Theory Flashcards

1
Q

In a Wye System, EL =

A

Ep x Root 3

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2
Q

In Wye, line current =

A

phase current

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3
Q

In Wye, Vab =

A

Van + Vnb

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4
Q

In Wye, Vbc =

A

Vbn + vnc

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5
Q

In Wye, Vca =

A

Vcn + Vna

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6
Q

In Wye, In =

A

Ia + Ib + Ic

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7
Q

In Wye, S =

A

Eline x Iline x Root 3

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8
Q

In Wye, P =

A

Eline x Iline x Root 3 x pf

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9
Q

In Wye, Q =

A

Eline x Iline x Root 3 x Sin Angle

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10
Q

In Wye, the phase voltage ____ the line voltage by ____ degrees

A

lags, 30 degrees

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11
Q

In an unbalanced three-phase wye system, positive rotation is in the ___ direction

A

counterclockwise

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12
Q

Define power factor

A

the displacement between phase voltage and phase current

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13
Q

What is the purpose of the neutral conductor in a three phase wye system?

A

To maintain proper phase voltage and to carry the unbalanced load

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14
Q

What determines the rotation of electrical generation

A

The prime mover

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15
Q

How many conductors are in a three phase system?

A

Three or four

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16
Q

When does voltage inducted reach its maximum?

A

At 90 degrees

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17
Q

How many mechanical degrees apart are coins placed?

A

120

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18
Q

How many electrical degrees apart are the voltages produced by a three-phase generator?

A

120

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19
Q

True or false: the CAB phase sequence is the same as the ABC phase sequence

A

True

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20
Q

The BAC sequence is the same as the ABC phase sequence. True or False?

A

False

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21
Q

What can be used to observe phase sequence?

A

Oscilloscope

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22
Q

Name two ways that we can change the phase sequence?

A

Swap two leads or change the direction of the prime mover

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23
Q

What are three advantages of a three phase system over a single phase system?

A

Constant voltage, smaller equipment (conductors), more efficient

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24
Q

Would you expect less vibration in a three phase or single phase system?

A

Three phase

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25
Q

How much more copper is used in a single phase system than a three phase system and why?

A

25% because the conductors must be bigger to get the same supply

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26
Q

What requirements must be met for a three phase system to be balanced?

A

same impedance, same power factor

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27
Q

In what order do the phase voltages follow one another for positive phase sequence in a three phase system?

A

ABCABCABC

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28
Q

In a balanced system, each line current is separated by how many electrical degrees?

A

120

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29
Q

When the neutral is broken, the highest volt drop will be across the load with the _____ impedance

A

highest

30
Q

State the mathematical relationship between phase and line voltage in a wye connection

A

E line = e phase x root three

31
Q

What is the line voltage if the phase voltage is 120 in a wye system?

A

208V

32
Q

What is the angle between line and phase voltages in a wye system?

A

30 degrees

33
Q

What is the angle between phase voltages in a wye system?

A

120 degrees

34
Q

In wye, iline =

A

i phase

35
Q

What two conditions must be met for a three phase wye system to be balanced?

A

Same current, same power factor

36
Q

In an unbalanced system, are impedance and power factor equal?

A

no

37
Q

Is the neutral current always equal to the phasor sum of the three line currents?

A

yes

38
Q

Does the neutral current always have current?

A

It has no current in a balanced system

39
Q

Are the phase voltages equal in an unbalanced system?

A

no

40
Q

What is a phantom phasor and why do we need it?

A

It is the opposite of the real phase (180 degrees) and we ened it to add up our currents in an unbalanced delta

41
Q

What needs to be true for a system to be considered balanced?

A

same impedance, same pf

42
Q

Does I Line = I Phase x root 3 always apply in a delta system?

A

Only if it is balanced

43
Q

In an unbalanced delta, how do we find the current of IA for example?

A

IA = IAB + IAC (phantom phasor)

44
Q

In a delta, is line current split evenly across both coils? Why or why not? Let’s say that ILine = 100A, what happens?

A

It doesn’t split evenly because each coil is at a different time displacement. if the system is balanced, it splits by root three, so in the case of ILine = 100A, each coil would see 57A

45
Q

Describe current flow in a delta system

A

The current that flows through each coil combines with the current in the other coil to add up to the line current

46
Q

In Delta, e phase =

A

e line

47
Q

In delta, does E line = e phase always applY?

A

yes

48
Q

Are phasor diagrams for the source or for the load?

A

the load

49
Q

How do you know if a delta is connected correctly using an open corner test?

A

V = 0

50
Q

How do you know if a delta is not connected correctly using an open corner test?

A

There is a lot of voltage

51
Q

What is an open corner test?

A

It is a test conducted to confirm that a three phase delta secondary is connected correctly

52
Q

When a balanced three phase wye connected load is reconnected as a delta, the volt drop across each load will be _____

A

root three times greater

53
Q

When a balanced three phase wye connected load is reconnected as a delta, the current through each load will be

A

root 3 times greater

54
Q

When a balanced three phase wye connected load is reconnected as a delta, the line current supplying the delta loads will be

A

three times greater

55
Q

If there are 0 amps of current on the neutral, is the circuit balanced or unbalanced?

A

balanced

56
Q

If we add two phasors with the same magnitude and 120 degrees apart from each other, what will the result be?

A

The same magnitude, sixty degrees apart

57
Q

When reconnecting a wye load into a delta, the line voltage will

A

stay the same

58
Q

When reconnecting a wye load into a delta, the phase voltage will

A

increase by root 3

59
Q

When reconnecting a wye load into a delta, the phase current will

A

increase by root 3

60
Q

When reconnecting a wye load into a delta, the line current will

A

increase by 3x

61
Q

When reconnecting a delta load into a wye, the line voltage will

A

stay the same

62
Q

When reconnecting a delta load into a wye, the phase voltage will

A

decrease by root 3

63
Q

When reconnecting a delta load into a wye, the phase current will

A

decrease by root 3

64
Q

When reconnecting a delta load into a wye, the line current will

A

decrease by 3x

65
Q

When reconnecting a three phase supply from a wye to a delta, the output line voltage will be

A

reduced by a factor of root 3

66
Q

When working with both wye and delta in the same question, what must be done?

A

Shift our phasors -30 degrees to make V AB the reference point at 0 degrees

67
Q

When a delta is reconnected as a wye, the volt drop across each load will be ____ times _____

A

root 3, less

68
Q

When a delta is reconnected as a wye, the current through each load will be ____ times _____

A

root 3, less

69
Q

When a delta is reconnected as a wye, the line current supplying the wye loads will be ____ times _____

A

3, less

70
Q

Will a delta or a wye provide greater current?

A

delta

71
Q

If the neutral reads zero amperes, what can be said about the loads?

A

they are balanced