Machines; Direct Current Machines Objective 1 Flashcards by Christine Rogers

Name 3 basic parts of a DC generator

Set of permanent magnets/ electromagnets
Armature winding
Commutator and brush set

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Which component is rotated within the stationary field?

Armature winding

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What are 5 properties of flux?

1) Oriented FROM north TO south in the external circuit
2) form complete loops
3) repel eachother & never cross
4) become as short as possible
5) follow path of least reluctance

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What is reluctance?

Opposition to flux in a magnetic circuit

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What is the basic unit of measurement of magnetic flux?

Weber

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The Weber represents a (small/large) number of flux lines

Large

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What is the quantity symbol for flux density?

Beta (B)

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What is the unit of flux density?

Tesla (T)

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What is electromagnetism?

Relationship between magnetism and electricity

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When there is a(n) ___________________ in a conductor, a(n) _______________ sets up around that conductor

Electric current; magnetic field

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What is the density of the field surrounding the conductor proportional to?

Amount of current flowing in the conductor.

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What is the orientation of the lines of flux dependent upon?

Direction of current

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What type of current flow is used in the left hand rule for conductors?

Electron flow from negative to positive

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What do the fingers and thumb represent in the left-hand rule for conductors?

Thumb: current direction
Fingers: direction of magnetic lines of force

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What symbol represents current out of the page, toward the reader?

Dot in center of cross-sectional view of conductor

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What is the usual definition of flux density?

Flux per unit area

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What is the polarity of a current carrying coil (electromagnet) determined?

The direction of current around the coil
And
The direction the coil is wound

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What is the left hand coil rule used for?

Determine magnetic polarity of a coil

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What do the fingers and thumb represent in the left hand coil rule?

Fingers- around the coil in the direction the current (using electron flow) is flowing

Thumb- indicates north end of the coil

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What does coil flux density depend on?

1) Value of current in coil conductors
2) Number of turns of conductor
3) material composition of coils core

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Will an iron core result in higher or lower flux density compared to air core?

Greater flux density.

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True or false: number of turns in a coil is directly proportional to strength of magnetic field.

True

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What term defines the coil’s ability to produce flux?

Magnomotive force (mmf)

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How do we calculate magnomotive force?

Product of:
Current in the coil and
Number of turns in the coil

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How do we measure magnomotive force?

Ampere-turns

What does a B-H curve illustrate?

Relationship between: Applied magnetic field intensity (H) and Resulting flux density (B) in the core

As magnetic field increases, what is the respective change in the flowing properties?: 1) applied magnomotive force 2) amount of aligned magnetic domains within the core 3) flux density of the core 4) amount of mmf needed to increase flux density

1) increased mmf 2) greater percentage of aligned magnetic domains 3) flux density increases 4) greater mmf required to produce small increases in flux density

What kind of coils can become saturated?

Coils that have a core made of magnetic material

What kind of relationship is seen between feild density and flux density for coils with cores of air or other non-magnetic materials?

Linear

True or false: all coil types become saturated

False; non-magnetic core coils do not

What components of a DC generator comprise the magnetic circuit?

The pole pieces (pole core and shoe) The yoke (frame) Armature core Air gap

Why is the pole shoe made larger than the main body of the pole core?

Reduce reluctance of the air gap

Where is the field winding located?

Around the pole core

What are two different types of feild windings?

1) few turns of large-guage wire | 2) many turns of small-guage wire

What type of field winding requires high value of current to produce sufficient flux?

Few turns of large guage wire

What does the polarity of the magnetic field within a generator depend on? How could you change this polarity?

Depends on direction of current through the field winding | Can change field polarity by reversing polarity of DC VOLTAGE applied to the feild winding.

What aspect of a DC machine keeps the armature coils in the correct polarity to interact with the main field?

Commutator

In a DC generator, the commutator connects the windings to the __________________ via the ________

Electrical load; brushes

In a DC motor, the commutator connects the windings to ________________ via the ________

The electrical supply; brushes

What machine component makes the connection between the load or supply and the windings?

Brushes

True or false: the field polarity of a DC machine never changes

True

What effect does having more segments on a commutator have on output voltage?

Makes a smoother output voltage.

What is commutation?

Process in which a DC voltage output is taken from an armature that has an ac voltage induced into it.

The commutator _________________ (mechanically/electrically) reverses the armature loop connections to the external circuit.

Mechanically

What happens at the same instant a commutator mechanically reverses the armature loop connections to the external circuit?

