Chapter 3 : Study Guide Summary

Question 1

Why is alternating current desirable from a practical standpoint?

Answer 1

It is supplied directly to homes and factories through power lines

In the United States, commercially supplied alternating current reverses its polarity 60 times per second (60 Hz).

Question 2

List the methods to provide direct current.

Answer 2

• Rectifiers
• Batteries
• Direct current generators

Each method has its own applications in generating direct current.

Question 3

What is a limitation of Alternating Current in metal penetration?

Answer 3

Does not penetrate deeply into metal

This limitation emphasizes fatigue cracks, which start at the surface.

Question 4

What is the result of full-wave direct current rectification?

Answer 4

Inverts negative current to positive current, leaving some ripple

The current is then filtered to remove most or all of the ripple.

Question 5

Answer 5

Question 6

What is the typical current range provided by three-phase full-wave rectified alternating current?

Answer 6

3000 A to 10 000 A

This high current range is suitable for industrial applications.

Question 7

What are the types of current used to produce magnetic fields within ferromagnetic materials?

Answer 7

• Alternating current
• Direct current
• Half-wave rectified current
• Full-wave rectified current
• Three-phase full-wave rectified current

These currents are essential for magnetizing ferromagnetic materials.

Question 8

Answer 8

Question 9

What is a feature of Half-Wave Direct Current equipment regarding magnetization?

Answer 9

Leaves a part in a partially magnetized state

Most equipment provides demagnetization options, either alternating current, reversing direct current, or both.

Question 10

What type of power do most full-wave rectified alternating current systems use?

Answer 10

Three-phase power

Three-phase power provides very low ripple direct current.

Question 11

For maximum visibility of an indication, it should be located where?

Answer 11

In the area between the two pole pieces of the magnet

The long axis of the indication should be positioned 90° ± 45° with a line between the poles.

Question 12

What is the main advantage of permanent magnets in testing?

Answer 12

Portability with no need for external power

This makes them suitable for testing small areas of parts where fatigue cracks frequently occur.

Question 13

What is the phase angle between currents in a three-phase current?

Answer 13

60°

This configuration provides three currents superimposed that follow each other at intervals.

Question 14

What is the primary advantage of full-wave rectified alternating current?

Answer 14

Most efficient conversion of alternating current power to direct current

Full-wave rectification provides a smoother output compared to half-wave rectification.

Question 15

What is the skin effect in relation to alternating current?

Answer 15

The inductive effect of current reversal produces a high density of electric current near the surface of a ferromagnetic conductor

This characteristic makes alternating current excellent for detecting fatigue cracks.

Question 16

What does a half-wave rectifier do to negative voltage peaks?

Answer 16

Clips off all negative voltage peaks, leaving only positive peaks

This results in a pulsed direct current that retains the advantages of direct current.

Question 17

What should the magnetic particles used with permanent magnets be suspended in?

Answer 17

An approved liquid carrier

This is necessary because a permanent magnet provides a static magnetic field.

Question 18

What is direct current?

Answer 18

Current that flows continuously in one direction at a constant voltage

Direct current establishes strong magnetic fields in conductors and coils.

Question 19

How does alternating current flow during one cycle?

Answer 19

From 0 to positive maximum voltage, back to 0, to negative voltage peak, and back to 0

This process completes one cycle and is commonly illustrated as a sine wave.

Question 20

What is the primary advantage of alternating current?

Answer 20

Its availability

Alternating current is widely accessible for use in various applications.

Question 21

What is the primary disadvantage of alternating current?

Answer 21

Limited depth of penetration

This limits its effectiveness in certain applications compared to direct current.

Question 22

What must the ASNT NDT Level III consider when recommending inspection equipment?

Answer 22

• Advantages of different power supplies
• Power available at inspection sites
• Need for portability
• Projection of power required for future production

The type of power required influences all these considerations.

Question 23

What are the advantages of using Half-Wave Direct Current?

