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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. A solenoid coil is...
Lower current and higher voltage
An electromagnet.
150 turns.
It provides good speed control.

2. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
At least two phases
100 turns.
15 feet off the ground.
277 volts.

3. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Kilovolt ampere (KVA).
At least two phases
Hyperventilation.
They must be installed at least 7 feet above the ground.

4. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
The sum of the two capacitors.
Zero amps.
480 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

5. 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.
The capacitors should be discharged.
An electrical shock hazard may exist.
Two speeds
5:1

6. 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.
Current that changes constantly in magnitude and periodically in direction.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
277 volts.

7. At what rate should mouth-to-mouth resuscitation be applied?
Reverse any two-phase leads.
12 to 15 times a minute.
Current that changes constantly in magnitude and periodically in direction.
3 -979 Volts.

8. 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...
Draw less current.
800 volts.
1 amp.
Tied together

9. 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.
Inductance.
The capacitors should be discharged.
The motor rotation is reversed.

10. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
A device used to store electrical energy.
Short-circuited after power has been removed.
Transformer

11. When not connected to a load - the secondary leads of a current transformer should be...
800 volts.
Shunted and taped
A device used to store electrical energy.
Circuit Breaker.

12. Current transformers - or CT's - are used to...
Capacitance.
1 amp.
An electrical shock hazard may exist.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

13. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Carotid pulse - which is located in the neck next to the Adam's apple.
277 volts.
Tied together
X.

14. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Zero amps.
480 volts.
Draw less current.
100 turns.

15. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
A device used to store electrical energy.
The chest should rise or inflate.
The current will read zero.
The capacitors should be discharged.

16. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Grounded phase and ground fault devices were not operating.
Allow for metering voltage and Current.
Reverse its rotation.
Inductance.

17. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
100 turns.
Circuit Breaker.
Provides single-phase power to smaller loads such as lights - heaters - etc.

18. 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?
Capacitance.
Full-load - three-phase voltage.
100 turns.
Inductance.

19. The purpose of a zig-zag transformer is to...
Two speeds
Reverse its rotation.
Derive a neutral for grounding purposes.
60 cycles per second (60 Hertz).

20. A control power transformer...
A device used to store electrical energy.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Current that changes constantly in magnitude and periodically in direction.
At the source transformer.

21. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
It provides good speed control.
Kilovolt ampere (KVA).
7 -630 volts.
Current changing constantly in magnitude and periodically in direction.

22. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Kilovolt ampere (KVA).
Reverse any two-phase leads.
Zero amps.
Full-load - three-phase voltage.

23. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
1 amp.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
No.6 AWG.

24. Compared to the secondary of a loaded - step-down transformer - the primary has...
60 Hertz
A grounding resistor.
Lower current and higher voltage
An electrical shock hazard may exist.

25. How do you change the direction of a 3-phase AC motor?
Capacitance.
Current that changes constantly in magnitude and periodically in direction.
Reverse any two-phase leads.
Potential transformer.

26. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
Continuous.
Potential transformer.
7 -630 volts.

27. What type of 3-phase AC motor is used most often around a mine?
Isolation transformer.
Squirrel-cage induction motor.
Full-load - three-phase voltage.
A.C.

28. What type motor is a wound-rotor induction motor?
H terminals.
X.
A.C.
Disconnect the frame-grounding conductor.

29. The secondary windings on a 10:1 step-down transformer will have a wire size...
Lower current and higher voltage
Larger than that found in the primary winding.
A.C.
800 volts.

30. 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.
200 volts AC.
25 feet.
100 turns.

31. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Potential transformer.
The chest should rise or inflate.
60 Hertz
277 volts.

32. Current that changes periodically in direction is...
Alternating current.
60 Hertz
7 -630 volts.
Isolation transformer.

33. All transformers step voltage up or down by...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Cut around clothing that has adhered to the burned area.
Inductance.
Hyperventilation.

34. The transformer used only to measure voltage is a...
Clamp-on ammeter.
Capacitance.
Squirrel-cage induction motor.
Potential transformer.

35. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
Hyperventilation.
3 -979 Volts.
75 turns.

36. 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.
Disconnect the frame-grounding conductor.
It provides good speed control.
At the source transformer.

37. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
They should be made by the same manufacturer.
Grounded phase and ground fault devices were not operating.
Draw less current.
Short-circuited after power has been removed.

38. Not true about AC power?
5:1.
Shunted and taped
It provides good speed control.
Because less current flowing at high voltage causes less line loss per kilowatt.

39. The size of a transformer is measured in...
Lower current and higher voltage
H terminals.
Kilovolt ampere (KVA).
Short-circuited after power has been removed.

40. Which of the following does a squirrel cage induction motor have?
Stator windings.
Continuous.
Squirrel-cage rotor.
Inductance.

41. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
Electro-magnetism for their operation.
Four times less than that of the secondary.
3 -979 Volts.

42. The primary leads of a single-phase transformer should be connected to the...
Full-load - three-phase voltage.
H terminals.
100 volts.
An electromagnet.

43. Current in an AC circuit may be measured by...
Continuous.
Full-load - three-phase voltage.
75 turns.
Clamp-on ammeter.

44. Not an acceptable method for the installations of surface transformers?
A step-up transformer.
An electrical shock hazard may exist.
An electromagnet.
They must be installed at least 7 feet above the ground.

45. 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.
12 to 15 times a minute.
60 cycles per second (60 Hertz).
Continuous.

46. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Two speeds
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

47. AC voltage is raised or lowered by means of...
Transformers.
Cut around clothing that has adhered to the burned area.
15 feet off the ground.
An electromagnet.

48. Where should grounding resistors be located?
Hyperventilation.
H terminals.
Isolation transformer.
At the source transformer.

49. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
2000 watts
Hyperventilation.
Capacitance.
Continuous.

50. Define alternating current...
Check to make sure that the victim is not still in contact with the electricity.
Current changing constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
Continuous.