Rad 160 - Radiographic Physics

Unit 7

Transformers - Production and Control of High Voltage

*Electric power is supplied to homes at 120 to 240 volts. A much higher voltage is needed to produce x-rays.

I. Transformer – also called x-ray generator

A. Definition - an electromagnetic device that changes AC from low to high voltage or	from high to low voltage without much energy loss (less than 5%).

1. Can increase or decrease the voltage from one circuit and transfer it to another	circuit.

2. Uses no moving parts, only the principle of electromagnetic mutual induction.

B. Components of a Transformer: made up of wire coils

1. Primary Coil

a. Called the input side of the transformer

b. Supplied with AC

2. Secondary Coil

a. Called the output side of the transformer

b. Develops AC by mutual induction

3. Example: A Transformer (see handout)

C. Operation of a Transformer:

1. AC is supplied to the primary coil (current is constantly changing).

2. A constantly changing magnetic field is set-up around the primary coil.

3. An AC is induced in the secondary coil by electromagnetic mutual induction	(electric current will be induced to flow in a circuit if some part of the	circuit is placed in a changing magnetic field).

*A transformer will work only with an AC source.

4. The strength of the induced voltage in the secondary coil is directly	proportional to the number of turns it has in the coil.	Remember, the more coils, the stronger the induced EMF.

* See handout.

D. Turns Ratio: The number of turns of the secondary coil to the number of turns of the	primary coil.

Example: 4:2 (4 to 2) means 4 turns on the secondary coil and 2 turns on the	primary. Can also be written as 4/2.

II. Transformer Law

A. Definition - The induced EMF 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.

Vs = Voltage in secondary

Vp = Voltage in primary

Ns = Number of turns in secondary

Np = Number of turns in primary

Vs = Ns

Vp Np

B. If the number of turns in the secondary coil is double to those of the primary coil, the	induced voltage will be doubled.

III. Types of Transformers

A. Step-Up Transformer – puts out a higher voltage than is supplied to it.

1. There are more turns on the secondary coil than the primary coil.

2. Has a turn’s ratio greater than 1.

B. Step-Down Transformer – Puts out a lower voltage than is supplied to it.

1. There are less turns in the secondary coil than the primary.

2. Has a turn’s ratio less than 1.

IV. Law of the Conservation of Energy: Remember energy cannot be created or destroyed but	can be converted to another type of energy.

A. So what is going on with these transformers?

1. Starting with 120 to 240 volts and ending up with 30,000 (30kV) to 150,000	(150kV) volts. How can that be?

Power = Voltage x Amperage (P = V x I)

So if voltage changes (as in step-up or step-down transformer), amperage	must also change in order to keep total power constant.

Stated in the following formula:

Is= current in amps in secondary

Ip= current in amps in primary

Vp= voltage in primary

Vs= voltage in secondary

2. Again, a step-up transformer takes 120 or 240 volts and increases it to 30,000	to 150,000 volts which is sufficient high voltage to drive electrons through	the tube. At the same time, amperage is reduced drastically.

B. How to determine the amperage of a transformer

Given the following info:

1. Primary turns

2. Secondary turns

3. Primary amps (current)

Use this formula to determine amperage:

C. Summary

1. Step-up transformer:

a. Voltage on the secondary side is greater than on the primary side.

b. Current on the secondary side is less than on the primary side.

2. Step-down transformer:

a. Voltage on the secondary side is less than on the primary side

b. Current on the secondary side is greater than on the primary side

3. A change of voltage is indirectly proportional to a change in amperage	(increase one, decrease the other).

V. Four Different Constructions of Transformers

A. Air Core – simplest (C & A, p 79, Fig 4-22)

1. 2 coils of wire lying side by side.

2. AC is supplied to the primary coil.

3. The secondary coil develops AC by mutual induction.

B. Open Core

1. 2 coils, each with an iron core, lying side by side.

2. AC is supplied to the primary coil.

3. The secondary coil develops AC by mutual induction.

4. The iron cores intensify the magnetic flux of the coils thus making

the induced current stronger.

* Disadvantage of the Air and Open Core Transformers: some of the magnetic flux	energy is lost out of the open ends of the coils.

