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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. Alternating current may be defined as...
60 Hertz
Tied together
Short-circuited after power has been removed.
Current that changes constantly in magnitude and periodically in direction.

2. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
A step-up transformer.
Current changing constantly in magnitude and periodically in direction.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

3. 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.
H terminals.
Clamp-on ammeter.
The sum of the two capacitors.

4. A control power transformer...
The motor rotation is reversed.
Provides single-phase power to smaller loads such as lights - heaters - etc.
No.6 AWG.
15 feet off the ground.

5. The speed of an alternating current - squirrel-cage induction motor is...
An electromagnet.
Nearly constant.
Inductance.
At the source transformer.

6. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
100 volts.
The sum of the two capacitors.
Cut around clothing that has adhered to the burned area.
7 -630 volts.

7. 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?
Second-degree burn.
Transformer
It provides good speed control.
3 -979 Volts.

8. The purpose of a zig-zag transformer is to...
Draw less current.
A.C.
Check to make sure that the victim is not still in contact with the electricity.
Derive a neutral for grounding purposes.

9. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Derive a neutral for grounding purposes.
They should be made by the same manufacturer.
Check to make sure that the victim is not still in contact with the electricity.
X.

10. The transformer used only to measure voltage is a...
Clamp-on ammeter.
Isolation transformer.
Potential transformer.
Two speeds

11. Not true about AC power?
It provides good speed control.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Two speeds
Disconnect the frame-grounding conductor.

12. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
The chest should rise or inflate.
They should be made by the same manufacturer.
75 turns.
600 volts.

13. 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.
Cut around clothing that has adhered to the burned area.
60 cycles per second (60 Hertz).
Lower current and higher voltage.

14. Current transformers - or CT's - are used to...
277 volts.
Draw less current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
60 cycles per second (60 Hertz).

15. Where should grounding resistors be located?
X.
At the source transformer.
1 amp.
They should be made by the same manufacturer.

16. AC voltage is raised or lowered by means of...
Lower current and higher voltage
277 volts.
It provides good speed control.
Transformers.

17. High voltage is used to transmit power over long distances...
The capacitors should be discharged.
150 turns.
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage

18. Not a symptom of electrical shock?.
Isolation transformer.
Current that changes constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.
Hyperventilation.

19. Compared to the secondary of a loaded - step-down transformer - the primary has...
No.6 AWG.
Stator windings.
Lower current and higher voltage.
100 volts.

20. Compared to the secondary of a loaded - step-down transformer - the primary has...
3 -979 Volts.
No.6 AWG.
An electromagnet.
Lower current and higher voltage

21. A wound-rotor induction motor has all the following EXCEPT...
Hyperventilation.
1 amp.
Squirrel-cage rotor.
Inductance.

22. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
2000 watts
Inductance.
The sum of the two capacitors.

23. Not an acceptable method for the installations of surface transformers?
X.
Four times less than that of the secondary.
They must be installed at least 7 feet above the ground.
The current will read zero.

24. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
1 amp.
Four times less than that of the secondary.
100 volts.
Disconnect the frame-grounding conductor.

25. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
A.C.
They should be made by the same manufacturer.
15 feet off the ground.
Check to make sure that the victim is not still in contact with the electricity.

26. 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...
The capacitors should be discharged.
H terminals.
Tied together
Draw less current.

27. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Larger than that found in the primary winding.
They must be installed at least 7 feet above the ground.
Zero amps.
277 volts.

28. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
800 volts.
The motor rotation is reversed.
Reverse any two-phase leads.
12 to 15 times a minute.

29. 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...
Short-circuited after power has been removed.
Shunted and taped
3 -979 Volts.
Tied together

30. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
No.6 AWG.
Inductance.
800 volts.

31. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
H terminals.
Reverse any two-phase leads.
2000 watts
They must be installed at least 7 feet above the ground.

32. What is a capacitor?
Kilovolt ampere (KVA).
A device used to store electrical energy.
Draw less current.
Transformers.

33. What is the danger when working around capacitors?
Current changing constantly in magnitude and periodically in direction.
Cut around clothing that has adhered to the burned area.
Nearly constant.
An electrical shock hazard may exist.

34. If any two-phase wires are reversed on a three-phase motor - the motor will...
Transformer
Stator windings.
60 cycles per second (60 Hertz).
Reverse its rotation.

35. 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?
H terminals.
It provides good speed control.
100 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.

36. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Draw less current.
Reverse its rotation.

37. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
They should be made by the same manufacturer.
The capacitors should be discharged.
Transformer
Circuit Breaker.

38. Three-phase AC underground cables must be provided with overcurrent protection using a...
2000 watts
Allow for metering voltage and Current.
Electro-magnetism for their operation.
Circuit Breaker.

39. A solenoid coil is...
Hyperventilation.
An electromagnet.
The sum of the two capacitors.
Larger than that found in the primary winding.

40. 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?
Larger than that found in the primary winding.
600 volts.
Capacitance.
Stator windings.

41. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Nearly constant.
Short-circuited after power has been removed.
Four times less than that of the secondary.
Capacitance.

42. The primary function of a power transformer is to provide...
An electromagnet.
Full-load - three-phase voltage.
Nearly constant.
Disconnect the frame-grounding conductor.

43. A burn causing a blister is classified as a...
Second-degree burn.
1 amp.
5:1
Hyperventilation.

44. 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...
800 volts.
A grounding resistor.
100 volts.
A step-up transformer.

45. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
No.6 AWG.
Clamp-on ammeter.
A grounding resistor.
Isolation transformer.

46. The standard frequency of alternating current used at a mine is...
5:1
Two speeds
60 Hertz
Continuous.

47. The property of an electrical circuit that opposes any change in voltage is called...
Four times less than that of the secondary.
7 -630 volts.
Capacitance.
Derive a neutral for grounding purposes.

48. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.
Draw less current.
Derive a neutral for grounding purposes.

49. Current in an AC circuit may be measured by...
600 volts.
Clamp-on ammeter.
Current changing constantly in magnitude and periodically in direction.
277 volts.

50. How do you change the direction of a 3-phase AC motor?
They should be made by the same manufacturer.
Reverse any two-phase leads.
2000 watts
Kilovolt ampere (KVA).