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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. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
60 Hertz
An electromagnet.
Transformer

2. 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.
Electro-magnetism for their operation.
Full-load - three-phase voltage.
Second-degree burn.

3. Not true about AC power?
It provides good speed control.
Disconnect the frame-grounding conductor.
60 cycles per second (60 Hertz).
H terminals.

4. 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?
7 -630 volts.
150 turns.
X.
Two speeds

5. The standard frequency of alternating current used at a mine is...
Transformers.
60 Hertz
7 -630 volts.
A grounding resistor.

6. 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.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Disconnect the frame-grounding conductor.

7. The transformer used only to measure voltage is a...
Disconnect the frame-grounding conductor.
Four times less than that of the secondary.
A.C.
Potential transformer.

8. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
60 Hertz
Capacitance.
Clamp-on ammeter.
A grounding resistor.

9. The secondary windings on a 10:1 step-down transformer will have a wire size...
Lower current and higher voltage
75 turns.
Larger than that found in the primary winding.
At least two phases

10. Define alternating current...
Larger than that found in the primary winding.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Clamp-on ammeter.
Current changing constantly in magnitude and periodically in direction.

11. The speed of an alternating current - squirrel-cage induction motor is...
Reverse any two-phase leads.
Draw less current.
Nearly constant.
12 to 15 times a minute.

12. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Derive a neutral for grounding purposes.
Clamp-on ammeter.
Disconnect the frame-grounding conductor.
Electro-magnetism for their operation.

13. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Tied together
60 Hertz
25 feet.
100 volts.

14. Which of the following does a squirrel cage induction motor have?
25 feet.
A step-up transformer.
Stator windings.
800 volts.

15. If any two-phase wires are reversed on a three-phase motor - the motor will...
60 cycles per second (60 Hertz).
Reverse its rotation.
Nearly constant.
An electromagnet.

16. 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?
Current that changes constantly in magnitude and periodically in direction.
600 volts.
Lower current and higher voltage
Transformers.

17. 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?
Larger than that found in the primary winding.
2000 watts
The capacitors should be discharged.
75 turns.

18. 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.
At the source transformer.
75 turns.
Four times less than that of the secondary.

19. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Check to make sure that the victim is not still in contact with the electricity.
Potential transformer.
7 -630 volts.
Zero amps.

20. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Alternating current.
Draw less current.
Zero amps.

21. 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.
Second-degree burn.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Check to make sure that the victim is not still in contact with the electricity.

22. The primary function of a power transformer is to provide...
75 turns.
Full-load - three-phase voltage.
Grounded phase and ground fault devices were not operating.
Continuous.

23. Operating coils - motors - transformers - relays and solenoids depend upon...
Isolation transformer.
A grounding resistor.
Electro-magnetism for their operation.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

24. What is the danger when working around capacitors?
Check to make sure that the victim is not still in contact with the electricity.
H terminals.
Short-circuited after power has been removed.
An electrical shock hazard may exist.

25. To protect against over-current in a three-phase circuit over-current protection must be provided for...
A device used to store electrical energy.
At least two phases
75 turns.
480 volts.

26. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Current that changes constantly in magnitude and periodically in direction.
Second-degree burn.
Draw less current.

27. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
It provides good speed control.
15 feet off the ground.
The current will read zero.
2000 watts

28. Current that changes periodically in direction is...
Alternating current.
A device used to store electrical energy.
Current changing constantly in magnitude and periodically in direction.
Nearly constant.

29. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
At the source transformer.
A step-up transformer.
800 volts.
The chest should rise or inflate.

30. The standard frequency of power supplied by the power company is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
60 cycles per second (60 Hertz).
2000 watts
X.

31. The size of a transformer is measured in...
Alternating current.
25 feet.
Kilovolt ampere (KVA).
X.

32. 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.
An electromagnet.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Draw less current.

33. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Squirrel-cage rotor.
The motor rotation is reversed.
Shunted and taped

34. Alternating current may be defined as...
Grounded phase and ground fault devices were not operating.
Squirrel-cage rotor.
Current that changes constantly in magnitude and periodically in direction.
100 volts.

35. A burn causing a blister is classified as a...
H terminals.
Grounded phase and ground fault devices were not operating.
Second-degree burn.
12 to 15 times a minute.

36. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
The motor rotation is reversed.
75 turns.
Disconnect the frame-grounding conductor.
Short-circuited after power has been removed.

37. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Nearly constant.
Second-degree burn.
Stator windings.

38. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
1 amp.
Draw less current.
Two speeds

39. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Full-load - three-phase voltage.
A step-up transformer.
Alternating current.
Provides single-phase power to smaller loads such as lights - heaters - etc.

40. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
Cut around clothing that has adhered to the burned area.
Check to make sure that the victim is not still in contact with the electricity.
H terminals.

41. How should clothing be removed from a burn?
5:1
Cut around clothing that has adhered to the burned area.
Nearly constant.
150 turns.

42. 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.
Current that changes constantly in magnitude and periodically in direction.
Capacitance.
Squirrel-cage induction motor.

43. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
A grounding resistor.
Second-degree burn.
Squirrel-cage rotor.

44. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
The current will read zero.
Draw less current.
No.6 AWG.
Second-degree burn.

45. 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.
Continuous.
Squirrel-cage induction motor.
5:1
Short-circuited after power has been removed.

46. 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...
Continuous.
Tied together
75 turns.
Draw less current.

47. A solenoid coil is...
Two speeds
An electromagnet.
Full-load - three-phase voltage.
277 volts.

48. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
5:1.
Draw less current.
X.
Circuit Breaker.

49. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
At least two phases
The chest should rise or inflate.
5:1.
Grounded phase and ground fault devices were not operating.

50. 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.
Two speeds
H terminals.
An electromagnet.