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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. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Allow for metering voltage and Current.
Capacitance.
A grounding resistor.
An electromagnet.

2. 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...
100 volts.
Tied together
The motor rotation is reversed.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

3. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Four times less than that of the secondary.
Because less current flowing at high voltage causes less line loss per kilowatt.
Two speeds
Capacitance.

4. At what rate should mouth-to-mouth resuscitation be applied?
A.C.
12 to 15 times a minute.
600 volts.
Capacitance.

5. What type of 3-phase AC motor is used most often around a mine?
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage induction motor.
A step-up transformer.

6. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
100 turns.
12 to 15 times a minute.
200 volts AC.
Grounded phase and ground fault devices were not operating.

7. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
5:1
A.C.
Clamp-on ammeter.

8. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
An electrical shock hazard may exist.
An electromagnet.
1 amp.
Alternating current.

9. 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?
75 turns.
A.C.
Kilovolt ampere (KVA).
Tied together

10. 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?
Stator windings.
277 volts.
Transformers.
800 volts.

11. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Tied together
Nearly constant.
H terminals.

12. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
The capacitors should be discharged.
Draw less current.
A step-up transformer.

13. What should the duty rating of a grounding resistor be?
60 cycles per second (60 Hertz).
Continuous.
Current that changes constantly in magnitude and periodically in direction.
15 feet off the ground.

14. The property of an electric circuit that opposes any change in current is called...
H terminals.
Inductance.
The motor rotation is reversed.
Check to make sure that the victim is not still in contact with the electricity.

15. Current that changes periodically in direction is...
Alternating current.
Shunted and taped
Check to make sure that the victim is not still in contact with the electricity.
150 turns.

16. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Potential transformer.
Tied together
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.

17. The purpose of a zig-zag transformer is to...
A grounding resistor.
Nearly constant.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Derive a neutral for grounding purposes.

18. 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...
Draw less current.
Full-load - three-phase voltage.
15 feet off the ground.
Inductance.

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

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20. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The chest should rise or inflate.
The capacitors should be discharged.
100 turns.
They must be installed at least 7 feet above the ground.

21. 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?
100 turns.
Two speeds
Inductance.
Lower current and higher voltage.

22. 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?
150 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
480 volts.
A device used to store electrical energy.

23. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Lower current and higher voltage.
Disconnect the frame-grounding conductor.
2000 watts
100 volts.

24. The property of an electrical circuit that opposes any change in voltage is called...
They should be made by the same manufacturer.
600 volts.
Capacitance.
75 turns.

25. What type motor is a wound-rotor induction motor?
Lower current and higher voltage
A.C.
12 to 15 times a minute.
Allow for metering voltage and Current.

26. The primary leads of a single-phase transformer should be connected to the...
Current that changes constantly in magnitude and periodically in direction.
H terminals.
600 volts.
Four times less than that of the secondary.

27. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Four times less than that of the secondary.
The motor rotation is reversed.
No.6 AWG.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

28. The transformer used only to measure voltage is a...
Tied together
60 cycles per second (60 Hertz).
Potential transformer.
Inductance.

29. AC voltage is raised or lowered by means of...
Circuit Breaker.
Capacitance.
7 -630 volts.
Transformers.

30. 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...
At the source transformer.
Draw less current.
5:1.
100 volts.

31. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Four times less than that of the secondary.
277 volts.
5:1.
They should be made by the same manufacturer.

32. What is a capacitor?
A device used to store electrical energy.
Lower current and higher voltage.
Two speeds
Electro-magnetism for their operation.

33. Define alternating current...
Transformer
3 -979 Volts.
Current changing constantly in magnitude and periodically in direction.
Current that changes constantly in magnitude and periodically in direction.

34. Current in an AC circuit may be measured by...
Clamp-on ammeter.
800 volts.
At the source transformer.
15 feet off the ground.

35. The secondary windings on a 10:1 step-down transformer will have a wire size...
Zero amps.
Transformers.
Short-circuited after power has been removed.
Larger than that found in the primary winding.

36. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Because less current flowing at high voltage causes less line loss per kilowatt.
Reverse any two-phase leads.
A step-up transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

37. Compared to the secondary of a loaded - step-down transformer - the primary has...
Larger than that found in the primary winding.
At the source transformer.
Carotid pulse - which is located in the neck next to the Adam's apple.
Lower current and higher voltage.

38. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
A device used to store electrical energy.
The current will read zero.
Clamp-on ammeter.
Zero amps.

39. The size of a transformer is measured in...
Kilovolt ampere (KVA).
15 feet off the ground.
7 -630 volts.
No.6 AWG.

40. A solenoid coil is...
100 volts.
Squirrel-cage rotor.
480 volts.
An electromagnet.

41. 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?
Nearly constant.
Grounded phase and ground fault devices were not operating.
A device used to store electrical energy.
600 volts.

42. 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?
The motor rotation is reversed.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
75 turns.
Potential transformer.

43. Which of the following does a squirrel cage induction motor have?
25 feet.
Stator windings.
5:1.
Zero amps.

44. Not a symptom of electrical shock?.
Squirrel-cage induction motor.
Hyperventilation.
The sum of the two capacitors.
277 volts.

45. If any two-phase wires are reversed on a three-phase motor - the motor will...
Short-circuited after power has been removed.
Zero amps.
Inductance.
Reverse its rotation.

46. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
5:1
Derive a neutral for grounding purposes.
X.
Two speeds

47. High voltage is used to transmit power over long distances...
277 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
Alternating current.
They must be installed at least 7 feet above the ground.

48. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Reverse its rotation.
H terminals.
Grounded phase and ground fault devices were not operating.
15 feet off the ground.

49. 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.
Zero amps.
At the source transformer.
A device used to store electrical energy.

50. A control power transformer...
Isolation transformer.
A grounding resistor.
Provides single-phase power to smaller loads such as lights - heaters - etc.
An electromagnet.