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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. 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.
15 feet off the ground.
Lower current and higher voltage.
Tied together

2. Where should grounding resistors be located?
At the source transformer.
An electrical shock hazard may exist.
60 cycles per second (60 Hertz).
Short-circuited after power has been removed.

3. Current transformers - or CT's - are used to...
Reverse any two-phase leads.
Circuit Breaker.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
5:1.

4. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
60 Hertz
2000 watts
The chest should rise or inflate.
75 turns.

5. AC voltage is raised or lowered by means of...
Continuous.
The current will read zero.
A grounding resistor.
Transformers.

6. 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?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.
7 -630 volts.
75 turns.

7. 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.
Hyperventilation.
5:1.
A.C.
Stator windings.

8. 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?
Derive a neutral for grounding purposes.
Continuous.
800 volts.
Hyperventilation.

9. The purpose of a zig-zag transformer is to...
Inductance.
The motor rotation is reversed.
A step-up transformer.
Derive a neutral for grounding purposes.

10. 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...
Transformer
Capacitance.
Lower current and higher voltage
Draw less current.

11. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
200 volts AC.
Squirrel-cage rotor.
Alternating current.

12. Current that changes periodically in direction is...
3 -979 Volts.
15 feet off the ground.
Alternating current.
800 volts.

13. What type of 3-phase AC motor is used most often around a mine?
75 turns.
Derive a neutral for grounding purposes.
Squirrel-cage induction motor.
15 feet off the ground.

14. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
600 volts.
Full-load - three-phase voltage.
5:1.
277 volts.

15. The speed of an alternating current - squirrel-cage induction motor is...
Short-circuited after power has been removed.
An electrical shock hazard may exist.
Nearly constant.
At the source transformer.

16. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Lower current and higher voltage
Two speeds
150 turns.
Lower current and higher voltage.

17. The property of an electric circuit that opposes any change in current is called...
Four times less than that of the secondary.
Second-degree burn.
Tied together
Inductance.

18. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Lower current and higher voltage.
75 turns.
A grounding resistor.
Full-load - three-phase voltage.

19. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The sum of the two capacitors.
The current will read zero.
Reverse any two-phase leads.
2000 watts

20. 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?
It provides good speed control.
Continuous.
X.
200 volts AC.

21. What type motor is a wound-rotor induction motor?
100 volts.
Larger than that found in the primary winding.
A.C.
Stator windings.

22. Not an acceptable method for the installations of surface transformers?
Disconnect the frame-grounding conductor.
They must be installed at least 7 feet above the ground.
Cut around clothing that has adhered to the burned area.
The chest should rise or inflate.

23. A solenoid coil is...
60 cycles per second (60 Hertz).
A step-up transformer.
1 amp.
An electromagnet.

24. What should the duty rating of a grounding resistor be?
Hyperventilation.
At least two phases
Continuous.
The sum of the two capacitors.

25. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
12 to 15 times a minute.
The motor rotation is reversed.
3 -979 Volts.
480 volts.

26. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
They should be made by the same manufacturer.
75 turns.
The chest should rise or inflate.

27. What is the danger when working around capacitors?
Lower current and higher voltage
An electrical shock hazard may exist.
They must be installed at least 7 feet above the ground.
Circuit Breaker.

28. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Continuous.
A grounding resistor.
No.6 AWG.
Provides single-phase power to smaller loads such as lights - heaters - etc.

29. 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?
A step-up transformer.
A.C.
3 -979 Volts.
7 -630 volts.

30. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
5:1.
2000 watts
5:1
Carotid pulse - which is located in the neck next to the Adam's apple.

31. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Grounded phase and ground fault devices were not operating.
Zero amps.
The sum of the two capacitors.
The capacitors should be discharged.

32. Which of the following does a squirrel cage induction motor have?
Transformer
Stator windings.
Cut around clothing that has adhered to the burned area.
The current will read zero.

33. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Carotid pulse - which is located in the neck next to the Adam's apple.
A device used to store electrical energy.
Provides single-phase power to smaller loads such as lights - heaters - etc.

34. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Disconnect the frame-grounding conductor.
The sum of the two capacitors.
7 -630 volts.
150 turns.

35. Compared to the secondary of a loaded - step-down transformer - the primary has...
600 volts.
60 Hertz
Lower current and higher voltage.
Larger than that found in the primary winding.

36. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Disconnect the frame-grounding conductor.
Grounded phase and ground fault devices were not operating.
Four times less than that of the secondary.
Carotid pulse - which is located in the neck next to the Adam's apple.

37. When not connected to a load - the secondary leads of a current transformer should be...
Lower current and higher voltage
Shunted and taped
Stator windings.
277 volts.

38. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Squirrel-cage induction motor.
They should be made by the same manufacturer.
800 volts.
Circuit Breaker.

39. All transformers step voltage up or down by...
Inductance.
X.
Lower current and higher voltage.
The current will read zero.

40. The standard frequency of power supplied by the power company is...
60 cycles per second (60 Hertz).
3 -979 Volts.
Two speeds
Current that changes constantly in magnitude and periodically in direction.

41. 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?
Transformer
An electrical shock hazard may exist.
15 feet off the ground.
75 turns.

42. Operating coils - motors - transformers - relays and solenoids depend upon...
Check to make sure that the victim is not still in contact with the electricity.
Electro-magnetism for their operation.
2000 watts
15 feet off the ground.

43. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
12 to 15 times a minute.
Draw less current.
Second-degree burn.

44. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The chest should rise or inflate.
Lower current and higher voltage.
The capacitors should be discharged.
Short-circuited after power has been removed.

45. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
It provides good speed control.
15 feet off the ground.
100 volts.
Current that changes constantly in magnitude and periodically in direction.

46. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Allow for metering voltage and Current.
Lower current and higher voltage.
Disconnect the frame-grounding conductor.
Alternating current.

47. The standard frequency of alternating current used at a mine is...
Check to make sure that the victim is not still in contact with the electricity.
60 Hertz
Provides single-phase power to smaller loads such as lights - heaters - etc.
Short-circuited after power has been removed.

48. What is the first step to be followed in aiding a victim of electrical shock?
X.
At least two phases
150 turns.
Check to make sure that the victim is not still in contact with the electricity.

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

50. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
5:1
Potential transformer.
Zero amps.
Four times less than that of the secondary.