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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. What steps up or steps down AC voltage?
The current will read zero.
Lower current and higher voltage
100 turns.
Transformer

2. 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.
X.
Potential transformer.
5:1

3. The primary leads of a single-phase transformer should be connected to the...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The current will read zero.
H terminals.
Squirrel-cage rotor.

4. The property of an electric circuit that opposes any change in current is called...
Derive a neutral for grounding purposes.
Inductance.
800 volts.
75 turns.

5. 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?
A.C.
Allow for metering voltage and Current.
Reverse any two-phase leads.
3 -979 Volts.

6. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Stator windings.
Two speeds
At least two phases
X.

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

8. Which of the following does a squirrel cage induction motor have?
Squirrel-cage rotor.
Stator windings.
Check to make sure that the victim is not still in contact with the electricity.
Nearly constant.

9. 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.
Two speeds
Nearly constant.
They must be installed at least 7 feet above the ground.
5:1.

10. The secondary windings on a 10:1 step-down transformer will have a wire size...
It provides good speed control.
Larger than that found in the primary winding.
Zero amps.
An electromagnet.

11. 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?
Nearly constant.
Hyperventilation.
75 turns.
15 feet off the ground.

12. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Lower current and higher voltage
The current will read zero.
Alternating current.
It provides good speed control.

13. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
They should be made by the same manufacturer.
Reverse any two-phase leads.
150 turns.

14. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Short-circuited after power has been removed.
It provides good speed control.
At least two phases
X.

15. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Stator windings.
Draw less current.
Short-circuited after power has been removed.

16. A wound-rotor induction motor has all the following EXCEPT...
480 volts.
Full-load - three-phase voltage.
Squirrel-cage rotor.
The chest should rise or inflate.

17. The property of an electrical circuit that opposes any change in voltage is called...
Full-load - three-phase voltage.
Capacitance.
5:1.
The current will read zero.

18. What is the danger when working around capacitors?
480 volts.
Electro-magnetism for their operation.
Nearly constant.
An electrical shock hazard may exist.

19. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
The capacitors should be discharged.
Squirrel-cage rotor.
15 feet off the ground.
480 volts.

20. 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.
It provides good speed control.
Alternating current.
5:1
A step-up transformer.

21. AC voltage is raised or lowered by means of...
Transformers.
Squirrel-cage rotor.
Stator windings.
A.C.

22. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
150 turns.
75 turns.
A grounding resistor.
An electrical shock hazard may exist.

23. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Clamp-on ammeter.
A grounding resistor.
15 feet off the ground.
2000 watts

24. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
X.
75 turns.
Isolation transformer.
1 amp.

25. To protect against over-current in a three-phase circuit over-current protection must be provided for...
15 feet off the ground.
Squirrel-cage induction motor.
At least two phases
Squirrel-cage rotor.

26. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
At the source transformer.
Draw less current.
5:1

27. Not true about AC power?
25 feet.
It provides good speed control.
Kilovolt ampere (KVA).
150 turns.

28. What type of 3-phase AC motor is used most often around a mine?
No.6 AWG.
A step-up transformer.
Squirrel-cage induction motor.
Derive a neutral for grounding purposes.

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...
100 volts.
Potential transformer.
They must be installed at least 7 feet above the ground.
12 to 15 times a minute.

30. The combined capacitance of two capacitors in parallel is...
1 amp.
Grounded phase and ground fault devices were not operating.
75 turns.
The sum of the two capacitors.

31. What should the duty rating of a grounding resistor be?
Continuous.
Allow for metering voltage and Current.
Isolation transformer.
At least two phases

32. 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.
Provides single-phase power to smaller loads such as lights - heaters - etc.
60 Hertz
Grounded phase and ground fault devices were not operating.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
800 volts.
Lower current and higher voltage.
The capacitors should be discharged.
Lower current and higher voltage

34. 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...
Lower current and higher voltage.
Draw less current.
A grounding resistor.
Current changing constantly in magnitude and periodically in direction.

35. 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.
75 turns.
2000 watts
Squirrel-cage induction motor.

36. Current in an AC circuit may be measured by...
Because less current flowing at high voltage causes less line loss per kilowatt.
Clamp-on ammeter.
800 volts.
Inductance.

37. 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?
Check to make sure that the victim is not still in contact with the electricity.
Hyperventilation.
600 volts.
Isolation transformer.

38. Operating coils - motors - transformers - relays and solenoids depend upon...
3 -979 Volts.
Current that changes constantly in magnitude and periodically in direction.
Electro-magnetism for their operation.
Circuit Breaker.

39. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
25 feet.
A grounding resistor.
The motor rotation is reversed.
Zero amps.

40. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Grounded phase and ground fault devices were not operating.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Potential transformer.
The motor rotation is reversed.

41. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Transformer
No.6 AWG.
60 cycles per second (60 Hertz).

42. Not a symptom of electrical shock?.
Continuous.
7 -630 volts.
Hyperventilation.
600 volts.

43. The main function of CT's and PT's on a main substation is to...
Draw less current.
Allow for metering voltage and Current.
A.C.
5:1.

44. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
75 turns.
Reverse any two-phase leads.
Short-circuited after power has been removed.
Nearly constant.

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.
It provides good speed control.
H terminals.
Lower current and higher voltage.

46. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Lower current and higher voltage
A device used to store electrical energy.
Zero amps.

47. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
An electromagnet.
They should be made by the same manufacturer.
Draw less current.
The chest should rise or inflate.

48. What is a capacitor?
Draw less current.
3 -979 Volts.
A device used to store electrical energy.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

49. Where should grounding resistors be located?
Larger than that found in the primary winding.
At the source transformer.
480 volts.
Isolation transformer.

50. Current transformers - or CT's - are used to...
100 volts.
Isolation transformer.
Four times less than that of the secondary.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.