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AC Theory

Instructions:

Answer 50 questions in 15 minutes.
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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. The standard frequency of power supplied by the power company is...
Check to make sure that the victim is not still in contact with the electricity.
60 cycles per second (60 Hertz).
15 feet off the ground.
200 volts AC.

2. What type motor is a wound-rotor induction motor?
Current changing constantly in magnitude and periodically in direction.
A.C.
A device used to store electrical energy.
Provides single-phase power to smaller loads such as lights - heaters - etc.

3. If any two-phase wires are reversed on a three-phase motor - the motor will...
Squirrel-cage induction motor.
Current changing constantly in magnitude and periodically in direction.
Kilovolt ampere (KVA).
Reverse its rotation.

4. 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.
100 volts.
Transformers.
Two speeds

5. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
At least two phases
The current will read zero.
25 feet.
An electromagnet.

6. A burn causing a blister is classified as a...
25 feet.
At least two phases
Second-degree burn.
Continuous.

7. 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.
Check to make sure that the victim is not still in contact with the electricity.
Full-load - three-phase voltage.
Squirrel-cage rotor.

8. The size of a transformer is measured in...
A step-up transformer.
2000 watts
600 volts.
Kilovolt ampere (KVA).

9. Compared to the secondary of a loaded - step-down transformer - the primary has...
Grounded phase and ground fault devices were not operating.
No.6 AWG.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Lower current and higher voltage

10. What steps up or steps down AC voltage?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Transformer
Provides single-phase power to smaller loads such as lights - heaters - etc.
Current changing constantly in magnitude and periodically in direction.

11. 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...
Because less current flowing at high voltage causes less line loss per kilowatt.
Tied together
2000 watts
Full-load - three-phase voltage.

12. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
At least two phases
600 volts.
A device used to store electrical energy.
7 -630 volts.

13. 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?
Nearly constant.
Larger than that found in the primary winding.
100 turns.
Clamp-on ammeter.

14. 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...
Continuous.
100 volts.
They should be made by the same manufacturer.
Lower current and higher voltage

15. Current that changes periodically in direction is...
75 turns.
The sum of the two capacitors.
Alternating current.
Nearly constant.

16. 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?
They should be made by the same manufacturer.
Nearly constant.
It provides good speed control.
800 volts.

17. 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?
Zero amps.
200 volts AC.
75 turns.
Lower current and higher voltage.

18. Not true about AC power?
25 feet.
It provides good speed control.
800 volts.
Hyperventilation.

19. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
No.6 AWG.
Kilovolt ampere (KVA).
Circuit Breaker.
A step-up transformer.

20. What type of 3-phase AC motor is used most often around a mine?
Provides single-phase power to smaller loads such as lights - heaters - etc.
The motor rotation is reversed.
3 -979 Volts.
Squirrel-cage induction motor.

21. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Draw less current.
Reverse its rotation.
Stator windings.

22. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Continuous.
75 turns.
They should be made by the same manufacturer.
Disconnect the frame-grounding conductor.

23. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
They should be made by the same manufacturer.
Electro-magnetism for their operation.
Allow for metering voltage and Current.
Nearly constant.

24. 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?
Current changing constantly in magnitude and periodically in direction.
Current that changes constantly in magnitude and periodically in direction.
75 turns.
Draw less current.

25. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Reverse any two-phase leads.
Squirrel-cage induction motor.
The chest should rise or inflate.

26. 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?
60 Hertz
75 turns.
200 volts AC.
A grounding resistor.

27. 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...
5:1.
Clamp-on ammeter.
Potential transformer.
Draw less current.

28. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Derive a neutral for grounding purposes.
480 volts.
The current will read zero.
An electrical shock hazard may exist.

29. The primary leads of a single-phase transformer should be connected to the...
Nearly constant.
An electrical shock hazard may exist.
H terminals.
Allow for metering voltage and Current.

30. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
It provides good speed control.
Four times less than that of the secondary.
Capacitance.
The motor rotation is reversed.

31. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
The chest should rise or inflate.
Allow for metering voltage and Current.
Transformer

32. The standard frequency of alternating current used at a mine is...
Hyperventilation.
60 Hertz
Allow for metering voltage and Current.
Shunted and taped

33. The property of an electrical circuit that opposes any change in voltage is called...
60 cycles per second (60 Hertz).
75 turns.
A step-up transformer.
Capacitance.

34. 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.
5:1.
Reverse its rotation.
Kilovolt ampere (KVA).
3 -979 Volts.

35. 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
Circuit Breaker.
At least two phases
Electro-magnetism for their operation.

36. The main function of CT's and PT's on a main substation is to...
Capacitance.
Stator windings.
Allow for metering voltage and Current.
The sum of the two capacitors.

37. The property of an electric circuit that opposes any change in current is called...
Inductance.
Four times less than that of the secondary.
Grounded phase and ground fault devices were not operating.
A step-up transformer.

38. The primary function of a power transformer is to provide...
Disconnect the frame-grounding conductor.
At least two phases
Because less current flowing at high voltage causes less line loss per kilowatt.
Full-load - three-phase voltage.

39. 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.
A device used to store electrical energy.
Isolation transformer.
Circuit Breaker.
5:1

40. 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?
X.
Second-degree burn.
Provides single-phase power to smaller loads such as lights - heaters - etc.
3 -979 Volts.

41. Not an acceptable method for the installations of surface transformers?
An electromagnet.
They must be installed at least 7 feet above the ground.
Current changing constantly in magnitude and periodically in direction.
At the source transformer.

42. To protect against over-current in a three-phase circuit over-current protection must be provided for...
60 Hertz
75 turns.
Carotid pulse - which is located in the neck next to the Adam's apple.
At least two phases

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

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44. Current in an AC circuit may be measured by...
The current will read zero.
The sum of the two capacitors.
277 volts.
Clamp-on ammeter.

45. All transformers step voltage up or down by...
Provides single-phase power to smaller loads such as lights - heaters - etc.
The motor rotation is reversed.
The sum of the two capacitors.
Inductance.

46. Which of the following does a squirrel cage induction motor have?
Four times less than that of the secondary.
Stator windings.
12 to 15 times a minute.
Kilovolt ampere (KVA).

47. Where should grounding resistors be located?
Squirrel-cage rotor.
At the source transformer.
2000 watts
Zero amps.

48. What is a capacitor?
A.C.
Alternating current.
Provides single-phase power to smaller loads such as lights - heaters - etc.
A device used to store electrical energy.

49. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
Continuous.
Tied together
12 to 15 times a minute.

50. High voltage is used to transmit power over long distances...
Four times less than that of the secondary.
They must be installed at least 7 feet above the ground.
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage.

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AC Theory

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