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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. The purpose of a zig-zag transformer is to...
600 volts.
Derive a neutral for grounding purposes.
Current changing constantly in magnitude and periodically in direction.
Draw less current.

2. 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?
Continuous.
Squirrel-cage rotor.
5:1.
600 volts.

3. Which of the following does a squirrel cage induction motor have?
Kilovolt ampere (KVA).
Disconnect the frame-grounding conductor.
Stator windings.
An electromagnet.

4. The combined capacitance of two capacitors in parallel is...
100 turns.
1 amp.
The sum of the two capacitors.
Isolation transformer.

5. Alternating current may be defined as...
480 volts.
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
600 volts.

6. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Four times less than that of the secondary.
No.6 AWG.
Zero amps.
Inductance.

7. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Draw less current.
Stator windings.
X.
An electromagnet.

8. Not true about AC power?
No.6 AWG.
It provides good speed control.
Nearly constant.
60 cycles per second (60 Hertz).

9. 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?
Current that changes constantly in magnitude and periodically in direction.
Circuit Breaker.
Electro-magnetism for their operation.
200 volts AC.

10. If any two-phase wires are reversed on a three-phase motor - the motor will...
Capacitance.
Reverse its rotation.
3 -979 Volts.
Transformers.

11. The transformer used only to measure voltage is a...
Potential transformer.
The current will read zero.
X.
Circuit Breaker.

12. A solenoid coil is...
An electromagnet.
800 volts.
The sum of the two capacitors.
Isolation transformer.

13. 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...
At the source transformer.
Draw less current.
Because less current flowing at high voltage causes less line loss per kilowatt.
Cut around clothing that has adhered to the burned area.

14. A control power transformer...
60 Hertz
Provides single-phase power to smaller loads such as lights - heaters - etc.
The sum of the two capacitors.
No.6 AWG.

15. Define alternating current...
They should be made by the same manufacturer.
Current changing constantly in magnitude and periodically in direction.
12 to 15 times a minute.
Larger than that found in the primary winding.

16. The speed of an alternating current - squirrel-cage induction motor is...
7 -630 volts.
Nearly constant.
The current will read zero.
An electrical shock hazard may exist.

17. What type of 3-phase AC motor is used most often around a mine?
Lower current and higher voltage
Hyperventilation.
Squirrel-cage induction motor.
Cut around clothing that has adhered to the burned area.

18. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Four times less than that of the secondary.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
7 -630 volts.
Larger than that found in the primary winding.

19. What should the duty rating of a grounding resistor be?
The current will read zero.
Tied together
Allow for metering voltage and Current.
Continuous.

20. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
60 cycles per second (60 Hertz).
60 Hertz
Grounded phase and ground fault devices were not operating.
Current that changes constantly in magnitude and periodically in direction.

21. How do you change the direction of a 3-phase AC motor?
The chest should rise or inflate.
At the source transformer.
15 feet off the ground.
Reverse any two-phase leads.

22. What steps up or steps down AC voltage?
Transformer
Squirrel-cage induction motor.
Squirrel-cage rotor.
The chest should rise or inflate.

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...
The capacitors should be discharged.
Inductance.
A grounding resistor.
100 volts.

24. Not an acceptable method for the installations of surface transformers?
Reverse its rotation.
2000 watts
They must be installed at least 7 feet above the ground.
At the source transformer.

25. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Reverse its rotation.
1 amp.
15 feet off the ground.
Isolation transformer.

26. Not a symptom of electrical shock?.
The sum of the two capacitors.
5:1
150 turns.
Hyperventilation.

27. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Second-degree burn.
Inductance.
Transformers.
15 feet off the ground.

28. At what rate should mouth-to-mouth resuscitation be applied?
100 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A device used to store electrical energy.
12 to 15 times a minute.

29. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
A step-up transformer.
The current will read zero.
150 turns.
Draw less current.

30. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Cut around clothing that has adhered to the burned area.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Inductance.

31. The main function of CT's and PT's on a main substation is to...
Squirrel-cage induction motor.
Capacitance.
Allow for metering voltage and Current.
25 feet.

32. Current that changes periodically in direction is...
2000 watts
200 volts AC.
Tied together
Alternating current.

33. What is the first step to be followed in aiding a victim of electrical shock?
Full-load - three-phase voltage.
Short-circuited after power has been removed.
It provides good speed control.
Check to make sure that the victim is not still in contact with the electricity.

34. How should clothing be removed from a burn?
Electro-magnetism for their operation.
Cut around clothing that has adhered to the burned area.
Squirrel-cage induction motor.
They must be installed at least 7 feet above the ground.

35. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
A grounding resistor.
Kilovolt ampere (KVA).
25 feet.
They should be made by the same manufacturer.

36. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Grounded phase and ground fault devices were not operating.
277 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

37. When not connected to a load - the secondary leads of a current transformer should be...
Isolation transformer.
Shunted and taped
Current changing constantly in magnitude and periodically in direction.
Draw less current.

38. The standard frequency of power supplied by the power company is...
60 cycles per second (60 Hertz).
No.6 AWG.
Draw less current.
Continuous.

39. 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...
Four times less than that of the secondary.
Draw less current.
12 to 15 times a minute.
Disconnect the frame-grounding conductor.

40. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
7 -630 volts.
12 to 15 times a minute.
Stator windings.
The capacitors should be discharged.

41. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
H terminals.
Lower current and higher voltage

42. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Tied together
Cut around clothing that has adhered to the burned area.
Potential transformer.
At least two phases

43. The standard frequency of alternating current used at a mine is...
60 Hertz
Reverse its rotation.
A.C.
A step-up transformer.

44. A wound-rotor induction motor has all the following EXCEPT...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Squirrel-cage rotor.
An electrical shock hazard may exist.
12 to 15 times a minute.

45. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
480 volts.
The sum of the two capacitors.
Stator windings.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

46. 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?
Current that changes constantly in magnitude and periodically in direction.
800 volts.
Isolation transformer.
Potential transformer.

47. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
Current changing constantly in magnitude and periodically in direction.
Hyperventilation.
An electromagnet.

48. What type motor is a wound-rotor induction motor?
Potential transformer.
Two speeds
Current changing constantly in magnitude and periodically in direction.
A.C.

49. 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.
They should be made by the same manufacturer.
Zero amps.
5:1
Because less current flowing at high voltage causes less line loss per kilowatt.

50. The primary leads of a single-phase transformer should be connected to the...
Provides single-phase power to smaller loads such as lights - heaters - etc.
H terminals.
75 turns.
Disconnect the frame-grounding conductor.