Deck 27: Three-Phase Circuits

Question 1

A three-phase connection in which three separate inductive loads have been connected together is a(n) _____ connection.

A) delta
B) wye
C) open delta
D) open wye

Answer 1

A

Question 2

In a three-phase circuit, the true power (watts) is found by _____.

A) multiplying line voltage by line current
B) multiplying apparent power by the power factor
C) dividing power factor by the apparent power
D) multiplying phase voltage by phase current

Answer 2

B

Question 3

A wye-connected alternator that has phase voltages of 277 V is feeding two loads. The first load is wye-connected inductors that have 40 Ω of X_L. The second load is delta-connected capacitors that have X_C of 60 Ω . How much is the alternator phase current?

A) 6.93 A
B) 14.93 A
C) 24.62 A
D) 28 A

Answer 3

A

Question 4

In a three-phase connection, what is the voltage measured across a single winding?

A) line voltage
B) phase voltage

Question 5

A delta-connected set of windings has a phase current of 10 A. What is the line current?

A) 5.77 A
B) 10 A
C) 17.32 A
D) 30 A

Question 6

In calculating apparent power (VA) in a three-phase circuit, the phase values of voltage and current are multiplied by _____.

A) three
B) four
C) five
D) six

Question 7

If a three-phase load is a pure reactive load, then the power calculated has units of _____.

A) VA
B) W
C) VARs

Question 8

The three-phase connection in which one end of each of the windings are connected together is called a(n) _____ connection.

A) delta
B) wye
C) open delta
D) open wye

Question 9

A delta connected set of windings has a phase voltage of 100 V. What is the line voltage?

A) 57.7 V
B) 100 V
C) 173.2 V
D) 300 V

Question 10

In a wye-connected system, the line voltage is higher than the phase voltage by a factor of _____.

A) square root of  two
B) square root of three
C) two
D) three

Question 11

The horsepower rating of three-phase motors and the kilovolt-ampere rating of three-phase transformers are about _____ greater than for single-phase motors or transformers with a similar frame size.

A) 75 percent
B) 100 percent
C) 150 percent
D) 200 percent

Question 12

Line current and phase current are the same in the _____ configuration.

A) delta
B) wye

Question 13

In a balanced three-phase system, the conductors need be only about _____ the size of conductors for a single-phase, two-wire system of the same kilovolt-ampere (kVA) rating.

A) 25 percent
B) 30 percent
C) 50 percent
D) 75 percent

Question 14

When calculating the power of a pure resistive load, the voltage and current are in phase with each other and the power factor is _____.

A) one
B) two
C) three
D) four

Question 15

A delta-connected three-phase alternator is driving a wye-connected resistive load. The alternator output voltage is 480 V. The load resistors are 12 Ω each. What is the phase current in the alternator?

A) 13.33 A
B) 23.09 A
C) 39.28 A
D) 40 A

Question 16

Line voltage and phase voltage are the same in the _____ connection.

A) delta
B) wye

Question 17

In three-phase power systems, the delivered power _____.

A) falls to zero three times per cycle
B) falls to zero once per cycle
C) never falls to zero
D) falls to zero at least three times per cycle

Question 18

A delta connected set of windings has a phase voltage of 480 V. What is the line voltage?

A) 277.14 V
B) 480 V
C) 831.36 V
D) 1440 V

Question 19

Three-phase voltages are _____ out of phase with each other.

A) 80°
B) 90°
C) 100°
D) 120°

Question 20

In a three-phase connection, what is the voltage measured between the lines?

A) line voltage
B) phase voltage

Question 21

The alternator must supply the _____ for each of the loads.

A) capacitance
B) line current
C) inductance
D) voltage

Question 22

Another name for the three-phase wye connection is the _______________ connection.