Deck 7: Three-Phase Circuits

Question 1

The reason for the difference between line current and phase current in a delta connection is that current flows through different windings at different times in a three-phase circuit.

Answer 1

True

Question 2

A three-phase motor is connected to a 480V, 60-Hz line. A clamp-on ammeter indicates a running current of 68A at full load, and a three-phase wattmeter indicates a true power of 40,277W. To solve this problem, first find the amount of apparent power in the circuit using the formula ____.

A) VA = 1.732 / (EL x IL)
B) VA = (EL x IL) / 1.732
C) VA = 1.732(EL + IL)
D) VA = 1.732 x EL x IL

Answer 2

D

Question 3

A formula used to compute the voltage in a wye-connected system is ____.

A) EPhase = ELine / 1.732
B) EPhase = ELine x 1.732
C) EPhase = 1.732 / ELine
D) ELine = EPhase / 1.732

Answer 3

A

Question 4

The line current of a delta connection is higher than the phase current by a factor of the square root of 3 (1.732).

Question 5

The wye, or ____, connection is made by connecting one end of each of the three-phase windings together.

A) triangle
B) square
C) star
D) rectangle

Question 6

Correcting the power factor of a three-phase circuit is similar to the procedure used to correct the power factor of a single-phase circuit.

Question 7

The amount of phase current can now be determined using the Ohm's law equation, IP(Load) = Z / EP(Load).

Question 8

In a wye-connected system, the line voltage is lower than the phase voltage by a factor of the square root of 3 (1.732).

Question 9

The phase windings of an alternator are connected in wye. The alternator produces a line voltage of 440V and supplies power to two resistive loads. One load contains resistors with a value of 4 ohm each, connected in wye. The second load contains resistors with a value of 6 ohm each, connected in delta. Both loads are connected directly to the output of the alternator. To determine the amount of line current needed for load 2, use the following formula: IL(Load 2) = IP(Load 2) x ____.

A) 1.732
B) 3
C) 73.33
D) 440

Question 10

The horsepower rating of three-phase motors and the kilovolt-amp rating of three-phase transformers are about 75% greater than for single-phase motors or transformers with a similar frame size.

Question 11

A delta-connected alternator is connected to a wye-connected resistive load. The alternator produces a line voltage of 240V, and the resistors have a value of 6 ohm each. If EL(Load) = 240V, then EP(Load) = ____V.

A) 80
B) 138.57
C) 160
D) 415.68

Question 12

If the phase values of voltage and current in a three-phase circuit are known, the apparent power can be computed using the formula VA = ____.

A) 3 x EPhase x IPhase
B) (EPhase x IPhase) / 3
C) 3 / (EPhase x IPhase)
D) EPhase x IPhase

Question 13

A wye-connected three-phase alternator supplies power to a delta-connected resistive load. The alternator has a line voltage of 480V. Each resistor of the delta load has 8 ohm of resistance. The load is connected directly to the alternator. Therefore, the line voltage supplied by the alternator is 160V.

Question 14

One reason why three-phase power is superior to single-phase power is that in a balanced three-phase system, the conductors need be only about ____% of the size of conductors for a single-phase two-wire system of the same kVA (kilovolt-amp) rating.

A) 33.3
B) 60
C) 75
D) 85

Question 15

The phase windings of an alternator are connected in wye. The alternator produces a line voltage of 440V and supplies power to two resistive loads. One load contains resistors with a value of 4 ohm each, connected in wye. The second load contains resistors with a value of 6 ohm each, connected in delta. Both loads are connected directly to the output of the alternator. The line voltage for both loads 1 and 2 will be the same, ____V.

A) 73.33
B) 219.09
C) 220
D) 440

Question 16

Motor power factor can be computed using this formula: PF = VA / P.

Question 17

The inductive VARs in a three-phase circuit can be computed using the formula VARsL equals the square root of VA2 - P2.

Question 18

A delta-connected alternator is connected to a wye-connected resistive load. The alternator produces a line voltage of 240V, and the resistors have a value of 6 ohm each. Using Ohm's law, we determine the amount of phase current, IP(Load) = ____A.

A) 13.3
B) 23.1
C) 26.6
D) 69.28

Question 19

In a(n) ____ connection, line voltage and phase voltage are the same.

A) alpha
B) beta
C) delta
D) gamma

Question 20

In a wye-connected system, phase current and line current are the same.