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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. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
60 Hertz
100 volts.
Inductance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

2. 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?
7 -630 volts.
X.
It provides good speed control.
600 volts.

3. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A grounding resistor.
A step-up transformer.
25 feet.
75 turns.

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

5. What should the duty rating of a grounding resistor be?
Full-load - three-phase voltage.
Draw less current.
200 volts AC.
Continuous.

6. The purpose of a zig-zag transformer is to...
Reverse its rotation.
Continuous.
Circuit Breaker.
Derive a neutral for grounding purposes.

7. Compared to the secondary of a loaded - step-down transformer - the primary has...
Transformer
Lower current and higher voltage.
At least two phases
Disconnect the frame-grounding conductor.

8. All transformers step voltage up or down by...
Inductance.
They should be made by the same manufacturer.
75 turns.
Four times less than that of the secondary.

9. Current that changes periodically in direction is...
Alternating current.
Tied together
No.6 AWG.
Reverse any two-phase leads.

10. The transformer used only to measure voltage is a...
Potential transformer.
5:1
600 volts.
5:1.

11. The standard frequency of power supplied by the power company is...
It provides good speed control.
60 cycles per second (60 Hertz).
Squirrel-cage induction motor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

12. What type of 3-phase AC motor is used most often around a mine?
The chest should rise or inflate.
Current that changes constantly in magnitude and periodically in direction.
75 turns.
Squirrel-cage induction motor.

13. At what rate should mouth-to-mouth resuscitation be applied?
Allow for metering voltage and Current.
150 turns.
12 to 15 times a minute.
5:1.

14. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Potential transformer.
They should be made by the same manufacturer.
Current changing constantly in magnitude and periodically in direction.

15. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Zero amps.
480 volts.
Isolation transformer.
Reverse any two-phase leads.

16. What type motor is a wound-rotor induction motor?
They must be installed at least 7 feet above the ground.
Circuit Breaker.
A.C.
A step-up transformer.

17. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
The current will read zero.
1 amp.
7 -630 volts.
Zero amps.

18. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
3 -979 Volts.
A grounding resistor.
Stator windings.
Four times less than that of the secondary.

19. Which of the following does a squirrel cage induction motor have?
277 volts.
Current that changes constantly in magnitude and periodically in direction.
Stator windings.
At least two phases

20. 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.
A grounding resistor.
Continuous.
Inductance.
5:1

21. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
Tied together
100 volts.
Alternating current.

22. A control power transformer...
480 volts.
15 feet off the ground.
Circuit Breaker.
Provides single-phase power to smaller loads such as lights - heaters - etc.

23. A burn causing a blister is classified as a...
Inductance.
Second-degree burn.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Capacitance.

24. High voltage is used to transmit power over long distances...
They must be installed at least 7 feet above the ground.
Circuit Breaker.
Kilovolt ampere (KVA).
Because less current flowing at high voltage causes less line loss per kilowatt.

25. The primary leads of a single-phase transformer should be connected to the...
A device used to store electrical energy.
H terminals.
Nearly constant.
It provides good speed control.

26. The size of a transformer is measured in...
3 -979 Volts.
Larger than that found in the primary winding.
Kilovolt ampere (KVA).
Continuous.

27. 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...
Two speeds
Inductance.
An electromagnet.
100 volts.

28. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Potential transformer.
The motor rotation is reversed.
No.6 AWG.
Tied together

29. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
At the source transformer.
15 feet off the ground.
They must be installed at least 7 feet above the ground.
Shunted and taped

30. What steps up or steps down AC voltage?
Transformer
75 turns.
Lower current and higher voltage.
12 to 15 times a minute.

31. Define alternating current...
It provides good speed control.
800 volts.
Current changing constantly in magnitude and periodically in direction.
12 to 15 times a minute.

32. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
It provides good speed control.
Inductance.
The chest should rise or inflate.

33. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Check to make sure that the victim is not still in contact with the electricity.
25 feet.
An electrical shock hazard may exist.
H terminals.

34. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
A step-up transformer.
Carotid pulse - which is located in the neck next to the Adam's apple.
Squirrel-cage rotor.
The capacitors should be discharged.

35. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Potential transformer.
Clamp-on ammeter.
The current will read zero.
Derive a neutral for grounding purposes.

36. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Grounded phase and ground fault devices were not operating.
Draw less current.
The chest should rise or inflate.

37. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Derive a neutral for grounding purposes.
2000 watts
Transformer
Potential transformer.

38. The secondary windings on a 10:1 step-down transformer will have a wire size...
The current will read zero.
Larger than that found in the primary winding.
Transformers.
Carotid pulse - which is located in the neck next to the Adam's apple.

39. Not a symptom of electrical shock?.
Isolation transformer.
Hyperventilation.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
2000 watts

40. 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?
Lower current and higher voltage.
Nearly constant.
3 -979 Volts.
A grounding resistor.

41. Not true about AC power?
The capacitors should be discharged.
Squirrel-cage rotor.
It provides good speed control.
Shunted and taped

42. If any two-phase wires are reversed on a three-phase motor - the motor will...
Derive a neutral for grounding purposes.
A step-up transformer.
A device used to store electrical energy.
Reverse its rotation.

43. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
7 -630 volts.
Short-circuited after power has been removed.
A grounding resistor.
An electrical shock hazard may exist.

44. Current in an AC circuit may be measured by...
Nearly constant.
5:1.
Clamp-on ammeter.
Because less current flowing at high voltage causes less line loss per kilowatt.

45. Alternating current may be defined as...
100 turns.
75 turns.
Current that changes constantly in magnitude and periodically in direction.
480 volts.

46. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Draw less current.
Transformers.
75 turns.

47. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Because less current flowing at high voltage causes less line loss per kilowatt.
It provides good speed control.
Allow for metering voltage and Current.

48. The property of an electric circuit that opposes any change in current is called...
Zero amps.
No.6 AWG.
Inductance.
Tied together

49. A transformer with 1 -500 turns in the primary winding has a primary voltage of 2 -400 volts and a secondary voltage of 120 volts. How many turns are in the secondary winding?
75 turns.
Stator windings.
Electro-magnetism for their operation.
Potential transformer.

50. The property of an electrical circuit that opposes any change in voltage is called...
Nearly constant.
75 turns.
Kilovolt ampere (KVA).
Capacitance.