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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. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
X.
They should be made by the same manufacturer.
Lower current and higher voltage.
A device used to store electrical energy.

2. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
An electromagnet.
5:1
Circuit Breaker.

3. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
75 turns.
An electromagnet.

4. The property of an electrical circuit that opposes any change in voltage is called...
Draw less current.
Capacitance.
Inductance.
5:1.

5. High voltage is used to transmit power over long distances...
Stator windings.
Because less current flowing at high voltage causes less line loss per kilowatt.
Derive a neutral for grounding purposes.
Check to make sure that the victim is not still in contact with the electricity.

6. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
15 feet off the ground.
An electrical shock hazard may exist.
The current will read zero.
Squirrel-cage rotor.

7. 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.
Current changing constantly in magnitude and periodically in direction.
Potential transformer.
Nearly constant.

8. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Grounded phase and ground fault devices were not operating.
480 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
The chest should rise or inflate.

9. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Potential transformer.
Allow for metering voltage and Current.
A step-up transformer.

10. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Draw less current.
Squirrel-cage rotor.
Continuous.

11. The secondary windings on a 10:1 step-down transformer will have a wire size...
800 volts.
100 volts.
An electromagnet.
Larger than that found in the primary winding.

12. AC voltage is raised or lowered by means of...
Clamp-on ammeter.
Transformers.
75 turns.
2000 watts

13. 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.
Grounded phase and ground fault devices were not operating.
5:1.
Current changing constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.

14. When not connected to a load - the secondary leads of a current transformer should be...
3 -979 Volts.
Shunted and taped
800 volts.
Four times less than that of the secondary.

15. What type of 3-phase AC motor is used most often around a mine?
Carotid pulse - which is located in the neck next to the Adam's apple.
Two speeds
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage induction motor.

16. 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.
3 -979 Volts.
277 volts.
480 volts.

17. The primary leads of a single-phase transformer should be connected to the...
12 to 15 times a minute.
Squirrel-cage induction motor.
Shunted and taped
H terminals.

18. The standard frequency of alternating current used at a mine is...
They must be installed at least 7 feet above the ground.
The current will read zero.
A.C.
60 Hertz

19. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
15 feet off the ground.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Current changing constantly in magnitude and periodically in direction.
800 volts.

20. What is the first step to be followed in aiding a victim of electrical shock?
Allow for metering voltage and Current.
Check to make sure that the victim is not still in contact with the electricity.
75 turns.
Alternating current.

21. The primary function of a power transformer is to provide...
Provides single-phase power to smaller loads such as lights - heaters - etc.
60 Hertz
A step-up transformer.
Full-load - three-phase voltage.

22. If any two-phase wires are reversed on a three-phase motor - the motor will...
5:1.
Reverse its rotation.
Derive a neutral for grounding purposes.
100 turns.

23. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Because less current flowing at high voltage causes less line loss per kilowatt.
2000 watts
Transformers.
75 turns.

24. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
Derive a neutral for grounding purposes.
Disconnect the frame-grounding conductor.
The current will read zero.

25. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
An electromagnet.
A step-up transformer.
Full-load - three-phase voltage.
Lower current and higher voltage

26. How do you change the direction of a 3-phase AC motor?
Draw less current.
Carotid pulse - which is located in the neck next to the Adam's apple.
Reverse any two-phase leads.
Check to make sure that the victim is not still in contact with the electricity.

27. Current that changes periodically in direction is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Alternating current.
Short-circuited after power has been removed.
An electrical shock hazard may exist.

28. Current in an AC circuit may be measured by...
They must be installed at least 7 feet above the ground.
The motor rotation is reversed.
Stator windings.
Clamp-on ammeter.

29. 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...
Continuous.
100 volts.
The motor rotation is reversed.
Reverse any two-phase leads.

30. The property of an electric circuit that opposes any change in current is called...
Transformers.
Inductance.
5:1
A device used to store electrical energy.

31. What is the danger when working around capacitors?
Lower current and higher voltage
Hyperventilation.
They should be made by the same manufacturer.
An electrical shock hazard may exist.

32. 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.
Continuous.
At the source transformer.
5:1
Grounded phase and ground fault devices were not operating.

33. 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?
An electromagnet.
200 volts AC.
1 amp.
Reverse any two-phase leads.

34. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Check to make sure that the victim is not still in contact with the electricity.
12 to 15 times a minute.
Derive a neutral for grounding purposes.
1 amp.

35. A burn causing a blister is classified as a...
1 amp.
Second-degree burn.
At the source transformer.
Check to make sure that the victim is not still in contact with the electricity.

36. The speed of an alternating current - squirrel-cage induction motor is...
60 cycles per second (60 Hertz).
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Nearly constant.
Larger than that found in the primary winding.

37. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Isolation transformer.
Draw less current.
No.6 AWG.
A.C.

38. Three-phase AC underground cables must be provided with overcurrent protection using a...
Because less current flowing at high voltage causes less line loss per kilowatt.
The motor rotation is reversed.
Circuit Breaker.
Lower current and higher voltage.

39. The transformer used only to measure voltage is a...
Lower current and higher voltage.
60 Hertz
Potential transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

40. Define alternating current...
Circuit Breaker.
Current changing constantly in magnitude and periodically in direction.
A step-up transformer.
Tied together

41. 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.
Capacitance.
150 turns.
Current that changes constantly in magnitude and periodically in direction.

42. 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?
800 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
480 volts.
Derive a neutral for grounding purposes.

43. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Derive a neutral for grounding purposes.
A device used to store electrical energy.
The motor rotation is reversed.
Cut around clothing that has adhered to the burned area.

44. The combined capacitance of two capacitors in parallel is...
Four times less than that of the secondary.
Cut around clothing that has adhered to the burned area.
The sum of the two capacitors.
H terminals.

45. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Shunted and taped
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Nearly constant.

46. Current transformers - or CT's - are used to...
Carotid pulse - which is located in the neck next to the Adam's apple.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
150 turns.
Shunted and taped

47. The main function of CT's and PT's on a main substation is to...
Current changing constantly in magnitude and periodically in direction.
Allow for metering voltage and Current.
5:1.
Reverse its rotation.

48. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
H terminals.
Draw less current.
Because less current flowing at high voltage causes less line loss per kilowatt.

49. 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?
Hyperventilation.
H terminals.
Lower current and higher voltage
100 turns.

50. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Transformer
Second-degree burn.
X.
75 turns.