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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. The transformer used only to measure voltage is a...
15 feet off the ground.
3 -979 Volts.
Potential transformer.
The current will read zero.

2. What type motor is a wound-rotor induction motor?
75 turns.
15 feet off the ground.
A.C.
Short-circuited after power has been removed.

3. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Squirrel-cage induction motor.
Full-load - three-phase voltage.
An electrical shock hazard may exist.
Two speeds

4. The property of an electrical circuit that opposes any change in voltage is called...
Transformers.
Capacitance.
200 volts AC.
Tied together

5. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Four times less than that of the secondary.
75 turns.
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.

6. 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...
The capacitors should be discharged.
75 turns.
60 cycles per second (60 Hertz).
Draw less current.

7. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
It provides good speed control.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
No.6 AWG.
Squirrel-cage rotor.

8. The combined capacitance of two capacitors in parallel is...
60 Hertz
Hyperventilation.
Provides single-phase power to smaller loads such as lights - heaters - etc.
The sum of the two capacitors.

9. What steps up or steps down AC voltage?
An electrical shock hazard may exist.
Transformers.
1 amp.
Transformer

10. 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?
Current that changes constantly in magnitude and periodically in direction.
Cut around clothing that has adhered to the burned area.
100 turns.
Draw less current.

11. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
H terminals.
100 volts.
25 feet.
A step-up transformer.

12. The secondary windings on a 10:1 step-down transformer will have a wire size...
X.
Larger than that found in the primary winding.
15 feet off the ground.
Alternating current.

13. At what rate should mouth-to-mouth resuscitation be applied?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
800 volts.
12 to 15 times a minute.
Continuous.

14. Not true about AC power?
Transformers.
It provides good speed control.
Short-circuited after power has been removed.
7 -630 volts.

15. A burn causing a blister is classified as a...
At the source transformer.
Squirrel-cage rotor.
12 to 15 times a minute.
Second-degree burn.

16. 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?
75 turns.
Reverse any two-phase leads.
They should be made by the same manufacturer.
100 turns.

17. What type of 3-phase AC motor is used most often around a mine?
Alternating current.
12 to 15 times a minute.
Inductance.
Squirrel-cage induction motor.

18. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Current that changes constantly in magnitude and periodically in direction.
An electromagnet.
12 to 15 times a minute.
480 volts.

19. To protect against over-current in a three-phase circuit over-current protection must be provided for...
12 to 15 times a minute.
At least two phases
They should be made by the same manufacturer.
1 amp.

20. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Squirrel-cage induction motor.
Current changing constantly in magnitude and periodically in direction.
60 Hertz
7 -630 volts.

21. 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?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Four times less than that of the secondary.
Kilovolt ampere (KVA).
600 volts.

22. The property of an electric circuit that opposes any change in current is called...
277 volts.
They must be installed at least 7 feet above the ground.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Inductance.

23. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
5:1.
Lower current and higher voltage.
Squirrel-cage induction motor.

24. The standard frequency of alternating current used at a mine is...
Short-circuited after power has been removed.
1 amp.
60 Hertz
It provides good speed control.

25. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Hyperventilation.
X.
No.6 AWG.
5:1

26. The speed of an alternating current - squirrel-cage induction motor is...
Shunted and taped
Nearly constant.
Transformer
Provides single-phase power to smaller loads such as lights - heaters - etc.

27. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Larger than that found in the primary winding.
Kilovolt ampere (KVA).
Disconnect the frame-grounding conductor.

28. Which of the following does a squirrel cage induction motor have?
15 feet off the ground.
75 turns.
Stator windings.
Allow for metering voltage and Current.

29. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
It provides good speed control.
1 amp.
12 to 15 times a minute.
25 feet.

30. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
It provides good speed control.
A.C.
Two speeds

31. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
75 turns.
They should be made by the same manufacturer.
Full-load - three-phase voltage.
480 volts.

32. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
No.6 AWG.
Larger than that found in the primary winding.
Nearly constant.
Zero amps.

33. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
The current will read zero.
Allow for metering voltage and Current.
Grounded phase and ground fault devices were not operating.
150 turns.

34. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
150 turns.
15 feet off the ground.
Cut around clothing that has adhered to the burned area.
12 to 15 times a minute.

35. 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.
Hyperventilation.
Circuit Breaker.
5:1.
5:1

36. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Reverse any two-phase leads.
800 volts.
Current changing constantly in magnitude and periodically in direction.

37. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
Current changing constantly in magnitude and periodically in direction.
Reverse its rotation.
600 volts.

38. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
5:1
Because less current flowing at high voltage causes less line loss per kilowatt.
Squirrel-cage rotor.
The motor rotation is reversed.

39. The standard frequency of power supplied by the power company is...
Potential transformer.
Current changing constantly in magnitude and periodically in direction.
Clamp-on ammeter.
60 cycles per second (60 Hertz).

40. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Nearly constant.
Second-degree burn.
Derive a neutral for grounding purposes.

41. When not connected to a load - the secondary leads of a current transformer should be...
An electrical shock hazard may exist.
Shunted and taped
5:1
Derive a neutral for grounding purposes.

42. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
The capacitors should be discharged.
2000 watts
Isolation transformer.
800 volts.

43. 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...
Draw less current.
They should be made by the same manufacturer.
Draw less current.
100 volts.

44. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
A grounding resistor.
60 Hertz
Cut around clothing that has adhered to the burned area.
Derive a neutral for grounding purposes.

45. Where should grounding resistors be located?
Hyperventilation.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
At the source transformer.
3 -979 Volts.

46. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Electro-magnetism for their operation.
A step-up transformer.
277 volts.
Full-load - three-phase voltage.

47. Current that changes periodically in direction is...
1 amp.
Allow for metering voltage and Current.
An electromagnet.
Alternating current.

48. 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?
Full-load - three-phase voltage.
A grounding resistor.
800 volts.
Inductance.

49. How should clothing be removed from a burn?
Carotid pulse - which is located in the neck next to the Adam's apple.
Transformers.
Cut around clothing that has adhered to the burned area.
Squirrel-cage induction motor.

50. Not a symptom of electrical shock?.
Two speeds
3 -979 Volts.
Hyperventilation.
5:1