C. Closed Core – most commonly used in x-ray equipment

1. Coils (also called windings) are wound around a square or circular iron core.

2. The iron core is closed (or continuous) providing a continuous path for the	magnetic flux.

3. Only a small percentage of magnetic energy is lost because of leakage.

D. Shell Type – the most advanced, used commercially.

1. Primary and secondary windings are wrapped around the same core.

VI. Transformer Efficiency – ideally, the amount of power going into the primary should equal	the amount coming out of the secondary. However, some energy is lost as HEAT.

A. A transformer is usually about 95% efficient.

B. Ways Electricity is lost in a Transformer

1. Copper losses – the resistance in the copper conductor results in some energy	loss.

Solution: use wire of adequate diameter (thicker wire = less resistance)

2. Eddy Current losses – the magnetic flux set up around the coil induces “eddy	current” (swirling currents) in the iron core which causes heat.

Solution: use layered steel plates for the core to reduce these currents.

(C & A, p 78)

3. Hysteresis Losses – the constant rearranging of the magnetic domains in the	core (because of the AC) produces heat.

Solution: use of laminated steel core which has a higher resistance to	magnetic energy.

C. Regulation of High Voltage

The voltage coming into the hospital is high voltage (120 or 240 volts) and it must	be varied so x-rays can be obtained of appropriate penetrability, depending on the	exam being done.

VII. Autotransformer

A. Definition - a device connected into an x-ray circuit that is located between the	source of AC (the incoming electricity) and the primary side of the high voltage	transformer.

1. Varies the voltage going to the transformer.

2. Takes the 120 or 240 volts coming in and adjusts them to what voltage is	needed depending on what exam is being done.

B. Example: Peds hand and need about 50 Kv (50,000 v). The high voltage transformer	ratio is 500 to 1 (500:1). Then the autotransformer must adjust the voltage going	to the transformer to be 100 volts (from the original 120 or 240).

1. Why?

2. Because the primary coil of the high voltage transformer takes the 100 volts	from the autotransformer and steps it up 500 times.

So: 500 x 100=50,000 volts or 50 Kv

C. Construction of an Autotransformer

1. A coil of wire wrapped around a large iron core (this single coil acts as both the	primary and secondary coil and operates on the principle of self	induction).

2. A movable contactor selects how many turns of the coil on the secondary side	are to be included in the incoming voltage adjustment. These are called	“taps.”

3. Only 2 turns have been chosen by the contactor on the secondary side and	therefore the turns ratio is 2:7 meaning the voltage is being stepped down	before going to the high voltage step-up transformer.

4. Therefore, the autotransformer serves indirectly as a Kv selector.

5. On handout, there are 7 primary turns and a possible 9 turns on the secondary	side. If turn 8 or 9 is chosen on the secondary side, voltage will be	stepped up.

VIII. Voltage can be adjusted by autotransformers and transformers.

A. How can amperage and current be varied?

1. Amperage – the electron flow

a. Amperage is responsible for heating the filament of the x-ray tube

b. Need a way to vary amperage so the right amount of electrons burns off	the filament. (Filament – part of the cathode that emits electrons.)

B. Two of the Main Circuits of X-Ray Generating Systems

1. Filament Circuit – carries the amperage needed to heat the filament

a. Need an appropriate amount of amperage going to the filament in order	to burn off the electrons.

b. A step down transformer is placed in the filament circuit to increase the	amperage going to the filament. (At the same time, voltage is	decreased because of the Law of Conservation of Energy.)

2. Tube Circuit - carries the current (electrons from the filament) from the	cathode to the anode of the tube and produces x-rays.

a. A small change in filament current produces a large change in tube	current. WHY?

b. An increase in the filament current results in an increase in the number	of electrons “burned off” the filament which results in an increased	tube current because more electrons are speeding across the tube	and more x-rays are being emitted.

c. Therefore, regulating filament current will regulate tube current.

Objectives

Explain the principle and function of a transformer.

Describe the transformer law by use of equation.

Understand and be able to diagram a simple step-up and step-down transformer.

Understand principle of an autotransformer.

Explain why it is necessary to regulate the filament current of an x-ray tube.

Readings

C & A, Chapter 4, pgs 76-80.

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