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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. The standard frequency of power supplied by the power company is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Current changing constantly in magnitude and periodically in direction.
60 cycles per second (60 Hertz).
Inductance.

2. 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?
Tied together
Larger than that found in the primary winding.
800 volts.
Potential transformer.

3. Where should grounding resistors be located?
Allow for metering voltage and Current.
A device used to store electrical energy.
At the source transformer.
The current will read zero.

4. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
A device used to store electrical energy.
They must be installed at least 7 feet above the ground.
Allow for metering voltage and Current.
Short-circuited after power has been removed.

5. How should clothing be removed from a burn?
100 turns.
Cut around clothing that has adhered to the burned area.
Stator windings.
2000 watts

6. 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?
Current that changes constantly in magnitude and periodically in direction.
Grounded phase and ground fault devices were not operating.
200 volts AC.
25 feet.

7. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
They must be installed at least 7 feet above the ground.
25 feet.
100 volts.

8. AC voltage is raised or lowered by means of...
Transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
1 amp.
Cut around clothing that has adhered to the burned area.

9. Not an acceptable method for the installations of surface transformers?
They should be made by the same manufacturer.
They must be installed at least 7 feet above the ground.
The motor rotation is reversed.
Continuous.

10. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
480 volts.
Reverse its rotation.
The current will read zero.
Draw less current.

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

12. Not true about AC power?
They should be made by the same manufacturer.
The current will read zero.
It provides good speed control.
75 turns.

13. 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...
No.6 AWG.
100 volts.
Draw less current.
Four times less than that of the secondary.

14. Not a symptom of electrical shock?.
75 turns.
At the source transformer.
Hyperventilation.
They should be made by the same manufacturer.

15. What type of 3-phase AC motor is used most often around a mine?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage induction motor.
A step-up transformer.
75 turns.

16. What is the danger when working around capacitors?
Zero amps.
Check to make sure that the victim is not still in contact with the electricity.
Grounded phase and ground fault devices were not operating.
An electrical shock hazard may exist.

17. 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.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1
Full-load - three-phase voltage.
Inductance.

18. Current in an AC circuit may be measured by...
Larger than that found in the primary winding.
Check to make sure that the victim is not still in contact with the electricity.
A grounding resistor.
Clamp-on ammeter.

19. If any two-phase wires are reversed on a three-phase motor - the motor will...
Inductance.
60 cycles per second (60 Hertz).
3 -979 Volts.
Reverse its rotation.

20. The property of an electric circuit that opposes any change in current is called...
Inductance.
Reverse any two-phase leads.
Larger than that found in the primary winding.
1 amp.

21. The transformer used only to measure voltage is a...
100 turns.
X.
Potential transformer.
Stator windings.

22. 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?
Zero amps.
It provides good speed control.
3 -979 Volts.
X.

23. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
600 volts.
15 feet off the ground.
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

24. 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 440-volt power draws less current so cables and control circuit wiring can be smaller.
Grounded phase and ground fault devices were not operating.
The current will read zero.

25. Alternating current may be defined as...
12 to 15 times a minute.
Current that changes constantly in magnitude and periodically in direction.
Disconnect the frame-grounding conductor.
Isolation transformer.

26. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Reverse its rotation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
It provides good speed control.
An electromagnet.

27. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
The current will read zero.
Kilovolt ampere (KVA).
5:1.

28. Three-phase AC underground cables must be provided with overcurrent protection using a...
At least two phases
Circuit Breaker.
277 volts.
Transformer

29. Define alternating current...
25 feet.
The sum of the two capacitors.
Current changing constantly in magnitude and periodically in direction.
Inductance.

30. 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?
Shunted and taped
Current changing constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.
75 turns.

31. 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?
75 turns.
Zero amps.
100 turns.
A step-up transformer.

32. The size of a transformer is measured in...
Hyperventilation.
Kilovolt ampere (KVA).
5:1.
The capacitors should be discharged.

33. A burn causing a blister is classified as a...
800 volts.
Nearly constant.
At the source transformer.
Second-degree burn.

34. 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...
12 to 15 times a minute.
Zero amps.
5:1
Tied together

35. Compared to the secondary of a loaded - step-down transformer - the primary has...
They should be made by the same manufacturer.
Four times less than that of the secondary.
Lower current and higher voltage.
Transformer

36. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
Nearly constant.
75 turns.
An electromagnet.

37. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Nearly constant.
A step-up transformer.
They should be made by the same manufacturer.
Short-circuited after power has been removed.

38. The property of an electrical circuit that opposes any change in voltage is called...
1 amp.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Capacitance.
Disconnect the frame-grounding conductor.

39. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Carotid pulse - which is located in the neck next to the Adam's apple.
They should be made by the same manufacturer.
The current will read zero.
A grounding resistor.

40. Current that changes periodically in direction is...
At least two phases
Alternating current.
Inductance.
Second-degree burn.

41. How do you change the direction of a 3-phase AC motor?
A step-up transformer.
600 volts.
Reverse any two-phase leads.
75 turns.

42. What is a capacitor?
Squirrel-cage induction motor.
The motor rotation is reversed.
A device used to store electrical energy.
Isolation transformer.

43. All transformers step voltage up or down by...
Inductance.
12 to 15 times a minute.
It provides good speed control.
Because less current flowing at high voltage causes less line loss per kilowatt.

44. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Carotid pulse - which is located in the neck next to the Adam's apple.
At least two phases
1 amp.
Lower current and higher voltage

45. The primary function of a power transformer is to provide...
Squirrel-cage induction motor.
Circuit Breaker.
Full-load - three-phase voltage.
100 volts.

46. 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...
Current changing constantly in magnitude and periodically in direction.
At the source transformer.
Draw less current.
Carotid pulse - which is located in the neck next to the Adam's apple.

47. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Allow for metering voltage and Current.
Transformer
Disconnect the frame-grounding conductor.
Inductance.

48. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The motor rotation is reversed.
An electrical shock hazard may exist.
No.6 AWG.
15 feet off the ground.

49. The standard frequency of alternating current used at a mine is...
Alternating current.
X.
60 Hertz
Electro-magnetism for their operation.

50. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Current that changes constantly in magnitude and periodically in direction.
2000 watts
Draw less current.