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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. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
An electrical shock hazard may exist.
Circuit Breaker.
Tied together
Zero amps.

2. The transformer used only to measure voltage is a...
Inductance.
They must be installed at least 7 feet above the ground.
Potential transformer.
A.C.

3. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Kilovolt ampere (KVA).
Two speeds
277 volts.
5:1

4. What type of 3-phase AC motor is used most often around a mine?
60 cycles per second (60 Hertz).
A grounding resistor.
Squirrel-cage induction motor.
The current will read zero.

5. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
277 volts.
X.
Lower current and higher voltage.
200 volts AC.

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

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7. What is the danger when working around capacitors?
The capacitors should be discharged.
Inductance.
An electrical shock hazard may exist.
Tied together

8. A wound-rotor induction motor has all the following EXCEPT...
Four times less than that of the secondary.
Current that changes constantly in magnitude and periodically in direction.
Alternating current.
Squirrel-cage rotor.

9. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Derive a neutral for grounding purposes.
5:1.
A step-up transformer.
2000 watts

10. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
1 amp.
A grounding resistor.
100 volts.
480 volts.

11. 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.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
480 volts.
5:1
Reverse any two-phase leads.

12. At what rate should mouth-to-mouth resuscitation be applied?
No.6 AWG.
Draw less current.
12 to 15 times a minute.
60 Hertz

13. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Current changing constantly in magnitude and periodically in direction.
12 to 15 times a minute.
25 feet.
100 volts.

14. Current in an AC circuit may be measured by...
Current that changes constantly in magnitude and periodically in direction.
Clamp-on ammeter.
X.
75 turns.

15. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Short-circuited after power has been removed.
600 volts.
Larger than that found in the primary winding.
They should be made by the same manufacturer.

16. 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?
They should be made by the same manufacturer.
A device used to store electrical energy.
75 turns.
200 volts AC.

17. 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?
X.
Lower current and higher voltage
Hyperventilation.
150 turns.

18. The standard frequency of alternating current used at a mine is...
3 -979 Volts.
60 Hertz
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
1 amp.

19. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
They must be installed at least 7 feet above the ground.
At the source transformer.
The motor rotation is reversed.
Nearly constant.

20. AC voltage is raised or lowered by means of...
Electro-magnetism for their operation.
At least two phases
Nearly constant.
Transformers.

21. 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.
It provides good speed control.
Draw less current.
Potential transformer.

22. 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.
Full-load - three-phase voltage.
2000 watts
277 volts.

23. 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?
100 volts.
75 turns.
The chest should rise or inflate.
Derive a neutral for grounding purposes.

24. A solenoid coil is...
60 Hertz
An electromagnet.
2000 watts
A device used to store electrical energy.

25. All transformers step voltage up or down by...
Potential transformer.
Inductance.
Larger than that found in the primary winding.
Draw less current.

26. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Reverse any two-phase leads.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

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...
100 volts.
12 to 15 times a minute.
At least two phases
Continuous.

28. What steps up or steps down AC voltage?
480 volts.
Stator windings.
Transformer
X.

29. 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?
150 turns.
Two speeds
75 turns.
600 volts.

30. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Two speeds
480 volts.
60 Hertz
Draw less current.

31. The property of an electric circuit that opposes any change in current is called...
Hyperventilation.
Inductance.
Full-load - three-phase voltage.
Reverse its rotation.

32. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
60 cycles per second (60 Hertz).
480 volts.
H terminals.

33. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Because less current flowing at high voltage causes less line loss per kilowatt.
5:1
Disconnect the frame-grounding conductor.
Two speeds

34. A transformer with a 1:1 ratio is usually a (an)...
Lower current and higher voltage.
Two speeds
75 turns.
Isolation transformer.

35. Not a symptom of electrical shock?.
7 -630 volts.
Hyperventilation.
X.
Second-degree burn.

36. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
Inductance.
Potential transformer.
An electrical shock hazard may exist.

37. A burn causing a blister is classified as a...
The chest should rise or inflate.
Second-degree burn.
At the source transformer.
Hyperventilation.

38. 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...
A step-up transformer.
Draw less current.
Second-degree burn.
7 -630 volts.

39. The primary function of a power transformer is to provide...
Electro-magnetism for their operation.
Potential transformer.
They should be made by the same manufacturer.
Full-load - three-phase voltage.

40. 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?
An electromagnet.
800 volts.
Clamp-on ammeter.
Second-degree burn.

41. 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.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
X.
1 amp.

42. 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...
Tied together
X.
Capacitance.
Inductance.

43. Which of the following does a squirrel cage induction motor have?
Stator windings.
Kilovolt ampere (KVA).
Check to make sure that the victim is not still in contact with the electricity.
Second-degree burn.

44. Define alternating current...
The sum of the two capacitors.
A device used to store electrical energy.
Current changing constantly in magnitude and periodically in direction.
12 to 15 times a minute.

45. Compared to the secondary of a loaded - step-down transformer - the primary has...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
800 volts.
7 -630 volts.
Lower current and higher voltage

46. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Circuit Breaker.
Allow for metering voltage and Current.
The capacitors should be discharged.
No.6 AWG.

47. Alternating current may be defined as...
The capacitors should be discharged.
480 volts.
Current that changes constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.

48. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
H terminals.
Derive a neutral for grounding purposes.
Lower current and higher voltage

49. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Hyperventilation.
A step-up transformer.
5:1.
Grounded phase and ground fault devices were not operating.

50. 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?
200 volts AC.
480 volts.
X.
3 -979 Volts.

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AC Theory

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