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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. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.
75 turns.
The sum of the two capacitors.

2. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
The current will read zero.
600 volts.
Clamp-on ammeter.

3. Not a symptom of electrical shock?.
Hyperventilation.
Potential transformer.
Zero amps.
Reverse its rotation.

4. 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.
Four times less than that of the secondary.
5:1
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Clamp-on ammeter.

5. 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...
100 turns.
Larger than that found in the primary winding.
Tied together
Clamp-on ammeter.

6. 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.
277 volts.
75 turns.
Squirrel-cage rotor.

7. 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?
Electro-magnetism for their operation.
Disconnect the frame-grounding conductor.
800 volts.
A device used to store electrical energy.

8. The size of a transformer is measured in...
3 -979 Volts.
Kilovolt ampere (KVA).
Short-circuited after power has been removed.
No.6 AWG.

9. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
They should be made by the same manufacturer.
The motor rotation is reversed.
The sum of the two capacitors.
15 feet off the ground.

10. 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?
Transformer
Alternating current.
3 -979 Volts.
An electrical shock hazard may exist.

11. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
200 volts AC.
Clamp-on ammeter.
Four times less than that of the secondary.
They should be made by the same manufacturer.

12. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Full-load - three-phase voltage.
An electromagnet.
The chest should rise or inflate.
Circuit Breaker.

13. The property of an electric circuit that opposes any change in current is called...
Inductance.
2000 watts
Stator windings.
It provides good speed control.

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?
Tied together
Squirrel-cage induction motor.
Grounded phase and ground fault devices were not operating.
100 turns.

15. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
They should be made by the same manufacturer.
60 Hertz
25 feet.

16. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Draw less current.
Grounded phase and ground fault devices were not operating.
A device used to store electrical energy.
The current will read zero.

17. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Inductance.
Alternating current.
60 Hertz

18. 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?
Potential transformer.
Lower current and higher voltage.
200 volts AC.
No.6 AWG.

19. Not true about AC power?
A grounding resistor.
It provides good speed control.
Larger than that found in the primary winding.
7 -630 volts.

20. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Alternating current.
480 volts.
Lower current and higher voltage
H terminals.

21. High voltage is used to transmit power over long distances...
200 volts AC.
The chest should rise or inflate.
Squirrel-cage induction motor.
Because less current flowing at high voltage causes less line loss per kilowatt.

22. A wound-rotor induction motor has all the following EXCEPT...
Nearly constant.
The capacitors should be discharged.
Squirrel-cage rotor.
A device used to store electrical energy.

23. What steps up or steps down AC voltage?
Current changing constantly in magnitude and periodically in direction.
Stator windings.
Alternating current.
Transformer

24. What is the danger when working around capacitors?
Electro-magnetism for their operation.
5:1.
Larger than that found in the primary winding.
An electrical shock hazard may exist.

25. What is a capacitor?
At least two phases
Reverse its rotation.
A grounding resistor.
A device used to store electrical energy.

26. A burn causing a blister is classified as a...
The capacitors should be discharged.
A device used to store electrical energy.
Second-degree burn.
12 to 15 times a minute.

27. To protect against over-current in a three-phase circuit over-current protection must be provided for...
The current will read zero.
At least two phases
Lower current and higher voltage.
75 turns.

28. The standard frequency of power supplied by the power company is...
277 volts.
An electrical shock hazard may exist.
Current changing constantly in magnitude and periodically in direction.
60 cycles per second (60 Hertz).

29. What should the duty rating of a grounding resistor be?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
They must be installed at least 7 feet above the ground.
A.C.
Continuous.

30. Current in an AC circuit may be measured by...
H terminals.
The sum of the two capacitors.
75 turns.
Clamp-on ammeter.

31. Not an acceptable method for the installations of surface transformers?
Lower current and higher voltage.
They must be installed at least 7 feet above the ground.
An electromagnet.
5:1.

32. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
The motor rotation is reversed.
At least two phases
5:1.
Disconnect the frame-grounding conductor.

33. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Isolation transformer.
Zero amps.
75 turns.

34. 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.
Lower current and higher voltage.
100 volts.
Full-load - three-phase voltage.

35. The secondary windings on a 10:1 step-down transformer will have a wire size...
Inductance.
Larger than that found in the primary winding.
Continuous.
Disconnect the frame-grounding conductor.

36. The primary leads of a single-phase transformer should be connected to the...
200 volts AC.
H terminals.
100 volts.
Current that changes constantly in magnitude and periodically in direction.

37. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Nearly constant.
Four times less than that of the secondary.
Lower current and higher voltage.
60 Hertz

38. Define alternating current...
Check to make sure that the victim is not still in contact with the electricity.
An electromagnet.
A device used to store electrical energy.
Current changing constantly in magnitude and periodically in direction.

39. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
15 feet off the ground.
The capacitors should be discharged.
Two speeds
Grounded phase and ground fault devices were not operating.

40. At what rate should mouth-to-mouth resuscitation be applied?
12 to 15 times a minute.
Stator windings.
Capacitance.
H terminals.

41. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Grounded phase and ground fault devices were not operating.
2000 watts
Two speeds

42. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Electro-magnetism for their operation.
Stator windings.
1 amp.
25 feet.

43. What type motor is a wound-rotor induction motor?
Two speeds
150 turns.
A.C.
Isolation transformer.

44. 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?
12 to 15 times a minute.
They should be made by the same manufacturer.
75 turns.
Squirrel-cage induction motor.

45. Compared to the secondary of a loaded - step-down transformer - the primary has...
3 -979 Volts.
Allow for metering voltage and Current.
Zero amps.
Lower current and higher voltage.

46. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Isolation transformer.
Squirrel-cage rotor.
A step-up transformer.
Zero amps.

47. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Inductance.
Reverse its rotation.

48. The main function of CT's and PT's on a main substation is to...
Lower current and higher voltage
Check to make sure that the victim is not still in contact with the electricity.
Allow for metering voltage and Current.
Nearly constant.

49. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
15 feet off the ground.
The current will read zero.
Continuous.

50. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.
Reverse any two-phase leads.
No.6 AWG.