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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. Three-phase AC underground cables must be provided with overcurrent protection using a...
25 feet.
Hyperventilation.
Tied together
Circuit Breaker.

2. The primary function of a power transformer is to provide...
Check to make sure that the victim is not still in contact with the electricity.
7 -630 volts.
Current changing constantly in magnitude and periodically in direction.
Full-load - three-phase voltage.

3. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Cut around clothing that has adhered to the burned area.
Transformer
Disconnect the frame-grounding conductor.
Reverse its rotation.

4. The primary leads of a single-phase transformer should be connected to the...
Full-load - three-phase voltage.
H terminals.
Four times less than that of the secondary.
Short-circuited after power has been removed.

5. 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...
200 volts AC.
Draw less current.
Four times less than that of the secondary.
2000 watts

6. 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.
Because less current flowing at high voltage causes less line loss per kilowatt.
The capacitors should be discharged.
Kilovolt ampere (KVA).

7. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
At the source transformer.
Kilovolt ampere (KVA).
800 volts.

8. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
150 turns.
5:1.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

9. What should the duty rating of a grounding resistor be?
3 -979 Volts.
Two speeds
The capacitors should be discharged.
Continuous.

10. Not an acceptable method for the installations of surface transformers?
Lower current and higher voltage
150 turns.
75 turns.
They must be installed at least 7 feet above the ground.

11. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
Shunted and taped
Current that changes constantly in magnitude and periodically in direction.
The current will read zero.

12. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
Kilovolt ampere (KVA).
Transformers.
Full-load - three-phase voltage.

13. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Grounded phase and ground fault devices were not operating.
The chest should rise or inflate.
An electromagnet.
X.

14. The standard frequency of power supplied by the power company is...
A step-up transformer.
Isolation transformer.
60 cycles per second (60 Hertz).
The current will read zero.

15. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
No.6 AWG.
Derive a neutral for grounding purposes.
The chest should rise or inflate.
Second-degree burn.

16. Which of the following does a squirrel cage induction motor have?
Transformer
Stator windings.
They should be made by the same manufacturer.
Larger than that found in the primary winding.

17. Current transformers - or CT's - are used to...
Continuous.
Squirrel-cage induction motor.
Draw less current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

18. The standard frequency of alternating current used at a mine is...
60 Hertz
A grounding resistor.
The sum of the two capacitors.
The motor rotation is reversed.

19. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
60 cycles per second (60 Hertz).
Isolation transformer.
3 -979 Volts.
2000 watts

20. 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...
An electromagnet.
Draw less current.
At the source transformer.
60 cycles per second (60 Hertz).

21. What type of 3-phase AC motor is used most often around a mine?
It provides good speed control.
Kilovolt ampere (KVA).
Squirrel-cage induction motor.
Derive a neutral for grounding purposes.

22. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Current changing constantly in magnitude and periodically in direction.
15 feet off the ground.
75 turns.

23. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
60 cycles per second (60 Hertz).
Short-circuited after power has been removed.
480 volts.
Kilovolt ampere (KVA).

24. The speed of an alternating current - squirrel-cage induction motor is...
1 amp.
A device used to store electrical energy.
Nearly constant.
15 feet off the ground.

25. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Cut around clothing that has adhered to the burned area.
Allow for metering voltage and Current.
Stator windings.
Short-circuited after power has been removed.

26. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Grounded phase and ground fault devices were not operating.
Circuit Breaker.
Two speeds
They must be installed at least 7 feet above the ground.

27. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Capacitance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
They should be made by the same manufacturer.
25 feet.

28. 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?
Cut around clothing that has adhered to the burned area.
600 volts.
An electromagnet.
3 -979 Volts.

29. 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?
75 turns.
1 amp.
12 to 15 times a minute.
Inductance.

30. The property of an electric circuit that opposes any change in current is called...
Inductance.
Draw less current.
Provides single-phase power to smaller loads such as lights - heaters - etc.
60 Hertz

31. 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?
Shunted and taped
Isolation transformer.
3 -979 Volts.
Derive a neutral for grounding purposes.

32. High voltage is used to transmit power over long distances...
The motor rotation is reversed.
5:1
5:1.
Because less current flowing at high voltage causes less line loss per kilowatt.

33. The transformer used only to measure voltage is a...
Potential transformer.
Allow for metering voltage and Current.
Derive a neutral for grounding purposes.
Full-load - three-phase voltage.

34. How do you change the direction of a 3-phase AC motor?
No.6 AWG.
Reverse any two-phase leads.
Full-load - three-phase voltage.
Current that changes constantly in magnitude and periodically in direction.

35. The combined capacitance of two capacitors in parallel is...
Cut around clothing that has adhered to the burned area.
60 cycles per second (60 Hertz).
The sum of the two capacitors.
Shunted and taped

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

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37. What is the first step to be followed in aiding a victim of electrical shock?
Tied together
A step-up transformer.
Clamp-on ammeter.
Check to make sure that the victim is not still in contact with the electricity.

38. Define alternating current...
Short-circuited after power has been removed.
Current changing constantly in magnitude and periodically in direction.
The capacitors should be discharged.
Lower current and higher voltage.

39. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Inductance.
7 -630 volts.
100 volts.
150 turns.

40. How should clothing be removed from a burn?
100 turns.
Alternating current.
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage.

41. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
At the source transformer.
5:1.
An electromagnet.

42. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Draw less current.
Isolation transformer.
The motor rotation is reversed.
15 feet off the ground.

43. 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.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Alternating current.
Nearly constant.

44. A control power transformer...
Zero amps.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Continuous.
Inductance.

45. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
60 Hertz
No.6 AWG.
60 cycles per second (60 Hertz).
X.

46. All transformers step voltage up or down by...
Inductance.
600 volts.
A step-up transformer.
Two speeds

47. 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?
800 volts.
2000 watts
Shunted and taped
7 -630 volts.

48. 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...
Transformer
Full-load - three-phase voltage.
100 volts.
Circuit Breaker.

49. A transformer has a primary voltage of 2 -800 volts and a secondary reading of 140 volts. If there are 2000 turns on the primary - how many turns are on the secondary?
H terminals.
100 turns.
At the source transformer.
A grounding resistor.

50. AC voltage is raised or lowered by means of...
Transformers.
150 turns.
Squirrel-cage induction motor.
The capacitors should be discharged.

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AC Theory

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