Line A-5 Flashcards
What does the term polyphase mean?
Polyphase systems are systems that have two or more phases.
List four advantages of a three-phase system.
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
What is the phase angle between the three generated voltages of a three-phase alternator?
120 degrees
What are the two possible configurations for the leads of a three-phase alternator?
Wye (Y), Delta
What does the term phase sequence mean?
Phase sequence is the order in which the three-phase voltages reach their positive peaks.
How is the reversal of phase sequence normally accomplished?
Interchange any two line leads.
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
Phase A = red
Phase B = black
Phase C = blue
Neutral = white or natural grey
The length of the phasor is most often used to represent the ________________ value of the sine wave.
Effective (Rms)
The positive direction of phasor rotation is ___________________
Counter-clockwise
Is it important that all the sine waves represented on the same phasor diagram have the same frequency?
Yes. Two sine waves having the same frequency add to form a third sine wave with the same frequency.
- Voltage phasors are generally drawn with __________ arrowheads.
Open
When subscript notation is used to represent the voltage across a source, the first subscript has ____________ polarity with respect to the second.
Positive
When subscript notation is used to represent the voltage across a load, the first subscript has ____________ polarity with respect to the second.
Negative
The first subscript always represents the end of the circuit component at which the current ____________ the circuit component.
Enters
Reversing the order of the subscripts causes the direction of the phasor to change by ____________ degrees
180
In a purely resistive, balanced Y-connected system, what is the magnitude of the phase angles between the line voltages and the line currents?
30 degrees
What is the mathematical relationship between the line currents and the phase currents in a Y-connected system?
I line = I phase
What is the mathematical relationship between the line voltages and the phase voltages in a Y-connected system?
E line = Root 3 × Ε phase
List four reasons for grounding the neutral in a three-phase, four-wire, Y-connected system
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
In a purely resistive, balanced, Δ-connected system, what is the magnitude of the phase angles between the line voltages and the line currents?
30 degrees
What is the mathematical relationship between the line currents and the phase currents in a Δ-connected system?
I line = Root 3 × I phase
What is the mathematical relationship between the line voltages and the phase voltages in a Δ-connected system?
E line = E phase
Are three-wire, Δ systems normally grounded or ungrounded?
Ungrounded
In a three-phase, four-wire, grounded Δ system, which phase conductor is required to be the one with the higher voltage to ground?
Phase A
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?
No. The CEC requires that the high leg be separated from the grounded conductor.
Briefly explain what the term balanced load. What could cause a three-phase load to be unbalanced?
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.
When a three-wire, wye-connected load single phases, what percentage of power is developed compared to its normal, three-phase value?
50%
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. 50%
b. 66.7%
Which method for connecting capacitors is preferred and why?
Individual units at the load so that the correcting capacitor is switched with the inductive load to prevent over-correcting the system.
Which method for connecting capacitors is the most economical?
A large bank at the main service
Three-phase PF correction capacitors can be either wye or delta-connected internally. Which connection is preferred for higher voltage installations?
Wye connections
Three-phase PF correction capacitors can be either wye or delta-connected internally. Which connection is preferred for higher current installations?
Delta connections
When connecting a PF correction capacitor directly at the motor, what important consideration must be observed for the motor starter?
The overload relay protection (heaters) must be derated to the reduced line current value as a result of PF correction.
What safety precautions must be observed before working on capacitors or their circuits?
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.
For a disconnect switch supplying a capacitor only, what important marking is required?
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.
To what value of PF are most inductive loads usually corrected?
Between unity 90% lagging
