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Level 3 Electrical, BCIT > Three Phase Systems > Flashcards

Three Phase Systems Flashcards

1
Q

Why are three phase systems necessary?

A

For power distribution to larger systems, transmission, and generation

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2
Q

What is the main advantage of using a three phase system over a single phase system?

A

More constant power distribution

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3
Q

What are the three advantages of three phase systems compared to single phase systems?

A
  1. Less material than a single phase system
  2. Three phase motors are smaller and more simple
  3. Delivers more constant power distribution
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4
Q

Theee phase motors provide how much more HP than single phase motors?

A

About 150% more

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5
Q

Coils are ____% apart

A

120°

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6
Q

What is voltage measured across a single winding or phase called?

A

Phase voltage

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7
Q

What is voltage measured across the load side lines called?

A

Line voltage

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8
Q

How is the Wye system connected

A

By connecting lower subscripts together.

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9
Q

How is the delta system connected?

A

By connecting windings end to end to form a triangle

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10
Q

El is higher than Ep by a factor of root 3 in which system?

A

Wye

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11
Q

What is the formula for finding El in a Wye system?

A

El=Ep x root 3

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12
Q

El and Ep are equal in which system?

A

Delta

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13
Q

What is the formula for finding El in a Delta system?

A

El=Ep

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14
Q

In which system are Il and Ip equal?

A

Wye

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15
Q

In which system is Il higher than Ip by root 3?

A

Delta

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16
Q

What is the formula for finding Il in a Delta system?

A

Il=Ip x root 3

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17
Q

What is the formula for finding VAt on both Delta and Wye systems?

A

Vat=El x Il x root 3

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18
Q

What is the formula to find total power on both a Wye and Delta system?

A

TP = El x Il x root 3 x Power Factor

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19
Q

In a delta connected load, the line current lags the phase current by how many degrees?

A

30%

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20
Q

In a delta connected supply, line voltage leads phase voltage by 90° T or F?

A

False

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21
Q

What is the relationship between the current flow in all three line conductors and the current in the neutral conductor.

A

The neutral conductor only carries unbalanced current.

In = Ia + Ib + Ic

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22
Q

What are the four reasons for grounding the common point in a Wye connected system?

A
  1. Limit V to ground
  2. Facilitate O/C protection
  3. Lightning protection is improved
  4. Prevents floating neutral
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23
Q

What are the two methods to indicate a ground fault in an ungrounded system?

What is the code rule?

A

Audible, and visual. Buzzer/horn

10-106

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24
Q

How is voltage to ground different in the high leg compared to other line voltages to ground?

A

The high leg is larger by a factor of root 3

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25
Q

What is the formula to find power out?

A

I^2 x R

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26
Q

When does VAt = Pt?

A

When PF is 100% or at unity

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27
Q

Difference between total circuit power and total output power?

A

Total circuit power is rated, total output is what is output.

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28
Q

If only one voltage is given, assume it is ____

A

Line voltage

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29
Q

Formula for impedence given resistance and pf

30
Q

What formula would you use to find total power delivered?

A

Pt = El x Il x root 3 x PF

31
Q

How much more is a delta connected load kVA than a wye connected load?

A

The VAt of a delta connected load will be three times as much as a wye.

32
Q

Total circuit power is phase or line power?

33
Q

When an ohmmeter is placed across two windings of a Wye system, what will the resistance read?

A

The two resistances added inversely.

34
Q

When an ohmmeter is placed across two windings of a Delta system, what will the resistance read?

A

Two of the resistances added inversely, plus the remaining resistance added in parallel *number system

35
Q

What is motor FLA?

A

Full load amps. Current at rated load. Iline.

36
Q

Watts is _____ power

37
Q

VARs is ______ power

38
Q

VA is _____ power

39
Q

Do all Delta and Wye systems need to be grounded?

A

No, only 4 wires. Ground the neutral.

40
Q

What is the purpose of grounding a delta or wye system?

A
  1. Limit voltage to ground
  2. Facilitate O/C protection
  3. Maintain phase voltage
  4. Prevents floating neutral.
41
Q

In an equation, when only one voltage is provided, assume it is 1. Line or 2. Phase?

42
Q

Ground fault indicating lights are connected in delta or wye configuration?

43
Q

In a delta supply, if there is a ground fault, will loads still run?

44
Q

In a wye supply, if there is a ground fault, will loads still run?

45
Q

1 wattmeter method is used for measuring ____ circuits

46
Q

What is the formula for the 1 wattmeter method?

A

Pt = W1 x 3

All lines will have equal power in a balanced system.

47
Q

The 3 wattmeter method is used for measuring ______ loads

A

Unbalanced

48
Q

What is the formula for using the 3 wattmeter method?

A

Pt = W1 + W2 + W3

49
Q

The 2 wattmeter method measures ____

A

Il and El per wattmeter

50
Q

What is the formula for using the 2 wattmeter method?

A

Pt = W1 + W2

51
Q

Which wattmeter method can be used to determine PF?

A

2 wattmeter method

52
Q

When using the 2 wattmeter method, if one wattmeter kicks downscale, the PF is _____

A

Less than 50%

53
Q

What happens in the 2 wattmeter method if PF is less than 50%?

A

One wattmeter will kick downscale

54
Q

When using the 2 wattmeter method, if one wattmeter reads zero, the PF is _____

55
Q

What happens in the 2 wattmeter method if PF is equal to 50%?

A

One wattmeter will read zero

56
Q

When using the 2 wattmeter method, if both wattmeters read positive, the PF is _____

57
Q

What happens in the 2 wattmeter method if PF is over 50%?

A

Both wattmeters will read positive

58
Q

What happens in the 2 wattmeter method if PF is equal to 100%?

A

Both wattmeters read equal positive values

59
Q

When using the 2 wattmeter method, if both wattmeters read the same value, the PF is _____

60
Q

In a 3 phase 3 wire Wye connection, when there is an open in the line, VAt will be ____ of rated VAt?

61
Q

In a 3 phase 3 wire Wye connection, when there is an open in the line, the circuit is considered to be doing what?

A

Single phasing

62
Q

In a 3 phase 3 wire Wye connection, when there is an open in the phase, VAt will be ____ of rated VAt?

63
Q

In a 3 phase 4 wire system, when there is an open in line or phase, power will be _____

A

2/3 of rated

64
Q

What are three signs of a motor single phasing?

A
  1. Motor will sound different
  2. Motors may stall
  3. Motor will not be able to start
65
Q

What is a floating neutral?

A

When neutral connection or ground connection becomes open.

66
Q

In a delta 3 phase 3 wire system, when there is an open line, power will be ____ of rated

A

1/2 of rated

67
Q

In a delta 3 phase 3 wire system, if there is an open phase, power will be _____ of rated

68
Q

What is the angular displacement between phase voltage and line voltage in a balanced delta connection?

69
Q

How do you find angle theta of a delta system when given PF?

A

Inverse cos.PF

70
Q

How do you find PF of a delta system when given angle theta?

Level 3 Electrical, BCIT (7 decks)

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