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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 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.
25 feet.
75 turns.
A.C.

2. High voltage is used to transmit power over long distances...
They should be made by the same manufacturer.
Because less current flowing at high voltage causes less line loss per kilowatt.
Current that changes constantly in magnitude and periodically in direction.
Shunted and taped

3. How do you change the direction of a 3-phase AC motor?
Two speeds
Reverse any two-phase leads.
Stator windings.
800 volts.

4. Current that changes periodically in direction is...
H terminals.
Disconnect the frame-grounding conductor.
Current that changes constantly in magnitude and periodically in direction.
Alternating current.

5. Not an acceptable method for the installations of surface transformers?
480 volts.
1 amp.
Because less current flowing at high voltage causes less line loss per kilowatt.
They must be installed at least 7 feet above the ground.

6. AC voltage is raised or lowered by means of...
An electromagnet.
Transformers.
5:1
The capacitors should be discharged.

7. Not true about AC power?
Reverse its rotation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1
It provides good speed control.

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...
Tied together
Circuit Breaker.
Alternating current.
Draw less current.

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.
12 to 15 times a minute.
Tied together
Shunted and taped

10. The primary function of a power transformer is to provide...
12 to 15 times a minute.
The chest should rise or inflate.
A grounding resistor.
Full-load - three-phase voltage.

11. Not a symptom of electrical shock?.
1 amp.
Hyperventilation.
7 -630 volts.
A grounding resistor.

12. The property of an electrical circuit that opposes any change in voltage is called...
The capacitors should be discharged.
1 amp.
Capacitance.
Clamp-on ammeter.

13. Where should grounding resistors be located?
Provides single-phase power to smaller loads such as lights - heaters - etc.
15 feet off the ground.
At the source transformer.
Derive a neutral for grounding purposes.

14. 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
Current changing constantly in magnitude and periodically in direction.
75 turns.
Nearly constant.

15. What is a capacitor?
A device used to store electrical energy.
The motor rotation is reversed.
12 to 15 times a minute.
Tied together

16. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
An electrical shock hazard may exist.
The motor rotation is reversed.
800 volts.

17. A solenoid coil is...
Potential transformer.
It provides good speed control.
An electromagnet.
The motor rotation is reversed.

18. 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.
Squirrel-cage induction motor.
Disconnect the frame-grounding conductor.
No.6 AWG.

19. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
100 volts.
A device used to store electrical energy.
3 -979 Volts.

20. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Lower current and higher voltage.
Second-degree burn.
Because less current flowing at high voltage causes less line loss per kilowatt.

21. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
X.
Short-circuited after power has been removed.
1 amp.
Hyperventilation.

22. 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?
X.
The capacitors should be discharged.
200 volts AC.
At the source transformer.

23. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Shunted and taped
100 volts.
Isolation transformer.
25 feet.

24. 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.
Allow for metering voltage and Current.
Current changing constantly in magnitude and periodically in direction.
5:1
Two speeds

25. At what rate should mouth-to-mouth resuscitation be applied?
Two speeds
12 to 15 times a minute.
7 -630 volts.
Check to make sure that the victim is not still in contact with the electricity.

26. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Because less current flowing at high voltage causes less line loss per kilowatt.
Hyperventilation.
Short-circuited after power has been removed.
The capacitors should be discharged.

27. A wound-rotor induction motor has all the following EXCEPT...
480 volts.
Cut around clothing that has adhered to the burned area.
Squirrel-cage rotor.
7 -630 volts.

28. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
2000 watts
Transformer
Inductance.

29. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
A grounding resistor.
Shunted and taped
15 feet off the ground.

30. 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?
Larger than that found in the primary winding.
A.C.
Inductance.
100 turns.

31. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Disconnect the frame-grounding conductor.
Inductance.
Electro-magnetism for their operation.
Grounded phase and ground fault devices were not operating.

32. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Two speeds
They should be made by the same manufacturer.
Allow for metering voltage and Current.
Draw less current.

33. When not connected to a load - the secondary leads of a current transformer should be...
Inductance.
The chest should rise or inflate.
Shunted and taped
A step-up transformer.

34. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
75 turns.
A step-up transformer.
Continuous.
Second-degree burn.

35. The standard frequency of alternating current used at a mine is...
60 Hertz
Circuit Breaker.
Stator windings.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

36. Alternating current may be defined as...
Transformers.
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.
7 -630 volts.

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

38. The purpose of a zig-zag transformer is to...
They should be made by the same manufacturer.
It provides good speed control.
Draw less current.
Derive a neutral for grounding purposes.

39. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Continuous.
The chest should rise or inflate.
Capacitance.
3 -979 Volts.

40. A transformer with a 1:1 ratio is usually a (an)...
Potential transformer.
277 volts.
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.

41. What is the first step to be followed in aiding a victim of electrical shock?
They must be installed at least 7 feet above the ground.
An electrical shock hazard may exist.
X.
Check to make sure that the victim is not still in contact with the electricity.

42. 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...
100 turns.
Draw less current.
Nearly constant.
The current will read zero.

43. 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?
Shunted and taped
600 volts.
X.
They should be made by the same manufacturer.

44. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Lower current and higher voltage
2000 watts
Two speeds
Allow for metering voltage and Current.

45. Current in an AC circuit may be measured by...
Clamp-on ammeter.
H terminals.
Transformer
Reverse its rotation.

46. The primary leads of a single-phase transformer should be connected to the...
Larger than that found in the primary winding.
H terminals.
An electromagnet.
277 volts.

47. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Four times less than that of the secondary.
Kilovolt ampere (KVA).
Inductance.
Alternating current.

48. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Four times less than that of the secondary.
100 volts.

49. All transformers step voltage up or down by...
A grounding resistor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Four times less than that of the secondary.
Inductance.

50. The transformer used only to measure voltage is a...
Nearly constant.
Because less current flowing at high voltage causes less line loss per kilowatt.
Potential transformer.
Current changing constantly in magnitude and periodically in direction.