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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. A transformer with a 1:1 ratio is usually a (an)...
Capacitance.
Allow for metering voltage and Current.
The chest should rise or inflate.
Isolation transformer.

2. The size of a transformer is measured in...
Reverse any two-phase leads.
Kilovolt ampere (KVA).
Squirrel-cage induction motor.
60 Hertz

3. 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.
Capacitance.
Disconnect the frame-grounding conductor.
H terminals.

4. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The motor rotation is reversed.
Full-load - three-phase voltage.
5:1
The current will read zero.

5. 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.
Inductance.
5:1.
Capacitance.
Second-degree burn.

6. What type of 3-phase AC motor is used most often around a mine?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage induction motor.
A device used to store electrical energy.
Disconnect the frame-grounding conductor.

7. Define alternating current...
75 turns.
Kilovolt ampere (KVA).
Zero amps.
Current changing constantly in magnitude and periodically in direction.

8. The property of an electrical circuit that opposes any change in voltage is called...
An electrical shock hazard may exist.
Capacitance.
Isolation transformer.
They should be made by the same manufacturer.

9. 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?
Larger than that found in the primary winding.
600 volts.
150 turns.
Continuous.

10. Not a symptom of electrical shock?.
Hyperventilation.
Short-circuited after power has been removed.
277 volts.
Shunted and taped

11. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Draw less current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
No.6 AWG.
3 -979 Volts.

12. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
200 volts AC.
An electrical shock hazard may exist.
Circuit Breaker.

13. 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...
No.6 AWG.
Draw less current.
60 cycles per second (60 Hertz).
7 -630 volts.

14. 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.
Transformers.
75 turns.
At the source transformer.
5:1

15. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Stator windings.
Derive a neutral for grounding purposes.
The motor rotation is reversed.
A.C.

16. Operating coils - motors - transformers - relays and solenoids depend upon...
12 to 15 times a minute.
Electro-magnetism for their operation.
5:1.
No.6 AWG.

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?
Shunted and taped
At the source transformer.
3 -979 Volts.
Kilovolt ampere (KVA).

18. 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?
Reverse its rotation.
600 volts.
Clamp-on ammeter.
Two speeds

19. Not an acceptable method for the installations of surface transformers?
Transformers.
Squirrel-cage rotor.
Reverse its rotation.
They must be installed at least 7 feet above the ground.

20. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Squirrel-cage rotor.
Circuit Breaker.
Draw less current.
A grounding resistor.

21. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Two speeds
Inductance.
150 turns.

22. How do you change the direction of a 3-phase AC motor?
75 turns.
At the source transformer.
Reverse any two-phase leads.
A step-up transformer.

23. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
200 volts AC.
2000 watts
Transformers.
150 turns.

24. Current in an AC circuit may be measured by...
277 volts.
5:1
Clamp-on ammeter.
3 -979 Volts.

25. 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...
100 volts.
Allow for metering voltage and Current.
At least two phases
Transformer

26. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
At the source transformer.
Continuous.
Clamp-on ammeter.

27. A burn causing a blister is classified as a...
75 turns.
Larger than that found in the primary winding.
Squirrel-cage induction motor.
Second-degree burn.

28. High voltage is used to transmit power over long distances...
7 -630 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
X.
At least two phases

29. AC voltage is raised or lowered by means of...
Transformers.
At the source transformer.
A step-up transformer.
2000 watts

30. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
A grounding resistor.
Hyperventilation.
The chest should rise or inflate.
60 Hertz

31. 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...
1 amp.
Grounded phase and ground fault devices were not operating.
Draw less current.
An electromagnet.

32. What should the duty rating of a grounding resistor be?
Carotid pulse - which is located in the neck next to the Adam's apple.
200 volts AC.
Continuous.
At least two phases

33. At what rate should mouth-to-mouth resuscitation be applied?
12 to 15 times a minute.
Inductance.
An electrical shock hazard may exist.
75 turns.

34. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
Reverse any two-phase leads.
The capacitors should be discharged.
Lower current and higher voltage.

35. Not true about AC power?
Derive a neutral for grounding purposes.
A.C.
It provides good speed control.
15 feet off the ground.

36. What is the danger when working around capacitors?
H terminals.
An electromagnet.
Because less current flowing at high voltage causes less line loss per kilowatt.
An electrical shock hazard may exist.

37. To protect against over-current in a three-phase circuit over-current protection must be provided for...
60 cycles per second (60 Hertz).
Derive a neutral for grounding purposes.
At least two phases
Provides single-phase power to smaller loads such as lights - heaters - etc.

38. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
The chest should rise or inflate.
An electromagnet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Four times less than that of the secondary.

39. If any two-phase wires are reversed on a three-phase motor - the motor will...
An electrical shock hazard may exist.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Reverse its rotation.
At least two phases

40. What is a capacitor?
25 feet.
A device used to store electrical energy.
An electromagnet.
Check to make sure that the victim is not still in contact with the electricity.

41. What position of a selector switch using a voltmeter would you select to check a 120-volt AC circuit to obtain the most accurate reading?
Cut around clothing that has adhered to the burned area.
Hyperventilation.
The current will read zero.
200 volts AC.

42. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
277 volts.
Four times less than that of the secondary.
At least two phases
Potential transformer.

43. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Squirrel-cage rotor.
A device used to store electrical energy.
Squirrel-cage induction motor.
7 -630 volts.

44. The transformer used only to measure voltage is a...
Potential transformer.
They must be installed at least 7 feet above the ground.
They should be made by the same manufacturer.
Short-circuited after power has been removed.

45. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A device used to store electrical energy.
A step-up transformer.
480 volts.
Current that changes constantly in magnitude and periodically in direction.

46. The speed of an alternating current - squirrel-cage induction motor is...
Derive a neutral for grounding purposes.
Nearly constant.
The sum of the two capacitors.
Continuous.

47. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
They must be installed at least 7 feet above the ground.
The capacitors should be discharged.
An electromagnet.
15 feet off the ground.

48. The purpose of a zig-zag transformer is to...
Squirrel-cage rotor.
No.6 AWG.
Derive a neutral for grounding purposes.
75 turns.

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

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50. 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.
Alternating current.
They must be installed at least 7 feet above the ground.
Larger than that found in the primary winding.

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AC Theory

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