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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. 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.
800 volts.
Alternating current.
Current changing constantly in magnitude and periodically in direction.

2. How do you change the direction of a 3-phase AC motor?
Lower current and higher voltage
Reverse any two-phase leads.
Short-circuited after power has been removed.
60 Hertz

3. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Two speeds
7 -630 volts.
Disconnect the frame-grounding conductor.
60 cycles per second (60 Hertz).

4. 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.
75 turns.
2000 watts
Potential transformer.

5. 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...
277 volts.
60 cycles per second (60 Hertz).
Check to make sure that the victim is not still in contact with the electricity.
100 volts.

6. 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
Provides single-phase power to smaller loads such as lights - heaters - etc.
Short-circuited after power has been removed.
25 feet.

7. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Alternating current.
100 volts.
25 feet.
The chest should rise or inflate.

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

9. Alternating current may be defined as...
The current will read zero.
Current changing constantly in magnitude and periodically in direction.
Continuous.
Current that changes constantly in magnitude and periodically in direction.

10. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Shunted and taped
Squirrel-cage induction motor.
Disconnect the frame-grounding conductor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

11. Three-phase AC underground cables must be provided with overcurrent protection using a...
Because less current flowing at high voltage causes less line loss per kilowatt.
Isolation transformer.
75 turns.
Circuit Breaker.

12. Current that changes periodically in direction is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
60 Hertz
Alternating current.
Full-load - three-phase voltage.

13. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
5:1
Tied together
100 volts.

14. 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?
Inductance.
200 volts AC.
100 volts.
Short-circuited after power has been removed.

15. What type motor is a wound-rotor induction motor?
A.C.
Transformers.
Lower current and higher voltage.
The motor rotation is reversed.

16. What steps up or steps down AC voltage?
Transformer
The motor rotation is reversed.
Circuit Breaker.
5:1

17. The speed of an alternating current - squirrel-cage induction motor is...
480 volts.
The chest should rise or inflate.
Nearly constant.
75 turns.

18. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
480 volts.
A grounding resistor.
The chest should rise or inflate.

19. 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?
The current will read zero.
150 turns.
Hyperventilation.
The capacitors should be discharged.

20. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Transformers.
60 Hertz
At least two phases
X.

21. All transformers step voltage up or down by...
Because less current flowing at high voltage causes less line loss per kilowatt.
They should be made by the same manufacturer.
Inductance.
Current changing constantly in magnitude and periodically in direction.

22. 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...
Continuous.
Because less current flowing at high voltage causes less line loss per kilowatt.
Disconnect the frame-grounding conductor.
Draw less current.

23. If any two-phase wires are reversed on a three-phase motor - the motor will...
1 amp.
Shunted and taped
Reverse its rotation.
H terminals.

24. Compared to the secondary of a loaded - step-down transformer - the primary has...
Kilovolt ampere (KVA).
Larger than that found in the primary winding.
200 volts AC.
Lower current and higher voltage.

25. To protect against over-current in a three-phase circuit over-current protection must be provided for...
They should be made by the same manufacturer.
600 volts.
Draw less current.
At least two phases

26. Current in an AC circuit may be measured by...
Clamp-on ammeter.
At least two phases
They must be installed at least 7 feet above the ground.
It provides good speed control.

27. Define alternating current...
100 turns.
25 feet.
Current changing constantly in magnitude and periodically in direction.
Allow for metering voltage and Current.

28. A control power transformer...
A device used to store electrical energy.
Second-degree burn.
The chest should rise or inflate.
Provides single-phase power to smaller loads such as lights - heaters - etc.

29. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Tied together
Kilovolt ampere (KVA).
Grounded phase and ground fault devices were not operating.
Disconnect the frame-grounding conductor.

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?
Draw less current.
X.
100 turns.
1 amp.

31. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Transformers.
Stator windings.
480 volts.
They should be made by the same manufacturer.

32. 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?
Provides single-phase power to smaller loads such as lights - heaters - etc.
600 volts.
Circuit Breaker.
Allow for metering voltage and Current.

33. Which of the following does a squirrel cage induction motor have?
480 volts.
Stator windings.
25 feet.
1 amp.

34. What is the danger when working around capacitors?
A device used to store electrical energy.
12 to 15 times a minute.
Circuit Breaker.
An electrical shock hazard may exist.

35. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Second-degree burn.
Current changing constantly in magnitude and periodically in direction.
Stator windings.

36. How should clothing be removed from a burn?
Larger than that found in the primary winding.
Cut around clothing that has adhered to the burned area.
Because less current flowing at high voltage causes less line loss per kilowatt.
75 turns.

37. 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...
Full-load - three-phase voltage.
Draw less current.
The chest should rise or inflate.
Two speeds

38. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Clamp-on ammeter.
Current changing constantly in magnitude and periodically in direction.
100 volts.
Short-circuited after power has been removed.

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.
Circuit Breaker.
100 turns.
5:1
The capacitors should be discharged.

40. The primary function of a power transformer is to provide...
3 -979 Volts.
Circuit Breaker.
Full-load - three-phase voltage.
Derive a neutral for grounding purposes.

41. High voltage is used to transmit power over long distances...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Because less current flowing at high voltage causes less line loss per kilowatt.
200 volts AC.
A.C.

42. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Draw less current.
The capacitors should be discharged.
Potential transformer.
X.

43. Not a symptom of electrical shock?.
Hyperventilation.
800 volts.
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

44. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Full-load - three-phase voltage.
They must be installed at least 7 feet above the ground.
100 turns.
7 -630 volts.

45. What is the first step to be followed in aiding a victim of electrical shock?
H terminals.
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Alternating current.

46. 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
They should be made by the same manufacturer.
150 turns.
At the source transformer.

47. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Four times less than that of the secondary.
An electrical shock hazard may exist.
No.6 AWG.
H terminals.

48. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
A grounding resistor.
Two speeds
100 turns.

49. A wound-rotor induction motor has all the following EXCEPT...
Grounded phase and ground fault devices were not operating.
5:1
Hyperventilation.
Squirrel-cage rotor.

50. The purpose of a zig-zag transformer is to...
60 Hertz
It provides good speed control.
Tied together
Derive a neutral for grounding purposes.