Transformers in X-ray Machines

Principle of operation

• Based on mutual induction
• An electromotive force (EMF) is induced in a coil
• When current in a neighboring coil changes

Constructional detail: Shell type

• Windings are wrapped around the center leg of a laminated core

Core type

• Windings are wrapped around two sides of a laminated square core

Transformers in x-ray machine

• High tension transformer:
  • Step-up transformer
  • Primary winding number lower than secondary winding
  • Supplies the anode circuit
  • Responsible for producing high voltages needed for X-ray production
  • Typical turns ratio 1:500 or 1:1000
• Filament transformer:
  • Step-down transformer
  • Secondary winding number lower than primary winding
  • Supplies the filament circuit
  • Low voltage and high current
  • Allows thermionic emission to occur at the filament
• Autotransformer:
  • Single-coil transformer
  • Provides means for kVp selection
  • Supplies power to other parts of the X-ray circuit
  • Tube current is approximately 3 to 5 amps, depending on the filament temperature

Autotransformer – Example Calculation

• Transformer input voltage (vin)
• Output voltage (vout) can be switched between 0 and 120 VRMS
• Turns ratio is 1500
• Formula: vout/vin = NS/NP = turns ratio
• Rearranging the formula: vout = vin × turns ratio
• Substitute: vout-min = 0 V × 1500, vout-max = 120V × 1500
• Answer: vout has a range of 0 V to 180,000 V (RMS)

Autotransformer Continued

• Consists of a single coil of wire wrapped around an iron core

Operating Console

• Console controls voltage (kVp) and current (mAs)
• Radiation quantity:
  • Total number of X-ray photons in the diagnostic beam
  • Depends on kVp and mAs
  • Usually expressed in mR or mR/ mAs, where R = roentgen (for US systems)
  • Also expressed as ionizing charge per unit mass of air (SI unit), with 1 R = 2.58  10-4 C/kg
• Radiation quality:
  • Refers to the penetrating ability of the X-ray photons
  • Determined by kVp
  • Measured by HVL (Half Value Layer)

Mains Power

• Mains power in the UAE is 220 VRMS
• An X-ray set requires a large DC voltage
• Generated from mains by rectification, amplification, and smoothing
• Power system can be single-phase or 3-phase

Transformers in X-ray Machine

• High tension transformer and filament transformer are inside the high tension tank
• Autotransformer located in the control panel

Transformer

• An AC device used to change high voltage, low current AC into low voltage, high current AC and vice versa without changing the frequency
• It transfers electrical power from one circuit to another without a change of frequency
• It accomplishes this by electromagnetic induction
• There is no voltage loss in the remaining parts of the circuit
• Formula: vout/vin = NS/NP = turns ratio
• To power a 125 kVp x-ray tube, the transformer windings ratio is 403

Rectification

• Converts AC voltage to a unidirectional voltage (all positive)
• Done using a full-wave rectifier (diode bridge)
• In the UAE, the period of the mains power is 1/50 seconds

Process of Rectification

• Conversion of AC to DC
• Half-wave rectification
• Full-wave rectification
• Dedicated circuits
  • Contain diodes:
    • Semiconductor
    • Thermionic
  • Power rating: Supply current to circuits

Half-wave Rectification

• Only the positive half-cycle of the AC supply is allowed to pass

Half-wave Rectifier Circuit

• Simple! Single diode
• ‘Load’: DC circuit; not truly getting DC