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FEexam > Power > Flashcards

Power Flashcards

1
Q

The power factor Θ is measured between voltage and current . True or false?

A

True. Refer to NCEES FE Reference Handbook , v. 101 p. 363

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2
Q

If current (I) leads voltage (V), the power factor
9 (pf) is considered to be:

A

Leading

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3
Q

When a 1.5 k resistor and a coil with a 2.2 k inductive reactance are connected in series across an 18 V source, what will be the power factor?

A. 564
B. 0.564
C. 6.76
D. 55.7

A

B = The power factor will be 0.564

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4
Q

Complex power is the complex sum of real and reactive power. True or False?

A

True

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5
Q

A balance three-phase delta-connected load impedance can be converted to an equivalent wye-connected load by multiplying by root of 3. True or False?

A

False: A balanced three phase delta-connected load impedance can be converted to an equivalent wye-connected by dividing the delta-connected load by 3.

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6
Q

The synchronous speed of AC motors is given by:

A. 120 P/F
B. 120F/P
C. 120xp
D. p/120f

A

B

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7
Q

Which of the following can be used to control a DC shunt motor above normal speed?

A. Armature voltage control method
B. Flux control method
C. Both A& B
D. Shunt motor

A

B

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8
Q

Voltage regulation of a power supply can be calculated if the voltage under no load conditions and voltage under full load conditions (assuming that the source voltage remains constant) are know. True or false?

A

True

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9
Q

What are servomotors

A

Servomotors are electrical motors tied to a feedback system to obtain precise control

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10
Q

Which of the following is not a type of loss in rotating machine?
A. Core
B. Copper
C. Insulation
D. Friction

A

C

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11
Q

What can be used to control the armature voltage of a DC motor?

A. Cycloconverters
B. Inverters
C. AC-DC converters
D. Bridge rectifier circuit with fixed input

A

C

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12
Q

The turns ratio of a transformer cannot be determined by the number of turns in the primary & secondary winding. True or false?

A

False

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13
Q

By varying the voltage input to stator, you can impact the speed control of an induction motor. True or false?

A

True

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14
Q

The efficiency of a machine depends on both the input and output power. True or false?

A

True

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15
Q

A single-phase transformer can be connected into delta-wye configuration. True or false?

A

False

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16
Q

A Delta - Delta connection in three-phase transformer does not have a neutral. True or false?

A

True

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17
Q

A purely resistive circuit power. True or false?

A

False

18
Q

Which types of metal is high frequency induction heating used for?

A

Ferrours and non-ferrours metals. Ferrous and non-ferrous metals both use high frequency induction heating

19
Q

How are the capacitor for power factor correction rated?

A. KW
B. KV
C. KVA
D. KVAR

A

D

20
Q

The slip of a machine depends on the synchronous speed. True or False?

A

True

21
Q

In a three-phase wye-circuit, the line current is equal to the phase current. True or false?

A

True

22
Q

What will happen to the speed of the motor if the field coils get opened in a DC motor?

A. Decrease
B. Come to a stop
C. Increase
D. Constant

A

C = if the field coils get opened . The speed will increase

23
Q

In a three-phase Delta connection, line voltage is equal to phase voltage. True or false?

A

True

24
Q

The primary impedance of the transformer can be determined if we know the turns ratio and the secondary impedance. True or false?

A

True

25
Q

The balanced delta load shown is connected to a stable, three-phase voltage source. The rms line voltage is 120 V. What is most nearly the rms magnitude of the line current?

A

C
Solution:
Calculate the magnitude of the impedance for the phase.

Zp = a + jb

26
Q

A 240 V < 0⁰ source supplies a balanced, three-phase load in a delta configuration. The line current is 3 A < 45⁰. What is most nearly the magnitude of the complex power measured across the load?

A. 420 VA

B. 720 VA

C. 1,200 VA

D. 2,200 VA

A

C

27
Q

A balanced, three-phase, delta-connected system has three identical phase-impedances. Line-line voltage is 240V, and line current is 41A. Find magnitudes of phase voltage and phase current.

A. 138V, 11.8 A

B. 138V, 23.7 A

C. 240V, 11.8 A

D. 240V, 23.7 A

A

For a delta connection, the phase voltage is the line-to-line voltage.

