Theory Flashcards
In a Wye System, EL =
Ep x Root 3
In Wye, line current =
phase current
In Wye, Vab =
Van + Vnb
In Wye, Vbc =
Vbn + vnc
In Wye, Vca =
Vcn + Vna
In Wye, In =
Ia + Ib + Ic
In Wye, S =
Eline x Iline x Root 3
In Wye, P =
Eline x Iline x Root 3 x pf
In Wye, Q =
Eline x Iline x Root 3 x Sin Angle
In Wye, the phase voltage ____ the line voltage by ____ degrees
lags, 30 degrees
In an unbalanced three-phase wye system, positive rotation is in the ___ direction
counterclockwise
Define power factor
the displacement between phase voltage and phase current
What is the purpose of the neutral conductor in a three phase wye system?
To maintain proper phase voltage and to carry the unbalanced load
What determines the rotation of electrical generation
The prime mover
How many conductors are in a three phase system?
Three or four
When does voltage inducted reach its maximum?
At 90 degrees
How many mechanical degrees apart are coins placed?
120
How many electrical degrees apart are the voltages produced by a three-phase generator?
120
True or false: the CAB phase sequence is the same as the ABC phase sequence
True
The BAC sequence is the same as the ABC phase sequence. True or False?
False
What can be used to observe phase sequence?
Oscilloscope
Name two ways that we can change the phase sequence?
Swap two leads or change the direction of the prime mover
What are three advantages of a three phase system over a single phase system?
Constant voltage, smaller equipment (conductors), more efficient
Would you expect less vibration in a three phase or single phase system?
Three phase
How much more copper is used in a single phase system than a three phase system and why?
25% because the conductors must be bigger to get the same supply
What requirements must be met for a three phase system to be balanced?
same impedance, same power factor
In what order do the phase voltages follow one another for positive phase sequence in a three phase system?
ABCABCABC
In a balanced system, each line current is separated by how many electrical degrees?
120
When the neutral is broken, the highest volt drop will be across the load with the _____ impedance
highest
State the mathematical relationship between phase and line voltage in a wye connection
E line = e phase x root three
What is the line voltage if the phase voltage is 120 in a wye system?
208V
What is the angle between line and phase voltages in a wye system?
30 degrees
What is the angle between phase voltages in a wye system?
120 degrees
In wye, iline =
i phase
What two conditions must be met for a three phase wye system to be balanced?
Same current, same power factor
In an unbalanced system, are impedance and power factor equal?
no
Is the neutral current always equal to the phasor sum of the three line currents?
yes
Does the neutral current always have current?
It has no current in a balanced system
Are the phase voltages equal in an unbalanced system?
no
What is a phantom phasor and why do we need it?
It is the opposite of the real phase (180 degrees) and we ened it to add up our currents in an unbalanced delta
What needs to be true for a system to be considered balanced?
same impedance, same pf
Does I Line = I Phase x root 3 always apply in a delta system?
Only if it is balanced
In an unbalanced delta, how do we find the current of IA for example?
IA = IAB + IAC (phantom phasor)
In a delta, is line current split evenly across both coils? Why or why not? Let’s say that ILine = 100A, what happens?
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
Describe current flow in a delta system
The current that flows through each coil combines with the current in the other coil to add up to the line current
In Delta, e phase =
e line
In delta, does E line = e phase always applY?
yes
Are phasor diagrams for the source or for the load?
the load
How do you know if a delta is connected correctly using an open corner test?
V = 0
How do you know if a delta is not connected correctly using an open corner test?
There is a lot of voltage
What is an open corner test?
It is a test conducted to confirm that a three phase delta secondary is connected correctly
When a balanced three phase wye connected load is reconnected as a delta, the volt drop across each load will be _____
root three times greater
When a balanced three phase wye connected load is reconnected as a delta, the current through each load will be
root 3 times greater
When a balanced three phase wye connected load is reconnected as a delta, the line current supplying the delta loads will be
three times greater
If there are 0 amps of current on the neutral, is the circuit balanced or unbalanced?
balanced
If we add two phasors with the same magnitude and 120 degrees apart from each other, what will the result be?
The same magnitude, sixty degrees apart
When reconnecting a wye load into a delta, the line voltage will
stay the same
When reconnecting a wye load into a delta, the phase voltage will
increase by root 3
When reconnecting a wye load into a delta, the phase current will
increase by root 3
When reconnecting a wye load into a delta, the line current will
increase by 3x
When reconnecting a delta load into a wye, the line voltage will
stay the same
When reconnecting a delta load into a wye, the phase voltage will
decrease by root 3
When reconnecting a delta load into a wye, the phase current will
decrease by root 3
When reconnecting a delta load into a wye, the line current will
decrease by 3x
When reconnecting a three phase supply from a wye to a delta, the output line voltage will be
reduced by a factor of root 3
When working with both wye and delta in the same question, what must be done?
Shift our phasors -30 degrees to make V AB the reference point at 0 degrees
When a delta is reconnected as a wye, the volt drop across each load will be ____ times _____
root 3, less
When a delta is reconnected as a wye, the current through each load will be ____ times _____
root 3, less
When a delta is reconnected as a wye, the line current supplying the wye loads will be ____ times _____
3, less
Will a delta or a wye provide greater current?
delta
If the neutral reads zero amperes, what can be said about the loads?
they are balanced
