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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. 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.
5:1
Full-load - three-phase voltage.
A device used to store electrical energy.
100 volts.

2. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Transformer
An electromagnet.
100 volts.

3. Operating coils - motors - transformers - relays and solenoids depend upon...
800 volts.
Zero amps.
Electro-magnetism for their operation.
2000 watts

4. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Disconnect the frame-grounding conductor.
At least two phases
Lower current and higher voltage.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

5. The primary function of a power transformer is to provide...
A grounding resistor.
Full-load - three-phase voltage.
Tied together
Shunted and taped

6. What steps up or steps down AC voltage?
Transformer
Zero amps.
Allow for metering voltage and Current.
Circuit Breaker.

7. 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
15 feet off the ground.
Disconnect the frame-grounding conductor.
No.6 AWG.

8. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Current changing constantly in magnitude and periodically in direction.
Carotid pulse - which is located in the neck next to the Adam's apple.
X.

9. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Current that changes constantly in magnitude and periodically in direction.
The capacitors should be discharged.
Potential transformer.
Cut around clothing that has adhered to the burned area.

10. 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.
An electromagnet.
Reverse any two-phase leads.
Transformers.

11. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Because less current flowing at high voltage causes less line loss per kilowatt.
Second-degree burn.
The chest should rise or inflate.
60 cycles per second (60 Hertz).

12. What should the duty rating of a grounding resistor be?
Disconnect the frame-grounding conductor.
Electro-magnetism for their operation.
A grounding resistor.
Continuous.

13. 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?
3 -979 Volts.
150 turns.
2000 watts
H terminals.

14. The property of an electric circuit that opposes any change in current is called...
60 cycles per second (60 Hertz).
Inductance.
Four times less than that of the secondary.
Stator windings.

15. Where should grounding resistors be located?
At the source transformer.
Two speeds
Transformer
7 -630 volts.

16. What type of 3-phase AC motor is used most often around a mine?
Stator windings.
Cut around clothing that has adhered to the burned area.
Clamp-on ammeter.
Squirrel-cage induction motor.

17. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
150 turns.
The motor rotation is reversed.
60 cycles per second (60 Hertz).

18. Three-phase AC underground cables must be provided with overcurrent protection using a...
Potential transformer.
12 to 15 times a minute.
Circuit Breaker.
No.6 AWG.

19. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Stator windings.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Second-degree burn.

20. The transformer used only to measure voltage is a...
150 turns.
Alternating current.
Grounded phase and ground fault devices were not operating.
Potential transformer.

21. 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.
Nearly constant.
480 volts.
2000 watts

22. Not true about AC power?
It provides good speed control.
Current that changes constantly in magnitude and periodically in direction.
600 volts.
Inductance.

23. The standard frequency of alternating current used at a mine is...
Transformer
The capacitors should be discharged.
60 Hertz
Allow for metering voltage and Current.

24. Current that changes periodically in direction is...
Alternating current.
Cut around clothing that has adhered to the burned area.
The sum of the two capacitors.
Electro-magnetism for their operation.

25. 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.
75 turns.
Larger than that found in the primary winding.
The current will read zero.

26. A burn causing a blister is classified as a...
Second-degree burn.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Larger than that found in the primary winding.
Short-circuited after power has been removed.

27. Not an acceptable method for the installations of surface transformers?
Inductance.
They must be installed at least 7 feet above the ground.
600 volts.
A device used to store electrical energy.

28. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Current that changes constantly in magnitude and periodically in direction.
15 feet off the ground.
7 -630 volts.

29. Which of the following does a squirrel cage induction motor have?
Short-circuited after power has been removed.
100 volts.
Stator windings.
No.6 AWG.

30. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
A device used to store electrical energy.
At least two phases
Allow for metering voltage and Current.
Short-circuited after power has been removed.

31. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
The chest should rise or inflate.
Full-load - three-phase voltage.

32. The primary leads of a single-phase transformer should be connected to the...
Alternating current.
5:1
H terminals.
Inductance.

33. What type motor is a wound-rotor induction motor?
A.C.
A grounding resistor.
Grounded phase and ground fault devices were not operating.
Shunted and taped

34. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
The chest should rise or inflate.
Four times less than that of the secondary.
1 amp.

35. Not a symptom of electrical shock?.
Nearly constant.
Hyperventilation.
Disconnect the frame-grounding conductor.
200 volts AC.

36. 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?
60 Hertz
75 turns.
Circuit Breaker.
Kilovolt ampere (KVA).

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...
At least two phases
Draw less current.
Circuit Breaker.
Alternating current.

38. 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.
Lower current and higher voltage
Capacitance.
An electromagnet.

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

40. 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?
Circuit Breaker.
A.C.
600 volts.
At least two phases

41. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The sum of the two capacitors.
60 cycles per second (60 Hertz).
The motor rotation is reversed.
H terminals.

42. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Alternating current.
5:1
At the source transformer.

43. The size of a transformer is measured in...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.
7 -630 volts.

44. At what rate should mouth-to-mouth resuscitation be applied?
75 turns.
Second-degree burn.
12 to 15 times a minute.
200 volts AC.

45. The purpose of a zig-zag transformer is to...
Lower current and higher voltage.
Capacitance.
Lower current and higher voltage
Derive a neutral for grounding purposes.

46. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The chest should rise or inflate.
Disconnect the frame-grounding conductor.
No.6 AWG.
1 amp.

47. What is the first step to be followed in aiding a victim of electrical shock?
Two speeds
Check to make sure that the victim is not still in contact with the electricity.
Allow for metering voltage and Current.
Lower current and higher voltage.

48. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
The motor rotation is reversed.
Inductance.
Grounded phase and ground fault devices were not operating.

49. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Clamp-on ammeter.
Allow for metering voltage and Current.
2000 watts

50. 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?
15 feet off the ground.
Circuit Breaker.
Disconnect the frame-grounding conductor.
100 turns.