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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. 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...
60 cycles per second (60 Hertz).
100 volts.
Continuous.
Carotid pulse - which is located in the neck next to the Adam's apple.

2. To protect against over-current in a three-phase circuit over-current protection must be provided for...
800 volts.
A grounding resistor.
At least two phases
Reverse its rotation.

3. The combined capacitance of two capacitors in parallel is...
75 turns.
A step-up transformer.
The sum of the two capacitors.
Inductance.

4. The secondary windings on a 10:1 step-down transformer will have a wire size...
Inductance.
5:1.
Larger than that found in the primary winding.
Reverse its rotation.

5. What steps up or steps down AC voltage?
Transformer
Draw less current.
Lower current and higher voltage
At least two phases

6. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Four times less than that of the secondary.
277 volts.
Short-circuited after power has been removed.
Clamp-on ammeter.

7. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Draw less current.
X.
Transformer

8. The main function of CT's and PT's on a main substation is to...
Lower current and higher voltage.
Allow for metering voltage and Current.
A grounding resistor.
Reverse its rotation.

9. Operating coils - motors - transformers - relays and solenoids depend upon...
Isolation transformer.
Potential transformer.
Short-circuited after power has been removed.
Electro-magnetism for their operation.

10. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
15 feet off the ground.
Draw less current.
Grounded phase and ground fault devices were not operating.
Carotid pulse - which is located in the neck next to the Adam's apple.

11. What is the first step to be followed in aiding a victim of electrical shock?
It provides good speed control.
Current that changes constantly in magnitude and periodically in direction.
Check to make sure that the victim is not still in contact with the electricity.
Isolation transformer.

12. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Disconnect the frame-grounding conductor.
100 turns.
No.6 AWG.
150 turns.

13. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
277 volts.
800 volts.
The current will read zero.

14. The primary leads of a single-phase transformer should be connected to the...
60 cycles per second (60 Hertz).
5:1
H terminals.
Alternating current.

15. How should clothing be removed from a burn?
The capacitors should be discharged.
Inductance.
Cut around clothing that has adhered to the burned area.
Current changing constantly in magnitude and periodically in direction.

16. Current transformers - or CT's - are used to...
25 feet.
A step-up transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Continuous.

17. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
An electrical shock hazard may exist.
Check to make sure that the victim is not still in contact with the electricity.
5:1
480 volts.

18. The size of a transformer is measured in...
Kilovolt ampere (KVA).
5:1
X.
Four times less than that of the secondary.

19. 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?
Provides single-phase power to smaller loads such as lights - heaters - etc.
150 turns.
25 feet.
Short-circuited after power has been removed.

20. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
At least two phases
100 volts.
They should be made by the same manufacturer.

21. What is a capacitor?
7 -630 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Clamp-on ammeter.
A device used to store electrical energy.

22. Which of the following does a squirrel cage induction motor have?
Stator windings.
A device used to store electrical energy.
At least two phases
A step-up transformer.

23. What type motor is a wound-rotor induction motor?
Reverse its rotation.
Clamp-on ammeter.
A.C.
It provides good speed control.

24. Current that changes periodically in direction is...
Lower current and higher voltage
Alternating current.
Lower current and higher voltage.
Potential transformer.

25. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Disconnect the frame-grounding conductor.
Check to make sure that the victim is not still in contact with the electricity.
The motor rotation is reversed.
5:1

26. 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...
100 turns.
Current that changes constantly in magnitude and periodically in direction.
Tied together
Inductance.

27. The speed of an alternating current - squirrel-cage induction motor is...
Draw less current.
15 feet off the ground.
Nearly constant.
800 volts.

28. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Circuit Breaker.
Squirrel-cage rotor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
277 volts.

29. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
They should be made by the same manufacturer.
Full-load - three-phase voltage.
60 Hertz
Derive a neutral for grounding purposes.

30. Define alternating current...
Inductance.
Current changing constantly in magnitude and periodically in direction.
600 volts.
Full-load - three-phase voltage.

31. 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...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Allow for metering voltage and Current.
1 amp.
Draw less current.

32. When not connected to a load - the secondary leads of a current transformer should be...
An electromagnet.
The capacitors should be discharged.
Shunted and taped
Cut around clothing that has adhered to the burned area.

33. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
Potential transformer.
100 turns.
Carotid pulse - which is located in the neck next to the Adam's apple.

34. The standard frequency of power supplied by the power company is...
Derive a neutral for grounding purposes.
60 cycles per second (60 Hertz).
100 volts.
Tied together

35. Not true about AC power?
They must be installed at least 7 feet above the ground.
The sum of the two capacitors.
15 feet off the ground.
It provides good speed control.

36. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
60 Hertz
2000 watts
Lower current and higher voltage
Two speeds

37. A control power transformer...
H terminals.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Hyperventilation.
60 cycles per second (60 Hertz).

38. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
An electromagnet.
Carotid pulse - which is located in the neck next to the Adam's apple.
Kilovolt ampere (KVA).
Two speeds

39. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
It provides good speed control.
The capacitors should be discharged.
Squirrel-cage induction motor.
100 volts.

40. At what rate should mouth-to-mouth resuscitation be applied?
Carotid pulse - which is located in the neck next to the Adam's apple.
A.C.
12 to 15 times a minute.
277 volts.

41. 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.
Draw less current.
Squirrel-cage rotor.

42. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
Lower current and higher voltage.
Because less current flowing at high voltage causes less line loss per kilowatt.
277 volts.

43. 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?
Second-degree burn.
3 -979 Volts.
Zero amps.
Two speeds

44. 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 chest should rise or inflate.
Draw less current.
An electromagnet.
Continuous.

45. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Squirrel-cage induction motor.
60 cycles per second (60 Hertz).
Nearly constant.

46. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
No.6 AWG.
Continuous.
12 to 15 times a minute.
A grounding resistor.

47. Not a symptom of electrical shock?.
Allow for metering voltage and Current.
They should be made by the same manufacturer.
Hyperventilation.
5:1

48. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Zero amps.
X.
The sum of the two capacitors.
The motor rotation is reversed.

49. What should the duty rating of a grounding resistor be?
They should be made by the same manufacturer.
Continuous.
Potential transformer.
Short-circuited after power has been removed.

50. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Transformer
The chest should rise or inflate.
15 feet off the ground.
Reverse its rotation.