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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. High voltage is used to transmit power over long distances...
3 -979 Volts.
A step-up transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.
150 turns.

2. The speed of an alternating current - squirrel-cage induction motor is...
No.6 AWG.
Nearly constant.
Hyperventilation.
Transformers.

3. Alternating current may be defined as...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Tied together
Current that changes constantly in magnitude and periodically in direction.
Draw less current.

4. What is the danger when working around capacitors?
Isolation transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
An electrical shock hazard may exist.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

5. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Cut around clothing that has adhered to the burned area.
480 volts.
Squirrel-cage rotor.
Check to make sure that the victim is not still in contact with the electricity.

6. The secondary windings on a 10:1 step-down transformer will have a wire size...
60 Hertz
The sum of the two capacitors.
Disconnect the frame-grounding conductor.
Larger than that found in the primary winding.

7. 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.
800 volts.
5:1
An electromagnet.

8. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
H terminals.
Short-circuited after power has been removed.
600 volts.
They should be made by the same manufacturer.

9. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Inductance.
A grounding resistor.
600 volts.

10. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Capacitance.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The motor rotation is reversed.
A grounding resistor.

11. 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.
15 feet off the ground.
Isolation transformer.
2000 watts
5:1

12. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
800 volts.
277 volts.
480 volts.
The chest should rise or inflate.

13. Operating coils - motors - transformers - relays and solenoids depend upon...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Clamp-on ammeter.
Electro-magnetism for their operation.
3 -979 Volts.

14. 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.
Alternating current.
5:1.
Reverse its rotation.
The chest should rise or inflate.

15. When not connected to a load - the secondary leads of a current transformer should be...
5:1.
Shunted and taped
A.C.
25 feet.

16. 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.
They must be installed at least 7 feet above the ground.
Zero amps.
600 volts.

17. Current transformers - or CT's - are used to...
Electro-magnetism for their operation.
Lower current and higher voltage
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Two speeds

18. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The motor rotation is reversed.
No.6 AWG.
277 volts.
5:1

19. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
It provides good speed control.
Disconnect the frame-grounding conductor.
Inductance.

20. 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...
H terminals.
12 to 15 times a minute.
Tied together
No.6 AWG.

21. Define alternating current...
A.C.
7 -630 volts.
15 feet off the ground.
Current changing constantly in magnitude and periodically in direction.

22. 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.
60 Hertz
Full-load - three-phase voltage.
The sum of the two capacitors.

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

24. The combined capacitance of two capacitors in parallel is...
5:1.
Hyperventilation.
Continuous.
The sum of the two capacitors.

25. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Stator windings.
A.C.
Potential transformer.

26. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
A step-up transformer.
Grounded phase and ground fault devices were not operating.
Lower current and higher voltage
15 feet off the ground.

27. What should the duty rating of a grounding resistor be?
Full-load - three-phase voltage.
Continuous.
15 feet off the ground.
At least two phases

28. 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?
75 turns.
A grounding resistor.
200 volts AC.
Stator windings.

29. Not an acceptable method for the installations of surface transformers?
At least two phases
No.6 AWG.
They must be installed at least 7 feet above the ground.
600 volts.

30. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
The sum of the two capacitors.
Zero amps.
At least two phases
100 volts.

31. Not a symptom of electrical shock?.
A device used to store electrical energy.
Hyperventilation.
150 turns.
Squirrel-cage rotor.

32. 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.
A grounding resistor.
75 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.

33. A burn causing a blister is classified as a...
Second-degree burn.
75 turns.
7 -630 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.

34. 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?
Alternating current.
Squirrel-cage induction motor.
480 volts.
3 -979 Volts.

35. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Larger than that found in the primary winding.
A step-up transformer.
7 -630 volts.
200 volts AC.

36. What type of 3-phase AC motor is used most often around a mine?
Transformer
Carotid pulse - which is located in the neck next to the Adam's apple.
Squirrel-cage induction motor.
The capacitors should be discharged.

37. What steps up or steps down AC voltage?
Larger than that found in the primary winding.
Transformer
Draw less current.
Stator windings.

38. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Short-circuited after power has been removed.
277 volts.
Derive a neutral for grounding purposes.

39. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Lower current and higher voltage.
X.
Capacitance.

40. Not true about AC power?
H terminals.
They must be installed at least 7 feet above the ground.
It provides good speed control.
Shunted and taped

41. The purpose of a zig-zag transformer is to...
Second-degree burn.
Derive a neutral for grounding purposes.
600 volts.
Shunted and taped

42. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
200 volts AC.
The capacitors should be discharged.
Alternating current.

43. Which of the following does a squirrel cage induction motor have?
480 volts.
Isolation transformer.
At the source transformer.
Stator windings.

44. 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?
75 turns.
Carotid pulse - which is located in the neck next to the Adam's apple.
100 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

45. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Derive a neutral for grounding purposes.
277 volts.
100 turns.

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

47. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
480 volts.
2000 watts
1 amp.
Check to make sure that the victim is not still in contact with the electricity.

48. Current in an AC circuit may be measured by...
100 turns.
Short-circuited after power has been removed.
Clamp-on ammeter.
Reverse any two-phase leads.

49. Compared to the secondary of a loaded - step-down transformer - the primary has...
Tied together
Inductance.
Lower current and higher voltage
Because less current flowing at high voltage causes less line loss per kilowatt.

50. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Electro-magnetism for their operation.
Short-circuited after power has been removed.
Four times less than that of the secondary.
The chest should rise or inflate.