Production and Control of High Voltage Transformers Production and Control of High Voltage Regulation of Current

The Three Types of Transformers Transformer – An electromagnetic device that changes an alternating current from low voltage to high voltage or vice versa Works on the principle of electromagnetic mutual induction The Three Types of Transformers 1) Step-Down 2) Step-Up 3) Autotransformer

The Transformer Law – The emf induced in the secondary coil is to the emf in the primary coil as the number of turns in the secondary coil is to the number of turns in the primary coil. Stated as and equation:

Current in a transformer – If the voltage increases in the secondary, amperage (current) will decrease due to the law of conservation of energy Stated as an equation:

Types of Transformer Construction Air Core Transformer Open Core Transformer Closed Core Transformer Shell Transformer

Transformer Power Loss Transformers: 95% efficient All power loss due to heat loss Three Forms of Power Loss: 1) Copper Loss - Caused by electrical resistance of wire - Decreased with larger diameter wire 2) Eddy Current Loss - Swirling currents in iron core - Produced by electromagnetic induction of windings - Reduced by laminated silicon plates 3) Hysteresis – Power loss caused by changing magnetic domains in AC core - Also reduced by laminated silicon plates

Autotransformer (Variable Transformer) Purpose - A device used to control kVp Varies input voltage to primary of step-up transformer Works on principle of self-induction and therefore requires AC voltage Components: 1) Single winding acting as both primary and secondary 2) Metal taps to adjust number of turns in secondary of autotransformer 3) Iron core (increases back emf)

Autotransformer Law Works identical to the transformer law Example: What is the voltage delivered to the primary of the step-up transformer if there is 120 volts in the primary of the autotransformer and 2 of 10 turns are tapped? X = 2 120 10 = 240 10x = 24 V

Control of Filament & Tube Current X-Ray Tube Circuits: 1) Filament Circuit – Carries current to heat the filament (typically 3-5 A, 10 – 12 V) 2) Tube Circuit – Current passing between the electrodes of the x-ray tube (stated in mA) A small change in filament circuit produces a large change in tube current.

Three Devices Used to Control Filament Current 1) Choke Coil Works on self-induction Requires AC Consists of coil of wire and iron core Can be used to control current and voltage drop 2) Rheostat Variable resistor Increases resistance to decrease current & voltage Operates on AC or DC

Three Devices Used to Control Filament Current 3) High Frequency Control – The use of high frequency generators (transformers) to control output. - Contains microprocessor feedback circuit for better control of kVp and mA From high ripple to low ripple means less variation in voltage and mA - More precise control of voltage and mA going to x-ray tube

Operation of High Frequency Generator and Feedback Circuit 1) Supplied with single or three phase power 2) AC is converted to high frequency DC wave by DC chopper 3) Inverter converts DC to high frequency AC in order to be used by transformer 4) High voltage from secondary of transformer is changed back to DC (rectified) and smoothed. High Freq Inverter DC Chopper

Advantages of High Frequency Generators 1) Less ripple (less than 2%) 2) Better control of kVp and mA 3) Increased tube output 4) Shorter exposure times 5) Improved image consistency 6) Less space required

Practice Problems

Answers to Transformer Problems 1) 22 kVp , 100 mA 2) 5.5 kVp, 400 mA 3) 50 volts, 20 A 4) 100 windings, 1 A 5) 7,000 V (7 kVp), 30 A