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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
The current will read zero.
Shunted and taped
Reverse any two-phase leads.

2. What is the danger when working around capacitors?
A.C.
100 volts.
An electrical shock hazard may exist.
2000 watts

3. A solenoid coil is...
A.C.
An electromagnet.
600 volts.
Squirrel-cage rotor.

4. 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.
Capacitance.
5:1.
Transformers.

5. The standard frequency of alternating current used at a mine is...
60 Hertz
Four times less than that of the secondary.
Circuit Breaker.
No.6 AWG.

6. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Potential transformer.
1 amp.
The chest should rise or inflate.
Kilovolt ampere (KVA).

7. The property of an electrical circuit that opposes any change in voltage is called...
Isolation transformer.
X.
Lower current and higher voltage.
Capacitance.

8. What type of 3-phase AC motor is used most often around a mine?
Lower current and higher voltage.
Draw less current.
Squirrel-cage induction motor.
They must be installed at least 7 feet above the ground.

9. Current that changes periodically in direction is...
They must be installed at least 7 feet above the ground.
60 cycles per second (60 Hertz).
Alternating current.
Allow for metering voltage and Current.

10. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Allow for metering voltage and Current.
277 volts.
Current changing constantly in magnitude and periodically in direction.
Full-load - three-phase voltage.

11. 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?
No.6 AWG.
A device used to store electrical energy.
3 -979 Volts.
The motor rotation is reversed.

12. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Cut around clothing that has adhered to the burned area.
Electro-magnetism for their operation.
1 amp.
It provides good speed control.

13. 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
Two speeds
Inductance.
Isolation transformer.

14. Not an acceptable method for the installations of surface transformers?
Nearly constant.
They must be installed at least 7 feet above the ground.
Because less current flowing at high voltage causes less line loss per kilowatt.
Allow for metering voltage and Current.

15. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
75 turns.
3 -979 Volts.
Alternating current.

16. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
The chest should rise or inflate.
At least two phases
Zero amps.

17. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Because less current flowing at high voltage causes less line loss per kilowatt.
The capacitors should be discharged.
Alternating current.
Transformers.

18. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Potential transformer.
12 to 15 times a minute.
60 Hertz

19. The primary leads of a single-phase transformer should be connected to the...
800 volts.
H terminals.
Four times less than that of the secondary.
Because less current flowing at high voltage causes less line loss per kilowatt.

20. 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?
Transformer
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.
150 turns.

21. The main function of CT's and PT's on a main substation is to...
They must be installed at least 7 feet above the ground.
Allow for metering voltage and Current.
Potential transformer.
A device used to store electrical energy.

22. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Electro-magnetism for their operation.
3 -979 Volts.
12 to 15 times a minute.
A step-up transformer.

23. 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?
100 volts.
75 turns.
Capacitance.
A device used to store electrical energy.

24. The standard frequency of power supplied by the power company is...
60 cycles per second (60 Hertz).
15 feet off the ground.
Transformer
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

25. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Squirrel-cage induction motor.
Kilovolt ampere (KVA).
Inductance.
Short-circuited after power has been removed.

26. 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?
25 feet.
200 volts AC.
Because less current flowing at high voltage causes less line loss per kilowatt.
100 turns.

27. If any two-phase wires are reversed on a three-phase motor - the motor will...
Inductance.
3 -979 Volts.
Reverse its rotation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

28. Which of the following does a squirrel cage induction motor have?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Short-circuited after power has been removed.
The motor rotation is reversed.
Stator windings.

29. To protect against over-current in a three-phase circuit over-current protection must be provided for...
No.6 AWG.
Isolation transformer.
At least two phases
100 volts.

30. The size of a transformer is measured in...
Squirrel-cage rotor.
The sum of the two capacitors.
Circuit Breaker.
Kilovolt ampere (KVA).

31. Where should grounding resistors be located?
A step-up transformer.
Allow for metering voltage and Current.
Disconnect the frame-grounding conductor.
At the source transformer.

32. At what rate should mouth-to-mouth resuscitation be applied?
12 to 15 times a minute.
100 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
The current will read zero.

33. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
They should be made by the same manufacturer.
5:1
3 -979 Volts.
The motor rotation is reversed.

34. Not a symptom of electrical shock?.
Derive a neutral for grounding purposes.
Because less current flowing at high voltage causes less line loss per kilowatt.
Carotid pulse - which is located in the neck next to the Adam's apple.
Hyperventilation.

35. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Reverse its rotation.
Continuous.
Two speeds
Inductance.

36. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
A device used to store electrical energy.
Grounded phase and ground fault devices were not operating.
Kilovolt ampere (KVA).
Continuous.

37. Define alternating current...
Nearly constant.
Current changing constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

38. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Electro-magnetism for their operation.
Provides single-phase power to smaller loads such as lights - heaters - etc.

39. Alternating current may be defined as...
The motor rotation is reversed.
15 feet off the ground.
25 feet.
Current that changes constantly in magnitude and periodically in direction.

40. What steps up or steps down AC voltage?
An electrical shock hazard may exist.
Lower current and higher voltage
1 amp.
Transformer

41. The secondary windings on a 10:1 step-down transformer will have a wire size...
Tied together
Larger than that found in the primary winding.
Transformer
Check to make sure that the victim is not still in contact with the electricity.

42. A control power transformer...
The capacitors should be discharged.
X.
Stator windings.
Provides single-phase power to smaller loads such as lights - heaters - etc.

43. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Reverse any two-phase leads.
Four times less than that of the secondary.
It provides good speed control.
Larger than that found in the primary winding.

44. 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.
200 volts AC.
3 -979 Volts.
277 volts.

45. High voltage is used to transmit power over long distances...
Allow for metering voltage and Current.
Kilovolt ampere (KVA).
A grounding resistor.
Because less current flowing at high voltage causes less line loss per kilowatt.

46. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Stator windings.
A grounding resistor.
They should be made by the same manufacturer.
Alternating current.

47. A transformer with a 1:1 ratio is usually a (an)...
15 feet off the ground.
200 volts AC.
Derive a neutral for grounding purposes.
Isolation transformer.

48. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
They must be installed at least 7 feet above the ground.
Nearly constant.
Because less current flowing at high voltage causes less line loss per kilowatt.

49. The property of an electric circuit that opposes any change in current is called...
Inductance.
Draw less current.
The chest should rise or inflate.
3 -979 Volts.

50. Current transformers - or CT's - are used to...
Zero amps.
Allow for metering voltage and Current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.

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