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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. A control power transformer...
Four times less than that of the secondary.
Reverse any two-phase leads.
Tied together
Provides single-phase power to smaller loads such as lights - heaters - etc.

2. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
1 amp.
Circuit Breaker.
Tied together

3. What steps up or steps down AC voltage?
A device used to store electrical energy.
The motor rotation is reversed.
480 volts.
Transformer

4. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
An electrical shock hazard may exist.
Carotid pulse - which is located in the neck next to the Adam's apple.
Because less current flowing at high voltage causes less line loss per kilowatt.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

5. Current that changes periodically in direction is...
Two speeds
Squirrel-cage rotor.
Alternating current.
60 cycles per second (60 Hertz).

6. A burn causing a blister is classified as a...
Second-degree burn.
5:1.
A step-up transformer.
Current changing constantly in magnitude and periodically in direction.

7. A transformer with a 1:1 ratio is usually a (an)...
An electromagnet.
7 -630 volts.
Isolation transformer.
3 -979 Volts.

8. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Second-degree burn.
Allow for metering voltage and Current.
1 amp.

9. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Squirrel-cage rotor.
Grounded phase and ground fault devices were not operating.
The motor rotation is reversed.
Second-degree burn.

10. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Full-load - three-phase voltage.
480 volts.
A grounding resistor.
Current changing constantly in magnitude and periodically in direction.

11. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Continuous.
Inductance.
12 to 15 times a minute.

12. Current transformers - or CT's - are used to...
X.
Transformer
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Reverse its rotation.

13. When not connected to a load - the secondary leads of a current transformer should be...
Lower current and higher voltage.
Tied together
200 volts AC.
Shunted and taped

14. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Cut around clothing that has adhered to the burned area.
A step-up transformer.
Stator windings.
Current changing constantly in magnitude and periodically in direction.

15. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Reverse its rotation.
15 feet off the ground.
Zero amps.
75 turns.

16. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Potential transformer.
1 amp.
Grounded phase and ground fault devices were not operating.
The capacitors should be discharged.

17. Alternating current may be defined as...
Four times less than that of the secondary.
1 amp.
Current that changes constantly in magnitude and periodically in direction.
The capacitors should be discharged.

18. The most reliable method to use to determine whether or not the heart has stopped is by checking the...

19. Which of the following does a squirrel cage induction motor have?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Alternating current.
Stator windings.
A device used to store electrical energy.

20. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Potential transformer.
7 -630 volts.
Inductance.
277 volts.

21. To protect against over-current in a three-phase circuit over-current protection must be provided for...
The chest should rise or inflate.
60 cycles per second (60 Hertz).
At least two phases
150 turns.

22. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
Disconnect the frame-grounding conductor.
Inductance.
Derive a neutral for grounding purposes.

23. 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...
Draw less current.
Transformers.
Larger than that found in the primary winding.
Cut around clothing that has adhered to the burned area.

24. What is the first step to be followed in aiding a victim of electrical shock?
An electromagnet.
Kilovolt ampere (KVA).
Check to make sure that the victim is not still in contact with the electricity.
A step-up transformer.

25. At what rate should mouth-to-mouth resuscitation be applied?
Short-circuited after power has been removed.
12 to 15 times a minute.
At the source transformer.
25 feet.

26. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
800 volts.
600 volts.
Disconnect the frame-grounding conductor.
200 volts AC.

27. The standard frequency of alternating current used at a mine is...
600 volts.
Second-degree burn.
60 Hertz
Lower current and higher voltage.

28. What should the duty rating of a grounding resistor be?
The capacitors should be discharged.
Clamp-on ammeter.
Hyperventilation.
Continuous.

29. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Electro-magnetism for their operation.
Tied together
Lower current and higher voltage

30. 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.
Transformers.
277 volts.
7 -630 volts.

31. AC voltage is raised or lowered by means of...
Current that changes constantly in magnitude and periodically in direction.
Transformers.
5:1
Draw less current.

32. Compared to the secondary of a loaded - step-down transformer - the primary has...
Grounded phase and ground fault devices were not operating.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Lower current and higher voltage.
Tied together

33. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
15 feet off the ground.
Provides single-phase power to smaller loads such as lights - heaters - etc.
An electromagnet.
60 Hertz

34. The property of an electric circuit that opposes any change in current is called...
A device used to store electrical energy.
Check to make sure that the victim is not still in contact with the electricity.
Tied together
Inductance.

35. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
A device used to store electrical energy.
Nearly constant.
The chest should rise or inflate.
Second-degree burn.

36. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
12 to 15 times a minute.
Four times less than that of the secondary.
Provides single-phase power to smaller loads such as lights - heaters - etc.

37. What type of 3-phase AC motor is used most often around a mine?
The current will read zero.
Squirrel-cage induction motor.
Allow for metering voltage and Current.
Short-circuited after power has been removed.

38. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
1 amp.
X.
The chest should rise or inflate.

39. 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?
Second-degree burn.
The capacitors should be discharged.
3 -979 Volts.
Electro-magnetism for their operation.

40. What is the danger when working around capacitors?
An electrical shock hazard may exist.
Full-load - three-phase voltage.
Grounded phase and ground fault devices were not operating.
The motor rotation is reversed.

41. The speed of an alternating current - squirrel-cage induction motor is...
277 volts.
Two speeds
Transformers.
Nearly constant.

42. Not an acceptable method for the installations of surface transformers?
Continuous.
They must be installed at least 7 feet above the ground.
At the source transformer.
Full-load - three-phase voltage.

43. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Nearly constant.
The capacitors should be discharged.
At the source transformer.

44. Where should grounding resistors be located?
Current that changes constantly in magnitude and periodically in direction.
At the source transformer.
Kilovolt ampere (KVA).
Disconnect the frame-grounding conductor.

45. A solenoid coil is...
An electromagnet.
Hyperventilation.
75 turns.
100 volts.

46. How do you change the direction of a 3-phase AC motor?
1 amp.
Reverse any two-phase leads.
A.C.
100 turns.

47. 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...
60 Hertz
12 to 15 times a minute.
100 volts.
Four times less than that of the secondary.

48. The primary function of a power transformer is to provide...
Draw less current.
Full-load - three-phase voltage.
60 Hertz
Circuit Breaker.

49. The combined capacitance of two capacitors in parallel is...
Hyperventilation.
The sum of the two capacitors.
Allow for metering voltage and Current.
Electro-magnetism for their operation.

50. 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.
The sum of the two capacitors.
5:1
Potential transformer.
12 to 15 times a minute.