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. 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.
800 volts.
They should be made by the same manufacturer.

2. A control power transformer...
1 amp.
Reverse its rotation.
75 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.

3. The speed of an alternating current - squirrel-cage induction motor is...
Short-circuited after power has been removed.
Full-load - three-phase voltage.
Nearly constant.
H terminals.

4. High voltage is used to transmit power over long distances...
A device used to store electrical energy.
Because less current flowing at high voltage causes less line loss per kilowatt.
Current changing constantly in magnitude and periodically in direction.
The motor rotation is reversed.

5. A wound-rotor induction motor has all the following EXCEPT...
Continuous.
7 -630 volts.
Squirrel-cage rotor.
No.6 AWG.

6. AC voltage is raised or lowered by means of...
Tied together
Cut around clothing that has adhered to the burned area.
Potential transformer.
Transformers.

7. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Continuous.
Two speeds
12 to 15 times a minute.
An electrical shock hazard may exist.

8. 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.
Reverse its rotation.
The capacitors should be discharged.
Grounded phase and ground fault devices were not operating.

9. 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?
Clamp-on ammeter.
Check to make sure that the victim is not still in contact with the electricity.
100 turns.
No.6 AWG.

10. 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.
Lower current and higher voltage
60 cycles per second (60 Hertz).
Check to make sure that the victim is not still in contact with the electricity.

11. 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?
100 turns.
Stator windings.
75 turns.
The sum of the two capacitors.

12. The standard frequency of alternating current used at a mine is...
Four times less than that of the secondary.
60 Hertz
A device used to store electrical energy.
Capacitance.

13. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
They must be installed at least 7 feet above the ground.
Disconnect the frame-grounding conductor.
Derive a neutral for grounding purposes.
Draw less current.

14. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
3 -979 Volts.
Two speeds
A grounding resistor.

15. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
5:1
Current that changes constantly in magnitude and periodically in direction.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Alternating current.

16. 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?
Clamp-on ammeter.
A.C.
150 turns.
Two speeds

17. Current that changes periodically in direction is...
480 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Alternating current.

18. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Potential transformer.
60 cycles per second (60 Hertz).
150 turns.

19. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Squirrel-cage induction motor.
Provides single-phase power to smaller loads such as lights - heaters - etc.
480 volts.
Short-circuited after power has been removed.

20. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Inductance.
15 feet off the ground.
Shunted and taped
No.6 AWG.

21. What should the duty rating of a grounding resistor be?
Inductance.
An electromagnet.
Carotid pulse - which is located in the neck next to the Adam's apple.
Continuous.

22. The purpose of a zig-zag transformer is to...
Circuit Breaker.
5:1.
Second-degree burn.
Derive a neutral for grounding purposes.

23. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
Allow for metering voltage and Current.
Grounded phase and ground fault devices were not operating.
Reverse any two-phase leads.

24. 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.
7 -630 volts.
5:1
A step-up transformer.
Isolation transformer.

25. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
A.C.
Current changing constantly in magnitude and periodically in direction.
Current that changes constantly in magnitude and periodically in direction.

26. Not true about AC power?
No.6 AWG.
Because less current flowing at high voltage causes less line loss per kilowatt.
60 cycles per second (60 Hertz).
It provides good speed control.

27. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
277 volts.
An electromagnet.
Capacitance.

28. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Derive a neutral for grounding purposes.
Second-degree burn.
Larger than that found in the primary winding.
Zero amps.

29. The size of a transformer is measured in...
They must be installed at least 7 feet above the ground.
At least two phases
Current changing constantly in magnitude and periodically in direction.
Kilovolt ampere (KVA).

30. What is the danger when working around capacitors?
Second-degree burn.
An electrical shock hazard may exist.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A device used to store electrical energy.

31. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Carotid pulse - which is located in the neck next to the Adam's apple.
An electromagnet.
A.C.

32. All transformers step voltage up or down by...
Inductance.
277 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Full-load - three-phase voltage.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
Clamp-on ammeter.
A grounding resistor.
2000 watts
Lower current and higher voltage.

34. Current in an AC circuit may be measured by...
Current changing constantly in magnitude and periodically in direction.
They must be installed at least 7 feet above the ground.
A device used to store electrical energy.
Clamp-on ammeter.

35. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The sum of the two capacitors.
The chest should rise or inflate.
Short-circuited after power has been removed.
Alternating current.

36. 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.
Current that changes constantly in magnitude and periodically in direction.
Full-load - three-phase voltage.
Squirrel-cage rotor.

37. 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?
3 -979 Volts.
Second-degree burn.
A device used to store electrical energy.
60 cycles per second (60 Hertz).

38. 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...
They must be installed at least 7 feet above the ground.
5:1
Draw less current.
Electro-magnetism for their operation.

39. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
12 to 15 times a minute.
A.C.
Four times less than that of the secondary.
Kilovolt ampere (KVA).

40. Define alternating current...
Short-circuited after power has been removed.
Current changing constantly in magnitude and periodically in direction.
Provides single-phase power to smaller loads such as lights - heaters - etc.
A grounding resistor.

41. Current transformers - or CT's - are used to...
Nearly constant.
An electromagnet.
A step-up transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

42. What steps up or steps down AC voltage?
Shunted and taped
Because less current flowing at high voltage causes less line loss per kilowatt.
Transformer
600 volts.

43. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Draw less current.
Isolation transformer.
100 turns.
1 amp.

44. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
Second-degree burn.
A device used to store electrical energy.
Four times less than that of the secondary.

45. 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?
2000 watts
1 amp.
7 -630 volts.
800 volts.

46. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Second-degree burn.
Continuous.
Provides single-phase power to smaller loads such as lights - heaters - etc.

47. The primary function of a power transformer is to provide...
Hyperventilation.
75 turns.
Full-load - three-phase voltage.
Potential transformer.

48. The transformer used only to measure voltage is a...
600 volts.
5:1
Potential transformer.
150 turns.

49. What type of 3-phase AC motor is used most often around a mine?
Alternating current.
277 volts.
100 turns.
Squirrel-cage induction motor.

50. A solenoid coil is...
Inductance.
Zero amps.
An electromagnet.
Provides single-phase power to smaller loads such as lights - heaters - etc.