – where AC enters
#2 Exposure switch – used to start the exposure
#3 Autotransfomer -- used to select the peak (kilo) voltage of the anode
#4 Timer circuit – stops the exposure after a specified time
#5 High-voltage step-up transformer – ensures an anode voltage high enough
to accelerate electrons so that they create
x-rays on impact with the anode.
#6 Full-wave rectifier – converts AC voltage to a DC voltage across the x-ray
tube.
#9 X-ray tube -- evacuated tube in which electrons are accelerated toward an
anode, where they create x-rays on impact.
#10 Rotor stator – rotates the anode in order to keep it form overheating. [Anode
is a disk. Electrons are focused on a spot at the edge of the
disk. By rotating the disk, the electrons impact energy is
spread out over the entire edge of the disk, which prevents
overheating.]
Filament circuit components
#7 Variable resistor – adjusts the current to the filament in order to adjust the
rate of (thermionic) electron emission from the cathode
filament
#8 Step-down transformer -- lowers voltage to the filament to ensure that it does
not burn out due to excessively high current