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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. High voltage is used to transmit power over long distances...
12 to 15 times a minute.
Because less current flowing at high voltage causes less line loss per kilowatt.
At the source transformer.
480 volts.

2. 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...
Alternating current.
15 feet off the ground.
Draw less current.
A device used to store electrical energy.

3. 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.
Check to make sure that the victim is not still in contact with the electricity.
75 turns.
60 cycles per second (60 Hertz).

4. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
An electrical shock hazard may exist.
2000 watts
Cut around clothing that has adhered to the burned area.
Disconnect the frame-grounding conductor.

5. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Alternating current.
Transformer
Four times less than that of the secondary.

6. 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.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Isolation transformer.
Current changing constantly in magnitude and periodically in direction.

7. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Transformers.
Lower current and higher voltage.
Current that changes constantly in magnitude and periodically in direction.

8. The purpose of a zig-zag transformer is to...
An electrical shock hazard may exist.
Transformers.
Derive a neutral for grounding purposes.
75 turns.

9. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
150 turns.
Squirrel-cage rotor.
15 feet off the ground.
Lower current and higher voltage

10. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Alternating current.
7 -630 volts.
Full-load - three-phase voltage.
A step-up transformer.

11. What type motor is a wound-rotor induction motor?
Tied together
200 volts AC.
12 to 15 times a minute.
A.C.

12. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Larger than that found in the primary winding.
The motor rotation is reversed.
Draw less current.
Alternating current.

13. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
7 -630 volts.
2000 watts
600 volts.
60 Hertz

14. Which of the following does a squirrel cage induction motor have?
Draw less current.
5:1.
Stator windings.
H terminals.

15. Current transformers - or CT's - are used to...
5:1
Nearly constant.
Second-degree burn.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

16. The secondary windings on a 10:1 step-down transformer will have a wire size...
Squirrel-cage rotor.
Larger than that found in the primary winding.
480 volts.
60 cycles per second (60 Hertz).

17. 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 step-up transformer.
Isolation transformer.
Circuit Breaker.
75 turns.

18. What steps up or steps down AC voltage?
Inductance.
Current that changes constantly in magnitude and periodically in direction.
Transformer
75 turns.

19. At what rate should mouth-to-mouth resuscitation be applied?
75 turns.
Stator windings.
Check to make sure that the victim is not still in contact with the electricity.
12 to 15 times a minute.

20. A wound-rotor induction motor has all the following EXCEPT...
X.
Cut around clothing that has adhered to the burned area.
The motor rotation is reversed.
Squirrel-cage rotor.

21. Not an acceptable method for the installations of surface transformers?
The chest should rise or inflate.
Disconnect the frame-grounding conductor.
They must be installed at least 7 feet above the ground.
Squirrel-cage induction motor.

22. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
60 cycles per second (60 Hertz).
3 -979 Volts.
Nearly constant.
They should be made by the same manufacturer.

23. The transformer used only to measure voltage is a...
Current that changes constantly in magnitude and periodically in direction.
Potential transformer.
Check to make sure that the victim is not still in contact with the electricity.
Current changing constantly in magnitude and periodically in direction.

24. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
A.C.
60 cycles per second (60 Hertz).
Inductance.

25. If any two-phase wires are reversed on a three-phase motor - the motor will...
Draw less current.
Reverse its rotation.
100 volts.
Check to make sure that the victim is not still in contact with the electricity.

26. 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.
5:1
An electrical shock hazard may exist.
75 turns.
A step-up transformer.

27. 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?
7 -630 volts.
The sum of the two capacitors.
800 volts.
It provides good speed control.

28. All transformers step voltage up or down by...
60 Hertz
Inductance.
A step-up transformer.
Second-degree burn.

29. The main function of CT's and PT's on a main substation is to...
Capacitance.
At the source transformer.
150 turns.
Allow for metering voltage and Current.

30. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Four times less than that of the secondary.
The chest should rise or inflate.
60 cycles per second (60 Hertz).
Inductance.

31. What type of 3-phase AC motor is used most often around a mine?
60 Hertz
Squirrel-cage induction motor.
Transformer
Tied together

32. 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.
75 turns.
Continuous.
Clamp-on ammeter.

33. A control power transformer...
100 turns.
2000 watts
Shunted and taped
Provides single-phase power to smaller loads such as lights - heaters - etc.

34. 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?
Inductance.
200 volts AC.
Disconnect the frame-grounding conductor.
5:1

35. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Disconnect the frame-grounding conductor.
2000 watts

36. The standard frequency of alternating current used at a mine is...
Grounded phase and ground fault devices were not operating.
277 volts.
Shunted and taped
60 Hertz

37. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
It provides good speed control.
Second-degree burn.
Transformers.

38. 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.
Larger than that found in the primary winding.
Hyperventilation.
A grounding resistor.

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?
3 -979 Volts.
277 volts.
Kilovolt ampere (KVA).
75 turns.

40. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
480 volts.
The capacitors should be discharged.
1 amp.
Current changing constantly in magnitude and periodically in direction.

41. The standard frequency of power supplied by the power company is...
The capacitors should be discharged.
Squirrel-cage rotor.
60 cycles per second (60 Hertz).
They should be made by the same manufacturer.

42. 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?
Two speeds
100 turns.
Lower current and higher voltage.
Stator windings.

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

44. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
60 cycles per second (60 Hertz).
Electro-magnetism for their operation.
Larger than that found in the primary winding.
No.6 AWG.

45. Define alternating current...
Potential transformer.
Current that changes constantly in magnitude and periodically in direction.
Electro-magnetism for their operation.
Current changing constantly in magnitude and periodically in direction.

46. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
60 cycles per second (60 Hertz).
The chest should rise or inflate.
60 Hertz

47. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
The chest should rise or inflate.
A device used to store electrical energy.
Zero amps.

48. Compared to the secondary of a loaded - step-down transformer - the primary has...
Zero amps.
Lower current and higher voltage
The capacitors should be discharged.
Hyperventilation.

49. What is the first step to be followed in aiding a victim of electrical shock?
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage rotor.
Tied together
Current changing constantly in magnitude and periodically in direction.

50. What is the danger when working around capacitors?
Draw less current.
Potential transformer.
Squirrel-cage induction motor.
An electrical shock hazard may exist.