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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. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
60 Hertz
480 volts.
Current changing constantly in magnitude and periodically in direction.

2. 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?
75 turns.
Inductance.
Reverse any two-phase leads.
Isolation transformer.

3. The property of an electric circuit that opposes any change in current is called...
Because less current flowing at high voltage causes less line loss per kilowatt.
25 feet.
Inductance.
X.

4. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
The capacitors should be discharged.
2000 watts
Carotid pulse - which is located in the neck next to the Adam's apple.

5. 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
The capacitors should be discharged.
A step-up transformer.
The chest should rise or inflate.

6. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
7 -630 volts.
25 feet.
100 volts.

7. 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?
Disconnect the frame-grounding conductor.
150 turns.
100 turns.
Alternating current.

8. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Current that changes constantly in magnitude and periodically in direction.
150 turns.
25 feet.

9. 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...
Inductance.
100 volts.
800 volts.
An electromagnet.

10. The size of a transformer is measured in...
Squirrel-cage rotor.
Kilovolt ampere (KVA).
Draw less current.
Inductance.

11. What steps up or steps down AC voltage?
5:1
Lower current and higher voltage
Transformer
Nearly constant.

12. What should the duty rating of a grounding resistor be?
60 cycles per second (60 Hertz).
Continuous.
The sum of the two capacitors.
Current that changes constantly in magnitude and periodically in direction.

13. What type motor is a wound-rotor induction motor?
Squirrel-cage induction motor.
A.C.
Check to make sure that the victim is not still in contact with the electricity.
5:1

14. 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.
Current changing constantly in magnitude and periodically in direction.
5:1
A device used to store electrical energy.
Kilovolt ampere (KVA).

15. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
100 volts.
The current will read zero.
The motor rotation is reversed.
480 volts.

16. The standard frequency of alternating current used at a mine is...
60 Hertz
800 volts.
Full-load - three-phase voltage.
Two speeds

17. When not connected to a load - the secondary leads of a current transformer should be...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The current will read zero.
Shunted and taped
Transformer

18. 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.
Transformer
2000 watts
Grounded phase and ground fault devices were not operating.

19. Not true about AC power?
It provides good speed control.
Full-load - three-phase voltage.
800 volts.
Draw less current.

20. The primary function of a power transformer is to provide...
A device used to store electrical energy.
Potential transformer.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Full-load - three-phase voltage.

21. Compared to the secondary of a loaded - step-down transformer - the primary has...
Squirrel-cage rotor.
Lower current and higher voltage.
Stator windings.
Circuit Breaker.

22. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
Potential transformer.
At least two phases
The capacitors should be discharged.

23. Current transformers - or CT's - are used to...
Two speeds
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Transformer
X.

24. What is the danger when working around capacitors?
Two speeds
75 turns.
An electrical shock hazard may exist.
Stator windings.

25. 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?
An electrical shock hazard may exist.
100 turns.
Reverse its rotation.
Full-load - three-phase voltage.

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...
3 -979 Volts.
Tied together
No.6 AWG.
Draw less current.

27. What is a capacitor?
At the source transformer.
480 volts.
A device used to store electrical energy.
800 volts.

28. 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?
Carotid pulse - which is located in the neck next to the Adam's apple.
277 volts.
7 -630 volts.
600 volts.

29. The purpose of a zig-zag transformer is to...
Continuous.
Because less current flowing at high voltage causes less line loss per kilowatt.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Derive a neutral for grounding purposes.

30. A transformer with a 1:1 ratio is usually a (an)...
The current will read zero.
Isolation transformer.
They must be installed at least 7 feet above the ground.
200 volts AC.

31. The property of an electrical circuit that opposes any change in voltage is called...
Inductance.
Grounded phase and ground fault devices were not operating.
Capacitance.
Second-degree burn.

32. The transformer used only to measure voltage is a...
800 volts.
Potential transformer.
Allow for metering voltage and Current.
A.C.

33. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
The capacitors should be discharged.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electromagnet.
600 volts.

34. 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?
A.C.
Lower current and higher voltage
200 volts AC.
A step-up transformer.

35. At what rate should mouth-to-mouth resuscitation be applied?
Kilovolt ampere (KVA).
Reverse any two-phase leads.
12 to 15 times a minute.
Current changing constantly in magnitude and periodically in direction.

36. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Disconnect the frame-grounding conductor.
Four times less than that of the secondary.
At least two phases

37. A control power transformer...
Reverse any two-phase leads.
200 volts AC.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Current that changes constantly in magnitude and periodically in direction.

38. What is the first step to be followed in aiding a victim of electrical shock?
Disconnect the frame-grounding conductor.
Check to make sure that the victim is not still in contact with the electricity.
100 volts.
75 turns.

39. What type of 3-phase AC motor is used most often around a mine?
Derive a neutral for grounding purposes.
5:1
Squirrel-cage induction motor.
277 volts.

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

41. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
480 volts.
Isolation transformer.
277 volts.
Grounded phase and ground fault devices were not operating.

42. Alternating current may be defined as...
Current changing constantly in magnitude and periodically in direction.
Current that changes constantly in magnitude and periodically in direction.
15 feet off the ground.
Kilovolt ampere (KVA).

43. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Cut around clothing that has adhered to the burned area.
60 cycles per second (60 Hertz).
Larger than that found in the primary winding.

44. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Clamp-on ammeter.
Isolation transformer.
Tied together

45. A solenoid coil is...
Inductance.
An electromagnet.
A device used to store electrical energy.
It provides good speed control.

46. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Derive a neutral for grounding purposes.
Current that changes constantly in magnitude and periodically in direction.
Electro-magnetism for their operation.

47. Operating coils - motors - transformers - relays and solenoids depend upon...
Continuous.
Larger than that found in the primary winding.
Electro-magnetism for their operation.
5:1

48. Current in an AC circuit may be measured by...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Provides single-phase power to smaller loads such as lights - heaters - etc.
They must be installed at least 7 feet above the ground.
Clamp-on ammeter.

49. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
At the source transformer.
An electrical shock hazard may exist.
Squirrel-cage rotor.

50. 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...
The motor rotation is reversed.
Draw less current.
Reverse its rotation.
A device used to store electrical energy.