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 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?
5:1.
15 feet off the ground.
100 turns.
277 volts.

2. Not a symptom of electrical shock?.
Hyperventilation.
1 amp.
15 feet off the ground.
480 volts.

3. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Larger than that found in the primary winding.
Tied together
They must be installed at least 7 feet above the ground.
15 feet off the ground.

4. Which of the following does a squirrel cage induction motor have?
Stator windings.
The capacitors should be discharged.
They should be made by the same manufacturer.
75 turns.

5. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
7 -630 volts.
At least two phases
600 volts.
Disconnect the frame-grounding conductor.

6. 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...
No.6 AWG.
3 -979 Volts.
Hyperventilation.
Tied together

7. The secondary windings on a 10:1 step-down transformer will have a wire size...
277 volts.
Draw less current.
Draw less current.
Larger than that found in the primary winding.

8. The standard frequency of power supplied by the power company is...
Squirrel-cage rotor.
60 cycles per second (60 Hertz).
The motor rotation is reversed.
Lower current and higher voltage

9. The standard frequency of alternating current used at a mine is...
Transformer
H terminals.
60 Hertz
7 -630 volts.

10. 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.
15 feet off the ground.
100 volts.
Current changing constantly in magnitude and periodically in direction.

11. Where should grounding resistors be located?
Hyperventilation.
200 volts AC.
At the source transformer.
Continuous.

12. 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.
Kilovolt ampere (KVA).
H terminals.
Stator windings.

13. 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?
Draw less current.
75 turns.
12 to 15 times a minute.
Squirrel-cage induction motor.

14. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
No.6 AWG.
Inductance.
The sum of the two capacitors.

15. What is a capacitor?
A device used to store electrical energy.
Two speeds
Zero amps.
150 turns.

16. What should the duty rating of a grounding resistor be?
A grounding resistor.
Continuous.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Allow for metering voltage and Current.

17. 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...
100 volts.
An electromagnet.
Electro-magnetism for their operation.
Grounded phase and ground fault devices were not operating.

18. 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...
Electro-magnetism for their operation.
No.6 AWG.
Transformers.
Draw less current.

19. Define alternating current...
Continuous.
Current changing constantly in magnitude and periodically in direction.
Stator windings.
75 turns.

20. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Inductance.
Grounded phase and ground fault devices were not operating.
5:1
No.6 AWG.

21. The main function of CT's and PT's on a main substation is to...
Inductance.
The capacitors should be discharged.
Grounded phase and ground fault devices were not operating.
Allow for metering voltage and Current.

22. Compared to the secondary of a loaded - step-down transformer - the primary has...
The capacitors should be discharged.
Lower current and higher voltage.
Lower current and higher voltage
1 amp.

23. What type of 3-phase AC motor is used most often around a mine?
A.C.
Potential transformer.
60 Hertz
Squirrel-cage induction motor.

24. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Draw less current.
7 -630 volts.
150 turns.
Shunted and taped

25. A control power transformer...
H terminals.
Provides single-phase power to smaller loads such as lights - heaters - etc.
X.
25 feet.

26. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Alternating current.
75 turns.
The chest should rise or inflate.

27. Not true about AC power?
It provides good speed control.
Clamp-on ammeter.
X.
Lower current and higher voltage.

28. The size of a transformer is measured in...
Kilovolt ampere (KVA).
100 turns.
Isolation transformer.
Check to make sure that the victim is not still in contact with the electricity.

29. At what rate should mouth-to-mouth resuscitation be applied?
They should be made by the same manufacturer.
12 to 15 times a minute.
Stator windings.
The motor rotation is reversed.

30. Find the turns ratio of a single-phase transformer with the following values: Primary power of 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
5:1.
Reverse any two-phase leads.
800 volts.
7 -630 volts.

31. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Inductance.
Four times less than that of the secondary.
Transformers.
The capacitors should be discharged.

32. Operating coils - motors - transformers - relays and solenoids depend upon...
Inductance.
They should be made by the same manufacturer.
Draw less current.
Electro-magnetism for their operation.

33. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
X.
Four times less than that of the secondary.
Grounded phase and ground fault devices were not operating.
At the source transformer.

34. 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...
Squirrel-cage rotor.
Reverse any two-phase leads.
At least two phases
Draw less current.

35. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
They must be installed at least 7 feet above the ground.
75 turns.
7 -630 volts.

36. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Squirrel-cage induction motor.
Squirrel-cage rotor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The capacitors should be discharged.

37. The combined capacitance of two capacitors in parallel is...
60 Hertz
Inductance.
The sum of the two capacitors.
Squirrel-cage induction motor.

38. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
1 amp.
Reverse any two-phase leads.
They should be made by the same manufacturer.
Squirrel-cage induction motor.

39. Current transformers - or CT's - are used to...
Lower current and higher voltage.
The chest should rise or inflate.
Potential transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

40. Alternating current may be defined as...
75 turns.
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Current that changes constantly in magnitude and periodically in direction.

41. A solenoid coil is...
An electromagnet.
Squirrel-cage induction motor.
12 to 15 times a minute.
Lower current and higher voltage.

42. To protect against over-current in a three-phase circuit over-current protection must be provided for...
100 turns.
At least two phases
Electro-magnetism for their operation.
277 volts.

43. 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.
480 volts.
Current changing constantly in magnitude and periodically in direction.
Four times less than that of the secondary.

44. High voltage is used to transmit power over long distances...
It provides good speed control.
Because less current flowing at high voltage causes less line loss per kilowatt.
They must be installed at least 7 feet above the ground.
Zero amps.

45. The purpose of a zig-zag transformer is to...
Squirrel-cage induction motor.
800 volts.
25 feet.
Derive a neutral for grounding purposes.

46. A wound-rotor induction motor has all the following EXCEPT...
Derive a neutral for grounding purposes.
Transformer
Short-circuited after power has been removed.
Squirrel-cage rotor.

47. Compared to the secondary of a loaded - step-down transformer - the primary has...
Stator windings.
Lower current and higher voltage
Current changing constantly in magnitude and periodically in direction.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

48. A transformer has 1 -600 turns in the primary and 400 turns in the secondary. The primary voltage is 2 -400 volts. What is the secondary voltage?
A device used to store electrical energy.
600 volts.
Disconnect the frame-grounding conductor.
Inductance.

49. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
60 Hertz
75 turns.
600 volts.

50. The property of an electric circuit that opposes any change in current is called...
Continuous.
Draw less current.
5:1.
Inductance.