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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 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
277 volts.
5:1.
Four times less than that of the secondary.
They should be made by the same manufacturer.

2. Current transformers - or CT's - are used to...
Electro-magnetism for their operation.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
100 turns.
An electromagnet.

3. If any two-phase wires are reversed on a three-phase motor - the motor will...
Grounded phase and ground fault devices were not operating.
Reverse its rotation.
Allow for metering voltage and Current.
277 volts.

4. Compared to the secondary of a loaded - step-down transformer - the primary has...
Disconnect the frame-grounding conductor.
Lower current and higher voltage.
Capacitance.
Nearly constant.

5. How should clothing be removed from a burn?
Reverse its rotation.
Disconnect the frame-grounding conductor.
Derive a neutral for grounding purposes.
Cut around clothing that has adhered to the burned area.

6. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
200 volts AC.
Continuous.
Zero amps.

7. What type of 3-phase AC motor is used most often around a mine?
Four times less than that of the secondary.
Squirrel-cage induction motor.
X.
Clamp-on ammeter.

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

9. 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?
600 volts.
Grounded phase and ground fault devices were not operating.
Cut around clothing that has adhered to the burned area.
Stator windings.

10. 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?
At least two phases
3 -979 Volts.
5:1
The capacitors should be discharged.

11. How do you change the direction of a 3-phase AC motor?
Draw less current.
Transformer
Reverse any two-phase leads.
800 volts.

12. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
75 turns.
At the source transformer.
The current will read zero.

13. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
Allow for metering voltage and Current.
600 volts.
The current will read zero.

14. Not a symptom of electrical shock?.
It provides good speed control.
5:1
Potential transformer.
Hyperventilation.

15. 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.
Zero amps.
60 cycles per second (60 Hertz).
2000 watts

16. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Hyperventilation.
Four times less than that of the secondary.
The chest should rise or inflate.
25 feet.

17. All transformers step voltage up or down by...
H terminals.
60 cycles per second (60 Hertz).
Allow for metering voltage and Current.
Inductance.

18. What is a capacitor?
7 -630 volts.
A device used to store electrical energy.
Current that changes constantly in magnitude and periodically in direction.
480 volts.

19. The speed of an alternating current - squirrel-cage induction motor is...
15 feet off the ground.
Nearly constant.
100 turns.
The current will read zero.

20. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
3 -979 Volts.
Reverse any two-phase leads.
60 cycles per second (60 Hertz).
1 amp.

21. Current in an AC circuit may be measured by...
Cut around clothing that has adhered to the burned area.
Zero amps.
1 amp.
Clamp-on ammeter.

22. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
480 volts.
At least two phases
12 to 15 times a minute.
Short-circuited after power has been removed.

23. A wound-rotor induction motor has all the following EXCEPT...
75 turns.
Squirrel-cage rotor.
Four times less than that of the secondary.
75 turns.

24. What type motor is a wound-rotor induction motor?
A.C.
25 feet.
Nearly constant.
Circuit Breaker.

25. Current that changes periodically in direction is...
Two speeds
Alternating current.
The capacitors should be discharged.
The chest should rise or inflate.

26. What should the duty rating of a grounding resistor be?
Continuous.
Transformer
Kilovolt ampere (KVA).
Larger than that found in the primary winding.

27. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Reverse its rotation.
A grounding resistor.
Inductance.

28. A burn causing a blister is classified as a...
A.C.
Carotid pulse - which is located in the neck next to the Adam's apple.
Current changing constantly in magnitude and periodically in direction.
Second-degree burn.

29. The standard frequency of power supplied by the power company is...
60 cycles per second (60 Hertz).
75 turns.
Disconnect the frame-grounding conductor.
Reverse its rotation.

30. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
Alternating current.
60 cycles per second (60 Hertz).
Shunted and taped

31. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
The current will read zero.
Alternating current.
7 -630 volts.

32. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Electro-magnetism for their operation.
200 volts AC.
Current that changes constantly in magnitude and periodically in direction.

33. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Two speeds
7 -630 volts.
Lower current and higher voltage
An electromagnet.

34. What is the danger when working around capacitors?
Disconnect the frame-grounding conductor.
100 turns.
At the source transformer.
An electrical shock hazard may exist.

35. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
The sum of the two capacitors.
Four times less than that of the secondary.
25 feet.
Continuous.

36. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
75 turns.
75 turns.
Larger than that found in the primary winding.
480 volts.

37. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
5:1
60 Hertz
Carotid pulse - which is located in the neck next to the Adam's apple.
Grounded phase and ground fault devices were not operating.

38. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
2000 watts
Two speeds
H terminals.

39. Not true about AC power?
A device used to store electrical energy.
It provides good speed control.
Squirrel-cage induction motor.
The capacitors should be discharged.

40. Three-phase AC underground cables must be provided with overcurrent protection using a...
75 turns.
Circuit Breaker.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
X.

41. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
277 volts.
A grounding resistor.
Nearly constant.
Full-load - three-phase voltage.

42. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Lower current and higher voltage
No.6 AWG.
Larger than that found in the primary winding.
277 volts.

43. 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
Reverse any two-phase leads.
Isolation transformer.
Squirrel-cage rotor.

44. 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?
Isolation transformer.
100 turns.
X.
Cut around clothing that has adhered to the burned area.

45. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Draw less current.
Stator windings.
600 volts.
They should be made by the same manufacturer.

46. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
3 -979 Volts.
X.
Potential transformer.

47. 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?
Current changing constantly in magnitude and periodically in direction.
The sum of the two capacitors.
A grounding resistor.
75 turns.

48. The standard frequency of alternating current used at a mine is...
60 Hertz
Disconnect the frame-grounding conductor.
The motor rotation is reversed.
Squirrel-cage induction motor.

49. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Current changing constantly in magnitude and periodically in direction.
Inductance.
X.
15 feet off the ground.

50. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.
12 to 15 times a minute.