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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. 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?
75 turns.
Draw less current.
Current changing constantly in magnitude and periodically in direction.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

2. What steps up or steps down AC voltage?
Electro-magnetism for their operation.
At the source transformer.
Transformer
25 feet.

3. When not connected to a load - the secondary leads of a current transformer should be...
15 feet off the ground.
Squirrel-cage rotor.
Shunted and taped
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

4. The primary leads of a single-phase transformer should be connected to the...
Stator windings.
H terminals.
They should be made by the same manufacturer.
15 feet off the ground.

5. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because less current flowing at high voltage causes less line loss per kilowatt.
It provides good speed control.
200 volts AC.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

6. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Circuit Breaker.
At the source transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.

7. Alternating current may be defined as...
480 volts.
Circuit Breaker.
1 amp.
Current that changes constantly in magnitude and periodically in direction.

8. The main function of CT's and PT's on a main substation is to...
A.C.
Allow for metering voltage and Current.
Draw less current.
The motor rotation is reversed.

9. 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...
Allow for metering voltage and Current.
Draw less current.
Continuous.
It provides good speed control.

10. What should the duty rating of a grounding resistor be?
Continuous.
277 volts.
They should be made by the same manufacturer.
Short-circuited after power has been removed.

11. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Lower current and higher voltage.
Zero amps.
5:1.
Four times less than that of the secondary.

12. 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.
5:1.
They must be installed at least 7 feet above the ground.
A device used to store electrical energy.
Reverse its rotation.

13. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Lower current and higher voltage
Shunted and taped
H terminals.

14. Three-phase AC underground cables must be provided with overcurrent protection using a...
5:1.
Circuit Breaker.
Two speeds
Second-degree burn.

15. At what rate should mouth-to-mouth resuscitation be applied?
2000 watts
X.
12 to 15 times a minute.
Cut around clothing that has adhered to the burned area.

16. A control power transformer...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Provides single-phase power to smaller loads such as lights - heaters - etc.
A.C.
Derive a neutral for grounding purposes.

17. The standard frequency of alternating current used at a mine is...
Disconnect the frame-grounding conductor.
5:1.
60 Hertz
25 feet.

18. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
An electromagnet.
2000 watts
60 cycles per second (60 Hertz).

19. The primary function of a power transformer is to provide...
Two speeds
Full-load - three-phase voltage.
Short-circuited after power has been removed.
Inductance.

20. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Tied together
Isolation transformer.
They should be made by the same manufacturer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

21. What is the first step to be followed in aiding a victim of electrical shock?
A device used to store electrical energy.
Check to make sure that the victim is not still in contact with the electricity.
X.
Current that changes constantly in magnitude and periodically in direction.

22. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Clamp-on ammeter.
At the source transformer.
7 -630 volts.
200 volts AC.

23. Not an acceptable method for the installations of surface transformers?
277 volts.
They must be installed at least 7 feet above the ground.
A grounding resistor.
100 turns.

24. 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.
800 volts.
2000 watts
The motor rotation is reversed.

25. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Provides single-phase power to smaller loads such as lights - heaters - etc.
1 amp.
25 feet.
Reverse its rotation.

26. What type of 3-phase AC motor is used most often around a mine?
Full-load - three-phase voltage.
Squirrel-cage induction motor.
At least two phases
X.

27. 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?
Allow for metering voltage and Current.
At least two phases
100 turns.
Reverse its rotation.

28. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Nearly constant.
Lower current and higher voltage.
At least two phases
1 amp.

29. Not a symptom of electrical shock?.
Zero amps.
The sum of the two capacitors.
Short-circuited after power has been removed.
Hyperventilation.

30. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
Zero amps.
Inductance.
X.

31. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
A grounding resistor.
They must be installed at least 7 feet above the ground.
At least two phases
Transformers.

32. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Kilovolt ampere (KVA).
480 volts.
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.

33. Not true about AC power?
Allow for metering voltage and Current.
Squirrel-cage rotor.
Cut around clothing that has adhered to the burned area.
It provides good speed control.

34. Where should grounding resistors be located?
At the source transformer.
Inductance.
The capacitors should be discharged.
5:1.

35. If any two-phase wires are reversed on a three-phase motor - the motor will...
No.6 AWG.
Allow for metering voltage and Current.
Reverse its rotation.
Draw less current.

36. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
480 volts.
The chest should rise or inflate.
Current changing constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.

37. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
60 cycles per second (60 Hertz).
5:1
Alternating current.

38. What is the danger when working around capacitors?
They should be made by the same manufacturer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
5:1
An electrical shock hazard may exist.

39. 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
Draw less current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
A device used to store electrical energy.

40. The property of an electric circuit that opposes any change in current is called...
Inductance.
They should be made by the same manufacturer.
Hyperventilation.
200 volts AC.

41. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Circuit Breaker.
A step-up transformer.
480 volts.
Disconnect the frame-grounding conductor.

42. Which of the following does a squirrel cage induction motor have?
Squirrel-cage induction motor.
1 amp.
Stator windings.
150 turns.

43. Define alternating current...
Draw less current.
Larger than that found in the primary winding.
Current changing constantly in magnitude and periodically in direction.
1 amp.

44. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
The chest should rise or inflate.
Grounded phase and ground fault devices were not operating.
480 volts.
Electro-magnetism for their operation.

45. Current in an AC circuit may be measured by...
A grounding resistor.
An electromagnet.
Clamp-on ammeter.
Stator windings.

46. 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...
Tied together
200 volts AC.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Electro-magnetism for their operation.

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

48. AC voltage is raised or lowered by means of...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The chest should rise or inflate.
Transformers.
15 feet off the ground.

49. The combined capacitance of two capacitors in parallel is...
100 turns.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The sum of the two capacitors.
Electro-magnetism for their operation.

50. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
75 turns.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
800 volts.
X.