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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 5KW - secondary current of 100 amps - and primary voltage of 250 volts.
Transformers.
Electro-magnetism for their operation.
A.C.
5:1

2. 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.
A grounding resistor.
480 volts.
Electro-magnetism for their operation.

3. What type of 3-phase AC motor is used most often around a mine?
At the source transformer.
Circuit Breaker.
Squirrel-cage induction motor.
Draw less current.

4. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
5:1
Draw less current.
100 volts.

5. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Squirrel-cage induction motor.
Draw less current.

6. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
No.6 AWG.
Lower current and higher voltage.
Continuous.

7. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage.
The current will read zero.
No.6 AWG.

8. Alternating current may be defined as...
Squirrel-cage induction motor.
Current that changes constantly in magnitude and periodically in direction.
No.6 AWG.
The motor rotation is reversed.

9. 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?
800 volts.
Shunted and taped
600 volts.
7 -630 volts.

10. All transformers step voltage up or down by...
A grounding resistor.
Inductance.
Draw less current.
Reverse its rotation.

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...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.
Draw less current.
3 -979 Volts.

12. Compared to the secondary of a loaded - step-down transformer - the primary has...
Stator windings.
Lower current and higher voltage
They should be made by the same manufacturer.
5:1.

13. The property of an electric circuit that opposes any change in current is called...
100 volts.
Inductance.
Larger than that found in the primary winding.
Lower current and higher voltage.

14. 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?
A.C.
100 turns.
Squirrel-cage induction motor.
Stator windings.

15. Not true about AC power?
A grounding resistor.
They should be made by the same manufacturer.
Cut around clothing that has adhered to the burned area.
It provides good speed control.

16. The main function of CT's and PT's on a main substation is to...
Check to make sure that the victim is not still in contact with the electricity.
600 volts.
Allow for metering voltage and Current.
25 feet.

17. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Draw less current.
480 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

18. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Hyperventilation.
Short-circuited after power has been removed.
An electrical shock hazard may exist.
They should be made by the same manufacturer.

19. 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.
Stator windings.
Potential transformer.
Alternating current.
5:1.

20. What type motor is a wound-rotor induction motor?
At the source transformer.
Stator windings.
5:1.
A.C.

21. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Potential transformer.
A grounding resistor.
Draw less current.
Grounded phase and ground fault devices were not operating.

22. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Four times less than that of the secondary.
X.
12 to 15 times a minute.
The capacitors should be discharged.

23. 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...
Two speeds
75 turns.
100 volts.
600 volts.

24. AC voltage is raised or lowered by means of...
It provides good speed control.
Transformers.
Lower current and higher voltage
Second-degree burn.

25. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
No.6 AWG.
An electrical shock hazard may exist.
Continuous.

26. A control power transformer...
15 feet off the ground.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Disconnect the frame-grounding conductor.
Clamp-on ammeter.

27. Where should grounding resistors be located?
Full-load - three-phase voltage.
X.
Second-degree burn.
At the source transformer.

28. Not an acceptable method for the installations of surface transformers?
Nearly constant.
5:1.
Zero amps.
They must be installed at least 7 feet above the ground.

29. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
15 feet off the ground.
7 -630 volts.
Transformer
Isolation transformer.

30. Compared to the secondary of a loaded - step-down transformer - the primary has...
Draw less current.
Lower current and higher voltage.
A grounding resistor.
15 feet off the ground.

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?
Stator windings.
150 turns.
Isolation transformer.
Clamp-on ammeter.

32. The standard frequency of alternating current used at a mine is...
Grounded phase and ground fault devices were not operating.
The current will read zero.
60 Hertz
Nearly constant.

33. A transformer with a 1:1 ratio is usually a (an)...
Allow for metering voltage and Current.
Draw less current.
At the source transformer.
Isolation transformer.

34. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
Electro-magnetism for their operation.
A.C.
At the source transformer.

35. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
The chest should rise or inflate.
At the source transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
1 amp.

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?
Four times less than that of the secondary.
5:1
3 -979 Volts.
5:1.

37. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Derive a neutral for grounding purposes.
An electrical shock hazard may exist.
Four times less than that of the secondary.

38. A solenoid coil is...
An electromagnet.
At least two phases
They should be made by the same manufacturer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

39. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Circuit Breaker.
Kilovolt ampere (KVA).
Nearly constant.

40. The size of a transformer is measured in...
5:1
Kilovolt ampere (KVA).
Cut around clothing that has adhered to the burned area.
They must be installed at least 7 feet above the ground.

41. The primary leads of a single-phase transformer should be connected to the...
A device used to store electrical energy.
Reverse its rotation.
Clamp-on ammeter.
H terminals.

42. Not a symptom of electrical shock?.
Transformer
Hyperventilation.
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage

43. 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?
480 volts.
75 turns.
Full-load - three-phase voltage.
They must be installed at least 7 feet above the ground.

44. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Draw less current.
Disconnect the frame-grounding conductor.
100 volts.
Clamp-on ammeter.

45. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
No.6 AWG.
1 amp.
Nearly constant.
Squirrel-cage induction motor.

46. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage.
200 volts AC.
Four times less than that of the secondary.

47. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Cut around clothing that has adhered to the burned area.
25 feet.
Lower current and higher voltage
Clamp-on ammeter.

48. The secondary windings on a 10:1 step-down transformer will have a wire size...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
200 volts AC.
Stator windings.
Larger than that found in the primary winding.

49. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
Squirrel-cage rotor.
150 turns.
60 cycles per second (60 Hertz).

50. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
A step-up transformer.
Full-load - three-phase voltage.
Lower current and higher voltage.