Deck 27: Three-Phase Circuits

Question 1

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

Answer 1

B

Question 2

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

A) line voltage
B) phase voltage

Answer 2

A

Question 3

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

A) delta
B) wye

Answer 3

B

Question 4

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 5

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

Question 6

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 7

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 8

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 9

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

A) 80°
B) 90°
C) 100°
D) 120°

Question 10

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

A) VA
B) W
C) VARs

Question 11

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 12

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

A) delta
B) wye

Question 13

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

Question 14

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 XL. The second load is delta-connected capacitors that have XC 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

Question 15

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 16

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 17

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 18

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 19

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 20

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

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.