Transformers

Transformer efficiency

How good it is at turning input power into output power. Formula: Power out ÷ Power in × 100. “Good transformer = less heat.”

Transformer law

Voltage ratio = Turns ratio. “More turns = more volts.”

Main losses

Copper loss (wires get hot), Core loss (eddy + hysteresis). “Copper cooks, Core swirls.”

Copper loss

Heat from wire resistance (I²R). Use thicker wire to fix.

Eddy currents

Tiny circles of current in the core that make heat. “Eddy = extra current that eats energy.”

Hysteresis loss

Heat from magnetic domains flipping each AC cycle. “Domains dance = heat.”

How to cool transformer

Use oil (cool + insulate) and laminated steel (blocks eddies).

Why laminate the core?

Stops eddy currents from forming.

Why oil?

Cools and insulates the transformer.

Common design in x-ray systems

Shell-type transformer (coils around same core, in oil).

Efficiency example

(13,750 ÷ 14,250) × 100 = 96.5% → very good!

Three sections of generator

Low-voltage → High-voltage → Tube section. “Control → Boost → Beam.”

Divider between primary and secondary

Step-up transformer (the big voltage booster).

Step-up transformer

Increases voltage for x-ray production (kVp).

Step-down transformer

Lowers voltage to heat filament (mA).

Autotransformer

kVp selector — sets how much voltage goes to the step-up.

Prereading kilovoltmeter

Meter that shows kVp before exposure (on primary side).

Main switch & fuse

Switch = power on; Fuse = protection (melts if current too high).

Timer

Controls how long the exposure lasts. (Electronic or AEC.)

Rheostat

Variable resistor that controls filament current (mA).

Filament transformer

Step-down → makes 6–12 V, 3–5 A for filament heating.

Focal spot selector

Chooses small or large filament → controls detail vs heat.

mA meter vs mAs meter

mA = current during exposure. mAs = total charge (for short exposures).

Rectification circuit

Turns AC into DC (so electrons only go one way).

X-ray tube

Turns electricity into x-rays (electrons hit anode target).

What’s active when main switch is ON?

kV meter + filament current show up. mA meter = zero until you take exposure.

What happens during exposure?

High voltage turns on → electrons cross tube → x-rays made.

How is mA picked?

Pushbutton → selects resistor → sets filament current.

What is a 3-phase generator?

Uses 3 AC waves (120° apart) → smoother, constant power.

Why 3-phase better than 1-phase?

No “zero” voltage dips → faster exposures, better output. “Never hits zero.”

Voltage ripple

Difference between peak & valley voltage. Low ripple = smoother beam.

Ripple comparison

1-phase = 70% ripple (bumpy). 3-phase = 3–5% ripple (smooth).

“Nearly constant potential” means…

Voltage stays close to peak all the time.

Types of 3-phase circuits

6-pulse & 12-pulse (6 = good, 12 = smoother).

Pulses per second (at 60 Hz)

6-pulse → 6×60 = 360 pulses/s. 12-pulse → 12×60 = 720 pulses/s.

Why does low ripple matter?

More consistent beam → better quality & shorter time.

Technique change rule

Use 40% less mAs or drop kVp by ~15% compared to single-phase.

Delta vs Wye windings

Delta = triangle (primary). Wye = star (secondary).