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Test your basic knowledge |

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...
Potential transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.

2. Compared to the secondary of a loaded - step-down transformer - the primary has...
Disconnect the frame-grounding conductor.
Lower current and higher voltage
Squirrel-cage induction motor.
25 feet.

3. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Transformers.
Lower current and higher voltage
Short-circuited after power has been removed.
An electromagnet.

4. What is the first step to be followed in aiding a victim of electrical shock?
An electromagnet.
Check to make sure that the victim is not still in contact with the electricity.
Isolation transformer.
Current that changes constantly in magnitude and periodically in direction.

5. How do you change the direction of a 3-phase AC motor?
A.C.
An electromagnet.
Reverse any two-phase leads.
Allow for metering voltage and Current.

6. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Reverse its rotation.
Circuit Breaker.
Lower current and higher voltage

7. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Circuit Breaker.
5:1
Reverse its rotation.

8. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Derive a neutral for grounding purposes.
75 turns.
5:1.
Four times less than that of the secondary.

9. Compared to the secondary of a loaded - step-down transformer - the primary has...
At least two phases
Lower current and higher voltage.
Capacitance.
Cut around clothing that has adhered to the burned area.

10. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Reverse its rotation.
Full-load - three-phase voltage.
25 feet.
A device used to store electrical energy.

11. A wound-rotor induction motor has all the following EXCEPT...
Reverse any two-phase leads.
Squirrel-cage rotor.
Clamp-on ammeter.
Draw less current.

12. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Current changing constantly in magnitude and periodically in direction.
A step-up transformer.
They should be made by the same manufacturer.
Disconnect the frame-grounding conductor.

13. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
An electrical shock hazard may exist.
A step-up transformer.
Allow for metering voltage and Current.

14. What type of 3-phase AC motor is used most often around a mine?
Isolation transformer.
60 Hertz
Squirrel-cage induction motor.
Disconnect the frame-grounding conductor.

15. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Electro-magnetism for their operation.
Reverse its rotation.
5:1.

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

17. 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...
Check to make sure that the victim is not still in contact with the electricity.
Tied together
2000 watts
Lower current and higher voltage

18. 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.
Kilovolt ampere (KVA).
Transformer
Continuous.

19. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Alternating current.
Continuous.
100 turns.

20. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
75 turns.
Transformer
A grounding resistor.

21. 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.
Continuous.
5:1
Current that changes constantly in magnitude and periodically in direction.
2000 watts

22. All transformers step voltage up or down by...
Transformer
No.6 AWG.
Carotid pulse - which is located in the neck next to the Adam's apple.
Inductance.

23. The property of an electric circuit that opposes any change in current is called...
75 turns.
100 volts.
Larger than that found in the primary winding.
Inductance.

24. 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...
3 -979 Volts.
100 volts.
X.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

25. 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...
H terminals.
Lower current and higher voltage
Draw less current.
A grounding resistor.

26. When not connected to a load - the secondary leads of a current transformer should be...
Tied together
Carotid pulse - which is located in the neck next to the Adam's apple.
Shunted and taped
Larger than that found in the primary winding.

27. Current that changes periodically in direction is...
5:1.
Allow for metering voltage and Current.
Alternating current.
75 turns.

28. 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...
X.
They should be made by the same manufacturer.
Draw less current.
75 turns.

29. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
Alternating current.
25 feet.
At the source transformer.

30. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
200 volts AC.
Zero amps.
Grounded phase and ground fault devices were not operating.
75 turns.

31. To protect against over-current in a three-phase circuit over-current protection must be provided for...
25 feet.
At least two phases
Because less current flowing at high voltage causes less line loss per kilowatt.
The current will read zero.

32. The standard frequency of power supplied by the power company is...
The motor rotation is reversed.
60 cycles per second (60 Hertz).
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Current that changes constantly in magnitude and periodically in direction.

33. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
The sum of the two capacitors.
15 feet off the ground.
7 -630 volts.
600 volts.

34. Current transformers - or CT's - are used to...
Current changing constantly in magnitude and periodically in direction.
Lower current and higher voltage
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
60 Hertz

35. 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?
H terminals.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Current changing constantly in magnitude and periodically in direction.
3 -979 Volts.

36. A solenoid coil is...
Hyperventilation.
3 -979 Volts.
Capacitance.
An electromagnet.

37. AC voltage is raised or lowered by means of...
The motor rotation is reversed.
480 volts.
At the source transformer.
Transformers.

38. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Lower current and higher voltage
600 volts.
The chest should rise or inflate.
60 cycles per second (60 Hertz).

39. What is the danger when working around capacitors?
Alternating current.
An electrical shock hazard may exist.
The capacitors should be discharged.
Reverse any two-phase leads.

40. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
Current that changes constantly in magnitude and periodically in direction.
100 turns.
Electro-magnetism for their operation.

41. 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?
100 turns.
Tied together
60 Hertz
Inductance.

42. The main function of CT's and PT's on a main substation is to...
Squirrel-cage induction motor.
800 volts.
An electrical shock hazard may exist.
Allow for metering voltage and Current.

43. The speed of an alternating current - squirrel-cage induction motor is...
480 volts.
Nearly constant.
The current will read zero.
Current changing constantly in magnitude and periodically in direction.

44. 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?
The sum of the two capacitors.
200 volts AC.
Current that changes constantly in magnitude and periodically in direction.
Inductance.

45. Which of the following does a squirrel cage induction motor have?
Stator windings.
Inductance.
The capacitors should be discharged.
800 volts.

46. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Short-circuited after power has been removed.
Squirrel-cage rotor.
7 -630 volts.
Shunted and taped

47. The property of an electrical circuit that opposes any change in voltage is called...
100 volts.
Capacitance.
They must be installed at least 7 feet above the ground.
H terminals.

48. What type motor is a wound-rotor induction motor?
Clamp-on ammeter.
75 turns.
No.6 AWG.
A.C.

49. The standard frequency of alternating current used at a mine is...
60 Hertz
25 feet.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.

50. Not true about AC power?
A.C.
Transformer
It provides good speed control.
15 feet off the ground.