Line A-5

Question 1

What does the term polyphase mean?

Answer 1

Polyphase systems are systems that have two or more phases.

Question 2

List four advantages of a three-phase system.

Answer 2

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

Question 3

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

Answer 3

120 degrees

Question 4

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

Answer 4

Wye (Y), Delta

Question 5

What does the term phase sequence mean?

Answer 5

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

Question 6

How is the reversal of phase sequence normally accomplished?

Answer 6

Interchange any two line leads.

Question 7

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

Answer 7

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

Question 8

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

Answer 8

Effective (Rms)

Question 9

The positive direction of phasor rotation is ___________________

Answer 9

Counter-clockwise

Question 10

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

Answer 10

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

Question 11

11. Voltage phasors are generally drawn with __________ arrowheads.

Answer 11

Open

Question 12

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

Answer 12

Positive

Question 13

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

Answer 13

Negative

Question 14

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

Answer 14

Enters

Question 15

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

Answer 15

180

Question 16

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

Answer 16

30 degrees

Question 17

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

Answer 17

I line = I phase

Question 18

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

Answer 18

E line = Root 3 × Ε phase

Question 19

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

Answer 19

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

Question 20

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

Answer 20

30 degrees

Question 21

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

Answer 21

I line = Root 3 × I phase

Question 22

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

Answer 22

E line = E phase

Question 23

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

Answer 23

Ungrounded

Question 24

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

Answer 24

Phase A

Question 25

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?

Answer 25

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

Question 26

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

Answer 26

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.

Question 27

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

Answer 27

50%

Question 28

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

Answer 28

a. 50% b. 66.7%

Question 29

Which method for connecting capacitors is preferred and why?

Answer 29

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

Question 30

Which method for connecting capacitors is the most economical?

Answer 30

A large bank at the main service

Question 31

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

Answer 31

Wye connections

Question 32

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

Answer 32

Delta connections

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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.

Question 35

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

Answer 35

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.

Question 36

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

Answer 36

Between unity 90% lagging