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AT-4 B > Portfolio Stufy > Flashcards

Portfolio Stufy Flashcards

1
Q

Draw a power triangle clearly labelling reactive power, apparent power and true power include units

A

True power watts, apparent power VA, reactive power VAR

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2
Q

State 3 effects of operating an installation with low or poor power factor and explain how to improve it

A

Higher cost, low efficiency, wasted power

Connect capacitors in parallel

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3
Q

The power that actually does work in a circuit is called

A

True power

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4
Q

The power that is a product of line voltage and current is called

A

Apparent power

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5
Q

The power stored and returned to a circuit is called

A

Reactive power

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6
Q

The ratio between true power and apparent power is called the

A

Cos - power factor

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7
Q

The common name for either the angle of leaf or lag is called

A

Phase angle

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8
Q

Name the instruments used to measure

Single and tree phase power
Energy consumption
Maximum demand
Power factor

A

Wattmeter
Kilowatt meter
Maximum demand meter
Power factor meter

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9
Q

Does operating a circuit with a low power factor significantly increase costs associated with running installation

A

Yes

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10
Q

State two methods to improve power factor

A

Operate motors at full load

Connecting capacitors

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11
Q

Name an instrument to measure resistance

A

Ohmeter

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12
Q

Name three measuring instruments required to enable the calculation of the power factor of a single phase circuit

A

Voltmeter, ammeter, wattmeter

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13
Q

Where on the manufacturers piece of equipment would you find the power factor requirements

A

Name plate

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14
Q

Do question 13

A

Check

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15
Q

Name one purpose for providing a neutral in an unbalanced three phase, four wire system

A

A return path for unbalanced current

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16
Q

State one effect of an open circuit neutral on an unbalanced three phase four wire MEN installation under load

A

It will creat unbalanced load voltages across the 3 phases

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17
Q

Harmonics can be defined as

A

Multiples of the fundamental frequency

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18
Q

State one effects of harmonics

A

Distortion of electricity supply voltage

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19
Q

Name an instrument used to test harmonics

A

Harmonic analyser

20
Q

State the dangers of operating a series circuit at resonance

A

XL and XC cancel each other out allowing high levels of current and voltage to flow across reactive components

21
Q

Name two common sources of harmonics

A

Electronic lighting equipment, computers

22
Q

Under what conditions will resonance occur in a series or parallel circuit

A

If inductive reactance and capacitive reactance are the same in a ac circuit

23
Q

State the dangers of operating a parallel circuit at resonance

A

Current will increase extremely high

24
Q

Name method used to reduce the problems caused by harmonics

A

Harmonic filters

25
Q

A delta connected 3 phase motor is supplied from a star connected alternator with a phase voltage of 240v. What is the line voltage of the delta motor

A

415v

26
Q

26

A

Use as 3000

27
Q

Define earth fault loop impedance

A

The total opposition to current flow

28
Q

Give one reason why it is important to measure energy, power, voltage, current and power factor of an installation or electrical load before alterations are made

A

So you can calculate max demand and make sure alterations will work

29
Q

29 in book

A

Check book

30
Q

The peak value per phase of a 3 phase alternator is 320v. Calculate the rms voltage per phase

A

0.707 x 320 = 226.24v

31
Q

The two advantages of a 3 phase distribution system compared to a single phase

A

more efficient
Gives 2 voltages

32
Q

State one effect of a three phase motor connected with one phase windings reversed

A

current would increase as Motors work against each other

33
Q

Do question 33

A

In book

34
Q

Give a brief explanation of how a 3 phase supply is generated inside a single alternator

A

3 coils are rotated at a constant speed tho a two pole magnetic field creating an EMF

35
Q

Do question 35

A

Book

36
Q

Explain the terms balanced and unbalanced loads give examples of each

A

Balanced - 3 phase motor
Unbalanced - network supply transformer

37
Q

37

A

Check book

38
Q

38

A

Book

39
Q

39

A

As/3000 nameplate

40
Q

40

A

Clause 3.5.2

41
Q

41

A

No, clause 2.3.2.1 (b)(I)

42
Q

42

A

415 x 2% = 8.3v

43
Q

43

A

You cannot exceed maximum current without overheating

Starting motors

44
Q

44

A

The supply system shall be no less than 0.8 lagging

45
Q

45

A

The device will overheat

46
Q

46

A

If one phase is reversed then it will likely burn out

47
Q

47

A

Balanced - 3 phase hot water system

Unbalanced - network supply transformer

AT-4 B (1 decks)

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