Download Current in a transformer

Production and Control of High Voltage
Regulation of Current
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