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Flashcards in Construction Features Of DC Machines Deck (36):
1

Define the term "dynamo"

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

2

What is the main difference between DC motors and generators?

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

3

Define "armature"

Rotating part, or rotor, of the machine

4

What does the armature core consist of?

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

5

Why are laminations slotted?

To house a set of insulated coil windings

6

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

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

7

What are armature coils generally wound with?

Varnish-insulated magnet wire

8

How are armature coils protected?

Slot-cell insulation

9

What holds the coils of the armature tightly in place?

A wedge driven the full length of the slot

10

What are the ends of each coil connected to?

Commutator bars

11

What are the two types of armature windings?

Lap windings, wave windings

12

What applications are lap windings used?

High current, low voltage

13

What applications are wave windings used?

Low current, high voltage

14

What is the action of the commutator?

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

15

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

Brushes, commutator, rotary selector switch

16

The commutator is an.......

Assembly of individually-insulated copper bars or segments

17

The copper bars in a commutator are.....

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

18

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

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

19

What materials are brushes usually made of?

One or more of the following:
- carbon
- electro-graphite
- graphite
- copper graphite

20

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

Application and cost

21

What is the brush holder designed to do?

Support the brush and provide suitable pressure against the commutator surface

22

What is the pressure applied by the brush holder?

1-2 psi

23

What is the purpose of field poles?

Provide the magnetic field in which the armature will rotate

24

How is the magnetic field provided in DC machines?

Permanent magnets or by direct current flowing through the field coils

25

What does the electromagnetic field poles consist of?

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

26

The frame or yoke...

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

27

End shields are....

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

28

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

Sleeve bearings and ball bearings

29

What do sleeve bearings consist of?

A shaft and oil ring

30

What do ball bearings consist of?

A shaft and an end plate

31

What are sleeve bearings lubricated with?

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

32

New sleeve bearings are....

Usually machined undersized and reamed for proper fit

33

What are ball bearings lubricated with?

Grease, and may be open or sealed

34

Sealed bearings are....

Factory lubricated for life of bearing

35

Open bearings require...

Grease nipples in the end shields

36

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

Produce heat and reduce bearing life