Transformers

Transformer

electromagnetic device that changes an alternating current from low voltage to high voltage or vice versa; no moving parts; very efficient but only works with AC and on principle of mutual induction

3 Types of Transformers

step down, step up, autotransformer

Transformer Law

emf induced in secondary coil is to emf in primary coil as to number of turns in secondary coil is to number of turns in primary coil

Transformer Law Equation

Vs/Vp=Ns/Np

Transformers Coils Ratio

500 to 1 between primary to secondary coils in x-ray

Transformer Current

if voltage increases in secondary, amperage(current) will decrease due to law of conservation of energy

Transformer Current Equation

Is/Ip=Vp/Vs

Transformer Construction

air core, open core, closed core, shell transformer

Transformer Oil Submertion

provide insulation to prevent arching between coils of transformer; provide cooling since considerable heat generated in iron or silicon core due to eddy currents and hysteresis

Transformer Power Loss

95% efficient, all power loss due to heat loss 5%

3 Forms of Power Loss

copper loss, eddy current loss, hysteresis

Copper Loss

caused by electrical resistance of wire; decreased with larger diameter wire

Eddy Current Loss

swirling currents in iron core; produced by electromagnetic induction of windings; reduced by laminated silicon plates

Hysteresis

caused by changing magnetic domains in AC core; reduced by laminated silicon plates

Autotransformer (Variale Transformer)

device used to control kVp; varies input voltage to primary of step up transformer; works on principle of self induction and requires AC voltage

Autotransformer Components

single winding acting as both primary and secondary; metal taps to adjust number of turns in seconday of autotransformer; iron core(increase back emf)

Autotransformer Law Equation

Vs/Vp=Nr/Np

X-Ray Tube Circuits

filament and tube

Filament Circuit

carries current to heat the filament; 3-5A and 10-12V; small change produces large change in tube current

Tube Circuit

current passing between the electrodes x-ray tube; stated in mA

3 Devices to Control Filament Current

choke coil, rheostat, high frequency control

Choke Coil

works on self induction; requires AC; consists of coil of wire and iron core; used to control current and voltage drop

Rheostat

variable resistor; increases resistance to decrease current and voltage; operates AC or DC

High Frequency Control

use of high frequency generators(transformers) to control output; contain microprocessor feedback circuit for better control of kVp and mA

High to Low Ripple

less variation and precise control of V/mA; tube uses more incoming voltage

Operation of High Frequency Generator and Feedback Circuit

supplied with single or three phase power; AC converted high frequency DV wave by DC chopper removing ripple from electrical sine wave; inverter convert DC to high frequency AC; high voltage from secondary of transformer changed back to DC and smoothed

Advantages of High Frequency Generators

less riple <2%, better control kVp/mA, increased tube output, shorter exposure times, improved image consistency, less space required(7×9)

Generator

step up transformer, step down transformer, insulating oil, rectifiers

Transformer Operation

only operate on AC; very efficient; strength of current varies by number of oils(doubling secondary coils doubles voltage but decreases amperage by 1/2

Step Up Transformer

increase voltage from primary source to secondary source, decrease amperage(more turns in secondary)

Step Down

decrease voltage from primary to secondary(less turns in secondary)

Transformer Principle

electromagnetic mutual induction

Leakage Flux

flux that takes path outside of windings rather than linking with secondary winding; caused by all flux by primary linking with all turns of energy winding

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