Week 5, Midterm, 1/14/2021

39. A three-phase transformer is constructed by winding three single-phase transformers on a single core. True False

True

18. *Assume that a single-phase transformer is a 2400/480 volt 15 kVA transformer. The full load current rating of the secondary winding is ____A. a. 31.25 b. 5 c. 3 d. 160

a. 31.25

1. 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-tapped b. tapped c. double-loaded d. center-tapped

d. center-tapped

13. When current transformers are used, the main power line is inserted through the opening in the transformer. The power line acts as the primary of the transformer and is considered to be two turns. True False

False

25. A total current-limiting effect is called impedance and is symbolized by the letter XL. True False

False

35. The exponential curve describes a rate of certain occurrences and is divided into four time constants. True False

False

41. A device that can be used for spike suppression in either direct- or alternating-current circuits is the diode. True False

False

43. A basic law of magnetism states that like poles attract and unlike poles repel. True False

False

6. In a delta-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

27. An inductor has an inductance of 0.8 H and is connected to a 120-V, 60-Hz line. How much current will flow in this circuit if the wire resistance of the inductor is negligible? a. 0.398 A b. 1.25 A c. 0.796 A d. 2.51 A

a. 0.398 A

24. 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 7 ohm each. Using Ohm's law, we determine the amount of phase current, IP(Load) = ____A. a. 19.8 b. 59.4 c. 34.3 d. 23.1

a. 19.8

7. If the primary section of a transformer is connected across part of a winding, and the secondary is connected across the entire winding, the secondary section contains more windings than the primary. This type of transformer is a step-down transformer. True False

False

32. An autotransformer has a 2:1 turns ratio. A voltage of 480 volts is connected across the entire winding. Since the transformer has a turns ratio of 2:1, a voltage of 240 volts will be supplied to the load. Ammeters connected in series with each winding indicate the current flow in the circuit. It is assumed that the load produces a current flow of 4 amperes on the secondary. A current flow of ____ amperes is supplied to the primary. a. 2 b. 12 c. 4 d. 6

a. 2

17. *In many larger installations, ____ are placed on each side of the transformer after it is deenergized to protect workers. a. grounds b. insulating materials c. wooden walkways d. plastic walkways

a. grounds

47. The current-limiting property of an inductor is called ______ and is symbolized by the letter___ a. reactance, X b. volts, V c. resistance, Ohms d. amps, A

a. reactance, X

11. A basic law of physics states that whenever an electric current flows through a conductor, a magnetic field is formed around the conductor. True False

True

19. The amount of current flow in a secondary circuit can be computed using the formula I = E / Z. True False

True

29. Harmonics are voltages or currents that operate at a frequency that is a multiple of the fundamental power frequency. If the fundamental power frequency is 60 Hz, for example, the second harmonic would be 120 Hz, and the third harmonic would be 180 Hz, and so on. True False

True

3. Transformer impedance is determined by the physical construction of the transformer. True False

True

30. Assume an inductor with a negligible resistance is connected to a 36-V, 400-Hz line. If the circuit has a current flow of 0.2 A, the inductance of the inductor is 0.0716 H. True False

True

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

a. 1.732

28. 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 ____. Select one: a. 5.196 b. 1.732 c. 0.577 d. 3

b. 1.732

33. 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. 480/277 c. 416/240 d. 440/220

b. 480/277

50. Three inductors are connected in parallel. Inductor 1 has an inductance of 2.8 H, inductor 2 has an inductance of 1.4 H, and inductor 3 has an inductance of 2.2 H. The total inductance of this circuit is ____H. a. 0.559 b. 1.525 c. 0.655 d. 6.4

c. 0.655

46. 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. 3 / (EPhase x IPhase) c. 3 x EPhase x IPhase d. (EPhase x IPhase) / 3

c. 3 x EPhase x IPhase

40. Assume that an isolation transformer has 240 turns of wire on the primary and 60 turns of wire on the secondary. This is a ratio of 4:1 (240/60 = 4). Now assume that 120 V is connected to the primary winding. The voltage of the secondary winding is ____V. a. 480 b. 240 c. 25 d. 30

