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AC Theory

Instructions:

Answer 50 questions in 15 minutes.
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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. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Stator windings.
Larger than that found in the primary winding.
Four times less than that of the secondary.
The current will read zero.

2. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Current that changes constantly in magnitude and periodically in direction.
No.6 AWG.
5:1.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

3. 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?
600 volts.
Circuit Breaker.
A.C.
Draw less current.

4. A transformer with a 1:1 ratio is usually a (an)...
7 -630 volts.
Isolation transformer.
Squirrel-cage induction motor.
Hyperventilation.

5. If any two-phase wires are reversed on a three-phase motor - the motor will...
277 volts.
No.6 AWG.
Shunted and taped
Reverse its rotation.

6. AC voltage is raised or lowered by means of...
Tied together
Squirrel-cage induction motor.
Transformers.
The motor rotation is reversed.

7. Alternating current may be defined as...
They must be installed at least 7 feet above the ground.
Full-load - three-phase voltage.
1 amp.
Current that changes constantly in magnitude and periodically in direction.

8. What type of 3-phase AC motor is used most often around a mine?
Tied together
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage rotor.
Squirrel-cage induction motor.

9. Which of the following does a squirrel cage induction motor have?
Stator windings.
Derive a neutral for grounding purposes.
The motor rotation is reversed.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

10. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Grounded phase and ground fault devices were not operating.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
It provides good speed control.

11. Compared to the secondary of a loaded - step-down transformer - the primary has...
Kilovolt ampere (KVA).
Lower current and higher voltage
They should be made by the same manufacturer.
Inductance.

12. 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?
5:1
Second-degree burn.
800 volts.
Larger than that found in the primary winding.

13. What type motor is a wound-rotor induction motor?
A.C.
Kilovolt ampere (KVA).
Squirrel-cage rotor.
Provides single-phase power to smaller loads such as lights - heaters - etc.

14. The size of a transformer is measured in...
Transformers.
Four times less than that of the secondary.
Kilovolt ampere (KVA).
They should be made by the same manufacturer.

15. Not a symptom of electrical shock?.
Short-circuited after power has been removed.
Hyperventilation.
Kilovolt ampere (KVA).
200 volts AC.

16. What is the first step to be followed in aiding a victim of electrical shock?
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Short-circuited after power has been removed.
Alternating current.

17. 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.
1 amp.
A step-up transformer.
Zero amps.

18. The property of an electrical circuit that opposes any change in voltage is called...
The motor rotation is reversed.
At least two phases
Capacitance.
480 volts.

19. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Circuit Breaker.
Inductance.
12 to 15 times a minute.

20. The secondary windings on a 10:1 step-down transformer will have a wire size...
Transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
277 volts.
Larger than that found in the primary winding.

21. Define alternating current...
Check to make sure that the victim is not still in contact with the electricity.
Reverse its rotation.
Transformers.
Current changing constantly in magnitude and periodically in direction.

22. Current transformers - or CT's - are used to...
Transformers.
An electrical shock hazard may exist.
The motor rotation is reversed.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

23. 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.
Carotid pulse - which is located in the neck next to the Adam's apple.
Tied together
5:1.
150 turns.

24. The most reliable method to use to determine whether or not the heart has stopped is by checking the...

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25. What steps up or steps down AC voltage?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Two speeds
Current changing constantly in magnitude and periodically in direction.
Transformer

26. 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?
Inductance.
75 turns.
Two speeds
It provides good speed control.

27. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Cut around clothing that has adhered to the burned area.
150 turns.
75 turns.
Zero amps.

28. A burn causing a blister is classified as a...
A device used to store electrical energy.
150 turns.
Second-degree burn.
H terminals.

29. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Nearly constant.
100 volts.
Derive a neutral for grounding purposes.
15 feet off the ground.

30. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
At the source transformer.
1 amp.
Carotid pulse - which is located in the neck next to the Adam's apple.
Clamp-on ammeter.

31. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Squirrel-cage rotor.
75 turns.
The chest should rise or inflate.
Current that changes constantly in magnitude and periodically in direction.

32. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
A.C.
Inductance.
1 amp.

33. 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...
Transformers.
Tied together
Kilovolt ampere (KVA).
Potential transformer.

34. At what rate should mouth-to-mouth resuscitation be applied?
Because less current flowing at high voltage causes less line loss per kilowatt.
12 to 15 times a minute.
Potential transformer.
Capacitance.

35. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Potential transformer.
Hyperventilation.
25 feet.
H terminals.

36. 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
The sum of the two capacitors.
Current that changes constantly in magnitude and periodically in direction.
Electro-magnetism for their operation.

37. Where should grounding resistors be located?
At the source transformer.
Larger than that found in the primary winding.
75 turns.
Hyperventilation.

38. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
5:1
The capacitors should be discharged.
At the source transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

39. What is a capacitor?
Provides single-phase power to smaller loads such as lights - heaters - etc.
At the source transformer.
A device used to store electrical energy.
Current that changes constantly in magnitude and periodically in direction.

40. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
At least two phases
A grounding resistor.
5:1.
Squirrel-cage rotor.

41. The standard frequency of power supplied by the power company is...
At the source transformer.
At least two phases
60 cycles per second (60 Hertz).
200 volts AC.

42. 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?
Inductance.
75 turns.
60 Hertz
Shunted and taped

43. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
X.
Inductance.
The chest should rise or inflate.
The current will read zero.

44. Current that changes periodically in direction is...
Alternating current.
Provides single-phase power to smaller loads such as lights - heaters - etc.
200 volts AC.
Current that changes constantly in magnitude and periodically in direction.

45. To protect against over-current in a three-phase circuit over-current protection must be provided for...
It provides good speed control.
Derive a neutral for grounding purposes.
At least two phases
15 feet off the ground.

46. The primary leads of a single-phase transformer should be connected to the...
Derive a neutral for grounding purposes.
H terminals.
Reverse its rotation.
Transformer

47. Three-phase AC underground cables must be provided with overcurrent protection using a...
A.C.
Current changing constantly in magnitude and periodically in direction.
Circuit Breaker.
The capacitors should be discharged.

48. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
A step-up transformer.
Capacitance.
The chest should rise or inflate.
2000 watts

49. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Draw less current.
Grounded phase and ground fault devices were not operating.
2000 watts
Two speeds

50. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Potential transformer.
600 volts.
X.
It provides good speed control.

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AC Theory

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