Answer 23

• Good mobility for dry magnetic particle powder
• Good sensitivity for revealing fine surface cracks
• Considered best for detecting subsurface discontinuities

It is a primary power source for weld and casting inspection.

Question 24

What are the advantages of Alternating Current in inspection equipment?

Answer 24

• No rectifiers required
• Reduced weight of equipment
• Used for demagnetizers
• Lighter and less expensive than other power supplies

Autotransformers and saturable rectifiers transform alternating current into various current requirements.

Question 25

What are the types of **rectified current**?

Answer 25

* Half-wave rectified * Full-wave rectified * Three-phase full-wave rectified ## Footnote These types refer to the methods of converting alternating current to direct current.

Question 26

What is the primary type of power used for **magnetization** in magnetic particle testing?

Answer 26

Direct Current ## Footnote Initially, wet-cell batteries were used to fulfill high current requirements.

Question 27

What type of rectifiers are used in **full-wave rectifiers**?

Answer 27

Dry plate rectifiers set up in a bridge circuit ## Footnote These rectifiers convert the negative half cycle to a positive half cycle.

Question 28

What is the primary limitation of using **permanent magnets** in magnetic particle testing?

Answer 28

The strength of the field that can be induced is relatively low ## Footnote Permanent magnets are limited in their applications due to the low strength of the induced magnetic field.

Question 29

Answer 29

Question 30

What should be maintained for effective testing with permanent magnets?

Answer 30

Maximum contact between the pole and the test surface ## Footnote This ensures better magnetic field induction and visibility of indications.

Question 31

What is the primary **power source** supplied by utility companies?

Answer 31

Alternating Current ## Footnote Much of the direct current is derived from alternating current.

Question 32

How is **direct current** produced from **alternating current**?

Answer 32

Through the use of **rectifiers** ## Footnote Rectifiers convert alternating current to direct current.

Question 33

What feature do some **permanent magnet yokes** have to vary the induced flux level?

Answer 33

Adjustable legs ## Footnote This allows the induced flux level to be varied by changing the distance between the contacts.

Question 34

Answer 34

Question 35

What is a major advantage of **Direct Current** for magnetization?

Answer 35

* Deep penetrating ability in metal * Reveals subsurface discontinuities * Leaves residual magnetism ## Footnote This allows more time for the inspector to detect and evaluate indications.

Question 36

What is the effect of **direct current** on a solid ferromagnetic conductor?

Answer 36

Produces a magnetic field that ranges from *0 flux at the center* to *maximum at the surface* ## Footnote This effect is due to the continuous flow of current in one direction.

Question 37

In magnetic particle testing, the induced field is **longitudinal** because the magnet is positioned how?

Answer 37

So the field between the magnet’s poles is perpendicular to the direction of the discontinuities ## Footnote This positioning is crucial for effective testing.

Question 38

How does a **bridge rectifier** function in full-wave rectification?

Answer 38

Converts the negative half cycle to a positive half cycle ## Footnote This allows for continuous current flow in one direction.

Question 39

Who is responsible for selecting the type of equipment used to induce **magnetism** in ASNT NDT Level III?

Answer 39

ASNT NDT Level III personnel ## Footnote This responsibility is crucial as proper inspection depends on the type, direction, and strength of the electric current used.

Question 40

What type of current is obtained after **full-wave rectification** of three-phase current?

Answer 40

A very smooth direct current ## Footnote This current can be filtered to eliminate any ripple.

Question 41

What is **Half-Wave Direct Current**?

Answer 41

Results from rectification of single-phase alternating current by clipping off negative voltage peaks ## Footnote It has the penetrating characteristics of direct current with added mobility for dry magnetic particle powder.

Question 42

Where is the magnetic **flux** stronger when using permanent magnets?

Answer 42

Near the poles ## Footnote The flux is weaker in the center of the gap.

Question 43

Why do **dry particles** require pulsed, changing, or alternating fields?

Answer 43

To become mobile ## Footnote This is necessary for effective testing as static fields do not provide mobility.