Math Applications Flashcards
Wye & Delta, Phase & Line, Power Calculation
The “phase angle” is the angle between _____ and _____
“phase current” and “phase voltage”
Coils in a 3-phase generator are mounted _____ apart from each other
120 mechanical degrees
The sine waves generated by a 3-ohase generator are _____ apart
120 electrical degrees
Phasor diagrams starting at a common point are called _____
Open phasor
Phasor diagrams where second phasor beginning at point where the first ended, are called _____
Tip-to-tail / closed phasor
The length of a phasor represents the _____ value of Voltage
Effective (RMS)
Line voltage in Wye is obtained by measuring _____
Any two lines
Line voltage V(ab) is the _____ sum of _____
Phasoral, V(an) + V(nb)
In a Wye connected load, there is a ____ phase angle between phase and line voltage
30 degrees
(ABC) is _____ phase sequence
Positive
The neutral in a Wye connected load is for what purpose? What would the current in the neutral be in a balanced circuit?
Carries the unbalanced current and maintains equal phase voltage. No current flows in balanced circuit.
If the neutral opens in a wye connection, what happens?
Total supply V applies across both loads. Becomes series circuit.
What is the result of an open neutral in Wye? Where will the highest voltage occur?
Unbalanced phase voltages proportional to the impedance of each phase. Highest V across largest impedance.
The unbalanced current in Wye will be the _____ sum of the _____
Phasor, three line currents
I(b) being 240 degrees from reference phasor I(a), it is located in a quadrant making the horizontal and vertical components _____
Negative
I(c) being 120 degrees from reference phasor I(a), located in the quadrant making the horizontal and vertical components _____
Horizontal = negative, Vertical = positive
In a balanced Wye load with a lagging power factor, line current and voltage are out of phase by _____
30 degrees plus phase angle
Phase and line voltage in Wye is _____
30 degrees out of phase
The relationship between line and phase current in Wye is _____
They are equal
The angle between line current and voltage in Wye at unity pf is _____
30 degrees
In Delta, the relationship between E(line) and E(phase) is _____
They are equal.
How would you confirm the correct connection of a 3-phase Delta transformer secondary? What would reversing the leads on a coil do?
Open corner test - Voltmeter shows 0 V.
Reversing would show voltmeter reading of twice the phase voltage.
Besides twice the phase voltage, what would reversed leads to a coil in a delta system cause?
High current flow around the loop, limited only by the impedance of the coils
What does the angle between a phasor and it’s reference indicate?
Time displacement
Phasor drawings are created for the load rather than source because _____
The load is what is being analyzed
If a delta connected load is balanced, the phase currents will be 120 degrees out of phase with each other phase regardless of the power factor. T or F
True
In Delta, to find line current I(a) you must phasorally add _____.
I(ab) + I(ac) … where IAC is created 180 degrees from phase current ICA
In an unbalanced Delta 3-phase load, the impedances in each phase _____
Are not equal value and/or don’t have the same power factor.
For an unbalanced Delta load, the following are true (3):
- Each phase current determined by specific phase V and Z
- Line currents calculated by phasor sum of appropriate phase currents
- I(line) = I(phase) x root3 cannot be used
The reverse of any current phasor in Delta may be indicated by _____
Reversing subscripts & flipping the phasor 180 degrees
In a balanced 3-phase Delta load, the phasor sum of I(a) + I(b) + I(c) is _____
0
In an unbalanced 3-phase Delta load, the impedances in each phase ______
Are not equal and/or have different power factors
In an unbalanced Delta load, I(a), I(b) and I(c) are not displaced from each other by 120 degrees. T or F
True
In an unbalanced Delta load, the phasor sum of the 3 line currents is 0. T or F
True
In a 4 wire Delta connection, where is the center tap placed?
Between line B and line C
A heater is connected in parallel to a single phase motor. If the heater is turned off, the overall phase angle of the circuit will _____
Increaae
The Fluke 434 PQA has ____ current probe connections, ____ voltage connections, and a ____ connection.
Four, four, ground
What generalization can be made in a leading PF series circuit when comparing capacitive and inductive reactance?
Xc is greater than Xl
What would happen to the power factor of a circuit if the ohmic value of the resister decreased?
Increase due to higher current and thus, higher powwr
If the X1 terminal is directly across from the H1 terminal, what polarity would the transformer have?
Subtractive
When does transformer action occur?
When a change of current in one circuit causes a voltage to be induced into another circuit.
How are the H and X terminals on a transformer oriented for additive polarity? And for subtractive?
Additive = diagonal , subtractive = across
The positive or negative value of the current when calculating horizontal and vertical components is not important to find the correct current values (true or false)
False
What equations can be used to determine the voltage of the high leg in a 4-wire Delta connection?
E(AN) = E(AC) + E(CN) or E(AN) = E(AB) + E(BN)
If the phasor sum of the line currents in an unbalanced Delta connected system equals zero then _____
The system is operating properly
For an unbalanced resistive Delta connection, the phase displacement between I(line A), I(line B) and I(line C) would be _____
Can’t determine without phase current values
How does line current change when reconnecting from Delta to Wye?
3 times smaller
How does phase current change when reconnecting from Delta to Wye?
√3 times smaller
When reconnecting a supply from Wye to Delta what happens to the voltage and current?
Output line voltage = √3 smaller (equal to Wye phase V)
Output line current = √3 larger (Delta phase current = the Wye line current)
What percentage of a Delta/delta line current is the phase current? Why is this advantageous?
57.8%. the conductors of each phase don’t have to be as large as the line conductors.
What are the 3 formulas for Power, Reactive Power, and Apparent Power for one phase of a 3 phase circuit?
P = V(line) x I(line) x pf
Q = V(l) x I(l) x sine(angle)
S = V(l) x I(l)
How do you determine total power values when the phase values are known?
Add them directly. Ie: S(a) + S(b) + S(c)
What must be done to calculate power components of unbalanced three phase systems?
Each phase must be analyzed separately.
Total apparent power in a balanced circuit can be found with what formulas?
S = 3 x V(ph) x I(ph)
S = √3 x V(l) x I(l)
Total power of three loads connected in Delta are _____ than that of the same loads in Wye
3 times larger
Adding capacitors to a circuit to correct the overall power factor will cause the _____
Line current to decrease
A synchronous motor used without any load connected is called a _____
Synchronous condenser
