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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. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
7 -630 volts.
5:1
At least two phases
The motor rotation is reversed.

2. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
25 feet.
Hyperventilation.
Cut around clothing that has adhered to the burned area.

3. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
Electro-magnetism for their operation.
1 amp.
Draw less current.

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?
They should be made by the same manufacturer.
3 -979 Volts.
Alternating current.
Tied together

5. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Isolation transformer.
Clamp-on ammeter.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Two speeds

6. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
A grounding resistor.
100 turns.
Transformer

7. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Carotid pulse - which is located in the neck next to the Adam's apple.
Current changing constantly in magnitude and periodically in direction.
Four times less than that of the secondary.
200 volts AC.

8. 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?
7 -630 volts.
600 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
277 volts.

9. 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.
Transformers.
25 feet.
Hyperventilation.

10. 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?
No.6 AWG.
150 turns.
Transformer
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

11. The secondary windings on a 10:1 step-down transformer will have a wire size...
The motor rotation is reversed.
480 volts.
Larger than that found in the primary winding.
75 turns.

12. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Isolation transformer.
Capacitance.
Squirrel-cage rotor.
2000 watts

13. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
200 volts AC.
150 turns.
X.
A grounding resistor.

14. The primary function of a power transformer is to provide...
Draw less current.
Full-load - three-phase voltage.
800 volts.
An electrical shock hazard may exist.

15. Operating coils - motors - transformers - relays and solenoids depend upon...
Circuit Breaker.
Electro-magnetism for their operation.
Four times less than that of the secondary.
Isolation transformer.

16. A transformer with a 1:1 ratio is usually a (an)...
Lower current and higher voltage.
Cut around clothing that has adhered to the burned area.
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.

17. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Alternating current.
7 -630 volts.
Four times less than that of the secondary.
Grounded phase and ground fault devices were not operating.

18. All transformers step voltage up or down by...
12 to 15 times a minute.
75 turns.
2000 watts
Inductance.

19. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Carotid pulse - which is located in the neck next to the Adam's apple.
Inductance.
Provides single-phase power to smaller loads such as lights - heaters - etc.

20. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Capacitance.
An electromagnet.
2000 watts
277 volts.

21. What steps up or steps down AC voltage?
Zero amps.
480 volts.
X.
Transformer

22. A burn causing a blister is classified as a...
They must be installed at least 7 feet above the ground.
Lower current and higher voltage.
Continuous.
Second-degree burn.

23. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
200 volts AC.
X.
At least two phases

24. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Nearly constant.
75 turns.
277 volts.

25. If any two-phase wires are reversed on a three-phase motor - the motor will...
Because less current flowing at high voltage causes less line loss per kilowatt.
It provides good speed control.
Transformer
Reverse its rotation.

26. What is a capacitor?
Zero amps.
An electromagnet.
A device used to store electrical energy.
Cut around clothing that has adhered to the burned area.

27. 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...
They must be installed at least 7 feet above the ground.
Draw less current.
100 volts.
75 turns.

28. 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...
Stator windings.
Reverse its rotation.
100 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.

29. The primary leads of a single-phase transformer should be connected to the...
Inductance.
H terminals.
Current changing constantly in magnitude and periodically in direction.
Inductance.

30. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Squirrel-cage rotor.
Larger than that found in the primary winding.
800 volts.

31. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Potential transformer.
An electromagnet.
At the source transformer.

32. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Inductance.
Transformer
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
1 amp.

33. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Two speeds
The current will read zero.
Allow for metering voltage and Current.
Reverse any two-phase leads.

34. 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.
The capacitors should be discharged.
Four times less than that of the secondary.
At least two phases

35. A solenoid coil is...
An electromagnet.
Two speeds
Shunted and taped
Kilovolt ampere (KVA).

36. The purpose of a zig-zag transformer is to...
No.6 AWG.
Potential transformer.
Derive a neutral for grounding purposes.
Inductance.

37. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
Lower current and higher voltage
Shunted and taped
Transformer

38. Current in an AC circuit may be measured by...
H terminals.
60 cycles per second (60 Hertz).
Clamp-on ammeter.
1 amp.

39. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Continuous.
The chest should rise or inflate.
No.6 AWG.
277 volts.

40. Alternating current may be defined as...
Circuit Breaker.
They should be made by the same manufacturer.
Current that changes constantly in magnitude and periodically in direction.
Larger than that found in the primary winding.

41. The standard frequency of alternating current used at a mine is...
Nearly constant.
60 Hertz
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Shunted and taped

42. Current that changes periodically in direction is...
No.6 AWG.
Alternating current.
The sum of the two capacitors.
75 turns.

43. A transformer has a primary voltage of 2 -400 volts and a secondary reading of 120 volts. If there are 1 -500 turns on the primary - how many turns are on the secondary?
75 turns.
480 volts.
Circuit Breaker.
Hyperventilation.

44. The property of an electric circuit that opposes any change in current is called...
Transformers.
Reverse its rotation.
Inductance.
15 feet off the ground.

45. To protect against over-current in a three-phase circuit over-current protection must be provided for...
A step-up transformer.
Continuous.
Reverse any two-phase leads.
At least two phases

46. 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...
They should be made by the same manufacturer.
Tied together
Draw less current.
Circuit Breaker.

47. The size of a transformer is measured in...
H terminals.
200 volts AC.
Isolation transformer.
Kilovolt ampere (KVA).

48. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Clamp-on ammeter.
Nearly constant.
Stator windings.

49. Not an acceptable method for the installations of surface transformers?
Capacitance.
They must be installed at least 7 feet above the ground.
2000 watts
No.6 AWG.

50. Which of the following does a squirrel cage induction motor have?
Disconnect the frame-grounding conductor.
Stator windings.
Check to make sure that the victim is not still in contact with the electricity.
5:1.