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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. 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.
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage induction motor.
800 volts.

2. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Current that changes constantly in magnitude and periodically in direction.
Inductance.
Stator windings.
No.6 AWG.

3. 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...
3 -979 Volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
They must be installed at least 7 feet above the ground.
Tied together

4. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
An electromagnet.
Carotid pulse - which is located in the neck next to the Adam's apple.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
The chest should rise or inflate.

5. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Potential transformer.
Reverse its rotation.
15 feet off the ground.
Allow for metering voltage and Current.

6. 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.
1 amp.
Reverse its rotation.
Draw less current.
5:1

7. Operating coils - motors - transformers - relays and solenoids depend upon...
Circuit Breaker.
No.6 AWG.
7 -630 volts.
Electro-magnetism for their operation.

8. AC voltage is raised or lowered by means of...
Because less current flowing at high voltage causes less line loss per kilowatt.
1 amp.
Isolation transformer.
Transformers.

9. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Check to make sure that the victim is not still in contact with the electricity.
Current changing constantly in magnitude and periodically in direction.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Reverse its rotation.

10. Alternating current may be defined as...
The chest should rise or inflate.
Current that changes constantly in magnitude and periodically in direction.
Current changing constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.

11. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
It provides good speed control.
Hyperventilation.
Isolation transformer.
The capacitors should be discharged.

12. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Stator windings.
A device used to store electrical energy.
Disconnect the frame-grounding conductor.
Isolation transformer.

13. A burn causing a blister is classified as a...
The capacitors should be discharged.
Second-degree burn.
At the source transformer.
800 volts.

14. Define alternating current...
Grounded phase and ground fault devices were not operating.
Lower current and higher voltage
Current changing constantly in magnitude and periodically in direction.
Derive a neutral for grounding purposes.

15. The standard frequency of alternating current used at a mine is...
H terminals.
Carotid pulse - which is located in the neck next to the Adam's apple.
X.
60 Hertz

16. What steps up or steps down AC voltage?
Transformer
Clamp-on ammeter.
150 turns.
Current changing constantly in magnitude and periodically in direction.

17. The main function of CT's and PT's on a main substation is to...
12 to 15 times a minute.
An electrical shock hazard may exist.
Allow for metering voltage and Current.
Zero amps.

18. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
Current changing constantly in magnitude and periodically in direction.
The chest should rise or inflate.
2000 watts

19. If any two-phase wires are reversed on a three-phase motor - the motor will...
A grounding resistor.
Current changing constantly in magnitude and periodically in direction.
150 turns.
Reverse its rotation.

20. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Zero amps.
Draw less current.
Capacitance.

21. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
They should be made by the same manufacturer.
A grounding resistor.
Transformer

22. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Tied together
100 turns.
Squirrel-cage induction motor.

23. The standard frequency of power supplied by the power company is...
A device used to store electrical energy.
At least two phases
Potential transformer.
60 cycles per second (60 Hertz).

24. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
The capacitors should be discharged.
Squirrel-cage rotor.
2000 watts
Short-circuited after power has been removed.

25. Current transformers - or CT's - are used to...
Clamp-on ammeter.
3 -979 Volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Draw less current.

26. 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
100 turns.
Potential transformer.
Grounded phase and ground fault devices were not operating.

27. 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...
At the source transformer.
Two speeds
Draw less current.
60 cycles per second (60 Hertz).

28. What is the danger when working around capacitors?
Potential transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
60 cycles per second (60 Hertz).
An electrical shock hazard may exist.

29. What is the first step to be followed in aiding a victim of electrical shock?
Nearly constant.
Check to make sure that the victim is not still in contact with the electricity.
The current will read zero.
5:1

30. How do you change the direction of a 3-phase AC motor?
A.C.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Transformer
Reverse any two-phase leads.

31. 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?
A grounding resistor.
Second-degree burn.
Continuous.
150 turns.

32. The property of an electric circuit that opposes any change in current is called...
Second-degree burn.
25 feet.
Inductance.
Draw less current.

33. High voltage is used to transmit power over long distances...
Continuous.
H terminals.
Because less current flowing at high voltage causes less line loss per kilowatt.
1 amp.

34. The combined capacitance of two capacitors in parallel is...
Inductance.
The sum of the two capacitors.
Derive a neutral for grounding purposes.
Transformers.

35. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
15 feet off the ground.
A step-up transformer.
At least two phases
Electro-magnetism for their operation.

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?
Isolation transformer.
An electromagnet.
Inductance.
3 -979 Volts.

37. 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?
200 volts AC.
25 feet.
Transformer
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

38. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Second-degree burn.
150 turns.
12 to 15 times a minute.
At least two phases

39. Not true about AC power?
Inductance.
At the source transformer.
A step-up transformer.
It provides good speed control.

40. Current in an AC circuit may be measured by...
Nearly constant.
Grounded phase and ground fault devices were not operating.
Clamp-on ammeter.
At the source transformer.

41. Current that changes periodically in direction is...
Stator windings.
75 turns.
Alternating current.
3 -979 Volts.

42. What is a capacitor?
1 amp.
At least two phases
It provides good speed control.
A device used to store electrical energy.

43. 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.
Reverse any two-phase leads.
Hyperventilation.
Kilovolt ampere (KVA).

44. The speed of an alternating current - squirrel-cage induction motor is...
60 cycles per second (60 Hertz).
Nearly constant.
Capacitance.
25 feet.

45. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
25 feet.
1 amp.
60 cycles per second (60 Hertz).
75 turns.

46. Which of the following does a squirrel cage induction motor have?
Nearly constant.
The capacitors should be discharged.
Stator windings.
15 feet off the ground.

47. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Continuous.
The motor rotation is reversed.
They must be installed at least 7 feet above the ground.
The current will read zero.

48. At what rate should mouth-to-mouth resuscitation be applied?
The motor rotation is reversed.
Provides single-phase power to smaller loads such as lights - heaters - etc.
12 to 15 times a minute.
Because less current flowing at high voltage causes less line loss per kilowatt.

49. 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?
Short-circuited after power has been removed.
Isolation transformer.
75 turns.
They must be installed at least 7 feet above the ground.

50. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
It provides good speed control.
Zero amps.
X.
Potential transformer.