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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. Define alternating current...
They must be installed at least 7 feet above the ground.
Potential transformer.
Lower current and higher voltage.
Current changing constantly in magnitude and periodically in direction.

2. 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...
Zero amps.
3 -979 Volts.
1 amp.
Tied together

3. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
100 volts.
No.6 AWG.
Full-load - three-phase voltage.
Allow for metering voltage and Current.

4. 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.
100 volts.
Continuous.
A grounding resistor.

5. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
H terminals.
200 volts AC.
Tied together
A step-up transformer.

6. Which of the following does a squirrel cage induction motor have?
The current will read zero.
The capacitors should be discharged.
Stator windings.
Short-circuited after power has been removed.

7. What type of 3-phase AC motor is used most often around a mine?
Reverse its rotation.
25 feet.
Derive a neutral for grounding purposes.
Squirrel-cage induction motor.

8. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
A device used to store electrical energy.
Two speeds
800 volts.

9. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
100 volts.
1 amp.
An electromagnet.
Zero amps.

10. The transformer used only to measure voltage is a...
They must be installed at least 7 feet above the ground.
200 volts AC.
Potential transformer.
Lower current and higher voltage

11. 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.
480 volts.
H terminals.
150 turns.

12. Alternating current may be defined as...
Hyperventilation.
At the source transformer.
Current that changes constantly in magnitude and periodically in direction.
It provides good speed control.

13. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
The chest should rise or inflate.
Full-load - three-phase voltage.
H terminals.

14. A wound-rotor induction motor has all the following EXCEPT...
75 turns.
800 volts.
Squirrel-cage rotor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

15. The standard frequency of power supplied by the power company is...
Clamp-on ammeter.
Draw less current.
60 cycles per second (60 Hertz).
Check to make sure that the victim is not still in contact with the electricity.

16. High voltage is used to transmit power over long distances...
Stator windings.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Because less current flowing at high voltage causes less line loss per kilowatt.
The sum of the two capacitors.

17. 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?
Grounded phase and ground fault devices were not operating.
200 volts AC.
100 volts.
Isolation transformer.

18. A burn causing a blister is classified as a...
Lower current and higher voltage.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Second-degree burn.
5:1

19. The property of an electric circuit that opposes any change in current is called...
Inductance.
Draw less current.
A step-up transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.

20. Current transformers - or CT's - are used to...
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.
No.6 AWG.
15 feet off the ground.

21. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
A grounding resistor.
7 -630 volts.
A step-up transformer.
It provides good speed control.

22. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
A.C.
2000 watts
5:1.
3 -979 Volts.

23. At what rate should mouth-to-mouth resuscitation be applied?
The chest should rise or inflate.
Draw less current.
12 to 15 times a minute.
5:1

24. AC voltage is raised or lowered by means of...
The motor rotation is reversed.
Short-circuited after power has been removed.
A step-up transformer.
Transformers.

25. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
75 turns.
The motor rotation is reversed.
The chest should rise or inflate.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

26. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
5:1
Disconnect the frame-grounding conductor.
Larger than that found in the primary winding.
Shunted and taped

27. When not connected to a load - the secondary leads of a current transformer should be...
60 Hertz
Shunted and taped
200 volts AC.
Reverse any two-phase leads.

28. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
Inductance.
Tied together
7 -630 volts.

29. How do you change the direction of a 3-phase AC motor?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Reverse any two-phase leads.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A.C.

30. A transformer with 1 -500 turns in the primary winding has a primary voltage of 2 -400 volts and a secondary voltage of 120 volts. How many turns are in the secondary winding?
Squirrel-cage induction motor.
Allow for metering voltage and Current.
H terminals.
75 turns.

31. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
200 volts AC.
Squirrel-cage rotor.
The motor rotation is reversed.
Four times less than that of the secondary.

32. Where should grounding resistors be located?
Because less current flowing at high voltage causes less line loss per kilowatt.
Electro-magnetism for their operation.
Current that changes constantly in magnitude and periodically in direction.
At the source transformer.

33. The property of an electrical circuit that opposes any change in voltage is called...
Transformer
They should be made by the same manufacturer.
Lower current and higher voltage.
Capacitance.

34. Operating coils - motors - transformers - relays and solenoids depend upon...
100 turns.
60 Hertz
5:1.
Electro-magnetism for their operation.

35. What is the danger when working around capacitors?
Lower current and higher voltage
100 volts.
Squirrel-cage rotor.
An electrical shock hazard may exist.

36. 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?
Four times less than that of the secondary.
75 turns.
Draw less current.
The sum of the two capacitors.

37. All transformers step voltage up or down by...
5:1
75 turns.
Inductance.
Tied together

38. A control power transformer...
Tied together
Provides single-phase power to smaller loads such as lights - heaters - etc.
Transformers.
7 -630 volts.

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

40. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
The chest should rise or inflate.
Current that changes constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.
15 feet off the ground.

41. 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.
They must be installed at least 7 feet above the ground.
Squirrel-cage rotor.
A step-up transformer.

42. What steps up or steps down AC voltage?
Current changing constantly in magnitude and periodically in direction.
Transformer
The motor rotation is reversed.
2000 watts

43. A solenoid coil is...
Reverse its rotation.
An electromagnet.
Electro-magnetism for their operation.
60 cycles per second (60 Hertz).

44. 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...
The capacitors should be discharged.
200 volts AC.
Continuous.
100 volts.

45. The standard frequency of alternating current used at a mine is...
Circuit Breaker.
75 turns.
60 Hertz
At least two phases

46. The primary leads of a single-phase transformer should be connected to the...
Isolation transformer.
150 turns.
H terminals.
480 volts.

47. What type motor is a wound-rotor induction motor?
800 volts.
It provides good speed control.
A.C.
Squirrel-cage induction motor.

48. 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.
5:1
800 volts.
Larger than that found in the primary winding.
Potential transformer.

49. Compared to the secondary of a loaded - step-down transformer - the primary has...
A grounding resistor.
Current that changes constantly in magnitude and periodically in direction.
Reverse its rotation.
Lower current and higher voltage

50. A transformer with a 1:1 ratio is usually a (an)...
Stator windings.
Draw less current.
Isolation transformer.
7 -630 volts.