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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. 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...
Draw less current.
Hyperventilation.
A grounding resistor.
5:1

2. The standard frequency of alternating current used at a mine is...
It provides good speed control.
Current changing constantly in magnitude and periodically in direction.
Isolation transformer.
60 Hertz

3. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Transformer
At least two phases
Draw less current.
Continuous.

4. A transformer with a 1:1 ratio is usually a (an)...
100 turns.
The capacitors should be discharged.
Isolation transformer.
600 volts.

5. Not true about AC power?
The chest should rise or inflate.
1 amp.
It provides good speed control.
Shunted and taped

6. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
A step-up transformer.
12 to 15 times a minute.
15 feet off the ground.

7. The size of a transformer is measured in...
Kilovolt ampere (KVA).
The sum of the two capacitors.
Tied together
Short-circuited after power has been removed.

8. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
A step-up transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Kilovolt ampere (KVA).
277 volts.

9. 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...
Second-degree burn.
5:1.
Reverse any two-phase leads.
Tied together

10. 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.
7 -630 volts.
A device used to store electrical energy.
Clamp-on ammeter.

11. The transformer used only to measure voltage is a...
A device used to store electrical energy.
At least two phases
2000 watts
Potential transformer.

12. What is the first step to be followed in aiding a victim of electrical shock?
3 -979 Volts.
Check to make sure that the victim is not still in contact with the electricity.
150 turns.
15 feet off the ground.

13. 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?
Alternating current.
Nearly constant.
200 volts AC.
Lower current and higher voltage.

14. When not connected to a load - the secondary leads of a current transformer should be...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Carotid pulse - which is located in the neck next to the Adam's apple.
75 turns.
Shunted and taped

15. AC voltage is raised or lowered by means of...
Transformers.
100 volts.
Four times less than that of the secondary.
Check to make sure that the victim is not still in contact with the electricity.

16. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
1 amp.
The current will read zero.
Isolation transformer.

17. 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.
5:1.
5:1
The chest should rise or inflate.
100 turns.

18. The combined capacitance of two capacitors in parallel is...
An electrical shock hazard may exist.
5:1.
The sum of the two capacitors.
They should be made by the same manufacturer.

19. The property of an electrical circuit that opposes any change in voltage is called...
15 feet off the ground.
Inductance.
Capacitance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

20. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Squirrel-cage rotor.
They must be installed at least 7 feet above the ground.
Continuous.
Zero amps.

21. A wound-rotor induction motor has all the following EXCEPT...
Draw less current.
Grounded phase and ground fault devices were not operating.
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage rotor.

22. Not a symptom of electrical shock?.
100 volts.
Hyperventilation.
The capacitors should be discharged.
Cut around clothing that has adhered to the burned area.

23. What type of 3-phase AC motor is used most often around a mine?
The chest should rise or inflate.
Clamp-on ammeter.
Draw less current.
Squirrel-cage induction motor.

24. Compared to the secondary of a loaded - step-down transformer - the primary has...
It provides good speed control.
They must be installed at least 7 feet above the ground.
Lower current and higher voltage.
60 Hertz

25. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Grounded phase and ground fault devices were not operating.
An electrical shock hazard may exist.
Disconnect the frame-grounding conductor.
Stator windings.

26. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
7 -630 volts.
Continuous.
Nearly constant.

27. 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?
Squirrel-cage rotor.
75 turns.
60 cycles per second (60 Hertz).
At the source transformer.

28. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
2000 watts
Reverse any two-phase leads.
25 feet.
800 volts.

29. The property of an electric circuit that opposes any change in current is called...
Inductance.
Capacitance.
A.C.
2000 watts

30. 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?
800 volts.
5:1
Zero amps.
200 volts AC.

31. 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.
At the source transformer.
Second-degree burn.
5:1.
They should be made by the same manufacturer.

32. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
2000 watts
They should be made by the same manufacturer.
Lower current and higher voltage
The sum of the two capacitors.

33. 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?
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The capacitors should be discharged.
75 turns.

34. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Short-circuited after power has been removed.
7 -630 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Second-degree burn.

35. The purpose of a zig-zag transformer is to...
Reverse its rotation.
Derive a neutral for grounding purposes.
15 feet off the ground.
Inductance.

36. 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.
Reverse any two-phase leads.
Tied together
Larger than that found in the primary winding.

37. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Inductance.
Reverse its rotation.
They must be installed at least 7 feet above the ground.

38. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Alternating current.
Transformer
An electrical shock hazard may exist.

39. How do you change the direction of a 3-phase AC motor?
60 cycles per second (60 Hertz).
Squirrel-cage rotor.
Reverse any two-phase leads.
Shunted and taped

40. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
12 to 15 times a minute.
Squirrel-cage rotor.
At least two phases
1 amp.

41. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Allow for metering voltage and Current.
480 volts.
Cut around clothing that has adhered to the burned area.
It provides good speed control.

42. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Reverse its rotation.
480 volts.
The sum of the two capacitors.
The capacitors should be discharged.

43. 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.
H terminals.
2000 watts
1 amp.

44. The primary function of a power transformer is to provide...
Larger than that found in the primary winding.
A device used to store electrical energy.
Full-load - three-phase voltage.
Stator windings.

45. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Full-load - three-phase voltage.
Inductance.
Capacitance.

46. Three-phase AC underground cables must be provided with overcurrent protection using a...
7 -630 volts.
Circuit Breaker.
They must be installed at least 7 feet above the ground.
The sum of the two capacitors.

47. A solenoid coil is...
Squirrel-cage induction motor.
Nearly constant.
A grounding resistor.
An electromagnet.

48. A control power transformer...
Continuous.
Hyperventilation.
The chest should rise or inflate.
Provides single-phase power to smaller loads such as lights - heaters - etc.

49. High voltage is used to transmit power over long distances...
Full-load - three-phase voltage.
At least two phases
Because less current flowing at high voltage causes less line loss per kilowatt.
Grounded phase and ground fault devices were not operating.

50. 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
100 turns.
1 amp.
75 turns.