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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. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Tied together
Potential transformer.
They should be made by the same manufacturer.
800 volts.

2. When not connected to a load - the secondary leads of a current transformer should be...
Continuous.
12 to 15 times a minute.
Full-load - three-phase voltage.
Shunted and taped

3. What is a capacitor?
At least two phases
Capacitance.
100 turns.
A device used to store electrical energy.

4. What should the duty rating of a grounding resistor be?
Potential transformer.
Continuous.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
At least two phases

5. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
600 volts.
Disconnect the frame-grounding conductor.
H terminals.

6. 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?
They must be installed at least 7 feet above the ground.
480 volts.
Lower current and higher voltage
200 volts AC.

7. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
150 turns.
Nearly constant.
3 -979 Volts.

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...
Full-load - three-phase voltage.
Draw less current.
5:1.
An electrical shock hazard may exist.

9. 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.
H terminals.
75 turns.
Because less current flowing at high voltage causes less line loss per kilowatt.

10. AC voltage is raised or lowered by means of...
An electrical shock hazard may exist.
Transformers.
1 amp.
Draw less current.

11. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
The current will read zero.
Zero amps.
It provides good speed control.

12. The secondary windings on a 10:1 step-down transformer will have a wire size...
H terminals.
15 feet off the ground.
Larger than that found in the primary winding.
Circuit Breaker.

13. Three-phase AC underground cables must be provided with overcurrent protection using a...
Cut around clothing that has adhered to the burned area.
Current that changes constantly in magnitude and periodically in direction.
No.6 AWG.
Circuit Breaker.

14. Current that changes periodically in direction is...
Current that changes constantly in magnitude and periodically in direction.
Shunted and taped
Alternating current.
60 Hertz

15. How do you change the direction of a 3-phase AC motor?
100 turns.
Reverse any two-phase leads.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
At least two phases

16. What type motor is a wound-rotor induction motor?
Reverse its rotation.
5:1.
Circuit Breaker.
A.C.

17. 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?
5:1.
75 turns.
7 -630 volts.
60 cycles per second (60 Hertz).

18. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Two speeds
Derive a neutral for grounding purposes.
A grounding resistor.
Alternating current.

19. 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
They must be installed at least 7 feet above the ground.
Draw less current.

20. The combined capacitance of two capacitors in parallel is...
Current that changes constantly in magnitude and periodically in direction.
The sum of the two capacitors.
Inductance.
15 feet off the ground.

21. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
25 feet.
12 to 15 times a minute.
15 feet off the ground.
Two speeds

22. The most reliable method to use to determine whether or not the heart has stopped is by checking the...

23. The property of an electrical circuit that opposes any change in voltage is called...
75 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.
The sum of the two capacitors.
Capacitance.

24. 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...
Capacitance.
Provides single-phase power to smaller loads such as lights - heaters - etc.
2000 watts
Draw less current.

25. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
Carotid pulse - which is located in the neck next to the Adam's apple.
75 turns.
Electro-magnetism for their operation.

26. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Draw less current.
Squirrel-cage induction motor.
Shunted and taped
277 volts.

27. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
A.C.
Reverse any two-phase leads.
X.
200 volts AC.

28. To protect against over-current in a three-phase circuit over-current protection must be provided for...
It provides good speed control.
Zero amps.
At least two phases
They must be installed at least 7 feet above the ground.

29. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
480 volts.

30. What is the first step to be followed in aiding a victim of electrical shock?
Capacitance.
Check to make sure that the victim is not still in contact with the electricity.
The chest should rise or inflate.
At the source transformer.

31. 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.
Cut around clothing that has adhered to the burned area.
5:1.
600 volts.

32. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Zero amps.
It provides good speed control.
60 Hertz

33. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Because less current flowing at high voltage causes less line loss per kilowatt.
Zero amps.
7 -630 volts.
Check to make sure that the victim is not still in contact with the electricity.

34. The standard frequency of power supplied by the power company is...
Two speeds
3 -979 Volts.
60 cycles per second (60 Hertz).
7 -630 volts.

35. The property of an electric circuit that opposes any change in current is called...
7 -630 volts.
25 feet.
1 amp.
Inductance.

36. 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...
The motor rotation is reversed.
Grounded phase and ground fault devices were not operating.
Tied together
Short-circuited after power has been removed.

37. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
75 turns.
An electrical shock hazard may exist.
15 feet off the ground.
Grounded phase and ground fault devices were not operating.

38. High voltage is used to transmit power over long distances...
The motor rotation is reversed.
Because less current flowing at high voltage causes less line loss per kilowatt.
Reverse its rotation.
A step-up transformer.

39. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
75 turns.
A device used to store electrical energy.
15 feet off the ground.
Transformers.

40. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Larger than that found in the primary winding.
At least two phases

41. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Circuit Breaker.
25 feet.
An electrical shock hazard may exist.
Because less current flowing at high voltage causes less line loss per kilowatt.

42. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Grounded phase and ground fault devices were not operating.
No.6 AWG.
Transformer
A step-up transformer.

43. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Transformers.
X.
600 volts.
Short-circuited after power has been removed.

44. 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?
Derive a neutral for grounding purposes.
Grounded phase and ground fault devices were not operating.
800 volts.
Check to make sure that the victim is not still in contact with the electricity.

45. The purpose of a zig-zag transformer is to...
Draw less current.
Isolation transformer.
7 -630 volts.
Derive a neutral for grounding purposes.

46. At what rate should mouth-to-mouth resuscitation be applied?
Continuous.
12 to 15 times a minute.
60 Hertz
Four times less than that of the secondary.

47. 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?
150 turns.
Because less current flowing at high voltage causes less line loss per kilowatt.
Cut around clothing that has adhered to the burned area.
The capacitors should be discharged.

48. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Full-load - three-phase voltage.
An electromagnet.
H terminals.
The motor rotation is reversed.

49. 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.
1 amp.
Shunted and taped
An electrical shock hazard may exist.

50. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
No.6 AWG.
The sum of the two capacitors.
15 feet off the ground.