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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. Not a symptom of electrical shock?.
5:1.
Check to make sure that the victim is not still in contact with the electricity.
Hyperventilation.
Reverse any two-phase leads.

2. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Larger than that found in the primary winding.
25 feet.
1 amp.
100 volts.

3. If any two-phase wires are reversed on a three-phase motor - the motor will...
Zero amps.
Reverse its rotation.
25 feet.
Second-degree burn.

4. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
Continuous.
Electro-magnetism for their operation.
H terminals.

5. The secondary windings on a 10:1 step-down transformer will have a wire size...
Derive a neutral for grounding purposes.
Larger than that found in the primary winding.
Alternating current.
60 cycles per second (60 Hertz).

6. Not an acceptable method for the installations of surface transformers?
Transformers.
They must be installed at least 7 feet above the ground.
Squirrel-cage induction motor.
Alternating current.

7. Current that changes periodically in direction is...
Alternating current.
Reverse any two-phase leads.
480 volts.
Current that changes constantly in magnitude and periodically in direction.

8. 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?
The motor rotation is reversed.
15 feet off the ground.
800 volts.
At the source transformer.

9. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
5:1
An electrical shock hazard may exist.
7 -630 volts.
Four times less than that of the secondary.

10. What type of 3-phase AC motor is used most often around a mine?
Carotid pulse - which is located in the neck next to the Adam's apple.
Squirrel-cage induction motor.
Nearly constant.
Grounded phase and ground fault devices were not operating.

11. AC voltage is raised or lowered by means of...
A step-up transformer.
Transformers.
Derive a neutral for grounding purposes.
Isolation transformer.

12. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
A grounding resistor.
Tied together
The motor rotation is reversed.
Lower current and higher voltage.

13. Current transformers - or CT's - are used to...
Kilovolt ampere (KVA).
Stator windings.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Transformers.

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.
At least two phases
3 -979 Volts.
5:1
Full-load - three-phase voltage.

15. 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?
Shunted and taped
600 volts.
Electro-magnetism for their operation.
Transformers.

16. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Transformer
No.6 AWG.
Circuit Breaker.
2000 watts

17. 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...
Current that changes constantly in magnitude and periodically in direction.
Draw less current.
12 to 15 times a minute.
100 turns.

18. A wound-rotor induction motor has all the following EXCEPT...
15 feet off the ground.
An electromagnet.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage rotor.

19. The primary function of a power transformer is to provide...
Lower current and higher voltage.
Second-degree burn.
Full-load - three-phase voltage.
Disconnect the frame-grounding conductor.

20. What steps up or steps down AC voltage?
60 Hertz
Squirrel-cage rotor.
Transformer
Reverse its rotation.

21. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
The sum of the two capacitors.
Check to make sure that the victim is not still in contact with the electricity.
A.C.

22. The speed of an alternating current - squirrel-cage induction motor is...
Stator windings.
600 volts.
Nearly constant.
Alternating current.

23. A transformer with a 1:1 ratio is usually a (an)...
No.6 AWG.
Isolation transformer.
Tied together
At the source transformer.

24. 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...
200 volts AC.
Two speeds
Tied together
100 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...
They must be installed at least 7 feet above the ground.
An electromagnet.
Tied together
100 volts.

26. 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.
480 volts.
X.
5:1.
The chest should rise or inflate.

27. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
The motor rotation is reversed.
The chest should rise or inflate.
Reverse its rotation.

28. Three-phase AC underground cables must be provided with overcurrent protection using a...
Allow for metering voltage and Current.
Transformer
Inductance.
Circuit Breaker.

29. At what rate should mouth-to-mouth resuscitation be applied?
100 volts.
12 to 15 times a minute.
At least two phases
Circuit Breaker.

30. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
60 cycles per second (60 Hertz).
60 Hertz
They should be made by the same manufacturer.

31. 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?
Grounded phase and ground fault devices were not operating.
200 volts AC.
Inductance.
Short-circuited after power has been removed.

32. The primary leads of a single-phase transformer should be connected to the...
H terminals.
12 to 15 times a minute.
Four times less than that of the secondary.
The sum of the two capacitors.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
At least two phases
They must be installed at least 7 feet above the ground.
Check to make sure that the victim is not still in contact with the electricity.

34. A solenoid coil is...
An electromagnet.
100 volts.
Two speeds
Electro-magnetism for their operation.

35. High voltage is used to transmit power over long distances...
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Because less current flowing at high voltage causes less line loss per kilowatt.

36. 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?
H terminals.
3 -979 Volts.
The motor rotation is reversed.
200 volts AC.

37. 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?
Current that changes constantly in magnitude and periodically in direction.
75 turns.
A device used to store electrical energy.
Capacitance.

38. The transformer used only to measure voltage is a...
Squirrel-cage induction motor.
150 turns.
Potential transformer.
An electromagnet.

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

40. How should clothing be removed from a burn?
15 feet off the ground.
Cut around clothing that has adhered to the burned area.
X.
They must be installed at least 7 feet above the ground.

41. 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...
Lower current and higher voltage.
Draw less current.
Current changing constantly in magnitude and periodically in direction.
5:1

42. 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.
75 turns.
At the source transformer.
600 volts.

43. Current in an AC circuit may be measured by...
Clamp-on ammeter.
100 volts.
Derive a neutral for grounding purposes.
Draw less current.

44. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Continuous.
The motor rotation is reversed.
Short-circuited after power has been removed.
Two speeds

45. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Provides single-phase power to smaller loads such as lights - heaters - etc.
Reverse any two-phase leads.
1 amp.
2000 watts

46. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Isolation transformer.
Draw less current.
277 volts.
Larger than that found in the primary winding.

47. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
At the source transformer.
Transformer
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Zero amps.

48. The purpose of a zig-zag transformer is to...
A step-up transformer.
277 volts.
Derive a neutral for grounding purposes.
Full-load - three-phase voltage.

49. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Inductance.
Kilovolt ampere (KVA).
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

50. The standard frequency of alternating current used at a mine is...
Derive a neutral for grounding purposes.
Draw less current.
60 Hertz
Because less current flowing at high voltage causes less line loss per kilowatt.