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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 neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Reverse any two-phase leads.
12 to 15 times a minute.
25 feet.
200 volts AC.

2. At what rate should mouth-to-mouth resuscitation be applied?
The current will read zero.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
H terminals.
12 to 15 times a minute.

3. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Electro-magnetism for their operation.
Four times less than that of the secondary.
100 volts.
The current will read zero.

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

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5. The speed of an alternating current - squirrel-cage induction motor is...
Tied together
Provides single-phase power to smaller loads such as lights - heaters - etc.
12 to 15 times a minute.
Nearly constant.

6. The secondary windings on a 10:1 step-down transformer will have a wire size...
Zero amps.
Current that changes constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.
Inductance.

7. The size of a transformer is measured in...
The sum of the two capacitors.
5:1.
Kilovolt ampere (KVA).
Provides single-phase power to smaller loads such as lights - heaters - etc.

8. The standard frequency of alternating current used at a mine is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
600 volts.
At least two phases
60 Hertz

9. Compared to the secondary of a loaded - step-down transformer - the primary has...
200 volts AC.
Zero amps.
Lower current and higher voltage.
The sum of the two capacitors.

10. Where should grounding resistors be located?
At the source transformer.
Reverse any two-phase leads.
No.6 AWG.
Kilovolt ampere (KVA).

11. How do you change the direction of a 3-phase AC motor?
Two speeds
Allow for metering voltage and Current.
Nearly constant.
Reverse any two-phase leads.

12. The primary leads of a single-phase transformer should be connected to the...
H terminals.
Squirrel-cage rotor.
Because less current flowing at high voltage causes less line loss per kilowatt.
They should be made by the same manufacturer.

13. What steps up or steps down AC voltage?
Transformer
Electro-magnetism for their operation.
Clamp-on ammeter.
Current that changes constantly in magnitude and periodically in direction.

14. What is the first step to be followed in aiding a victim of electrical shock?
No.6 AWG.
Tied together
An electrical shock hazard may exist.
Check to make sure that the victim is not still in contact with the electricity.

15. 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?
12 to 15 times a minute.
Isolation transformer.
The capacitors should be discharged.
800 volts.

16. 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...
60 Hertz
A step-up transformer.
Tied together
The capacitors should be discharged.

17. 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?
Alternating current.
Grounded phase and ground fault devices were not operating.
75 turns.
Larger than that found in the primary winding.

18. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
60 cycles per second (60 Hertz).
Stator windings.
Electro-magnetism for their operation.

19. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
2000 watts
Alternating current.
15 feet off the ground.

20. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
277 volts.
The current will read zero.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
At least two phases

21. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
600 volts.
Shunted and taped
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

22. The property of an electric circuit that opposes any change in current is called...
Shunted and taped
Disconnect the frame-grounding conductor.
Inductance.
800 volts.

23. Not true about AC power?
A grounding resistor.
60 cycles per second (60 Hertz).
It provides good speed control.
Reverse any two-phase leads.

24. A solenoid coil is...
An electromagnet.
Transformer
Isolation transformer.
Grounded phase and ground fault devices were not operating.

25. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
150 turns.
Check to make sure that the victim is not still in contact with the electricity.
25 feet.
480 volts.

26. What is the danger when working around capacitors?
60 Hertz
Second-degree burn.
H terminals.
An electrical shock hazard may exist.

27. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
No.6 AWG.
277 volts.
Current changing constantly in magnitude and periodically in direction.

28. Current in an AC circuit may be measured by...
A step-up transformer.
Four times less than that of the secondary.
800 volts.
Clamp-on ammeter.

29. 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.
X.
Continuous.
7 -630 volts.

30. When not connected to a load - the secondary leads of a current transformer should be...
X.
Reverse its rotation.
60 cycles per second (60 Hertz).
Shunted and taped

31. 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...
1 amp.
100 turns.
100 volts.
Inductance.

32. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
Short-circuited after power has been removed.
At the source transformer.
60 cycles per second (60 Hertz).

33. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
200 volts AC.
15 feet off the ground.
Disconnect the frame-grounding conductor.
Short-circuited after power has been removed.

34. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
A step-up transformer.
Current that changes constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.

35. A burn causing a blister is classified as a...
100 volts.
60 cycles per second (60 Hertz).
Second-degree burn.
25 feet.

36. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
A device used to store electrical energy.
Derive a neutral for grounding purposes.
Cut around clothing that has adhered to the burned area.
1 amp.

37. Which of the following does a squirrel cage induction motor have?
Reverse its rotation.
Lower current and higher voltage
Disconnect the frame-grounding conductor.
Stator windings.

38. 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?
100 turns.
Short-circuited after power has been removed.
75 turns.
The motor rotation is reversed.

39. A wound-rotor induction motor has all the following EXCEPT...
3 -979 Volts.
Cut around clothing that has adhered to the burned area.
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage rotor.

40. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
They must be installed at least 7 feet above the ground.
2000 watts
5:1
480 volts.

41. Three-phase AC underground cables must be provided with overcurrent protection using a...
Lower current and higher voltage
Current changing constantly in magnitude and periodically in direction.
75 turns.
Circuit Breaker.

42. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Transformers.
They must be installed at least 7 feet above the ground.
100 turns.
A step-up transformer.

43. Operating coils - motors - transformers - relays and solenoids depend upon...
They should be made by the same manufacturer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Because less current flowing at high voltage causes less line loss per kilowatt.
Electro-magnetism for their operation.

44. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
The current will read zero.
Transformer
Two speeds
2000 watts

45. Current transformers - or CT's - are used to...
Allow for metering voltage and Current.
Zero amps.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
25 feet.

46. AC voltage is raised or lowered by means of...
Transformers.
Squirrel-cage induction motor.
Larger than that found in the primary winding.
Short-circuited after power has been removed.

47. 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?
The motor rotation is reversed.
100 turns.
60 cycles per second (60 Hertz).
Transformer

48. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Cut around clothing that has adhered to the burned area.
Short-circuited after power has been removed.
Kilovolt ampere (KVA).

49. 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.
Cut around clothing that has adhered to the burned area.
They must be installed at least 7 feet above the ground.
5:1
Zero amps.

50. Define alternating current...
Short-circuited after power has been removed.
Current changing constantly in magnitude and periodically in direction.
Shunted and taped
Clamp-on ammeter.

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AC Theory

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