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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 10 KW - secondary current of 200 amps - and primary voltage of 250 volts.
Nearly constant.
5:1.
Current changing constantly in magnitude and periodically in direction.
No.6 AWG.

2. Three-phase AC underground cables must be provided with overcurrent protection using a...
At least two phases
An electrical shock hazard may exist.
200 volts AC.
Circuit Breaker.

3. At what rate should mouth-to-mouth resuscitation be applied?
5:1
Inductance.
Potential transformer.
12 to 15 times a minute.

4. A control power transformer...
Inductance.
Derive a neutral for grounding purposes.
Grounded phase and ground fault devices were not operating.
Provides single-phase power to smaller loads such as lights - heaters - etc.

5. 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?
Reverse its rotation.
75 turns.
A grounding resistor.
Squirrel-cage rotor.

6. Compared to the secondary of a loaded - step-down transformer - the primary has...
An electrical shock hazard may exist.
Lower current and higher voltage
An electromagnet.
The motor rotation is reversed.

7. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Alternating current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Circuit Breaker.
12 to 15 times a minute.

8. 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.
At the source transformer.
Potential transformer.
100 turns.

9. What steps up or steps down AC voltage?
A grounding resistor.
Kilovolt ampere (KVA).
The current will read zero.
Transformer

10. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
5:1
Circuit Breaker.
480 volts.
Allow for metering voltage and Current.

11. AC voltage is raised or lowered by means of...
The motor rotation is reversed.
Transformers.
X.
Potential transformer.

12. 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.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Shunted and taped

13. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
200 volts AC.
25 feet.
1 amp.
Hyperventilation.

14. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
The capacitors should be discharged.
Current that changes constantly in magnitude and periodically in direction.
Potential transformer.

15. Current in an AC circuit may be measured by...
Clamp-on ammeter.
150 turns.
Zero amps.
An electromagnet.

16. Not an acceptable method for the installations of surface transformers?
600 volts.
150 turns.
They must be installed at least 7 feet above the ground.
Hyperventilation.

17. The primary leads of a single-phase transformer should be connected to the...
Transformers.
1 amp.
H terminals.
150 turns.

18. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Stator windings.
Kilovolt ampere (KVA).
Lower current and higher voltage

19. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Potential transformer.
3 -979 Volts.
7 -630 volts.

20. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Current that changes constantly in magnitude and periodically in direction.
Derive a neutral for grounding purposes.
Stator windings.
The chest should rise or inflate.

21. The secondary windings on a 10:1 step-down transformer will have a wire size...
600 volts.
Lower current and higher voltage
Larger than that found in the primary winding.
Two speeds

22. A solenoid coil is...
An electromagnet.
Disconnect the frame-grounding conductor.
Zero amps.
Clamp-on ammeter.

23. 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?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
15 feet off the ground.
3 -979 Volts.
Tied together

24. The standard frequency of alternating current used at a mine is...
Inductance.
They should be made by the same manufacturer.
Potential transformer.
60 Hertz

25. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Cut around clothing that has adhered to the burned area.
At the source transformer.
A step-up transformer.
60 cycles per second (60 Hertz).

26. Current that changes periodically in direction is...
Larger than that found in the primary winding.
600 volts.
Alternating current.
Two speeds

27. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Inductance.
Squirrel-cage induction motor.
Nearly constant.

28. 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?
Current changing constantly in magnitude and periodically in direction.
150 turns.
Clamp-on ammeter.
Two speeds

29. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Larger than that found in the primary winding.
100 volts.
Allow for metering voltage and Current.
X.

30. The main function of CT's and PT's on a main substation is to...
100 volts.
Tied together
Allow for metering voltage and Current.
7 -630 volts.

31. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Two speeds
Disconnect the frame-grounding conductor.
75 turns.
At the source transformer.

32. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
X.
Cut around clothing that has adhered to the burned area.
Alternating current.

33. What should the duty rating of a grounding resistor be?
Zero amps.
600 volts.
Second-degree burn.
Continuous.

34. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
100 volts.
Reverse its rotation.
5:1.

35. How do you change the direction of a 3-phase AC motor?
Draw less current.
Reverse any two-phase leads.
60 Hertz
Grounded phase and ground fault devices were not operating.

36. The transformer used only to measure voltage is a...
Potential transformer.
100 volts.
A step-up transformer.
Larger than that found in the primary winding.

37. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Lower current and higher voltage
Zero amps.
X.
A device used to store electrical energy.

38. 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.
277 volts.
At least two phases
A.C.

39. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Draw less current.
15 feet off the ground.
A grounding resistor.
Two speeds

40. What is a capacitor?
A grounding resistor.
Two speeds
Inductance.
A device used to store electrical energy.

41. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
No.6 AWG.
Squirrel-cage rotor.
A grounding resistor.
The chest should rise or inflate.

42. A burn causing a blister is classified as a...
5:1.
Potential transformer.
Second-degree burn.
480 volts.

43. 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.
5:1
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Transformers.
The capacitors should be discharged.

44. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
7 -630 volts.
H terminals.
277 volts.
2000 watts

45. 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.
15 feet off the ground.
Reverse its rotation.
No.6 AWG.

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?
800 volts.
Allow for metering voltage and Current.
Alternating current.
Isolation transformer.

47. Not a symptom of electrical shock?.
150 turns.
Zero amps.
Grounded phase and ground fault devices were not operating.
Hyperventilation.

48. A wound-rotor induction motor has all the following EXCEPT...
277 volts.
3 -979 Volts.
Stator windings.
Squirrel-cage rotor.

49. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Squirrel-cage induction motor.
They should be made by the same manufacturer.
The sum of the two capacitors.
100 volts.

50. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
100 turns.
Check to make sure that the victim is not still in contact with the electricity.
The motor rotation is reversed.
1 amp.