VL = Vp = 240 V

28
Q

A balanced, three-phase, Y connected system has three same-phase impedances. Line-line voltage is 240V, line-neutral current is 41A, and power factor leading is 0.8. Find the apparent power.

A. 7.9 kw - j5.9 kVAR
B. 7.9 kW + j5.9 kVAR
C. 14 kW - j10kVAR
D. 14 kw + j10 kVAR

A

C

29
Q

Three identical loads of 4𝛺 – j3𝛺 are delta connected in a three-phase system. The line-to-line voltage is 120 V. Most nearly, the total apparent power in polar form is:

A. 2,880 VA < -36.87⁰

B. 2,880 VA < 36.87⁰.

C. 8,640 VA < -36.87⁰.

D. 8,640 VA < 36.87⁰.

A

C

30
Q

The A phase of a balanced, three-phase, positive sequence system is taken as the reference. The line-to-neutral voltage is 277 V. The magnitude and phase angle of the CA line-to-line phase voltage is most nearly:

A. 277 V ∠ -90⁰.

B. 277 V ∠ 150⁰.

C. 480 V ∠ -90⁰.

D. 480 V ∠ 150⁰

A

D

31
Q

Find the voltage regulation of an ideal three-phase transformer rated as 4160/480V, 1,000kVA, 60Hz delivering current at rated secondary voltage to a purely resistive load of 200Ω. The voltage drop on the outgoing cable is 5V.

A. 0%

B. 1%

C. 3%

D. 5%

A

A

32
Q

A 50 Hz, 120 V alternator having a synchronous reactance of 0.65 Ω feeds a line that is resistive with an impedance of 0.2 Ω. The line is terminated with a 2:1 step up transformer. What should be the secondary impedance of the transformer in order to have maximum power transfer to the load?

A. .0.8 – j2.6 Ω

B. 1.1 + j0.65Ω

C. 0.8 + j2.6 Ω

D. 1.1 – j0.65Ω

A

A. .0.8 – j2.6 Ω

33
Q

An emergency generator provides power to process, which is fed by a 600A distribution panel. There are four pumps with 100 hp and 85% efficiency. All motors are three phase and operate at 480V with power factor 0.8. What is the minimum size generator needed?

A. 110 kVA

B. 350 kVA

C. 440 kVA

D. 500 kVA

A

C. 440 kVA

34
Q

An ideal single-phase transformer is rated 440:110V, 440V primary voltage and load on secondary 5.5𝛺. Find the primary and secondary currents.

A. 5 A primary, 20 A secondary

B. 5 A primary, 80 A secondary

C. 20 A primary, 5 A secondary

D. 80 A primary, 20 A secondary

A

A. 5 A primary, 20 A secondary

35
Q

An ideal transformer contains 100 primary coil turns and 400 secondary coil turns. The impedance measured at the primary terminals of the transformer is 100 – j50𝛺. Most nearly, the impedance in the secondary circuit is

A. 100 – j50𝛺

B. 400 – j200𝛺

C. 1600 – j800𝛺

D. 1600 + j800𝛺

A

C

36
Q

A sinusoidal voltage source has max 120V, max current 4A, and power factor leading at 0.85. Find the reactive power.

A. -126VAR

B. -63.2VAR

C. 63.2VAR

D. 126VAR

A

Correct Option: A

37
Q

In a purely inductive circuit, the ­­­­­­­­­­­­­__________ lags the __________ by _______.

A. voltage, current, 120°

B. current, voltage, 120°

C. voltage, current, 90°

D. current, voltage, 90°

A

D = In a purely inductive circuit, the current lags the voltage by 90°.

38
Q

A source serves loads A and B. Load A consumes 10 kVAR at 0.70 leading. Load B consumes 5 kW at 0.80 lagging. Calculate the real and reactive power delivered by the source.

A. 14.8 kW, 6.25 kVAR

B. 14.8 kW, -6.25 kVAR

C. 4.8 kW, 6.25 kVAR

D. 4.8 kW, -6.25 kVAR

A

B. 14.8 kW, -6.25 kVAR

39
Q

Calculate the value of the purely resistive load resistance needed to maximize power delivered from the circuit shown below where Vsource = 50 + j 80 V, and Zsource = 30 – j 40 W.

A. 10 𝛺
B. 50 𝛺
C. 30 -j40
D. 30 + j40𝛺

A

B. 50 𝛺

40
Q
A

FEexam (10 decks)

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