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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?
Reverse any two-phase leads.
An electromagnet.
Current that changes constantly in magnitude and periodically in direction.
H terminals.

2. A control power transformer...
12 to 15 times a minute.
Larger than that found in the primary winding.
200 volts AC.
Provides single-phase power to smaller loads such as lights - heaters - etc.

3. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
No.6 AWG.
The capacitors should be discharged.
A grounding resistor.
Lower current and higher voltage.

4. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Circuit Breaker.
Transformers.
100 turns.
Zero amps.

5. The size of a transformer is measured in...
Squirrel-cage rotor.
A device used to store electrical energy.
480 volts.
Kilovolt ampere (KVA).

6. A wound-rotor induction motor has all the following EXCEPT...
60 Hertz
Four times less than that of the secondary.
The chest should rise or inflate.
Squirrel-cage rotor.

7. Compared to the secondary of a loaded - step-down transformer - the primary has...
15 feet off the ground.
The chest should rise or inflate.
Check to make sure that the victim is not still in contact with the electricity.
Lower current and higher voltage

8. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
They must be installed at least 7 feet above the ground.
Disconnect the frame-grounding conductor.
25 feet.
60 Hertz

9. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
60 Hertz
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Current changing constantly in magnitude and periodically in direction.
Transformers.

10. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Reverse its rotation.
15 feet off the ground.
Cut around clothing that has adhered to the burned area.

11. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Because less current flowing at high voltage causes less line loss per kilowatt.
Carotid pulse - which is located in the neck next to the Adam's apple.
5:1.

12. 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.
277 volts.
5:1
Draw less current.
Zero amps.

13. Compared to the secondary of a loaded - step-down transformer - the primary has...
Short-circuited after power has been removed.
Current changing constantly in magnitude and periodically in direction.
Lower current and higher voltage.
Check to make sure that the victim is not still in contact with the electricity.

14. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
It provides good speed control.
Zero amps.
Nearly constant.

15. The primary leads of a single-phase transformer should be connected to the...
Reverse its rotation.
At the source transformer.
H terminals.
Electro-magnetism for their operation.

16. Where should grounding resistors be located?
The current will read zero.
Stator windings.
Reverse its rotation.
At the source transformer.

17. 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?
Two speeds
Draw less current.
2000 watts
3 -979 Volts.

18. At what rate should mouth-to-mouth resuscitation be applied?
Current that changes constantly in magnitude and periodically in direction.
Cut around clothing that has adhered to the burned area.
A grounding resistor.
12 to 15 times a minute.

19. Current that changes periodically in direction is...
Potential transformer.
Squirrel-cage induction motor.
Capacitance.
Alternating current.

20. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
15 feet off the ground.
They must be installed at least 7 feet above the ground.
Lower current and higher voltage
A grounding resistor.

21. Current transformers - or CT's - are used to...
5:1.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
25 feet.
75 turns.

22. 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.
150 turns.
An electrical shock hazard may exist.
Alternating current.

23. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Potential transformer.
1 amp.
The motor rotation is reversed.

24. 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?
Isolation transformer.
75 turns.
An electrical shock hazard may exist.
Larger than that found in the primary winding.

25. The property of an electric circuit that opposes any change in current is called...
Check to make sure that the victim is not still in contact with the electricity.
Inductance.
Electro-magnetism for their operation.
Cut around clothing that has adhered to the burned area.

26. 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?
5:1.
600 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Potential transformer.

27. The main function of CT's and PT's on a main substation is to...
Inductance.
Hyperventilation.
No.6 AWG.
Allow for metering voltage and Current.

28. Not an acceptable method for the installations of surface transformers?
The current will read zero.
Provides single-phase power to smaller loads such as lights - heaters - etc.
75 turns.
They must be installed at least 7 feet above the ground.

29. 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...
Draw less current.
Transformer
The sum of the two capacitors.
Grounded phase and ground fault devices were not operating.

30. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
2000 watts
Kilovolt ampere (KVA).
They should be made by the same manufacturer.
100 volts.

31. Current in an AC circuit may be measured by...
Clamp-on ammeter.
5:1
60 Hertz
Allow for metering voltage and Current.

32. 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.
Draw less current.
Short-circuited after power has been removed.
Reverse any two-phase leads.

33. What is the first step to be followed in aiding a victim of electrical shock?
The motor rotation is reversed.
25 feet.
At the source transformer.
Check to make sure that the victim is not still in contact with the electricity.

34. Operating coils - motors - transformers - relays and solenoids depend upon...
Grounded phase and ground fault devices were not operating.
The chest should rise or inflate.
Electro-magnetism for their operation.
At least two phases

35. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
60 cycles per second (60 Hertz).
Allow for metering voltage and Current.

36. The standard frequency of alternating current used at a mine is...
60 Hertz
25 feet.
Short-circuited after power has been removed.
Carotid pulse - which is located in the neck next to the Adam's apple.

37. A burn causing a blister is classified as a...
Two speeds
Second-degree burn.
2000 watts
200 volts AC.

38. 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?
277 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Short-circuited after power has been removed.
150 turns.

39. Not true about AC power?
It provides good speed control.
Check to make sure that the victim is not still in contact with the electricity.
Potential transformer.
25 feet.

40. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Draw less current.
At least two phases
15 feet off the ground.
60 cycles per second (60 Hertz).

41. All transformers step voltage up or down by...
The chest should rise or inflate.
Inductance.
800 volts.
Allow for metering voltage and Current.

42. 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?
Lower current and higher voltage
Lower current and higher voltage.
800 volts.
Nearly constant.

43. What should the duty rating of a grounding resistor be?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
2000 watts
The capacitors should be discharged.
Continuous.

44. Not a symptom of electrical shock?.
Hyperventilation.
Inductance.
Carotid pulse - which is located in the neck next to the Adam's apple.
Nearly constant.

45. The speed of an alternating current - squirrel-cage induction motor is...
7 -630 volts.
600 volts.
Nearly constant.
Two speeds

46. What type of 3-phase AC motor is used most often around a mine?
The current will read zero.
Squirrel-cage induction motor.
Disconnect the frame-grounding conductor.
At the source transformer.

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

48. 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
100 volts.
H terminals.
Stator windings.

49. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Electro-magnetism for their operation.
Disconnect the frame-grounding conductor.
60 cycles per second (60 Hertz).
Draw less current.

50. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Disconnect the frame-grounding conductor.
The capacitors should be discharged.
Continuous.
Grounded phase and ground fault devices were not operating.