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AC Theory

Instructions:

Answer 50 questions in 20 minutes. 2 minutes extra for reading the instructions.
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. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
They should be made by the same manufacturer.
Derive a neutral for grounding purposes.
A grounding resistor.
X.

2. The primary leads of a single-phase transformer should be connected to the...
Four times less than that of the secondary.
H terminals.
Draw less current.
Continuous.

3. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
3 -979 Volts.
Draw less current.
15 feet off the ground.
The current will read zero.

4. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
1 amp.
Four times less than that of the secondary.
Transformers.
A device used to store electrical energy.

5. A control power transformer...
Electro-magnetism for their operation.
They should be made by the same manufacturer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
150 turns.

6. A burn causing a blister is classified as a...
Second-degree burn.
3 -979 Volts.
Hyperventilation.
Reverse its rotation.

7. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Larger than that found in the primary winding.
At the source transformer.
The current will read zero.
25 feet.

8. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Current that changes constantly in magnitude and periodically in direction.
Nearly constant.
Larger than that found in the primary winding.

9. 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...
Electro-magnetism for their operation.
Reverse any two-phase leads.
100 volts.
Two speeds

10. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
100 volts.
277 volts.
Squirrel-cage induction motor.
Nearly constant.

11. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
They should be made by the same manufacturer.
15 feet off the ground.
Transformers.

12. What is the danger when working around capacitors?
An electrical shock hazard may exist.
200 volts AC.
Electro-magnetism for their operation.
Short-circuited after power has been removed.

13. Current that changes periodically in direction is...
Inductance.
150 turns.
Lower current and higher voltage.
Alternating current.

14. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
100 turns.
800 volts.

15. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Transformers.
Current changing constantly in magnitude and periodically in direction.
A device used to store electrical energy.

16. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
An electrical shock hazard may exist.
Nearly constant.
Cut around clothing that has adhered to the burned area.

17. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Electro-magnetism for their operation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
7 -630 volts.
Capacitance.

18. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The current will read zero.
Stator windings.
Continuous.

19. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Cut around clothing that has adhered to the burned area.
100 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

20. What type of 3-phase AC motor is used most often around a mine?
480 volts.
The motor rotation is reversed.
Continuous.
Squirrel-cage induction motor.

21. What should the duty rating of a grounding resistor be?
Carotid pulse - which is located in the neck next to the Adam's apple.
At least two phases
Continuous.
Inductance.

22. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Zero amps.
Current that changes constantly in magnitude and periodically in direction.
Hyperventilation.

23. High voltage is used to transmit power over long distances...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
The capacitors should be discharged.
100 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.

24. Compared to the secondary of a loaded - step-down transformer - the primary has...
Full-load - three-phase voltage.
Lower current and higher voltage.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Stator windings.

25. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Draw less current.
Draw less current.
5:1.

26. 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.
5:1.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Nearly constant.
An electrical shock hazard may exist.

27. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
Allow for metering voltage and Current.
Kilovolt ampere (KVA).
480 volts.

28. 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?
X.
A device used to store electrical energy.
75 turns.
An electrical shock hazard may exist.

29. Current transformers - or CT's - are used to...
Second-degree burn.
Four times less than that of the secondary.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Nearly constant.

30. Where should grounding resistors be located?
The motor rotation is reversed.
150 turns.
60 Hertz
At the source transformer.

31. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
The sum of the two capacitors.
Squirrel-cage rotor.
Grounded phase and ground fault devices were not operating.

32. 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?
Allow for metering voltage and Current.
200 volts AC.
3 -979 Volts.
Tied together

33. 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?
Draw less current.
600 volts.
At the source transformer.
Disconnect the frame-grounding conductor.

34. The secondary windings on a 10:1 step-down transformer will have a wire size...
Nearly constant.
Larger than that found in the primary winding.
Transformer
800 volts.

35. The size of a transformer is measured in...
Disconnect the frame-grounding conductor.
75 turns.
Squirrel-cage induction motor.
Kilovolt ampere (KVA).

36. 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...
Draw less current.
Squirrel-cage induction motor.
2000 watts
Electro-magnetism for their operation.

37. A solenoid coil is...
Draw less current.
At the source transformer.
Allow for metering voltage and Current.
An electromagnet.

38. The primary function of a power transformer is to provide...
An electrical shock hazard may exist.
Full-load - three-phase voltage.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.

39. Alternating current may be defined as...
A grounding resistor.
60 Hertz
Current that changes constantly in magnitude and periodically in direction.
Two speeds

40. 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.
Transformers.
Allow for metering voltage and Current.
5:1
Reverse any two-phase leads.

41. The property of an electric circuit that opposes any change in current is called...
Larger than that found in the primary winding.
Short-circuited after power has been removed.
Zero amps.
Inductance.

42. The combined capacitance of two capacitors in parallel is...
A device used to store electrical energy.
The sum of the two capacitors.
Inductance.
Isolation transformer.

43. How do you change the direction of a 3-phase AC motor?
They must be installed at least 7 feet above the ground.
200 volts AC.
Zero amps.
Reverse any two-phase leads.

44. 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...
The chest should rise or inflate.
Tied together
Four times less than that of the secondary.
Potential transformer.

45. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
200 volts AC.
Inductance.
Provides single-phase power to smaller loads such as lights - heaters - etc.

46. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
A.C.
Capacitance.
60 Hertz

47. A transformer with a 1:1 ratio is usually a (an)...
Four times less than that of the secondary.
Isolation transformer.
1 amp.
75 turns.

48. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Squirrel-cage rotor.
Carotid pulse - which is located in the neck next to the Adam's apple.
Disconnect the frame-grounding conductor.

49. The standard frequency of alternating current used at a mine is...
Squirrel-cage rotor.
60 cycles per second (60 Hertz).
15 feet off the ground.
60 Hertz

50. What is the first step to be followed in aiding a victim of electrical shock?
The chest should rise or inflate.
At the source transformer.
Check to make sure that the victim is not still in contact with the electricity.
A.C.