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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 type of 3-phase AC motor is used most often around a mine?
It provides good speed control.
Disconnect the frame-grounding conductor.
277 volts.
Squirrel-cage induction motor.

2. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
1 amp.
Stator windings.
The capacitors should be discharged.
277 volts.

3. Alternating current may be defined as...
Full-load - three-phase voltage.
Tied together
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage

4. 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.
Full-load - three-phase voltage.
Four times less than that of the secondary.
5:1.

5. The combined capacitance of two capacitors in parallel is...
Short-circuited after power has been removed.
The sum of the two capacitors.
Nearly constant.
Allow for metering voltage and Current.

6. A burn causing a blister is classified as a...
Short-circuited after power has been removed.
Because less current flowing at high voltage causes less line loss per kilowatt.
Second-degree burn.
12 to 15 times a minute.

7. A solenoid coil is...
100 volts.
An electromagnet.
60 Hertz
Grounded phase and ground fault devices were not operating.

8. How do you change the direction of a 3-phase AC motor?
Reverse its rotation.
They must be installed at least 7 feet above the ground.
Reverse any two-phase leads.
Cut around clothing that has adhered to the burned area.

9. 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?
Inductance.
Hyperventilation.
800 volts.
3 -979 Volts.

10. 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?
25 feet.
12 to 15 times a minute.
An electrical shock hazard may exist.
150 turns.

11. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
7 -630 volts.
25 feet.
A step-up transformer.
The chest should rise or inflate.

12. All transformers step voltage up or down by...
Clamp-on ammeter.
Inductance.
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage.

13. 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...
Draw less current.
Clamp-on ammeter.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Shunted and taped

14. When not connected to a load - the secondary leads of a current transformer should be...
The current will read zero.
150 turns.
Shunted and taped
Derive a neutral for grounding purposes.

15. The property of an electric circuit that opposes any change in current is called...
X.
Inductance.
Because less current flowing at high voltage causes less line loss per kilowatt.
Potential transformer.

16. The main function of CT's and PT's on a main substation is to...
Cut around clothing that has adhered to the burned area.
25 feet.
No.6 AWG.
Allow for metering voltage and Current.

17. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The motor rotation is reversed.
Grounded phase and ground fault devices were not operating.
Second-degree burn.
No.6 AWG.

18. The transformer used only to measure voltage is a...
Potential transformer.
3 -979 Volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
12 to 15 times a minute.

19. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Circuit Breaker.
A grounding resistor.
An electromagnet.
Check to make sure that the victim is not still in contact with the electricity.

20. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Second-degree burn.
75 turns.
100 turns.

21. The secondary windings on a 10:1 step-down transformer will have a wire size...
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Larger than that found in the primary winding.
15 feet off the ground.

22. 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...
Derive a neutral for grounding purposes.
600 volts.
Short-circuited after power has been removed.
Draw less current.

23. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
They must be installed at least 7 feet above the ground.
2000 watts
The motor rotation is reversed.

24. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Squirrel-cage rotor.
Potential transformer.
It provides good speed control.

25. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
25 feet.
Zero amps.
Nearly constant.
The capacitors should be discharged.

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

27. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Disconnect the frame-grounding conductor.
Grounded phase and ground fault devices were not operating.
Circuit Breaker.
Nearly constant.

28. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Current that changes constantly in magnitude and periodically in direction.
At least two phases
They should be made by the same manufacturer.
Second-degree burn.

29. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
75 turns.
X.
Carotid pulse - which is located in the neck next to the Adam's apple.

30. 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?
Draw less current.
800 volts.
An electrical shock hazard may exist.
15 feet off the ground.

31. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Allow for metering voltage and Current.
25 feet.
Alternating current.
X.

32. What is a capacitor?
Inductance.
1 amp.
A device used to store electrical energy.
No.6 AWG.

33. Operating coils - motors - transformers - relays and solenoids depend upon...
Provides single-phase power to smaller loads such as lights - heaters - etc.
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Electro-magnetism for their operation.

34. The property of an electrical circuit that opposes any change in voltage is called...
Lower current and higher voltage
800 volts.
Capacitance.
100 volts.

35. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Four times less than that of the secondary.
A step-up transformer.
Reverse any two-phase leads.
100 volts.

36. Three-phase AC underground cables must be provided with overcurrent protection using a...
At least two phases
Isolation transformer.
15 feet off the ground.
Circuit Breaker.

37. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Draw less current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
60 cycles per second (60 Hertz).

38. 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.
Cut around clothing that has adhered to the burned area.
25 feet.
Isolation transformer.

39. Where should grounding resistors be located?
At the source transformer.
Larger than that found in the primary winding.
Reverse any two-phase leads.
It provides good speed control.

40. The standard frequency of alternating current used at a mine is...
Grounded phase and ground fault devices were not operating.
5:1.
60 Hertz
H terminals.

41. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Second-degree burn.
Check to make sure that the victim is not still in contact with the electricity.
7 -630 volts.
At least two phases

42. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Lower current and higher voltage
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Allow for metering voltage and Current.

43. Current transformers - or CT's - are used to...
Cut around clothing that has adhered to the burned area.
H terminals.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.

44. How should clothing be removed from a burn?
60 Hertz
Cut around clothing that has adhered to the burned area.
Reverse any two-phase leads.
A device used to store electrical energy.

45. 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?
200 volts AC.
Tied together
60 cycles per second (60 Hertz).
75 turns.

46. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
1 amp.
At least two phases
A grounding resistor.

47. Not a symptom of electrical shock?.
Capacitance.
Cut around clothing that has adhered to the burned area.
Alternating current.
Hyperventilation.

48. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Capacitance.
The chest should rise or inflate.
Disconnect the frame-grounding conductor.
Inductance.

49. Not true about AC power?
A step-up transformer.
It provides good speed control.
60 Hertz
No.6 AWG.

50. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
3 -979 Volts.
An electromagnet.
Reverse its rotation.