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AC Theory

Instructions:

Answer 50 questions in 15 minutes.
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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. What steps up or steps down AC voltage?
Transformer
Potential transformer.
Nearly constant.
Disconnect the frame-grounding conductor.

2. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Alternating current.
Nearly constant.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The capacitors should be discharged.

3. The standard frequency of alternating current used at a mine is...
An electromagnet.
7 -630 volts.
60 Hertz
Allow for metering voltage and Current.

4. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Carotid pulse - which is located in the neck next to the Adam's apple.
Reverse any two-phase leads.
X.
An electrical shock hazard may exist.

5. 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?
They must be installed at least 7 feet above the ground.
3 -979 Volts.
277 volts.
75 turns.

6. The property of an electric circuit that opposes any change in current is called...
25 feet.
Inductance.
Reverse its rotation.
5:1.

7. What is the first step to be followed in aiding a victim of electrical shock?
480 volts.
Check to make sure that the victim is not still in contact with the electricity.
The motor rotation is reversed.
An electromagnet.

8. The primary leads of a single-phase transformer should be connected to the...
They should be made by the same manufacturer.
H terminals.
Inductance.
Isolation transformer.

9. A transformer has 2000 turns in the primary and 500 turns in the secondary. The primary voltage is 3 -200 volts. What is the secondary voltage?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
800 volts.
Shunted and taped
5:1.

10. Compared to the secondary of a loaded - step-down transformer - the primary has...
A grounding resistor.
2000 watts
Reverse its rotation.
Lower current and higher voltage

11. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Transformer
480 volts.
Cut around clothing that has adhered to the burned area.
25 feet.

12. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Short-circuited after power has been removed.
They should be made by the same manufacturer.
Capacitance.
480 volts.

13. What type of 3-phase AC motor is used most often around a mine?
At the source transformer.
A device used to store electrical energy.
7 -630 volts.
Squirrel-cage induction motor.

14. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
75 turns.
Current changing constantly in magnitude and periodically in direction.
Inductance.

15. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Draw less current.
Four times less than that of the secondary.
Current changing constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.

16. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Circuit Breaker.
480 volts.
Second-degree burn.
15 feet off the ground.

17. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Derive a neutral for grounding purposes.
Full-load - three-phase voltage.
Second-degree burn.
The chest should rise or inflate.

18. The secondary windings on a 10:1 step-down transformer will have a wire size...
It provides good speed control.
200 volts AC.
Larger than that found in the primary winding.
Disconnect the frame-grounding conductor.

19. 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.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Alternating current.
Squirrel-cage induction motor.

20. The speed of an alternating current - squirrel-cage induction motor is...
The motor rotation is reversed.
60 Hertz
Kilovolt ampere (KVA).
Nearly constant.

21. Compared to the secondary of a loaded - step-down transformer - the primary has...
15 feet off the ground.
75 turns.
Lower current and higher voltage.
X.

22. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
At the source transformer.
X.
15 feet off the ground.
No.6 AWG.

23. Current transformers - or CT's - are used to...
The capacitors should be discharged.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
12 to 15 times a minute.
100 volts.

24. Where should grounding resistors be located?
150 turns.
At least two phases
At the source transformer.
Draw less current.

25. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
75 turns.
Capacitance.
A grounding resistor.
75 turns.

26. A transformer has 1 -600 turns in the primary and 400 turns in the secondary. The primary voltage is 2 -400 volts. What is the secondary voltage?
A device used to store electrical energy.
Isolation transformer.
600 volts.
Tied together

27. How do you change the direction of a 3-phase AC motor?
5:1
At the source transformer.
Reverse any two-phase leads.
150 turns.

28. The primary function of a power transformer is to provide...
Reverse its rotation.
A grounding resistor.
3 -979 Volts.
Full-load - three-phase voltage.

29. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
A step-up transformer.
The motor rotation is reversed.
Grounded phase and ground fault devices were not operating.
200 volts AC.

30. 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?
200 volts AC.
Cut around clothing that has adhered to the burned area.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A grounding resistor.

31. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Lower current and higher voltage.
Because less current flowing at high voltage causes less line loss per kilowatt.
Grounded phase and ground fault devices were not operating.
Isolation transformer.

32. What is a capacitor?
Squirrel-cage rotor.
Clamp-on ammeter.
A device used to store electrical energy.
A grounding resistor.

33. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
Alternating current.
Shunted and taped
Draw less current.

34. Not a symptom of electrical shock?.
800 volts.
Hyperventilation.
Nearly constant.
60 Hertz

35. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Draw less current.
100 turns.
Reverse its rotation.

36. Define alternating current...
They should be made by the same manufacturer.
Allow for metering voltage and Current.
100 turns.
Current changing constantly in magnitude and periodically in direction.

37. 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...
Tied together
A device used to store electrical energy.
150 turns.
75 turns.

38. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Clamp-on ammeter.
At the source transformer.
Reverse its rotation.

39. What type motor is a wound-rotor induction motor?
100 turns.
A.C.
No.6 AWG.
Electro-magnetism for their operation.

40. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Clamp-on ammeter.
3 -979 Volts.
15 feet off the ground.
Larger than that found in the primary winding.

41. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Derive a neutral for grounding purposes.
200 volts AC.
A step-up transformer.
277 volts.

42. What should the duty rating of a grounding resistor be?
Continuous.
The motor rotation is reversed.
Transformer
Current that changes constantly in magnitude and periodically in direction.

43. 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.
At the source transformer.
Inductance.
A.C.
5:1

44. All transformers step voltage up or down by...
Electro-magnetism for their operation.
100 volts.
It provides good speed control.
Inductance.

45. Current in an AC circuit may be measured by...
The motor rotation is reversed.
Clamp-on ammeter.
12 to 15 times a minute.
15 feet off the ground.

46. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Cut around clothing that has adhered to the burned area.

47. A wound-rotor induction motor has all the following EXCEPT...
60 cycles per second (60 Hertz).
5:1
Alternating current.
Squirrel-cage rotor.

48. 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?
Squirrel-cage induction motor.
Derive a neutral for grounding purposes.
The capacitors should be discharged.
3 -979 Volts.

49. The purpose of a zig-zag transformer is to...
A grounding resistor.
75 turns.
Derive a neutral for grounding purposes.
Full-load - three-phase voltage.

50. At what rate should mouth-to-mouth resuscitation be applied?
25 feet.
Tied together
Grounded phase and ground fault devices were not operating.
12 to 15 times a minute.

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AC Theory

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