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Magnetism > Portfolia Part B > Flashcards

Portfolia Part B Flashcards

1
Q

Name the major components of a rotating DC machine

A

Armature, frame, field coil

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2
Q

How many current paths does and 8 pole lap wound armature have

A

8 paths

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3
Q

The basic operating principle of any motor is

A

electrical to mechanical

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4
Q

When applying Flemings right hand rule for generators your thumb will point in the direction of

A

Motion of force

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5
Q

Describe the physical difference between the coils used for series-field and the coils used for shunt-field in a compound DC machine

A

Series - windings are very thick and less turns

Shunt- wires are thin with lots of turns

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6
Q

A 4-pole lap wound armature contains a total of 300 effective conductors. If the magnetic flux is 0.02 Webers per pole and the speed of rotation is 100RPM, what is the value of it’s generated voltage?

A

160V

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7
Q

What is the most likely cause of a DC generator failing to generate a voltage when rotated

A

There is no residual magnetism

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8
Q

2 methods used to determine losses in a D.C machine are by measurement and by calculation True or false

A

True

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9
Q

What is the amount of torque developed by a motor which exerts a force of 80 N over a perpendicular distance of 200mm?

A

16Nm

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10
Q

Why does the prime-mover for a DC generator have to work harder when the current drawn from the generator increases

A

It creates an opposing torque against the prime mover

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11
Q

When testing a 220V DC electrical motor for insulation resistance between coils on what setting would you set the instrument?

A

500V

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12
Q

Explain the purpose of a field Rheostat on a compound D.C motor

A

Allows you to change the resistance to control the motor

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13
Q

Describe the most common method used to regulate the output voltage of a DC generator

A

Rheostat - regulates current to the field coils

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14
Q

The methods used to create excitation in DC generators are

A

Separately excited, self excited

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15
Q

How will an over-compounded generator perform under load, compared to a level compounded generator

A

Level - will maintain constant voltage under full loads

Over- higher voltage is produced under full loads

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16
Q

What limits the current in a DC motor when it is running at normal operating speed

A

The opposing induced EMF

17
Q

Why is it that a series motor should not be operated under no load conditions

A

It will cause damage to itself as it speeds up

18
Q

A motor with an output of 5000 watts draws 25amps from 220volt supply. Calculate the motor efficiency

A

90.9%

19
Q

When reversing the direction of a cumulative compound motor you must

A

Swap the polarity of the armature or change the connections

20
Q

Name a method which can be used to reduce armature reaction

A

Compensating windings, inter-poles, brush shifts

21
Q

Name 1 way to start a DC motor

A

Progressively lower the resistance in series with the armature

22
Q

Which regulatory tool is used to ensure that Australians have efficient appliances and equipment

A

Minimum energy performance standards

23
Q

A four pole DC motor has a lap wound armature of 30 coils, each with 20 conductors. If the flux per pole is 0.02 Wb and the armature current is 19 Amps, how much torque is produced?

A

36.3Nm

24
Q

What is the most important safety factor associated with rotating machinery for electricians

A

No work shall be done on rotating machines if not locked out

25
Q

What do fingers indicate in Flemings left hand motor rule

A

Thumb is motion, pointer finger is flux and centre finger is current

26
Q

What electrical hand hel testing instrument can be used to test a DC armature for faults

A

Multimeter set to ohms

27
Q

What is the basic operating principle of a generator

A

Mechanical to electrical

28
Q

Which part of the DC generator produces the output voltage

A

The armature

29
Q

State the difference between a generator and a motor in terms of energy conversion

A

Motor - electrical to mechanical

Generator - mechanical to electrical

30
Q

Which electrical instrument can be used to compare the electrical measurements with nameplate details

A: voltmeter
B: watt-meter
C: Ammeter
D: all of the above

A

D

31
Q

What is the purpose of a prime-mover when related to a generator?

A

To turn the armature

32
Q

The voltage in a self excited generator may fail to build owing to

A

?

33
Q

Four safety precautions that should be observed when starting generators are

A

Properly earthed, emergency stop, must be guarded, coil to coil test

34
Q

A DC motor runs at 220v, draws a current of 22A, has a speed of 900 RPM and produces a torque of 50 Nm. calculate the output power

A

4712 watts

35
Q

Which DC motor has wide applications, especially within the traction industry?

A

Series excited motor

36
Q

What must DC motors have fitted to their windings to give better communications under all loads

A: Inter-poles
B: shunts
C: Rheostats
D: Permanent magnets

A

A

37
Q

Which of the following creates losses in a DC machine?

A: Copper losses
B: Iron losses
C: Friction
D: all of the above

A

D

38
Q

Does the speed characteristic of a shunt motor alter between no loads and full load?

A

Slightly on load

39
Q

Which component of a DC machine requires regular inspection and maintenance

A

Brushes

Magnetism (2 decks)

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