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Resident Physics Lectures • Christensen, Chapter 3A X-Ray Generators: Transformers & Rectification George David Associate Professor of Radiology Current Flow and Magnetic Fields • Whenever an electric current flows through a conductor, a magnetic field surrounds the conductor • Magnetic field can be concentrated by coiling conductor Magnetic Field N Magnetic Field S Current Flow Transformer Construction • Transformers have 2 coils of wire coils not in electrical contact with each another • When electric current passed through one coil magnetic field develops around first coil second coil near enough to feel magnetic field Magnetic Field Current Flow Transformer Core • Coils usually wrapped around core material such as steel • Improves magnetic coupling between coils Core Transformer Coil Designations Incoming AC Power • primary coil to which power is applied • secondary coil which feels magnetic field of primary coil Primary Coil Secondary Coil(s) Transformer Coil Designations Secondary can have >1 set of secondary coils Primary coil induces voltage in both secondary coils Transformer Coils • When secondary coil feels changing (increasing or decreasing) magnetic field of primary coil power is induced in secondary coil no physical connection Incoming AC Power Primary Coil Secondary Coil(s) Turns Ratio Definition • number of windings of secondary coil divided by number of windings of primary coil 850 / 1200 for transformer below NP = 1200 NS = 850 Transformer Theory • transformers do not work with direct (unchanging) current (DC) • Current induced in secondary coil only when primary coil current / magnetic field is changing Input DC Voltage & Current No Output Voltage Voltage Time Time Transformer Theory • Transformers alter both voltage & current of AC waveforms • Voltage in secondary can be > or < voltage in primary Input AC Voltage & Current Output AC Voltage & Current Transformer Law • Voltage Ratio = Turns Ratio # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ------------------# Prim. Coils Prim. Voltage Transformer Law # Sec. Coils Sec. Voltage Turns Ratio = ---------------- = ----------------# Prim. Coils Prim Voltage NS VS ----- = ----NP VP 240 VAC ?V NP = 1200 NS = 850 850 ? ------ = ----1200 240 If VPRIM = 240 Volts then VSEC = 170 Volts Transformer Types • Step down Transformer # primary coils > # secondary coils primary voltage > secondary voltage Step up Transformer • # primary coils < # secondary coils • primary voltage < secondary voltage Current Change Opposite of Voltage Change • If one goes up, the other goes down If Secondary voltage > primary voltage means » Secondary current < primary current If Secondary voltage < primary voltage means » Secondary current > primary current Input AC Voltage & Current Output AC Voltage & Current Voltage Current Transformer Law (cont.) • Current ratio is inverse of voltage ratio # Sec. Coils Sec. Voltage Prim. Current ----------------- = ------------------ = -----------------# Prim. Coils Prim. Voltage Sec. Current NS VS IP ----- = ----- = ----NP VP IS VPRIM X IPRIM = VSEC X ISEC Power Power is rate of energy usage Power defined as Voltage X Current Units Voltage => Volts Current => Amps Power => Watts •Voltage => Kilovolts •Current => milliamps •Power => Watts Power in Transformers • Transformer primary power = secondary power transformer neither creates nor consumes power PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC Power • Transformers do not change power • Transformers transform voltage / current mix High voltage --- Low current Low voltage --- High current PowerPRIM = PowerSEC VPRIM X IPRIM = VSEC X ISEC Autotransformer • Only one winding • incoming AC voltage connected across coils primary Taps Input • Output voltage proportional to # coils between taps NP NS secondary Primary Secondary Autotransformer • Voltage law for autotransformers same as for transformers • Secondary voltage adjustable by moving Input to a different tap changes # secondary coils NS NS VS ----- = ----NP VP NP Primary NS Autotransformer Input 100 80 50 20 Primary Rectification • Changes alternating current output of high voltage transformer to direct current • allows current flow in one direction only • x-ray tube is a rectifier because current will not flow from anode to cathode no source of free electrons at anode Rectifier • Also called diode • Diode is 2-component device cathode anode • Note symbol for diode below Cathode Anode Types of Rectifiers • Vacuum tubes (thermionic diode tubes) x-ray tube is one example • solid state (semiconductor) materials junction of p & n type semiconductors » contains impurities with excess or shortage of electrons materials » silicon • most common today • packaged in cylindrical stack called a “stick” » selenium Diodes are One Way Streets for Electric Current • Allows current to flow cathode to anode • Blocks current from flowing anode to cathode C A C A Biasing Semiconductor Rectifiers • forward biasing P N electron flow attempting to pass current (electrons) from Ntype to P-type semiconductor across barrier junction allows current to cross barrier • reverse biasing attempting to pass current (electrons) from Ptype to N-type semiconductor across barrier junction does not allow current to cross barrier X P N electron flow