d. 30

8. Each time constant in an exponential curve is equal to ____% of some value. a. 20.0 b. 33.3 c. 25.0 d. 63.2

d. 63.2

9. 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 ____. Select one: a. VA = (EL x IL) / 1.732 Incorrect b. VA = 1.732 / (EL x IL) c. VA = 1.732(EL + IL) d. VA = 1.732 x EL x IL

d. VA = 1.732 x EL x IL

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

d. delta

4. All values of a transformer are proportional to its turns ratio. True False

True

45. The secondary winding of a CT should never be opened when there is power applied to the primary. True False

True

16. *A transformer nameplate does not list the current rating of the windings. The current rating for a winding can be determined by dividing the kVA rating (listed in VA) by the ____. Select one: a. winding voltage b. secondary voltage c. primary voltage d. kilowatt rating

a. winding voltage

42. If a rotary switch were to be removed and replaced with a sliding tap that made contact directly with the transformer winding, the turns ratio could be adjusted continuously. This type of transformer is commonly referred to as a ____ depending on the manufacturer. a. Variate or Transtron b. Rotarystat c. Variate d. Variac or Powerstat

d. Variac or Powerstat

2. One of the most common three-phase connections to provide power for single phase loads is the _______. Typical voltages for this type of connection are 208/120 and 480/277.. a. Delta-Wye Connection with Neutral b. interconnected-wye c. Diametrical Connection d. interconnected-delta

a. Delta-Wye Connection with Neutral

49. ____ materials are either metal or nonmetal materials that cannot be magnetized. a. Diamagnetic b. Ferromagnetic c. Paramagnetic d. Antimagnetic

a. Diamagnetic

12. The percent impedance is measured by connecting a(n) ____ across the low-voltage terminals and a variable voltage source across the high-voltage terminals. Select one: a. voltmeter b. ammeter c. wattmeter d. KVARs meter

b. ammeter

5. *Some larger pole-mounted transformers have ____ welded to the outside of the tank to allow more surface area for the transfer of heat to the surrounding air. a. wings b. fins c. flaps d. fingers

b. fins

34. *A high-voltage tester, generally referred to as a "HiPot," develops a high voltage and measures any current leakage caused by weak or defective ____. a. switches b. insulation c. windings d. connections

b. insulation

36. In a pure inductive circuit, the current lags the voltage by ____. Select one: a. 45° b. 180° c. 90° d. 135°

c. 90°

22. The ____ or toroid core is one of the most efficient core designs available. a. control b. latent c. Tape-wound core d. distribution

c. Tape-wound core

23. *If ____ is not removed from a transformer, it will build up to an extremely high temperature and eventually destroy the insulation in the transformer. Select one: a. PCB b. water c. heat d. excess oil

c. heat

31. Harmonics cause an AC sine wave to become ____. a. almost flat b. regular c. none of the above (harmonics do not affect AC sine waves) d. distorted

d. distorted

37. *The ability of a fuse or circuit breaker to open under the ____ fault current is expressed as the interrupting rating of the device. a. required b. average c. minimum d. maximum

d. maximum

14. The ability of one coil to induce a voltage into another coil is called ________. a. inductive reactance b. cooperative inductance c. capacitive coupling d. mutual induction

d. mutual induction

15. The control transformer is used to_______ the line voltage. a. raise b. stabilize c. distort d. reduce

d. reduce

48. *NEC 210.19 (A)(1)(a) states that the rating of a branch-circuit shall not be ____. a. more than continuous load plus 125% of the noncontinuous load b. less than noncontinuous load plus 110% of the continuous load c. less than noncontinuous load plus 125% of the continuous load d. more than continuous load plus 150% of the noncontinuous load

c. less than noncontinuous load plus 125% of the continuous load

10. Three factors determine the amount of voltage that will be induced in a conductor: the number of turns of wire, the strength of the magnetic field (flux density), and the ____ of the cutting action. Select one: a. movement b. intensity c. speed Correct d. direction

c. speed

38. 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

c. star

26. The primary current can be computed by using the formula ____. a. ES / EP = IS / IP b. EP / ES = IS x IP c. EP x ES = IS / IP d. EP / ES = IS / IP

d. EP / ES = IS / IP

20. 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. triple consolidated b. temporary c. consolidated d. three-phase

d. three-phase