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DC Machines > Construction Features Of DC Machines > Flashcards

Construction Features Of DC Machines Flashcards

1
Q

Define the term “dynamo”

A

Term originally given to a rotating machine that converted mechanical energy into electrical energy, primarily in form of DC current

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2
Q

What is the main difference between DC motors and generators?

A

Surrounding enclosure. Generators = open frame, motors = closed frame

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3
Q

Define “armature”

A

Rotating part, or rotor, of the machine

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4
Q

What does the armature core consist of?

A

Stack of soft iron laminations which are slotted to house a set of insulated coil windings

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5
Q

Why are laminations slotted?

A

To house a set of insulated coil windings

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6
Q

Why is the steel shaft of the armature supported by bearings?

A

Allows for uniform rotation of the complete armature assembly within the air gap

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7
Q

What are armature coils generally wound with?

A

Varnish-insulated magnet wire

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8
Q

How are armature coils protected?

A

Slot-cell insulation

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9
Q

What holds the coils of the armature tightly in place?

A

A wedge driven the full length of the slot

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10
Q

What are the ends of each coil connected to?

A

Commutator bars

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11
Q

What are the two types of armature windings?

A

Lap windings, wave windings

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12
Q

What applications are lap windings used?

A

High current, low voltage

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13
Q

What applications are wave windings used?

A

Low current, high voltage

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14
Q

What is the action of the commutator?

A

Convert induced AC within the armature coils to a uni-directional current in the external circuit

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15
Q

The combination of _________ and _________ acts as a _________ _________ ________ between the armature coils and the external load

A

Brushes, commutator, rotary selector switch

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16
Q

The commutator is an…….

A

Assembly of individually-insulated copper bars or segments

17
Q

The copper bars in a commutator are…..

A

Wedge shaped segments with a V-shaped grove at each end for clamping together

18
Q

What is the purpose of the brushes in a DC generator?

A

Conduct the current to or from the armature coils via the commutator from or to the circuit

19
Q

What materials are brushes usually made of?

A

One or more of the following:

  • carbon
  • electro-graphite
  • graphite
  • copper graphite
20
Q

What is the choice of brush material in a DC generator based on?

A

Application and cost

21
Q

What is the brush holder designed to do?

A

Support the brush and provide suitable pressure against the commutator surface

22
Q

What is the pressure applied by the brush holder?

A

1-2 psi

23
Q

What is the purpose of field poles?

A

Provide the magnetic field in which the armature will rotate

24
Q

How is the magnetic field provided in DC machines?

A

Permanent magnets or by direct current flowing through the field coils

25
Q

What does the electromagnetic field poles consist of?

A

Soft iron, laminated core, field coils wound with varnish-insulated magnet wire

26
Q

The frame or yoke…

A

Provides mechanical support for the machine and forms part of the magnetic circuit between the two field poles

27
Q

End shields are….

A

Mounted at the end of the frame and contain recesses for the armature shaft bearings

28
Q

What are the two common classes of bearings used in motors and generators?

A

Sleeve bearings and ball bearings

29
Q

What do sleeve bearings consist of?

A

A shaft and oil ring

30
Q

What do ball bearings consist of?

A

A shaft and an end plate

31
Q

What are sleeve bearings lubricated with?

A

Oil, and applied with an oil ring or oil soaked ring

32
Q

New sleeve bearings are….

A

Usually machined undersized and reamed for proper fit

33
Q

What are ball bearings lubricated with?

A

Grease, and may be open or sealed

34
Q

Sealed bearings are….

A

Factory lubricated for life of bearing

35
Q

Open bearings require…

A

Grease nipples in the end shields

36
Q

What happens if you over grease the bearings of rotating machines?

A

Produce heat and reduce bearing life

DC Machines (10 decks)

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