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AC Theory

Instructions:

Answer 50 questions in 15 minutes.
If you are not ready to take this test, you can study here.
Match each statement with the correct term.
Don't refresh. All questions and answers are randomly picked and ordered every time you load a test.

This is a study tool. The 3 wrong answers for each question are randomly chosen from answers to other questions. So, you might find at times the answers obvious, but you will see it re-enforces your understanding as you take the test each time.

1. The primary function of a power transformer is to provide...
Second-degree burn.
100 turns.
Full-load - three-phase voltage.
75 turns.

2. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
The capacitors should be discharged.
100 volts.
600 volts.
Grounded phase and ground fault devices were not operating.

3. What value of voltage will be measured from phase-to-phase - in a wye-connected secondary - if 2 -300 volts is measured from phase-to-ground?
3 -979 Volts.
600 volts.
Continuous.
200 volts AC.

4. Compared to the secondary of a loaded - step-down transformer - the primary has...
Reverse any two-phase leads.
Lower current and higher voltage.
Continuous.
Inductance.

5. What should the duty rating of a grounding resistor be?
60 cycles per second (60 Hertz).
Check to make sure that the victim is not still in contact with the electricity.
Continuous.
Reverse its rotation.

6. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
2000 watts
At least two phases
The current will read zero.
Check to make sure that the victim is not still in contact with the electricity.

7. 460 volts AC is required to operate two motors - one connected in single-phase and the other connected in three-phase. The three-phase motor will...
Allow for metering voltage and Current.
Current changing constantly in magnitude and periodically in direction.
100 volts.
Draw less current.

8. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Inductance.
Electro-magnetism for their operation.
15 feet off the ground.
277 volts.

9. A burn causing a blister is classified as a...
12 to 15 times a minute.
Alternating current.
Because less current flowing at high voltage causes less line loss per kilowatt.
Second-degree burn.

10. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The capacitors should be discharged.
The chest should rise or inflate.
600 volts.
Shunted and taped

11. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
The current will read zero.
Two speeds
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
12 to 15 times a minute.

12. A transformer with a 1:1 ratio is usually a (an)...
Alternating current.
Draw less current.
An electrical shock hazard may exist.
Isolation transformer.

13. A transformer with 3000 turns in the primary winding has a primary voltage of 4 -800 volts and a secondary voltage of 240 volts. How many turns are in the secondary winding?
150 turns.
Zero amps.
60 cycles per second (60 Hertz).
At least two phases

14. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
480 volts.
Grounded phase and ground fault devices were not operating.
The capacitors should be discharged.

15. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Squirrel-cage induction motor.
Provides single-phase power to smaller loads such as lights - heaters - etc.
A step-up transformer.

16. The secondary windings on a 10:1 step-down transformer will have a wire size...
5:1
They should be made by the same manufacturer.
Kilovolt ampere (KVA).
Larger than that found in the primary winding.

17. A transformer with 1 -500 turns in the primary winding has a primary voltage of 2 -400 volts and a secondary voltage of 120 volts. How many turns are in the secondary winding?
75 turns.
Second-degree burn.
Because less current flowing at high voltage causes less line loss per kilowatt.
Disconnect the frame-grounding conductor.

18. If any two-phase wires are reversed on a three-phase motor - the motor will...
1 amp.
Reverse its rotation.
100 turns.
An electromagnet.

19. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
150 turns.
Draw less current.
Four times less than that of the secondary.

20. Define alternating current...
Circuit Breaker.
No.6 AWG.
Current changing constantly in magnitude and periodically in direction.
Clamp-on ammeter.

21. The property of an electric circuit that opposes any change in current is called...
At least two phases
Inductance.
Check to make sure that the victim is not still in contact with the electricity.
The capacitors should be discharged.

22. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Electro-magnetism for their operation.
They should be made by the same manufacturer.
The capacitors should be discharged.
60 cycles per second (60 Hertz).

23. The purpose of a zig-zag transformer is to...
Check to make sure that the victim is not still in contact with the electricity.
Tied together
Derive a neutral for grounding purposes.
7 -630 volts.

