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Year 3 Electrical > I3 Install DC Motors and Generators > Flashcards

I3 Install DC Motors and Generators Flashcards

1
Q

What is the term for a rotating machine that converts mechanical energy into electrical energy?

A

Dynamo

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2
Q

Define: armature

A

rotating part or rotor of the machine

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3
Q

List the two types of armature windings

A

Lap used for high current, low voltage applications

Wave used for low current, high voltage

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4
Q

What does the commutator do?

A

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

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5
Q

What is the function of the brushs

A

they conduct the current to or from the armature coils via the commutator from or to the external circuit.

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6
Q

What are brushes commonly made of?

A

carbon
graphite
copper graphite

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7
Q

How much pressure is on the brushes?

A

1 to 2 psi

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8
Q

Define: field poles

A

they provide the magnetic field in which the armature will rotate.

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9
Q

What type of bearing are used in DC Motors

A

sleeve bearings

ball bearings

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10
Q

Define: magnetos

A

small dc generators that use permanent magnets to provide the magnetic field.

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11
Q

what two ways can field poles be excited

A

DC source external to the machine

from the armature of the generator itself

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12
Q

Define: Series field windings

A

connected in series with the armature and carry full armature current

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13
Q

Define: Shunt field windings

A

connected in parallel with armature therfore are made up of large number of turns of small-guage wire.

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14
Q

Self-exited generators must have what to produce voltage?

A

residual magnetism of the field core

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15
Q

What is Faraday’s law of electromagnetic induction?

A

e = Blv

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16
Q

Explain the generated voltage formula

A 
Eg = k(flux)N
Eg = generated voltage in volts
k = constant
Flux 
N = rotational speed in rpm
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17
Q

How can an absence of residual magnetism be rectified in a self exciting DC motor

A

by flashing the field

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18
Q

If a DC motor is rotated backwards it generates a field current that ________

A

strips away the residual magnetism and voltage will fail to build-up

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19
Q

Critical field resistance

A

for voltage build-up to occur, the field resistance curve must lie below the saturation curve therefore set field rheostats to their minimum value.

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20
Q

Generated voltage is proportional to

A

the product of field flux and armature speed and some constant

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21
Q

Define armature reaction

A

when an armature is delivering current to a load, a magnetic field surrounds the armature conductors

22
Q

how does armature reaction affect the operation of a generator

A

reduced terminal voltage of the load

impairs commutation by shifting the mechanical neutral plane to the electric plane (sparking brushes)

23
Q

what two ways can armature reaction be fixed

A

using interpoles

compensating windings in the field poles

24
Q

Interpole coils consist of a

A

small number of turns of large guage wire and in series with the armature

25
Q

List three types of losses in a DC generator

A

Mechanical
Copper losses
Core losses

26
Q

List three types of DC generators

A

Series
Shunt
Compound

27
Q

What do the letters A,F,S stand for

A

armature
shunt field coils
series field coils

28
Q

Numerical subscripts are used to indicate current direction in the windings. Flow from even to odd

A

F2 to F1 and from S2 to S1

29
Q

Define cumulative compound

A

when the shunt and series field coils work together

30
Q

Define: differential compound

A

when the shunt and series field coil oppose each other.

31
Q

how is a long shunt connected?

A

across the line

32
Q

how is a short shunt connected?

A

across the armature

33
Q

List the three types of cumulative compound DC generators

A

Flat-compound
Over-compound
Under-compound

34
Q

what is the main application for a DC differential compound generator.

A

DC arc welding

35
Q

what do the three finger represent in the Right-hand motor rule?

A

Thumb is thrust
First finger is direction of flux from North to South
Middle finger is direction of electron flow

36
Q

Define: Torque

A

twisting or turning force which is capable of producing rotation about an axis

37
Q

Torque is directly proportional to

A

the product of flux and armature current and some constant

38
Q

Define counter emf

A

the motion of the armature conductors cutting the field in a DC motor results in an induced emf that opposes the applied emf that is producing the armature current

39
Q

Define effective voltage

A

the product of armature current and armature resistance

40
Q

Describe mechanical loading of a DC motor

A 
Speed decreases
counter emf decreases
increase in effective voltage
increase in armature current
increase in torque
41
Q

Describe how a DC motor is operated above base speed

A

by inserting rheostat in the shunt field cct.
field current will decrease
decrease in flux will cause counter emf to decrease
armature current to increase
armature current is greater than the decrease in excitation resulting in more torque.

42
Q

Describe how a DC motor is operated below base speed

A

reduce armature current by adding resistance in series with the armature ( not practical)
Done with electronic devices

43
Q

which way does the electrical plane rotate in a generator?

A

CW

44
Q

which way does the electrical plane rotate in a motor

A

CCW

45
Q

Shunt motor is commonly referred to as a

A

constant speed motor

46
Q

To prevent run-away in a shunt motor an

A

over-speed senor (loss of field relay) is used

47
Q

Always mechanically couple a series motor

A

directly to its load

48
Q

common application of series DC motor

A

automotive starters
locomotives
mine haul trucks

49
Q

Main characteristics of the series motor are

A

high starting torque

poor speed regulation

50
Q

Describe compounding or degree of compounding

A

the degree of compounding can produce torque characteristics ranging from series to shunt.
compounding is the ratio of resistances of the shunt coils to series coils. ( I think)

51
Q

Is it necessary to take precautions to prevent run-away of a compound motor?

A

No. shunt coils provide a no load base speed

Year 3 Electrical (14 decks)

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