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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. How do you change the direction of a 3-phase AC motor?
Electro-magnetism for their operation.
Reverse any two-phase leads.
Disconnect the frame-grounding conductor.
No.6 AWG.

2. What should the duty rating of a grounding resistor be?
The motor rotation is reversed.
75 turns.
Continuous.
Squirrel-cage rotor.

3. 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?
Zero amps.
100 turns.
Transformer
800 volts.

4. 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.
5:1.
Transformers.
A.C.

5. Three-phase AC underground cables must be provided with overcurrent protection using a...
Shunted and taped
Grounded phase and ground fault devices were not operating.
60 Hertz
Circuit Breaker.

6. At what rate should mouth-to-mouth resuscitation be applied?
Electro-magnetism for their operation.
150 turns.
12 to 15 times a minute.
Alternating current.

7. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
2000 watts
A.C.
The chest should rise or inflate.
Continuous.

8. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The motor rotation is reversed.
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage induction motor.

9. All transformers step voltage up or down by...
Stator windings.
Tied together
15 feet off the ground.
Inductance.

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?
Transformers.
X.
75 turns.
Full-load - three-phase voltage.

11. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
75 turns.
They should be made by the same manufacturer.
Grounded phase and ground fault devices were not operating.
277 volts.

12. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The chest should rise or inflate.
Alternating current.
60 cycles per second (60 Hertz).
The current will read zero.

13. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Inductance.
25 feet.
Squirrel-cage rotor.
Current changing constantly in magnitude and periodically in direction.

14. 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...
At least two phases
Lower current and higher voltage.
An electrical shock hazard may exist.
Tied together

15. Where should grounding resistors be located?
Full-load - three-phase voltage.
At the source transformer.
7 -630 volts.
Cut around clothing that has adhered to the burned area.

16. AC voltage is raised or lowered by means of...
Current that changes constantly in magnitude and periodically in direction.
Transformers.
At the source transformer.
A device used to store electrical energy.

17. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Hyperventilation.
Capacitance.
It provides good speed control.

18. When not connected to a load - the secondary leads of a current transformer should be...
100 turns.
A device used to store electrical energy.
Shunted and taped
Transformer

19. 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?
75 turns.
60 cycles per second (60 Hertz).
100 volts.
Draw less current.

20. 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...
Draw less current.
3 -979 Volts.
2000 watts
75 turns.

21. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
5:1
480 volts.
They must be installed at least 7 feet above the ground.
Grounded phase and ground fault devices were not operating.

22. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Draw less current.
Kilovolt ampere (KVA).
At least two phases
600 volts.

23. A burn causing a blister is classified as a...
Isolation transformer.
60 Hertz
Second-degree burn.
A step-up transformer.

24. High voltage is used to transmit power over long distances...
Circuit Breaker.
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage.
The current will read zero.

25. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
7 -630 volts.
The motor rotation is reversed.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Continuous.

26. 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.
60 cycles per second (60 Hertz).
Squirrel-cage induction motor.
The motor rotation is reversed.

27. Alternating current may be defined as...
Current changing constantly in magnitude and periodically in direction.
Current that changes constantly in magnitude and periodically in direction.
The sum of the two capacitors.
25 feet.

28. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
Transformer
25 feet.
No.6 AWG.

29. The secondary windings on a 10:1 step-down transformer will have a wire size...
60 cycles per second (60 Hertz).
Larger than that found in the primary winding.
At least two phases
The chest should rise or inflate.

30. The standard frequency of alternating current used at a mine is...
60 Hertz
100 turns.
A device used to store electrical energy.
The current will read zero.

31. The property of an electrical circuit that opposes any change in voltage is called...
Electro-magnetism for their operation.
A grounding resistor.
Potential transformer.
Capacitance.

32. 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.
Short-circuited after power has been removed.
At the source transformer.
No.6 AWG.

33. A control power transformer...
75 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.
X.
At the source transformer.

34. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Larger than that found in the primary winding.
Reverse its rotation.
An electrical shock hazard may exist.

35. A wound-rotor induction motor has all the following EXCEPT...
Larger than that found in the primary winding.
Squirrel-cage rotor.
A device used to store electrical energy.
Because less current flowing at high voltage causes less line loss per kilowatt.

36. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Four times less than that of the secondary.
A step-up transformer.
Lower current and higher voltage

37. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Allow for metering voltage and Current.
The chest should rise or inflate.
Derive a neutral for grounding purposes.
1 amp.

38. 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?
Reverse its rotation.
Second-degree burn.
100 turns.
Because less current flowing at high voltage causes less line loss per kilowatt.

39. Not an acceptable method for the installations of surface transformers?
Draw less current.
Four times less than that of the secondary.
They must be installed at least 7 feet above the ground.
A grounding resistor.

40. 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?
Stator windings.
200 volts AC.
75 turns.
Squirrel-cage induction motor.

41. A transformer with a 1:1 ratio is usually a (an)...
An electrical shock hazard may exist.
Full-load - three-phase voltage.
Current that changes constantly in magnitude and periodically in direction.
Isolation transformer.

42. What type motor is a wound-rotor induction motor?
12 to 15 times a minute.
25 feet.
A.C.
Inductance.

43. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
No.6 AWG.
100 volts.
Zero amps.
Squirrel-cage rotor.

44. If any two-phase wires are reversed on a three-phase motor - the motor will...
They must be installed at least 7 feet above the ground.
75 turns.
Reverse its rotation.
Potential transformer.

45. 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.
Inductance.
100 turns.

46. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
The capacitors should be discharged.
Four times less than that of the secondary.
Short-circuited after power has been removed.
The current will read zero.

47. Compared to the secondary of a loaded - step-down transformer - the primary has...
Potential transformer.
Four times less than that of the secondary.
Lower current and higher voltage.
Current that changes constantly in magnitude and periodically in direction.

48. The purpose of a zig-zag transformer is to...
Draw less current.
Derive a neutral for grounding purposes.
Draw less current.
Carotid pulse - which is located in the neck next to the Adam's apple.

49. Compared to the secondary of a loaded - step-down transformer - the primary has...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
They must be installed at least 7 feet above the ground.
Squirrel-cage rotor.
Lower current and higher voltage

50. 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.
15 feet off the ground.
At the source transformer.
Full-load - three-phase voltage.

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AC Theory

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