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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. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Squirrel-cage rotor.
Nearly constant.
Grounded phase and ground fault devices were not operating.
Allow for metering voltage and Current.

2. What type of 3-phase AC motor is used most often around a mine?
75 turns.
600 volts.
Squirrel-cage induction motor.
277 volts.

3. The primary leads of a single-phase transformer should be connected to the...
277 volts.
H terminals.
Circuit Breaker.
Zero amps.

4. Not an acceptable method for the installations of surface transformers?
No.6 AWG.
100 turns.
They must be installed at least 7 feet above the ground.
Second-degree burn.

5. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
75 turns.
No.6 AWG.
Squirrel-cage rotor.
5:1.

6. Operating coils - motors - transformers - relays and solenoids depend upon...
Lower current and higher voltage.
Electro-magnetism for their operation.
A device used to store electrical energy.
Potential transformer.

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...
Draw less current.
They should be made by the same manufacturer.
An electromagnet.
At the source transformer.

8. At what rate should mouth-to-mouth resuscitation be applied?
Current that changes constantly in magnitude and periodically in direction.
7 -630 volts.
Isolation transformer.
12 to 15 times a minute.

9. A burn causing a blister is classified as a...
Second-degree burn.
Because less current flowing at high voltage causes less line loss per kilowatt.
25 feet.
Stator windings.

10. What steps up or steps down AC voltage?
Transformer
Capacitance.
Grounded phase and ground fault devices were not operating.
75 turns.

11. 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?
A.C.
A step-up transformer.
3 -979 Volts.
60 cycles per second (60 Hertz).

12. Compared to the secondary of a loaded - step-down transformer - the primary has...
Capacitance.
Draw less current.
Lower current and higher voltage.
Lower current and higher voltage

13. The main function of CT's and PT's on a main substation is to...
Reverse any two-phase leads.
15 feet off the ground.
600 volts.
Allow for metering voltage and Current.

14. AC voltage is raised or lowered by means of...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Transformers.
A step-up transformer.
They must be installed at least 7 feet above the ground.

15. What is a capacitor?
Current changing constantly in magnitude and periodically in direction.
Draw less current.
Potential transformer.
A device used to store electrical energy.

16. Which of the following does a squirrel cage induction motor have?
75 turns.
Lower current and higher voltage.
Stator windings.
Potential transformer.

17. Current that changes periodically in direction is...
Alternating current.
277 volts.
Transformers.
They should be made by the same manufacturer.

18. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Potential transformer.
At least two phases
Circuit Breaker.
25 feet.

19. A control power transformer...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
600 volts.
It provides good speed control.
Provides single-phase power to smaller loads such as lights - heaters - etc.

20. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Transformer
Zero amps.
2000 watts
Reverse any two-phase leads.

21. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
1 amp.
Isolation transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.

22. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
60 cycles per second (60 Hertz).
It provides good speed control.
Zero amps.
Nearly constant.

23. What is the danger when working around capacitors?
An electrical shock hazard may exist.
Potential transformer.
Draw less current.
X.

24. What type motor is a wound-rotor induction motor?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
A.C.
Squirrel-cage rotor.
A step-up transformer.

25. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
It provides good speed control.
Grounded phase and ground fault devices were not operating.
7 -630 volts.
Lower current and higher voltage.

26. The property of an electric circuit that opposes any change in current is called...
Squirrel-cage induction motor.
Inductance.
5:1.
Larger than that found in the primary winding.

27. What should the duty rating of a grounding resistor be?
Two speeds
200 volts AC.
Continuous.
800 volts.

28. The size of a transformer is measured in...
75 turns.
Nearly constant.
Kilovolt ampere (KVA).
At the source transformer.

29. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
5:1
Derive a neutral for grounding purposes.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Lower current and higher voltage.

30. 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?
600 volts.
Capacitance.
1 amp.
Derive a neutral for grounding purposes.

31. Alternating current may be defined as...
25 feet.
Lower current and higher voltage
Isolation transformer.
Current that changes constantly in magnitude and periodically in direction.

32. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Second-degree burn.
X.

33. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
A device used to store electrical energy.
Short-circuited after power has been removed.
600 volts.
1 amp.

34. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
600 volts.
Kilovolt ampere (KVA).
Shunted and taped

35. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Transformers.
The chest should rise or inflate.
Lower current and higher voltage.
480 volts.

36. 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.
Allow for metering voltage and Current.
1 amp.
Grounded phase and ground fault devices were not operating.

37. High voltage is used to transmit power over long distances...
Isolation transformer.
They must be installed at least 7 feet above the ground.
An electrical shock hazard may exist.
Because less current flowing at high voltage causes less line loss per kilowatt.

38. How do you change the direction of a 3-phase AC motor?
Because less current flowing at high voltage causes less line loss per kilowatt.
Reverse its rotation.
2000 watts
Reverse any two-phase leads.

39. Define alternating current...
The motor rotation is reversed.
1 amp.
Current changing constantly in magnitude and periodically in direction.
Disconnect the frame-grounding conductor.

40. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
A device used to store electrical energy.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Continuous.
Four times less than that of the secondary.

41. The purpose of a zig-zag transformer is to...
Allow for metering voltage and Current.
A device used to store electrical energy.
Derive a neutral for grounding purposes.
Current that changes constantly in magnitude and periodically in direction.

42. 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.
Kilovolt ampere (KVA).
15 feet off the ground.
5:1.

43. The secondary windings on a 10:1 step-down transformer will have a wire size...
60 Hertz
A.C.
They must be installed at least 7 feet above the ground.
Larger than that found in the primary winding.

44. The property of an electrical circuit that opposes any change in voltage is called...
75 turns.
Capacitance.
Circuit Breaker.
7 -630 volts.

45. The transformer used only to measure voltage is a...
They should be made by the same manufacturer.
A grounding resistor.
12 to 15 times a minute.
Potential transformer.

46. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
2000 watts
Reverse its rotation.
480 volts.
Zero amps.

47. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
They should be made by the same manufacturer.
Cut around clothing that has adhered to the burned area.
A device used to store electrical energy.
Shunted and taped

48. 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...
Draw less current.
Transformers.
Lower current and higher voltage
5:1

49. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Kilovolt ampere (KVA).
They must be installed at least 7 feet above the ground.
Second-degree burn.
X.

50. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
Transformers.
7 -630 volts.
Nearly constant.