Voltage polarity in the armature loop reverses

What might cause severe arcing and overheating at the brushes?

If induced voltage into the coil at the instant of segment contact is not at or close to zero V, significant amount of current flows from the commutator segment to brush set, and to next commutator segment.

Where does the neutral plane exist with regards to the main field of flux of the machine?

Perpendicular to (at right angles)

At what point in its cycle does the winding have no voltage difference between coil ends? What does this mean for potential difference between commutator segments?

Neutral plane; ideally midway between the pole pieces. Similarly no voltage difference between commutator segments at the brush.

What effect does brush position have on the armature coil passing through the neutral plane?

Shorts out the armature coil

What sources of flux exist for a generator under load?

1) main field flux: from field coils | 2) armature flux: from current in armature winding

When does an armature winding of a DC generator produce magnetic flux?

When there is current in the armature winding (ie. Whenever a load is connected to the output of the operating generator)

What is armature reaction?

Distortion of flux pattern caused by the cross magnitizing effect of the armature-current flux

Which direction does the neutral plane shift when influenced by armature reaction?

In the same direction that the armature rotates

What does armature flux do to main flux?

Weakens and distorts it

What is the armature flux orientation with regards to main field flux orientation?

Armature flux is at right angles to main field flux

What two magnitizing forces act together to produce theRESULTANT FLUX through the armature?

Main field flux and | Armature flux

True or false: once main flux has been distorted by armature flux, the neutral plane is still at 90° (right angle) to the resultant flux.

True. No longer 90° to original main flux, but at 90° to distorted flux lines Ie. At 90° to "resultant vector" of resultant flux (pg. 12 for visual)

what properties are proportional to the amount the neutral plane shift from no-load position?

Amount of armature flux Which is proportional to Armature load current

In which areas of a generator does armature reaction lead to problems?

The interpolar zone - area between the poles of the machine | Polar zone - area directly under pole faces of the machine.

What methods are used to reduce the effects of armature reaction in the interpolar zone?

1) rotate position of brushes to new neutral plane 2) pole face design of main poles 3) add interpoles with commutating windings to the generator

Why is rotating the brushes to the new neutral plane, as a method to reduce the effects of armature reation, not practical for varying loads?

The brushes would have to be repositioned each time the load changes

True or false: using pole-face-design methods to address flux in interpolar zone is a way to eliminate the problem

False. Reduces but does not eliminate.

What is the relationship between interpole flux and armature flux?

Opposite direction

How effective are interpoles in eliminating effects of armature reaction?

Very effective; neutral plane becomes virtually fixed in the no-load position regardless of actual connected load

What must the interpole action be proportional to when used to reduce the effects of armature flux? Why?

Interpole action must be proportional to armature current. | Because armature reaction is directly proportional to the amount of armature current.

How is the commutating winding connected with respect to the armature winding? Why?

In series. So the commutating winding carries the full armature current.

What are the physical characteristics of a commutating winding conductor? Why?

Heavy guage with few turns. | To carry full armature current AND keep volt drop on commutating winding to a minimum.

What is the result of flux distortion in the polar zone of the DC generator?

Distorts waveform of generated voltage thereby reducing generated voltage.

What is the process by which emf is reduced due to flux distortion in the polar zone?

1) Flux is forced to one edge of the pole faces 2) this crowding leads to saturation in that area 3) saturation reduces flux 4) generated emf is reduced as it is proportional to amount of flux

True of false: the interpoles affect distortion in the polar zone

False. Interpoles have no effect on distortion of the field in this area.

What solution is available to reduce distortion in the polar zone?

Compensating windings

Explain the placement and current of compensating windings. Why does this effect polar zone distortion?

- Compensating windings placed in slots in MAIN POLE FACES of the pole shoe. - Run parallel to armature conductors (connected in series) - Carry current in OPPOSITE direction to armature windings adjacent to them. Therefore compensating mmf is in opposite direction to armature mmf If these two forces are EQUAL IN MAGNITUDE they cancel eachother out.

How is the compensating winding connected with regards to the armature winding and commutating winding? Why?

Series. So compensating winding carries same current as armature winding

What effect does connecting the compensating winding and armature winding in series have on the flux of each component?

Series connection allows for compensating winding flux to vary in proportion to armature flux (as current varies in each component proportionally).

True or false: compensating windings are common among motors smaller than 100hp

False. Compensating windings are not common. And they normally only seen on motors larger than 100hp

What do commutating windings look like?

Heavy guage conductors resembling a rotor bus bar