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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. The speed of an alternating current - squirrel-cage induction motor is...
Cut around clothing that has adhered to the burned area.
Nearly constant.
60 cycles per second (60 Hertz).
No.6 AWG.

2. 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
The sum of the two capacitors.
Because less current flowing at high voltage causes less line loss per kilowatt.
Alternating current.

3. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
The current will read zero.
A grounding resistor.
Lower current and higher voltage

4. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
3 -979 Volts.
Inductance.
Short-circuited after power has been removed.
Circuit Breaker.

5. Which of the following does a squirrel cage induction motor have?
480 volts.
They should be made by the same manufacturer.
An electrical shock hazard may exist.
Stator windings.

6. Current in an AC circuit may be measured by...
Isolation transformer.
Inductance.
Clamp-on ammeter.
Zero amps.

7. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
Check to make sure that the victim is not still in contact with the electricity.
A device used to store electrical energy.
The motor rotation is reversed.

8. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Second-degree burn.
Grounded phase and ground fault devices were not operating.
100 volts.
Two speeds

9. 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.
Lower current and higher voltage
The capacitors should be discharged.
277 volts.

10. What type motor is a wound-rotor induction motor?
A.C.
Carotid pulse - which is located in the neck next to the Adam's apple.
Because less current flowing at high voltage causes less line loss per kilowatt.
2000 watts

11. 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.
Carotid pulse - which is located in the neck next to the Adam's apple.
A grounding resistor.
Electro-magnetism for their operation.
5:1.

12. 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...
Larger than that found in the primary winding.
100 volts.
75 turns.
Continuous.

13. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
The sum of the two capacitors.
Capacitance.
Potential transformer.

14. The property of an electric circuit that opposes any change in current is called...
Isolation transformer.
Inductance.
Squirrel-cage induction motor.
Lower current and higher voltage.

15. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Circuit Breaker.
2000 watts
Lower current and higher voltage

16. All transformers step voltage up or down by...
12 to 15 times a minute.
Inductance.
2000 watts
Allow for metering voltage and Current.

17. The standard frequency of power supplied by the power company is...
Short-circuited after power has been removed.
Continuous.
60 cycles per second (60 Hertz).
75 turns.

18. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Four times less than that of the secondary.
7 -630 volts.
Transformer

19. 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.
A device used to store electrical energy.
Lower current and higher voltage
No.6 AWG.

20. A transformer with a 1:1 ratio is usually a (an)...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Lower current and higher voltage
75 turns.
Isolation transformer.

21. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
A.C.
Transformers.
The current will read zero.
They should be made by the same manufacturer.

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?
277 volts.
1 amp.
Clamp-on ammeter.
200 volts AC.

23. The size of a transformer is measured in...
Check to make sure that the victim is not still in contact with the electricity.
Kilovolt ampere (KVA).
277 volts.
H terminals.

24. What steps up or steps down AC voltage?
The chest should rise or inflate.
The capacitors should be discharged.
Transformer
Lower current and higher voltage

25. 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?
The current will read zero.
800 volts.
A device used to store electrical energy.
100 turns.

26. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Alternating current.
They should be made by the same manufacturer.
75 turns.

27. Three-phase AC underground cables must be provided with overcurrent protection using a...
Short-circuited after power has been removed.
600 volts.
Circuit Breaker.
Full-load - three-phase voltage.

28. The transformer used only to measure voltage is a...
Inductance.
Tied together
Potential transformer.
Current changing constantly in magnitude and periodically in direction.

29. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
A step-up transformer.
No.6 AWG.
5:1.
Zero amps.

30. When not connected to a load - the secondary leads of a current transformer should be...
Alternating current.
Because less current flowing at high voltage causes less line loss per kilowatt.
480 volts.
Shunted and taped

31. Compared to the secondary of a loaded - step-down transformer - the primary has...
Because less current flowing at high voltage causes less line loss per kilowatt.
Four times less than that of the secondary.
Lower current and higher voltage.
7 -630 volts.

32. What should the duty rating of a grounding resistor be?
Continuous.
Larger than that found in the primary winding.
277 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

33. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Allow for metering voltage and Current.
Lower current and higher voltage
15 feet off the ground.
Potential transformer.

34. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
A grounding resistor.
Stator windings.
7 -630 volts.
3 -979 Volts.

35. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
At least two phases
They should be made by the same manufacturer.
Draw less current.

36. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Allow for metering voltage and Current.
Shunted and taped
1 amp.
Two speeds

37. Not true about AC power?
It provides good speed control.
Squirrel-cage induction motor.
The chest should rise or inflate.
12 to 15 times a minute.

38. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Stator windings.
Grounded phase and ground fault devices were not operating.
Full-load - three-phase voltage.
Four times less than that of the secondary.

39. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
15 feet off the ground.
Transformers.
100 volts.
X.

40. If any two-phase wires are reversed on a three-phase motor - the motor will...
Disconnect the frame-grounding conductor.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Transformer
Reverse its rotation.

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

42. How do you change the direction of a 3-phase AC motor?
Check to make sure that the victim is not still in contact with the electricity.
12 to 15 times a minute.
They should be made by the same manufacturer.
Reverse any two-phase leads.

43. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Full-load - three-phase voltage.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
100 volts.

44. What is the danger when working around capacitors?
An electrical shock hazard may exist.
H terminals.
60 Hertz
Inductance.

45. The secondary windings on a 10:1 step-down transformer will have a wire size...
Isolation transformer.
Potential transformer.
Larger than that found in the primary winding.
75 turns.

46. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
The chest should rise or inflate.
Larger than that found in the primary winding.
Lower current and higher voltage.

47. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
At the source transformer.
The chest should rise or inflate.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Allow for metering voltage and Current.

48. What type of 3-phase AC motor is used most often around a mine?
Draw less current.
Squirrel-cage induction motor.
Two speeds
Potential transformer.

49. Not a symptom of electrical shock?.
Inductance.
Current that changes constantly in magnitude and periodically in direction.
It provides good speed control.
Hyperventilation.

50. 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?
100 turns.
3 -979 Volts.
Zero amps.
The capacitors should be discharged.