Lecture 2

Question 1

What are electrical transformers used to do to voltage?

Answer 1

They are used to transform voltage from one level to another

Question 2

Differentiate between a step up & step down transformer.

Answer 2

A step up transformer increases the output to input ratio. A step down transformer decreases the output to input ratio.

Question 3

When a varying magnetic field or flux cuts a conductor ____ is induced in the conductor

Answer 3

Emf

Question 4

What is magnetic flux?

Answer 4

Magnetic flux is as a result of AC current, as it keeps changing polarity and direction so will the magnitude and direction of the magnetic field keep changing.

Question 5

In a transformer:
1. The primary coil is connected to what?
2. The secondary coil is connected to what?

Answer 5

1. AC input from the socket connected to the substation
2. The circuit of the electrical equipment.

Question 6

When AC current from the socket flows through the primary coil, it will cause _____ _____ to be produced.

Answer 6

magnetic flux

Question 7

To increase the emf induced in the secondary coil, which principles make this possible? (according to Faraday’s 2nd Law)

Answer 7

1. Increase the no. of turns in the secondary coil.
2. Increase magnetic field strength
3. Increase the speed of relative motion between the coil & magnet

Question 8

What are the 3 main constructional parts of a transformer?

Answer 8

1. Primary Winding
2. Magnetic Core
3. Secondary Winding

Question 9

Is a high tension transformer a step up or step down transformer?

Answer 9

Step up transformer

Question 10

What is the function of the high tension transformer:
Hint: think voltage, x-ray

Answer 10

To provide high voltages (up to 150kV in some units) needed for the production of x-rays in the x-ray tube.

Question 11

The magnetic field associated with the current flowing in the primary winding is concentrated in the ______ ______.

Answer 11

metal core

Question 12

The core of the high tension transformer is made of a ____ ____. Such as ?

Answer 12

metal alloy; stalloy (steel & silicon)

Question 13

Definite “hysteresis”

Answer 13

How easily a conductor can magnetize & demagnetize

Question 14

Stalloy has ____ magnetic permeability with ____ hysteresis.

Stalloy also has ______ inherent electrical resistance, coupled with the core being laminated ______ eddy current loss.

If the core of the stalloy was of _____ resistance, then it would ______ current & _____ eddy current loss as well as become _______.

Answer 14

high; low

high; reduces

low; conduct; increase; overheated.

Question 15

What is the purpose of the oil when the entire transformer is placed in a metal tank that is filled with it?

Answer 15

It provides electrical insulation as well as convect heat away from the transformer.

Question 16

In the high tension transformer, the primary winding is made up of _______ turns of _______ ______ wire. This wire is coated with a tough insulation medium such as ______ or ______.

Answer 16

a few hundred; thick copper; shellac or enamel.

Question 17

In the high tension transformer, the secondary winding is made of _______ of turns of ____ ____ wire.

Answer 17

thousands; thin copper

Question 18

Potential difference between adjacent turns is __ ____ ____ however, potential difference between different layers of the windings may be up to _______.

What is the kV difference between inner & outer layer secondary windings?

What is located between the primary & secondary windings? hint: shielding

Answer 18

a few volts; thousands of volts

75kV

stress shield

Question 19

What does the stress shield in the high tension transformer prevent?
hint: tension, electrical insulation

Answer 19

The stress shield prevents the high tension current from discharging to the primary circuit in the event of breakdown of the electrical insulation between the windings.

Question 20

Copper losses in the high tension transformer vary in _______ proportion to what?

What occurs as a result of copper loss?

When does no copper loss or voltage drop occur?

Answer 20

Direct; to the resistance of the windings & the square of the current flowing through them.

As a result of copper loss, there is heat build up & increased resistance.

When the secondary winding is not connected in circuit.

Question 21

When maximum safe current is flowing in the secondary, it is said the circuit is operating under..?
a. “No Load” condition
b. “Full Load” condition

Answer 21

B. Full load condition

Question 22

Why is transformer regulation of importance in radiography?
hint: current, voltage

Answer 22

Because as current flow across the circuit increases, the voltage drops.

Question 23

The rating of a transformer is the statement of the ________ safe output of its ________ winding.

If exceeded, what may occur?
hint: insulation, winding

Answer 23

maximum; secondary

If exceeded, the transformer may overheat causing breakdown in insulation or burning out of the winding

Question 24

Name 3 things the rating of a high tension transformer in a diagnostic X-ray unit should include.

Answer 24

1. The maximum current which may flow continuously.
2. The percentage permissible overload.
3. The maximum voltage that can safely deliver under no load condition