24. A 3-phase - 2 speed - constant torque motor is running at fast speed. The motor windings are connected wye for fast speed. Motor leads T4 - T5 and T6 are connected to the line leads. Motor leads T1 - T2 and T3 should be...
Kilovolt ampere (KVA).
Inductance.
800 volts.
Tied together

25. Current in an AC circuit may be measured by...
Stator windings.
Two speeds
Shunted and taped
Clamp-on ammeter.

26. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Four times less than that of the secondary.
60 Hertz
Inductance.

27. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Potential transformer.
Stator windings.
75 turns.
7 -630 volts.

28. Find the turns ratio of a single-phase transformer with the following values: Primary power of 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
25 feet.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Short-circuited after power has been removed.
5:1.

29. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A.C.
Draw less current.
25 feet.

30. High voltage is used to transmit power over long distances...
480 volts.
A device used to store electrical energy.
The sum of the two capacitors.
Because less current flowing at high voltage causes less line loss per kilowatt.

31. What type of 3-phase AC motor is used most often around a mine?
No.6 AWG.
Squirrel-cage induction motor.
Inductance.
The sum of the two capacitors.

32. All transformers step voltage up or down by...
Inductance.
75 turns.
X.
Disconnect the frame-grounding conductor.

33. Not an acceptable method for the installations of surface transformers?
A device used to store electrical energy.
2000 watts
Capacitance.
They must be installed at least 7 feet above the ground.

34. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
25 feet.
100 volts.
The chest should rise or inflate.

35. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
100 turns.
Two speeds
1 amp.

36. The standard frequency of power supplied by the power company is...
They should be made by the same manufacturer.
60 cycles per second (60 Hertz).
Disconnect the frame-grounding conductor.
Reverse its rotation.

37. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
The motor rotation is reversed.
Squirrel-cage rotor.
At least two phases

38. A transformer has a primary voltage of 2 -400 volts and a secondary reading of 120 volts. If there are 1 -500 turns on the primary - how many turns are on the secondary?
A grounding resistor.
A device used to store electrical energy.
H terminals.
75 turns.

39. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
600 volts.
Four times less than that of the secondary.
X.

40. The property of an electrical circuit that opposes any change in voltage is called...
Potential transformer.
Capacitance.
Allow for metering voltage and Current.
Hyperventilation.

41. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
100 volts.
X.
Derive a neutral for grounding purposes.
800 volts.

42. Two 10-hp induction motors are operating on 460 volts AC. One motor is connected by single- phase; the other is connected in three-phase. The three-phase will...
Clamp-on ammeter.
60 cycles per second (60 Hertz).
Hyperventilation.
Draw less current.

43. Under fault conditions - the grounding resistor in a high voltage circuit limits the voltage drop in the grounding circuit external to the resistor to not more than...
5:1
100 volts.
Draw less current.
Capacitance.

44. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
The capacitors should be discharged.
15 feet off the ground.
The motor rotation is reversed.

45. Find the turns ratio of a single-phase transformer with the following values: Primary power of 5KW - secondary current of 100 amps - and primary voltage of 250 volts.
Short-circuited after power has been removed.
Continuous.
Current changing constantly in magnitude and periodically in direction.
5:1

46. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Reverse its rotation.
A step-up transformer.
Shunted and taped
No.6 AWG.

47. What is the first step to be followed in aiding a victim of electrical shock?
800 volts.
Grounded phase and ground fault devices were not operating.
Check to make sure that the victim is not still in contact with the electricity.
Derive a neutral for grounding purposes.

48. At what rate should mouth-to-mouth resuscitation be applied?
12 to 15 times a minute.
Four times less than that of the secondary.
5:1
Reverse its rotation.

49. What position of a selector switch using a voltmeter would you select to check a 120-volt AC circuit to obtain the most accurate reading?
Check to make sure that the victim is not still in contact with the electricity.
They must be installed at least 7 feet above the ground.
200 volts AC.
Alternating current.

50. The standard frequency of alternating current used at a mine is...
No.6 AWG.
Capacitance.
Transformer
60 Hertz