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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. The primary function of a power transformer is to provide...
12 to 15 times a minute.
Full-load - three-phase voltage.
800 volts.
An electrical shock hazard may exist.

2. A control power transformer...
Reverse its rotation.
3 -979 Volts.
Short-circuited after power has been removed.
Provides single-phase power to smaller loads such as lights - heaters - etc.

3. The property of an electric circuit that opposes any change in current is called...
Inductance.
A device used to store electrical energy.
X.
Transformers.

4. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
800 volts.
Transformer
A.C.
15 feet off the ground.

5. What is a capacitor?
Potential transformer.
A device used to store electrical energy.
277 volts.
1 amp.

6. To protect against over-current in a three-phase circuit over-current protection must be provided for...
75 turns.
A device used to store electrical energy.
Hyperventilation.
At least two phases

7. Compared to the secondary of a loaded - step-down transformer - the primary has...
480 volts.
Draw less current.
75 turns.
Lower current and higher voltage.

8. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
800 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Allow for metering voltage and Current.
60 Hertz

9. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Electro-magnetism for their operation.
2000 watts
Draw less current.
Squirrel-cage induction motor.

10. 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.
Inductance.
Reverse its rotation.
Disconnect the frame-grounding conductor.

11. Three-phase AC underground cables must be provided with overcurrent protection using a...
60 cycles per second (60 Hertz).
Circuit Breaker.
Draw less current.
Reverse any two-phase leads.

12. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Alternating current.
25 feet.
60 Hertz
800 volts.

13. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
3 -979 Volts.
The chest should rise or inflate.
600 volts.

14. The speed of an alternating current - squirrel-cage induction motor is...
Shunted and taped
Nearly constant.
Circuit Breaker.
They must be installed at least 7 feet above the ground.

15. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
A device used to store electrical energy.
H terminals.
Disconnect the frame-grounding conductor.
277 volts.

16. 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 current will read zero.
100 turns.
Current that changes constantly in magnitude and periodically in direction.
7 -630 volts.

17. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Shunted and taped
150 turns.
3 -979 Volts.

18. Current that changes periodically in direction is...
5:1
Continuous.
A device used to store electrical energy.
Alternating current.

19. 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...
277 volts.
It provides good speed control.
100 volts.
5:1

20. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
100 turns.
Short-circuited after power has been removed.
Disconnect the frame-grounding conductor.
Continuous.

21. 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?
Kilovolt ampere (KVA).
150 turns.
Capacitance.
Derive a neutral for grounding purposes.

22. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
75 turns.
Continuous.
Four times less than that of the secondary.

23. The purpose of a zig-zag transformer is to...
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Derive a neutral for grounding purposes.
Zero amps.

24. At what rate should mouth-to-mouth resuscitation be applied?
Check to make sure that the victim is not still in contact with the electricity.
A step-up transformer.
The current will read zero.
12 to 15 times a minute.

25. A burn causing a blister is classified as a...
Electro-magnetism for their operation.
Second-degree burn.
A grounding resistor.
Lower current and higher voltage

26. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Two speeds
Clamp-on ammeter.
100 volts.
277 volts.

27. AC voltage is raised or lowered by means of...
Allow for metering voltage and Current.
Transformers.
Cut around clothing that has adhered to the burned area.
Zero amps.

28. 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...
Zero amps.
Draw less current.
Hyperventilation.
Disconnect the frame-grounding conductor.

29. What steps up or steps down AC voltage?
No.6 AWG.
Transformer
The current will read zero.
600 volts.

30. All transformers step voltage up or down by...
Current changing constantly in magnitude and periodically in direction.
Circuit Breaker.
Inductance.
The current will read zero.

31. 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.
Electro-magnetism for their operation.
An electromagnet.
Current changing constantly in magnitude and periodically in direction.

32. High voltage is used to transmit power over long distances...
A device used to store electrical energy.
Because less current flowing at high voltage causes less line loss per kilowatt.
Nearly constant.
Draw less current.

33. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
The current will read zero.
75 turns.
Inductance.

34. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
They must be installed at least 7 feet above the ground.
5:1
A step-up transformer.
The motor rotation is reversed.

35. 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?
Inductance.
3 -979 Volts.
Alternating current.
60 Hertz

36. What type of 3-phase AC motor is used most often around a mine?
Potential transformer.
At the source transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage induction motor.

37. 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...
Tied together
Draw less current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Second-degree burn.

38. The size of a transformer is measured in...
277 volts.
Electro-magnetism for their operation.
Kilovolt ampere (KVA).
At the source transformer.

39. 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.
Allow for metering voltage and Current.
5:1.
An electromagnet.
150 turns.

40. 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
Reverse its rotation.
At the source transformer.
100 volts.

41. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
An electrical shock hazard may exist.
Because less current flowing at high voltage causes less line loss per kilowatt.
A.C.

42. Not an acceptable method for the installations of surface transformers?
100 turns.
75 turns.
The current will read zero.
They must be installed at least 7 feet above the ground.

43. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Potential transformer.
A grounding resistor.
Nearly constant.

44. What is the danger when working around capacitors?
Isolation transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electrical shock hazard may exist.
Draw less current.

45. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
Second-degree burn.
Full-load - three-phase voltage.
277 volts.

46. If any two-phase wires are reversed on a three-phase motor - the motor will...
60 cycles per second (60 Hertz).
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Reverse its rotation.
At least two phases

47. What should the duty rating of a grounding resistor be?
Capacitance.
Continuous.
Cut around clothing that has adhered to the burned area.
Inductance.

48. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
A.C.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Four times less than that of the secondary.
Clamp-on ammeter.

49. The standard frequency of alternating current used at a mine is...
Zero amps.
200 volts AC.
Second-degree burn.
60 Hertz

50. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Current that changes constantly in magnitude and periodically in direction.
75 turns.
480 volts.
At least two phases