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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?
Capacitance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Electro-magnetism for their operation.
Full-load - three-phase voltage.

2. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Grounded phase and ground fault devices were not operating.
The current will read zero.
Derive a neutral for grounding purposes.
The sum of the two capacitors.

3. AC voltage is raised or lowered by means of...
Transformers.
Allow for metering voltage and Current.
Reverse any two-phase leads.
At least two phases

4. 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.
Current that changes constantly in magnitude and periodically in direction.
5:1
Larger than that found in the primary winding.

5. The secondary windings on a 10:1 step-down transformer will have a wire size...
The capacitors should be discharged.
Two speeds
Larger than that found in the primary winding.
75 turns.

6. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Kilovolt ampere (KVA).
Derive a neutral for grounding purposes.
The capacitors should be discharged.
Inductance.

7. The speed of an alternating current - squirrel-cage induction motor is...
3 -979 Volts.
15 feet off the ground.
The chest should rise or inflate.
Nearly constant.

8. Not true about AC power?
Short-circuited after power has been removed.
They should be made by the same manufacturer.
12 to 15 times a minute.
It provides good speed control.

9. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
5:1.
It provides good speed control.
Grounded phase and ground fault devices were not operating.
Derive a neutral for grounding purposes.

10. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Lower current and higher voltage
Draw less current.
2000 watts
Two speeds

11. 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?
Current that changes constantly in magnitude and periodically in direction.
Transformers.
800 volts.
The current will read zero.

12. 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.
Shunted and taped
An electromagnet.
The motor rotation is reversed.

13. What steps up or steps down AC voltage?
Transformer
A device used to store electrical energy.
Draw less current.
The capacitors should be discharged.

14. The property of an electric circuit that opposes any change in current is called...
Inductance.
At the source transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Tied together

15. 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?
60 cycles per second (60 Hertz).
75 turns.
The current will read zero.
150 turns.

16. 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...
Hyperventilation.
100 volts.
60 Hertz
Short-circuited after power has been removed.

17. 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?
Tied together
The chest should rise or inflate.
3 -979 Volts.
Full-load - three-phase voltage.

18. How should clothing be removed from a burn?
800 volts.
Cut around clothing that has adhered to the burned area.
Capacitance.
Inductance.

19. Current that changes periodically in direction is...
The sum of the two capacitors.
Alternating current.
Two speeds
25 feet.

20. 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...
Full-load - three-phase voltage.
Short-circuited after power has been removed.
Draw less current.
75 turns.

21. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
An electromagnet.
Full-load - three-phase voltage.
600 volts.

22. What is a capacitor?
It provides good speed control.
A device used to store electrical energy.
480 volts.
Hyperventilation.

23. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
5:1.
Disconnect the frame-grounding conductor.
An electromagnet.
Two speeds

24. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Potential transformer.
480 volts.
An electromagnet.
75 turns.

25. Compared to the secondary of a loaded - step-down transformer - the primary has...
A step-up transformer.
60 Hertz
Provides single-phase power to smaller loads such as lights - heaters - etc.
Lower current and higher voltage.

26. Where should grounding resistors be located?
25 feet.
At the source transformer.
Isolation transformer.
Second-degree burn.

27. 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...
Current that changes constantly in magnitude and periodically in direction.
Tied together
A step-up transformer.
An electromagnet.

28. What should the duty rating of a grounding resistor be?
100 turns.
Continuous.
Grounded phase and ground fault devices were not operating.
Electro-magnetism for their operation.

29. A control power transformer...
Lower current and higher voltage.
Provides single-phase power to smaller loads such as lights - heaters - etc.
200 volts AC.
The motor rotation is reversed.

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?
Because less current flowing at high voltage causes less line loss per kilowatt.
75 turns.
12 to 15 times a minute.
Squirrel-cage induction motor.

31. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Shunted and taped
Second-degree burn.
A grounding resistor.
Short-circuited after power has been removed.

32. Define alternating current...
60 Hertz
Electro-magnetism for their operation.
Squirrel-cage induction motor.
Current changing constantly in magnitude and periodically in direction.

33. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Zero amps.
The motor rotation is reversed.
12 to 15 times a minute.
Larger than that found in the primary winding.

34. 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.
They should be made by the same manufacturer.
A step-up transformer.
At least two phases

35. Current transformers - or CT's - are used to...
Circuit Breaker.
Full-load - three-phase voltage.
800 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

36. What type of 3-phase AC motor is used most often around a mine?
Lower current and higher voltage.
X.
Tied together
Squirrel-cage induction motor.

37. The standard frequency of alternating current used at a mine is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
They must be installed at least 7 feet above the ground.
60 Hertz
100 volts.

38. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Kilovolt ampere (KVA).
Shunted and taped
Squirrel-cage induction motor.
7 -630 volts.

39. To protect against over-current in a three-phase circuit over-current protection must be provided for...
An electromagnet.
At least two phases
150 turns.
The motor rotation is reversed.

40. The size of a transformer is measured in...
3 -979 Volts.
Stator windings.
100 turns.
Kilovolt ampere (KVA).

41. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Potential transformer.
277 volts.
Reverse any two-phase leads.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

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?
25 feet.
The chest should rise or inflate.
100 turns.
Transformers.

43. The purpose of a zig-zag transformer is to...
A step-up transformer.
Derive a neutral for grounding purposes.
Transformers.
Inductance.

44. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Disconnect the frame-grounding conductor.
Capacitance.
The sum of the two capacitors.

45. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
100 volts.
Reverse its rotation.
Lower current and higher voltage.
X.

46. 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.
X.
5:1
Reverse its rotation.

47. The main function of CT's and PT's on a main substation is to...
Squirrel-cage induction motor.
Allow for metering voltage and Current.
Second-degree burn.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

48. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
100 turns.
60 Hertz
25 feet.

49. Which of the following does a squirrel cage induction motor have?
12 to 15 times a minute.
Lower current and higher voltage.
Stator windings.
A step-up transformer.

50. Alternating current may be defined as...
X.
7 -630 volts.
Current that changes constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.