Week 3 Quiz "Electricity" Units 26 & 28

9. 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). True False

False

4. If three-phase transformation is needed and a three-phase transformer of the proper size and turns ratio is not available, three single-phase transformers can be connected to form a ____ bank. a. three-phase b. triple consolidated c. temporary d. consolidated

a. three-phase

17. The primary windings of three single-phase transformers have been connected to form a wye connection. In a wye connection, the phase voltage is less than the line voltage by a factor of ____. a. 1.732 b. 5.196 c. 3 d. 0.577

a. 1.732

1. When a line is known as a high leg, it is because the voltage between this line and the neutral conductor will be higher than the voltage between the neutral and either of the other two conductors. True False

True

14. 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. True False

True

19. Three-phase transformer connections are often used to supply single-phase loads, which tend to unbalance the system. True False

True

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

True

7. A three-phase transformer is connected in a delta-delta configuration. The load is connected as a wye, and each phase has an impedance of 7 ohm. The primary is connected to a line voltage of 4160 volts, and the secondary line voltage is 440 volts. Therefore, the turns ratio is 9.45:1. True False

True

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

True

3. A dielectric oil provides electrical insulation between the windings and the ____. a. case b. three-phase bank c. transformer d. leads

a. case

16. In a(n) ____ connection, line voltage and phase voltage are the same. a. alpha b. delta c. gamma d. beta

b. delta

10. When a transformer bank must supply both three-phase and single-phase loads, the utility company will often provide an open-delta connection with one transformer ____. a. double-loaded b. tapped c. double-tapped d. center-tapped

d. center-tapped

15. Only phase values of ____ can be used when computing transformer values. a. voltage b. current c. wattage d. voltage and current

d. voltage and current

2. 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. 40 b. 23.1 c. 69.28 d. 13.3

b. 23.1

5. The wye, or ____, connection is made by connecting one end of each of the three-phase windings together. a. rectangle b. star c. triangle d. square

b. star

11. 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. EPhase x IPhase b. (EPhase x IPhase) / 3 c. 3 x EPhase x IPhase d. 3 / (EPhase x IPhase)

c. 3 x EPhase x IPhase

6. High-leg voltage can be computed by increasing the single-phase center-tapped voltage value by a factor of ____. a. 9 b. 5.196 c. 3 d. 1.732

d. 1.732

13. 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. 415.68 c. 160 d. 138.57

d. 138.57

18. Typical voltages for a three-phase transformer connection with a delta-connected primary and wye-connected secondary are 208/120 and ____. a. 104/180 b. 416/240 c. 440/220 d. 480/277

d. 480/277

12. A formula used to compute the voltage in a wye-connected system is ____. a. EPhase = 1.732 / ELine b. ELine = EPhase / 1.732 c. EPhase = ELine x 1.732 d. EPhase = ELine / 1.732

d. EPhase = ELine / 1.732