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Third Period Electrician > Math Applications > Flashcards

Math Applications Flashcards

Wye & Delta, Phase & Line, Power Calculation

1
Q

The “phase angle” is the angle between _____ and _____

A

“phase current” and “phase voltage”

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2
Q

Coils in a 3-phase generator are mounted _____ apart from each other

A

120 mechanical degrees

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3
Q

The sine waves generated by a 3-ohase generator are _____ apart

A

120 electrical degrees

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4
Q

Phasor diagrams starting at a common point are called _____

A

Open phasor

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5
Q

Phasor diagrams where second phasor beginning at point where the first ended, are called _____

A

Tip-to-tail / closed phasor

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6
Q

The length of a phasor represents the _____ value of Voltage

A

Effective (RMS)

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7
Q

Line voltage in Wye is obtained by measuring _____

A

Any two lines

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8
Q

Line voltage V(ab) is the _____ sum of _____

A

Phasoral, V(an) + V(nb)

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9
Q

In a Wye connected load, there is a ____ phase angle between phase and line voltage

A

30 degrees

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10
Q

(ABC) is _____ phase sequence

A

Positive

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11
Q

The neutral in a Wye connected load is for what purpose? What would the current in the neutral be in a balanced circuit?

A

Carries the unbalanced current and maintains equal phase voltage. No current flows in balanced circuit.

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12
Q

If the neutral opens in a wye connection, what happens?

A

Total supply V applies across both loads. Becomes series circuit.

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13
Q

What is the result of an open neutral in Wye? Where will the highest voltage occur?

A

Unbalanced phase voltages proportional to the impedance of each phase. Highest V across largest impedance.

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14
Q

The unbalanced current in Wye will be the _____ sum of the _____

A

Phasor, three line currents

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15
Q

I(b) being 240 degrees from reference phasor I(a), it is located in a quadrant making the horizontal and vertical components _____

A

Negative

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16
Q

I(c) being 120 degrees from reference phasor I(a), located in the quadrant making the horizontal and vertical components _____

A

Horizontal = negative, Vertical = positive

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17
Q

In a balanced Wye load with a lagging power factor, line current and voltage are out of phase by _____

A

30 degrees plus phase angle

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18
Q

Phase and line voltage in Wye is _____

A

30 degrees out of phase

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19
Q

The relationship between line and phase current in Wye is _____

A

They are equal

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20
Q

The angle between line current and voltage in Wye at unity pf is _____

A

30 degrees

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21
Q

In Delta, the relationship between E(line) and E(phase) is _____

A

They are equal.

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22
Q

How would you confirm the correct connection of a 3-phase Delta transformer secondary? What would reversing the leads on a coil do?

A

Open corner test - Voltmeter shows 0 V.

Reversing would show voltmeter reading of twice the phase voltage.

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23
Q

Besides twice the phase voltage, what would reversed leads to a coil in a delta system cause?

A

High current flow around the loop, limited only by the impedance of the coils

24
Q

What does the angle between a phasor and it’s reference indicate?

A

Time displacement

25
Q

Phasor drawings are created for the load rather than source because _____

A

The load is what is being analyzed

26
Q

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

A

True

27
Q

In Delta, to find line current I(a) you must phasorally add _____.

A

I(ab) + I(ac) … where IAC is created 180 degrees from phase current ICA

28
Q

In an unbalanced Delta 3-phase load, the impedances in each phase _____

A

Are not equal value and/or don’t have the same power factor.

29
Q

For an unbalanced Delta load, the following are true (3):

A
  • 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
30
Q

The reverse of any current phasor in Delta may be indicated by _____

A

Reversing subscripts & flipping the phasor 180 degrees

31
Q

In a balanced 3-phase Delta load, the phasor sum of I(a) + I(b) + I(c) is _____

A

0

32
Q

In an unbalanced 3-phase Delta load, the impedances in each phase ______

A

Are not equal and/or have different power factors

33
Q

In an unbalanced Delta load, I(a), I(b) and I(c) are not displaced from each other by 120 degrees. T or F

A

True

34
Q

In an unbalanced Delta load, the phasor sum of the 3 line currents is 0. T or F

A

True

35
Q

In a 4 wire Delta connection, where is the center tap placed?

A

Between line B and line C

36
Q

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 _____

A

Increaae

37
Q

The Fluke 434 PQA has ____ current probe connections, ____ voltage connections, and a ____ connection.

A

Four, four, ground

38
Q

What generalization can be made in a leading PF series circuit when comparing capacitive and inductive reactance?

A

Xc is greater than Xl

39
Q

What would happen to the power factor of a circuit if the ohmic value of the resister decreased?

A

Increase due to higher current and thus, higher powwr

40
Q

If the X1 terminal is directly across from the H1 terminal, what polarity would the transformer have?

A

Subtractive

41
Q

When does transformer action occur?

A

When a change of current in one circuit causes a voltage to be induced into another circuit.

42
Q

How are the H and X terminals on a transformer oriented for additive polarity? And for subtractive?

A

Additive = diagonal , subtractive = across

43
Q

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)

A

False

44
Q

What equations can be used to determine the voltage of the high leg in a 4-wire Delta connection?

A

E(AN) = E(AC) + E(CN) or E(AN) = E(AB) + E(BN)

45
Q

If the phasor sum of the line currents in an unbalanced Delta connected system equals zero then _____

A

The system is operating properly

46
Q

For an unbalanced resistive Delta connection, the phase displacement between I(line A), I(line B) and I(line C) would be _____

A

Can’t determine without phase current values

47
Q

How does line current change when reconnecting from Delta to Wye?

A

3 times smaller

48
Q

How does phase current change when reconnecting from Delta to Wye?

A

√3 times smaller

49
Q

When reconnecting a supply from Wye to Delta what happens to the voltage and current?

A

Output line voltage = √3 smaller (equal to Wye phase V)

Output line current = √3 larger (Delta phase current = the Wye line current)

50
Q

What percentage of a Delta/delta line current is the phase current? Why is this advantageous?

A

57.8%. the conductors of each phase don’t have to be as large as the line conductors.

51
Q

What are the 3 formulas for Power, Reactive Power, and Apparent Power for one phase of a 3 phase circuit?

A

P = V(line) x I(line) x pf

Q = V(l) x I(l) x sine(angle)

S = V(l) x I(l)

52
Q

How do you determine total power values when the phase values are known?

A

Add them directly. Ie: S(a) + S(b) + S(c)

53
Q

What must be done to calculate power components of unbalanced three phase systems?

A

Each phase must be analyzed separately.

54
Q

Total apparent power in a balanced circuit can be found with what formulas?

A

S = 3 x V(ph) x I(ph)

S = √3 x V(l) x I(l)

55
Q

Total power of three loads connected in Delta are _____ than that of the same loads in Wye

A

3 times larger

56
Q

Adding capacitors to a circuit to correct the overall power factor will cause the _____

A

Line current to decrease

57
Q

A synchronous motor used without any load connected is called a _____

A

Synchronous condenser

Third Period Electrician (4 decks)

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