Transformers (XFMR)
How do you calculate the efficiency of a transformer
% efficiency=power out/power in * 100% (ideally want 100%)
What are 4 devices that are used for controlling filament current
-A rheostat, a choke coil, a saturable reactor, and high frequency control
What is an autotransformer
-A special type of XFMR where the turns ratio can be varied to allow variable output voltages.
What kind of transformer has a turns ratio of 1000:1
-A step up transformer
What does a transformer do and what principle does it operate on
-A transformer transfers electrical energy from 1 circuit to another without the use of moving parts or any electrical contact between the 2 circuits. It makes use of the principle of electromagnetic induction
What is a rheostat
-A variable resistor
What are 4 main types of transformers used
-Air core, open core, closed core, and shell type
What are eddy current losses and how are they minimized
-Alternating flux in the iron core induces eddy currents in the core by electromagnetic induction. These currents produce heat => energy loss, it is minimized by laminated silicon steel plates. (Thin slices of steel)
What is the definition of a transformer
-An electromagnetic device that changes AC from either low voltage to high voltage or Vice Versa, without loss of an appreciable amount of electrical energy (they lose less than 5% of their energy [very efficient])
What enhances primary to secondary coupling in a transformer
-An iron core
What is the turns ratio that the constant high voltage transformer supplies to the x ray tube
-Approximately 500:1 (1 volt in on the primary side=500 volts out on the secondary side)
Why does the primary wire need to be larger than the secondary in a step up transformer
-Because the primary has more current
How can we control the amount of heat that the filament in an x ray tube produces
-By controlling the amount of current that goes through it
What transformer looks like a square donut
-Closed core
What are the three types of power losses
-Copper losses (heat losses), eddy current losses, and hysteresis losses
What does the law of conservation of energy state
-Energy can be neither created nor destroyed, only changed in form.
What is the main cause of power loss in transformers
-Heat
How do you know if a transformer is a step up or step down transformer
-If the turns ratio is greater than 1, voltage will increase making it a step up transformer. If the turns ratio is less than 1, the voltage will decrease making it a step down transformer (the voltage change across the transformer is proportional to the turns ratio)
What does the magnitude of the voltage induced depend on
-It depends on electromagnetic induction (Faraday's law) (speed, strength, angle, or number of turns-the induced EMF in each coil is additive)
What is a contactor on an autotransformer and what does it do
-It is a rotating knob that rotates to vary the number of turns on the secondary wire thereby varying its output
What is the power output equal to in a transformer
-It is equal to the power input (neglecting small losses)
What happens to electrical energy in a transformer
-It is just being transferred from one circuit to another. (Primary to secondary)
Is the output current in a step-up transformer higher or lower than the input current
-It is lower than the input current
What is an autotransformer made out of
-It is made up of a single coil wound around an iron core. Metal buttons (contacts) are connected at intervals to bare wire. (it works on the principle of self induction)
What is the main cause of copper losses (heat loss)
-It is mostly due to the resistance of the coils and it can be reduced by using adequate diameter wire
What is the autotransformer on the x ray equipment
-It is the kVp selector
What is the voltage that x ray equipment normally operates at and what is the power supplied to them at
-It operates anywhere from 50,000 volts (50 kVp) to 120,000 volts (120 kVp) but it is only supplied at 240 volts so a transformer is needed
What will happen when there is a small change in filament current
-It will produce a large change in tube current
What can be done to a transformer core to make it more efficient
-Laminate the transformer core
What law does a rheostat work on
-Ohms law V=IR (voltage drop)
What type of current do transformers operate on
-Only AC (never DC)
How many outputs are possible with only one constant input supplied to the x ray tube and how do we change this
-Only one but we can vary the output by means of an autotransformer
What is a shell type transformer
-Primary and secondary coils are wrapped around the central section of an iron core (maximum efficiency), the coils are highly insulated from each other, and the entire XFMR submerged in a tank filled with oil for added insulation and cooling.
What type of transformer design is used in x ray imaging systems
-Shell type
Do autotransformers usually act as step up or step down transformers
-Step down transformers and they only work on a relatively small voltage change (i.e. from 240 V to 100 V).
What is inversely proportional to the turns ratio when a transformer is designed
-The change in current
What happens when the voltage in the secondary goes up
-The current goes down
What happens when the voltage in the secondary goes down
-The current goes up
How does the magnitude of the current change as the voltage changes in a transformer
-The magnitude of the current change across a XFMR is in the opposite direction from the voltage change but in the same proportion or by the same factor (if one goes down by a factor of 4 the other goes up by a factor of 4)
What does the power of the primary side in a transformer equal to
-The power of the secondary side (Pp=Ps)
What are hysteresis losses
-The reversing of the magnetic domains due to AC. •Produces heat => energy loss •Minimized by laminated silicon steel plates
What voltage is greater when a step up transformer is used
-The secondary voltage is greater than the primary voltage
Will the turns ratio be greater or less than 1 if a transformer produces a large secondary current from a small primary current
-The turns ratio will be less than 1
What does the transformer law state
-The voltage induced into the secondary coil is to the voltage in the primary coil as the number of turns in the secondary coil is to the number of turns in the primary coil
What is the high voltage transformer in x ray equipment also referred to as
-The x ray generator
Where are the primary coils in relation to the transformer
-They are always on the left side
What is the function of the secondary coils and where are they in relation to the transformer
-They are always on the right side and they are the part that provides power to the load
What is the principal application of a transformer in an x-ray imaging system
-To change the voltage
Why do transformers have iron cores
-To intensify the magnetic field
What is the main difference between an electrical generator and a transformer
-Transformers have no moving parts
What is an air core transformer
-Two coils side by side
What is an open core transformer
-Two coils side-by-side, each wrapped around an iron core.
What is a closed core transformer
-Two coils wrapped around a square or circular iron ring or core which provides a continuous path for magnetic flux (there is a small amount of flux leakage)
What is another name for tube current
-mAs
How many circuits does the x-ray tube have
2, the filament circuit which is usually given in amps and tube circuit (mA)
What is the average efficiency of most transformers
95-98%
What is the autotransformer law
Autotransformer Vs /Autotransformer Vp=# of tapped turns/# of primary turns
If the power of the primary side in a transformer equals the power of the secondary side, and current is inversely proportional to it, then what equation can we use to show this
IpVp=IsVs (it can also be manipulated to Vs/Vp=Ip/Is) Where Ip=primary current Vp=primary voltage Is=secondary current Vs=secondary voltage
What is the turns ratio of a transformer
Ns/Np (number of secondary windings/number of primary windings)
What is the transformer law formula
Vs/Vp=Ns/Np Where Vs=Secondary voltage Vp=Primary voltage Ns=Number of turns in the secondary coil Np=Number of turns in the primary coil
If Vs/Vp=Ns/Np and Ns/Np=Ip/Is, then what is true about all of those ratios
Vs/Vp=Ns/Np=Ip/Is
