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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 not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
7 -630 volts.
No.6 AWG.
The sum of the two capacitors.

2. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Kilovolt ampere (KVA).
7 -630 volts.
60 cycles per second (60 Hertz).
An electrical shock hazard may exist.

3. What should the duty rating of a grounding resistor be?
Continuous.
12 to 15 times a minute.
X.
Squirrel-cage rotor.

4. The size of a transformer is measured in...
Kilovolt ampere (KVA).
They should be made by the same manufacturer.
At the source transformer.
Squirrel-cage rotor.

5. The main function of CT's and PT's on a main substation is to...
An electrical shock hazard may exist.
They must be installed at least 7 feet above the ground.
Electro-magnetism for their operation.
Allow for metering voltage and Current.

6. 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
60 cycles per second (60 Hertz).
Transformer
A.C.

7. If any two-phase wires are reversed on a three-phase motor - the motor will...
The current will read zero.
A device used to store electrical energy.
480 volts.
Reverse its rotation.

8. The speed of an alternating current - squirrel-cage induction motor is...
The sum of the two capacitors.
150 turns.
5:1
Nearly constant.

9. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Capacitance.
Transformers.
Reverse its rotation.
Disconnect the frame-grounding conductor.

10. The purpose of a zig-zag transformer is to...
It provides good speed control.
75 turns.
Derive a neutral for grounding purposes.
Current that changes constantly in magnitude and periodically in direction.

11. 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.
Current that changes constantly in magnitude and periodically in direction.
Shunted and taped

12. What is a capacitor?
Shunted and taped
Capacitance.
An electrical shock hazard may exist.
A device used to store electrical energy.

13. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
150 turns.
The current will read zero.
No.6 AWG.

14. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Lower current and higher voltage
480 volts.
Transformer
Larger than that found in the primary winding.

15. The combined capacitance of two capacitors in parallel is...
12 to 15 times a minute.
The sum of the two capacitors.
Larger than that found in the primary winding.
Inductance.

16. 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?
Inductance.
800 volts.
No.6 AWG.
Zero amps.

17. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Lower current and higher voltage
Allow for metering voltage and Current.
The current will read zero.
At least two phases

18. 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?
Second-degree burn.
600 volts.
Grounded phase and ground fault devices were not operating.
75 turns.

19. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Capacitance.
A step-up transformer.
100 turns.
No.6 AWG.

20. The secondary windings on a 10:1 step-down transformer will have a wire size...
Stator windings.
At the source transformer.
Hyperventilation.
Larger than that found in the primary winding.

21. Which of the following does a squirrel cage induction motor have?
200 volts AC.
3 -979 Volts.
Stator windings.
Draw less current.

22. 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.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Nearly constant.
Shunted and taped

23. What is the first step to be followed in aiding a victim of electrical shock?
Circuit Breaker.
Two speeds
Short-circuited after power has been removed.
Check to make sure that the victim is not still in contact with the electricity.

24. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
5:1
No.6 AWG.
3 -979 Volts.
Squirrel-cage induction motor.

25. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
25 feet.
A grounding resistor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electrical shock hazard may exist.

26. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Squirrel-cage rotor.
Check to make sure that the victim is not still in contact with the electricity.
Circuit Breaker.
2000 watts

27. 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.
480 volts.
Zero amps.
100 volts.

28. Current that changes periodically in direction is...
An electromagnet.
Draw less current.
Alternating current.
Transformers.

29. 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?
The current will read zero.
Cut around clothing that has adhered to the burned area.
3 -979 Volts.
Electro-magnetism for their operation.

30. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Alternating current.
100 volts.
A grounding resistor.
The capacitors should be discharged.

31. What type of 3-phase AC motor is used most often around a mine?
Four times less than that of the secondary.
100 volts.
Squirrel-cage induction motor.
It provides good speed control.

32. The property of an electric circuit that opposes any change in current is called...
Draw less current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Derive a neutral for grounding purposes.
Inductance.

33. Three-phase AC underground cables must be provided with overcurrent protection using a...
2000 watts
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Circuit Breaker.
A grounding resistor.

34. 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...
Draw less current.
Allow for metering voltage and Current.
Reverse any two-phase leads.
277 volts.

35. Not a symptom of electrical shock?.
Hyperventilation.
Electro-magnetism for their operation.
Squirrel-cage rotor.
1 amp.

36. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Alternating current.
Circuit Breaker.
They should be made by the same manufacturer.
X.

37. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Four times less than that of the secondary.
A grounding resistor.
75 turns.
100 volts.

38. Not an acceptable method for the installations of surface transformers?
Disconnect the frame-grounding conductor.
They must be installed at least 7 feet above the ground.
X.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

39. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
The motor rotation is reversed.
Lower current and higher voltage.
Isolation transformer.

40. Alternating current may be defined as...
Shunted and taped
Current that changes constantly in magnitude and periodically in direction.
A step-up transformer.
Disconnect the frame-grounding conductor.

41. 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.
60 Hertz
5:1.
A grounding resistor.
600 volts.

42. 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...
The chest should rise or inflate.
100 volts.
Reverse its rotation.
Stator windings.

43. The property of an electrical circuit that opposes any change in voltage is called...
H terminals.
Capacitance.
100 volts.
480 volts.

44. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Transformer
25 feet.
H terminals.
Two speeds

45. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Disconnect the frame-grounding conductor.
Two speeds
Larger than that found in the primary winding.
The chest should rise or inflate.

46. Compared to the secondary of a loaded - step-down transformer - the primary has...
Squirrel-cage rotor.
Current that changes constantly in magnitude and periodically in direction.
3 -979 Volts.
Lower current and higher voltage

47. What is the danger when working around capacitors?
Two speeds
An electrical shock hazard may exist.
X.
Kilovolt ampere (KVA).

48. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Larger than that found in the primary winding.
They should be made by the same manufacturer.
Transformers.
The sum of the two capacitors.

49. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Full-load - three-phase voltage.
They should be made by the same manufacturer.
15 feet off the ground.
Inductance.

50. The transformer used only to measure voltage is a...
Potential transformer.
15 feet off the ground.
The sum of the two capacitors.
Lower current and higher voltage