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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. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
1 amp.
Reverse any two-phase leads.
A.C.
A grounding resistor.

2. What is a capacitor?
H terminals.
A device used to store electrical energy.
277 volts.
2000 watts

3. Not an acceptable method for the installations of surface transformers?
Four times less than that of the secondary.
Draw less current.
They must be installed at least 7 feet above the ground.
60 Hertz

4. 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...
Lower current and higher voltage.
The chest should rise or inflate.
Four times less than that of the secondary.
Draw less current.

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?
Circuit Breaker.
The chest should rise or inflate.
60 Hertz
75 turns.

6. Compared to the secondary of a loaded - step-down transformer - the primary has...
600 volts.
Reverse any two-phase leads.
A.C.
Lower current and higher voltage.

7. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Reverse its rotation.
3 -979 Volts.
Nearly constant.
Two speeds

8. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Hyperventilation.
They should be made by the same manufacturer.
The current will read zero.
Short-circuited after power has been removed.

9. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Isolation transformer.
75 turns.
277 volts.
The motor rotation is reversed.

10. The primary leads of a single-phase transformer should be connected to the...
Squirrel-cage rotor.
H terminals.
Draw less current.
1 amp.

11. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Transformers.
Tied together
100 turns.

12. The transformer used only to measure voltage is a...
Four times less than that of the secondary.
Potential transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1

13. All transformers step voltage up or down by...
The sum of the two capacitors.
At the source transformer.
Inductance.
Derive a neutral for grounding purposes.

14. Where should grounding resistors be located?
At the source transformer.
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.
Reverse any two-phase leads.

15. Three-phase AC underground cables must be provided with overcurrent protection using a...
Alternating current.
Draw less current.
Circuit Breaker.
No.6 AWG.

16. 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?
75 turns.
Current that changes constantly in magnitude and periodically in direction.
100 turns.
It provides good speed control.

17. When not connected to a load - the secondary leads of a current transformer should be...
Reverse its rotation.
2000 watts
Shunted and taped
Hyperventilation.

18. 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
A step-up transformer.
The capacitors should be discharged.
Nearly constant.

19. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
1 amp.
Full-load - three-phase voltage.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Shunted and taped

20. 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.
Isolation transformer.
3 -979 Volts.
Full-load - three-phase voltage.

21. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
The capacitors should be discharged.
Stator windings.
2000 watts
The sum of the two capacitors.

22. If any two-phase wires are reversed on a three-phase motor - the motor will...
A grounding resistor.
Inductance.
Reverse its rotation.
Alternating current.

23. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
At the source transformer.
The motor rotation is reversed.
800 volts.

24. Current transformers - or CT's - are used to...
100 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Transformers.
3 -979 Volts.

25. 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...
Larger than that found in the primary winding.
Reverse its rotation.
Draw less current.
Lower current and higher voltage.

26. The standard frequency of power supplied by the power company is...
Inductance.
100 turns.
60 cycles per second (60 Hertz).
H terminals.

27. What type motor is a wound-rotor induction motor?
Draw less current.
Inductance.
A.C.
Kilovolt ampere (KVA).

28. Operating coils - motors - transformers - relays and solenoids depend upon...
Two speeds
Electro-magnetism for their operation.
100 turns.
Current changing constantly in magnitude and periodically in direction.

29. What should the duty rating of a grounding resistor be?
No.6 AWG.
Continuous.
25 feet.
Inductance.

30. What is the first step to be followed in aiding a victim of electrical shock?
Squirrel-cage induction motor.
Current that changes constantly in magnitude and periodically in direction.
Check to make sure that the victim is not still in contact with the electricity.
Inductance.

31. A burn causing a blister is classified as a...
Inductance.
Second-degree burn.
Disconnect the frame-grounding conductor.
The capacitors should be discharged.

32. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
Stator windings.
Shunted and taped
600 volts.

33. The primary function of a power transformer is to provide...
Shunted and taped
Grounded phase and ground fault devices were not operating.
The capacitors should be discharged.
Full-load - three-phase voltage.

34. The standard frequency of alternating current used at a mine is...
The sum of the two capacitors.
15 feet off the ground.
60 Hertz
277 volts.

35. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
5:1
The capacitors should be discharged.
Allow for metering voltage and Current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

36. 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?
60 cycles per second (60 Hertz).
3 -979 Volts.
75 turns.
No.6 AWG.

37. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
75 turns.
The chest should rise or inflate.
3 -979 Volts.

38. Current that changes periodically in direction is...
Alternating current.
Isolation transformer.
X.
Hyperventilation.

39. The secondary windings on a 10:1 step-down transformer will have a wire size...
Squirrel-cage induction motor.
Transformers.
Larger than that found in the primary winding.
A device used to store electrical energy.

40. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
X.
Squirrel-cage induction motor.
Capacitance.
The motor rotation is reversed.

41. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
A device used to store electrical energy.
1 amp.
Reverse its rotation.
The chest should rise or inflate.

42. The property of an electric circuit that opposes any change in current is called...
Inductance.
Larger than that found in the primary winding.
The sum of the two capacitors.
Full-load - three-phase voltage.

43. Which of the following does a squirrel cage induction motor have?
Potential transformer.
Stator windings.
A.C.
3 -979 Volts.

44. 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?
3 -979 Volts.
480 volts.
Current that changes constantly in magnitude and periodically in direction.
Potential transformer.

45. Not true about AC power?
It provides good speed control.
7 -630 volts.
Full-load - three-phase voltage.
Current that changes constantly in magnitude and periodically in direction.

46. How do you change the direction of a 3-phase AC motor?
5:1
Short-circuited after power has been removed.
Reverse any two-phase leads.
150 turns.

47. A transformer with a 1:1 ratio is usually a (an)...
100 volts.
Short-circuited after power has been removed.
277 volts.
Isolation transformer.

48. What is the danger when working around capacitors?
60 Hertz
Second-degree burn.
Squirrel-cage rotor.
An electrical shock hazard may exist.

49. A wound-rotor induction motor has all the following EXCEPT...
They must be installed at least 7 feet above the ground.
Squirrel-cage rotor.
Two speeds
800 volts.

50. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
At the source transformer.
7 -630 volts.
5:1
Tied together