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. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Kilovolt ampere (KVA).
Nearly constant.
1 amp.
5:1.

2. The primary leads of a single-phase transformer should be connected to the...
H terminals.
5:1.
Reverse its rotation.
5:1

3. 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.
480 volts.
At the source transformer.
Second-degree burn.

4. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
277 volts.
Nearly constant.
2000 watts
A grounding resistor.

5. 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?
An electrical shock hazard may exist.
3 -979 Volts.
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage induction motor.

6. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Reverse any two-phase leads.
Zero amps.
The motor rotation is reversed.
At the source transformer.

7. Compared to the secondary of a loaded - step-down transformer - the primary has...
75 turns.
Lower current and higher voltage.
An electromagnet.
60 cycles per second (60 Hertz).

8. The primary function of a power transformer is to provide...
Short-circuited after power has been removed.
Inductance.
Full-load - three-phase voltage.
Alternating current.

9. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
No.6 AWG.
Grounded phase and ground fault devices were not operating.
Inductance.
277 volts.

10. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Electro-magnetism for their operation.
The motor rotation is reversed.
The capacitors should be discharged.
The current will read zero.

11. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Reverse its rotation.
Second-degree burn.
2000 watts
15 feet off the ground.

12. Three-phase AC underground cables must be provided with overcurrent protection using a...
Transformers.
Continuous.
The motor rotation is reversed.
Circuit Breaker.

13. AC voltage is raised or lowered by means of...
Cut around clothing that has adhered to the burned area.
Kilovolt ampere (KVA).
Transformers.
Lower current and higher voltage

14. What is the danger when working around capacitors?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electrical shock hazard may exist.
Hyperventilation.
Allow for metering voltage and Current.

15. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
The chest should rise or inflate.
They should be made by the same manufacturer.
At the source transformer.

16. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
60 cycles per second (60 Hertz).
The sum of the two capacitors.
Electro-magnetism for their operation.
Disconnect the frame-grounding conductor.

17. Current that changes periodically in direction is...
Electro-magnetism for their operation.
Alternating current.
Hyperventilation.
Second-degree burn.

18. A transformer with a 1:1 ratio is usually a (an)...
An electromagnet.
H terminals.
Isolation transformer.
Draw less current.

19. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The capacitors should be discharged.
No.6 AWG.
60 Hertz
Check to make sure that the victim is not still in contact with the electricity.

20. 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?
150 turns.
A step-up transformer.
Tied together
The sum of the two capacitors.

21. Where should grounding resistors be located?
A step-up transformer.
Alternating current.
At the source transformer.
It provides good speed control.

22. 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.
Transformers.
12 to 15 times a minute.
Stator windings.

23. The property of an electrical circuit that opposes any change in voltage is called...
480 volts.
Check to make sure that the victim is not still in contact with the electricity.
Capacitance.
At least two phases

24. What should the duty rating of a grounding resistor be?
75 turns.
Transformers.
Larger than that found in the primary winding.
Continuous.

25. How do you change the direction of a 3-phase AC motor?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A device used to store electrical energy.
Cut around clothing that has adhered to the burned area.
Reverse any two-phase leads.

26. Alternating current may be defined as...
The capacitors should be discharged.
Current that changes constantly in magnitude and periodically in direction.
7 -630 volts.
Transformer

27. 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 volts.
12 to 15 times a minute.
Alternating current.
Tied together

28. The main function of CT's and PT's on a main substation is to...
Shunted and taped
3 -979 Volts.
Allow for metering voltage and Current.
Second-degree burn.

29. Current transformers - or CT's - are used to...
Second-degree burn.
Circuit Breaker.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Tied together

30. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Kilovolt ampere (KVA).
Zero amps.
Larger than that found in the primary winding.
Circuit Breaker.

31. The transformer used only to measure voltage is a...
No.6 AWG.
Current that changes constantly in magnitude and periodically in direction.
100 turns.
Potential transformer.

32. 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.
They should be made by the same manufacturer.
A grounding resistor.
Derive a neutral for grounding purposes.
5:1

33. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
5:1
Alternating current.
Transformer
480 volts.

34. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
100 turns.
Nearly constant.
Draw less current.

35. Not a symptom of electrical shock?.
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage induction motor.
Inductance.
Hyperventilation.

36. 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.
Second-degree burn.
7 -630 volts.
Clamp-on ammeter.

37. What type motor is a wound-rotor induction motor?
A.C.
Continuous.
15 feet off the ground.
Capacitance.

38. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Current changing constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
Full-load - three-phase voltage.

39. 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?
Four times less than that of the secondary.
200 volts AC.
A grounding resistor.
60 Hertz

40. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
100 volts.
Grounded phase and ground fault devices were not operating.
Hyperventilation.
Draw less current.

41. 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...
Draw less current.
5:1.
Inductance.
1 amp.

42. A control power transformer...
60 cycles per second (60 Hertz).
Four times less than that of the secondary.
1 amp.
Provides single-phase power to smaller loads such as lights - heaters - etc.

43. High voltage is used to transmit power over long distances...
Two speeds
Because less current flowing at high voltage causes less line loss per kilowatt.
150 turns.
1 amp.

44. Define alternating current...
800 volts.
100 turns.
60 cycles per second (60 Hertz).
Current changing constantly in magnitude and periodically in direction.

45. The secondary windings on a 10:1 step-down transformer will have a wire size...
277 volts.
Larger than that found in the primary winding.
Potential transformer.
Capacitance.

46. What steps up or steps down AC voltage?
Transformer
Circuit Breaker.
A step-up transformer.
3 -979 Volts.

47. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Transformers.
A device used to store electrical energy.
Current changing constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.

48. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Two speeds
Inductance.
Derive a neutral for grounding purposes.
The chest should rise or inflate.

49. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Squirrel-cage induction motor.
Lower current and higher voltage.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electromagnet.

50. 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?
Squirrel-cage induction motor.
100 turns.
Draw less current.
Alternating current.