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Electrical Level 3 > Line A-5 > Flashcards

Line A-5 Flashcards

1
Q

What does the term polyphase mean?

A

Polyphase systems are systems that have two or more phases.

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2
Q

List four advantages of a three-phase system.

A

Smaller machines for the same kVA capacity

Less copper required to supply the same energy

Constant power as opposed to pulsating power in single-phase systems

Simpler motor design

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3
Q

What is the phase angle between the three generated voltages of a three-phase alternator?

A

120 degrees

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4
Q

What are the two possible configurations for the leads of a three-phase alternator?

A

Wye (Y), Delta

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5
Q

What does the term phase sequence mean?

A

Phase sequence is the order in which the three-phase voltages reach their positive peaks.

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6
Q

How is the reversal of phase sequence normally accomplished?

A

Interchange any two line leads.

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7
Q

The CEC requires that the conductors of a three-phase system be identified with the
following colours:
a. Phase A
b. Phase B
c. Phase C
d. Neutral

A

Phase A = red
Phase B = black
Phase C = blue
Neutral = white or natural grey

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8
Q

The length of the phasor is most often used to represent the ________________ value of the sine wave.

A

Effective (Rms)

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9
Q

The positive direction of phasor rotation is ___________________

A

Counter-clockwise

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10
Q

Is it important that all the sine waves represented on the same phasor diagram have the same frequency?

A

Yes. Two sine waves having the same frequency add to form a third sine wave with the same frequency.

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11
Q
  1. Voltage phasors are generally drawn with __________ arrowheads.
A

Open

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12
Q

When subscript notation is used to represent the voltage across a source, the first subscript has ____________ polarity with respect to the second.

A

Positive

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13
Q

When subscript notation is used to represent the voltage across a load, the first subscript has ____________ polarity with respect to the second.

A

Negative

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14
Q

The first subscript always represents the end of the circuit component at which the current ____________ the circuit component.

A

Enters

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15
Q

Reversing the order of the subscripts causes the direction of the phasor to change by ____________ degrees

A

180

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16
Q

In a purely resistive, balanced Y-connected system, what is the magnitude of the phase angles between the line voltages and the line currents?

A

30 degrees

17
Q

What is the mathematical relationship between the line currents and the phase currents in a Y-connected system?

A

I line = I phase

18
Q

What is the mathematical relationship between the line voltages and the phase voltages in a Y-connected system?

A

E line = Root 3 × Ε phase

19
Q

List four reasons for grounding the neutral in a three-phase, four-wire, Y-connected system

A

Reduction in the magnitude of transient over-voltages

Improved lightning protection

Easier location and correction of ground faults

More effective system and equipment fault protection

20
Q

In a purely resistive, balanced, Δ-connected system, what is the magnitude of the phase angles between the line voltages and the line currents?

A

30 degrees

21
Q

What is the mathematical relationship between the line currents and the phase currents in a Δ-connected system?

A

I line = Root 3 × I phase

22
Q

What is the mathematical relationship between the line voltages and the phase voltages in a Δ-connected system?

A

E line = E phase

23
Q

Are three-wire, Δ systems normally grounded or ungrounded?

A

Ungrounded

24
Q

In a three-phase, four-wire, grounded Δ system, which phase conductor is required to be the one with the higher voltage to ground?

A

Phase A

25
Q

Is it permissible to put the grounded conductor and the high leg of a three-phase, four-wire, Δ system in the same panel compartment?

A

No. The CEC requires that the high leg be separated from the grounded conductor.

26
Q

Briefly explain what the term balanced load. What could cause a three-phase load to be unbalanced?

A

The three phases in a balanced load have the same values of impedance and power factor, and the same value of voltage impressed across them. Unbalanced loading can be caused by a fault condition or by the addition of a single-phase load.

27
Q

When a three-wire, wye-connected load single phases, what percentage of power is developed compared to its normal, three-phase value?

A

50%

28
Q

What percentage of the normal three-phase power value is developed in a delta-connected
load under each of the following open-fault conditions?

a. a blown line fuse or single-phasing
b. an open delta-load condition

A

a. 50%
b. 66.7%

29
Q

Which method for connecting capacitors is preferred and why?

A

Individual units at the load so that the correcting capacitor is switched with the inductive load to prevent over-correcting the system.

30
Q

Which method for connecting capacitors is the most economical?

A

A large bank at the main service

31
Q

Three-phase PF correction capacitors can be either wye or delta-connected internally. Which connection is preferred for higher voltage installations?

A

Wye connections

32
Q

Three-phase PF correction capacitors can be either wye or delta-connected internally. Which connection is preferred for higher current installations?

A

Delta connections

33
Q

When connecting a PF correction capacitor directly at the motor, what important consideration must be observed for the motor starter?

A

The overload relay protection (heaters) must be derated to the reduced line current value as a result of PF correction.

34
Q

What safety precautions must be observed before working on capacitors or their circuits?

A

Wait at least one minute (or five minutes for capacitor-only circuits); then test for any voltage
present; then short-circuit the capacitor terminals to ground.

35
Q

For a disconnect switch supplying a capacitor only, what important marking is required?

A

A “warning notice” must be affixed to the disconnecting means to indicate that the circuit has capacitors and that a waiting period of five minutes be observed, after which the
capacitors shall be discharged before handling.

36
Q

To what value of PF are most inductive loads usually corrected?

A

Between unity 90% lagging

Electrical Level 3 (9 decks)

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