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Transformers

A transformer changes the intensity of alternating voltage and current.

Mutual Induction

Transformers A device in which two coils are placed near one another without electrical connection. The number of turns in the coils differs, causing a change in currnet in the secondary coil; this serves to either increase or decrease the voltage.

Types of transformers

• Air core

• Open core

• Closed core

• Shell type

Air core transformer

Open core transformer

Closed core transformer

Shell type transformer

Step up vs Step-down

If you increase the number of turns on the right, the voltage coming off the transformer will increase in proportion.

Using the numbers in the example above, you can see that the right side has four times more turns. As a result, the voltage on the right has increased four times (from 100 V to 400 V). The voltage has been stepped up by a factor of four.

Because current is inversely proportional to voltage, you can see that stepping up the voltage pays a price ... the current on the right is only a quarter of what it was on the left. Step-up transformers increase the voltage, but decrease the current. In our example above, the current went from 10 A to 2.5 A, a reduction of by a factor of four Rad Physics Prof. Stelmark

If you decrease the number of turns on the right, the voltage coming off the transformer will decrease in proportion.

Using the numbers in the example above, you can see that the right side has one fifth the number of turns. As a result, the voltage on the right is only one-fifth as large. The voltage has been stepped down by a factor of five (1000 V down to 200 V).

Because current is inversely proportional to voltage, you can see that stepping down the voltage gives a bonus ... the current on the right is five times what it was on the left. Step-down transformers decrease the voltage, but increase the current. In our example above, the current went from 2 A to 10 A, an increase by a factor of five. Rad Physics Prof. Stelmark

If the turns ratio is greater than 1 the transformer is a step-up transformer.

If the turns ratio is less than 1 the transformer is a step-down transformer

Ns

Np

Autotransformer (kVp selector) self-induction

RECTIFICATION

X-RAY CIRCUIT

RECTIFICATION

PROCESS BY WHICH ALTERNATING CURRENT IS CHANGED TO PULSATING

DIRECT CURRENT

ALTERNATING CURRENT1 PHASE

A/C ( UNRECTIFIED)- FREQUENCY 60 Hz Rad Physics Prof. Stelmark

60 POSITIVE PULSES/SEC

60 NEGATIVE PULSES/SECRad Physics Prof. Stelmark

60 POSITIVEPULSES

60 NEGATIVEPULSES

120 PULSES60 FULL CYCLES

BATTERY – CURRENT OUTPUT

D/C HALF WAVE RECTIFIED

D/C HALF-WAVE – 60 POSITIVE PULSES/SECNEGATIVE PULSES ARE SUPPRESSED

1 PULSE/CYCLE Rad Physics Prof. Stelmark

D/C FULL WAVE RECTIFIED

D/C HALF-WAVE – 120 POSITIVE PULSES/SEC2 PULSES/CYCLE Rad Physics Prof. Stelmark

D/C -3 PHASE 6 PULSE CURRENT

D/C 3 PHASE 6 PULSE – 6 PULSES/CYCLE

D/C - 3 PHASE 12 PULSE

D/C 3 PHASE 12 PULSE – 12 PULSES/CYCLE Rad Physics Prof. Stelmark

DIODES OR VALVE TUBES HELP RECTIFY A/C CURRENT

DIODE

ELECTRONS

VALVE TUBE

X-RAY TUBE FORWARD BIAS

REVERSE BIAS

+ -

NOX-RAY

FORWARD vs REVERSE

RECTIFICATION TYPES

►HALF-WAVE

►FULL WAVE

►THREE PHASE SIX PULSE

►THREE PHASE TWELVE PULSE

►HIGH FREQUENCY SINGLE PHASE

RECTIFICATION

NO RECTIFICATION

HALF-WAVE

FULL-WAVE

3 PHASE RECTIFICATION

3 PHASE RECTIFICATION

HIGH FREQUENCY RECTIFICATION

HALF-WAVE RECTIFICATION

HALF-WAVE RECTIFICATION

FULL-WAVE RECTIFICATION

FULL-WAVE RECTIFICATION

FULL-WAVE RECTIFICATION

3 PHASE 6 PULSE

3 PHASE 12 PULSE

HIGH FREQUENCY

VOLTAGE RIPPLE

►HALF WAVE------------------- 100%

►FULL WAVE ------------------ 100%

►3 PHASE 6 PULSE ----------- 13%

►3 PHASE 12 PULSE ---------- 4%

►HIGH FREQUENCY ---------- <1%

RIPPLE

100%

100%

100%

RIPPLE-3 PHASE

13%

RIPPLE-HIGH FREQUENCY

<1%

CHECKING FAILURE OF RECTIFICATION SYSTEM

►SPINNING TOP TEST – 1 PHASE EQUIPMENT

►SYNCHRONOUS SPINNING TOP TEST-

3 PHASE WITH LONGER EXPOSURE TIMES

►OSCILLOSCOPE- 3 PHASE EQUIPMENT WITH SHORT EXPOSURE TIMES

SPINNING TOP TEST

RESULT OF SPINNING TOP TEST- 1 PHASE - DASHES

HALF- WAVE RECTIFICATION

# OF DASHES = TIME (SEC) x 60 PULSES

FULL- WAVE RECTIFICATION

# OF DASHES = TIME (SEC) x 120 PULSES

RESULT OF SYNCHRONOUS SPINNING TOP TEST-3PHASE

ARC

3 PHASE RECTIFICATION-LONGER EXPOSURE TIME

DEG. OF ARC = 360°/ sec x TIME (sec)

UTILIZING OSCILLOSCOPE

UTILIZING OSCILLOSCOPERECTIFIED SYSTEM

1. Main breaker - this is where the alternating current comes from to power the circuit.

2. Exposure switch - when you push the button to start an exposure this switch closes to start the exposure.

3. Autotransformer - this is where you adjust the kVp for the exposure.

4. Timer circuit - this part of the circuit stops the exposure.

5. High-voltage step-up transformer - this transformer bumps the voltage up so that the x-ray tube has very high voltage to make the electrons have enough energy to form x-rays.

6. Four-diode rectification circuit - this makes the current only go in one direction through the x-ray tube.

7. Filament circuit variable resistor - this variable resistor adjusts the current going to the filament.

8. Filament step-down transformer - this transformer steps the voltage down and therefore the current up.

9. X-ray tube - this is where the x-rays are created.

10. Rotor stator - this rotates the anode.

Control FactorElectrical Device and Location

in Circuit

kVp Selection kVp LevelAutotransformer (between incoming line and exposure

switch)

mA Selection Filament CurrentVariable resistor (in filament

circuit between incoming line and step-down transformer)

Time Selection Length of exposureTimer circuit (between exposure switch and step-up transformer)

Rotor Switch Speed of rotating anodeStator (separate circuit from

stator of anode motor)

Exposure Switch Moment of exposureSwitch (between autotransformer

and timer circuit)

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