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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. 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?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
100 turns.
Draw less current.
Current changing constantly in magnitude and periodically in direction.

2. What is the danger when working around capacitors?
Shunted and taped
An electrical shock hazard may exist.
Because less current flowing at high voltage causes less line loss per kilowatt.
Current changing constantly in magnitude and periodically in direction.

3. AC voltage is raised or lowered by means of...
Transformers.
The capacitors should be discharged.
Tied together
Draw less current.

4. Which of the following does a squirrel cage induction motor have?
H terminals.
Current that changes constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.
Stator windings.

5. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Allow for metering voltage and Current.
480 volts.
The motor rotation is reversed.
60 cycles per second (60 Hertz).

6. 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.
Short-circuited after power has been removed.
Larger than that found in the primary winding.
5:1.

7. High voltage is used to transmit power over long distances...
No.6 AWG.
The capacitors should be discharged.
Hyperventilation.
Because less current flowing at high voltage causes less line loss per kilowatt.

8. A solenoid coil is...
An electromagnet.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
600 volts.
Check to make sure that the victim is not still in contact with the electricity.

9. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Reverse any two-phase leads.
Disconnect the frame-grounding conductor.
The motor rotation is reversed.
Tied together

10. 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
Hyperventilation.
Current changing constantly in magnitude and periodically in direction.
A step-up transformer.

11. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
200 volts AC.
480 volts.
Full-load - three-phase voltage.

12. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
12 to 15 times a minute.
7 -630 volts.
Potential transformer.
Alternating current.

13. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Derive a neutral for grounding purposes.
An electromagnet.
15 feet off the ground.
Two speeds

14. The standard frequency of power supplied by the power company is...
The chest should rise or inflate.
Shunted and taped
Squirrel-cage rotor.
60 cycles per second (60 Hertz).

15. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
The current will read zero.
X.
The motor rotation is reversed.
12 to 15 times a minute.

16. 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
Squirrel-cage induction motor.
Four times less than that of the secondary.
H terminals.

17. The property of an electrical circuit that opposes any change in voltage is called...
75 turns.
Capacitance.
Derive a neutral for grounding purposes.
Electro-magnetism for their operation.

18. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Electro-magnetism for their operation.
5:1
Potential transformer.

19. Compared to the secondary of a loaded - step-down transformer - the primary has...
An electromagnet.
Clamp-on ammeter.
15 feet off the ground.
Lower current and higher voltage

20. 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.
Squirrel-cage induction motor.
No.6 AWG.

21. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
The current will read zero.
The motor rotation is reversed.
They must be installed at least 7 feet above the ground.

22. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
A grounding resistor.
Two speeds
75 turns.

23. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Tied together
Because less current flowing at high voltage causes less line loss per kilowatt.
Kilovolt ampere (KVA).

24. 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?
12 to 15 times a minute.
75 turns.
At least two phases
600 volts.

25. The speed of an alternating current - squirrel-cage induction motor is...
75 turns.
Squirrel-cage rotor.
A device used to store electrical energy.
Nearly constant.

26. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
Because less current flowing at high voltage causes less line loss per kilowatt.
Squirrel-cage induction motor.
Shunted and taped

27. 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?
2000 watts
75 turns.
5:1
60 Hertz

28. Current that changes periodically in direction is...
60 Hertz
Alternating current.
100 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.

29. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Squirrel-cage induction motor.
An electromagnet.
Provides single-phase power to smaller loads such as lights - heaters - etc.

30. The standard frequency of alternating current used at a mine is...
60 Hertz
An electrical shock hazard may exist.
60 cycles per second (60 Hertz).
The current will read zero.

31. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
X.
5:1
The sum of the two capacitors.
A grounding resistor.

32. What is the first step to be followed in aiding a victim of electrical shock?
Isolation transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.
A step-up transformer.
Check to make sure that the victim is not still in contact with the electricity.

33. The secondary windings on a 10:1 step-down transformer will have a wire size...
An electrical shock hazard may exist.
Continuous.
Larger than that found in the primary winding.
The capacitors should be discharged.

34. 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.
15 feet off the ground.
Squirrel-cage induction motor.
Full-load - three-phase voltage.

35. The transformer used only to measure voltage is a...
H terminals.
X.
Potential transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

36. Alternating current may be defined as...
Lower current and higher voltage.
1 amp.
Current that changes constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.

37. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Tied together
They should be made by the same manufacturer.
2000 watts
Hyperventilation.

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

39. How do you change the direction of a 3-phase AC motor?
12 to 15 times a minute.
15 feet off the ground.
The capacitors should be discharged.
Reverse any two-phase leads.

40. Find the turns ratio of a single-phase transformer with the following values: Primary power of 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
480 volts.
5:1.
1 amp.
A grounding resistor.

41. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
5:1.
The current will read zero.
Two speeds
Clamp-on ammeter.

42. What should the duty rating of a grounding resistor be?
150 turns.
600 volts.
Transformers.
Continuous.

43. 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?
1 amp.
H terminals.
100 turns.
800 volts.

44. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Check to make sure that the victim is not still in contact with the electricity.
Grounded phase and ground fault devices were not operating.
Full-load - three-phase voltage.
Continuous.

45. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
600 volts.
Continuous.
7 -630 volts.
Disconnect the frame-grounding conductor.

46. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Alternating current.
15 feet off the ground.
Current changing constantly in magnitude and periodically in direction.
No.6 AWG.

47. 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?
Draw less current.
5:1.
150 turns.
Larger than that found in the primary winding.

48. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
Four times less than that of the secondary.
An electrical shock hazard may exist.
Draw less current.

49. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Zero amps.
The chest should rise or inflate.
Shunted and taped

50. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
A.C.
Zero amps.
Because less current flowing at high voltage causes less line loss per kilowatt.