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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. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Because less current flowing at high voltage causes less line loss per kilowatt.
480 volts.
Hyperventilation.
They should be made by the same manufacturer.

2. The primary function of a power transformer is to provide...
Reverse its rotation.
150 turns.
Full-load - three-phase voltage.
100 volts.

3. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Disconnect the frame-grounding conductor.
Transformer
X.
They must be installed at least 7 feet above the ground.

4. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
3 -979 Volts.
Zero amps.
The chest should rise or inflate.
Reverse its rotation.

5. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
7 -630 volts.
Kilovolt ampere (KVA).
Nearly constant.

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
Because less current flowing at high voltage causes less line loss per kilowatt.
7 -630 volts.
Disconnect the frame-grounding conductor.

7. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Transformers.
Kilovolt ampere (KVA).
Zero amps.
The motor rotation is reversed.

8. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Circuit Breaker.
At the source transformer.
The current will read zero.
The capacitors should be discharged.

9. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
15 feet off the ground.
600 volts.
Allow for metering voltage and Current.
No.6 AWG.

10. The standard frequency of alternating current used at a mine is...
100 volts.
Zero amps.
60 Hertz
The motor rotation is reversed.

11. 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.
Second-degree burn.
5:1
Disconnect the frame-grounding conductor.
Because less current flowing at high voltage causes less line loss per kilowatt.

12. Not an acceptable method for the installations of surface transformers?
Because less current flowing at high voltage causes less line loss per kilowatt.
No.6 AWG.
Reverse its rotation.
They must be installed at least 7 feet above the ground.

13. The combined capacitance of two capacitors in parallel is...
They should be made by the same manufacturer.
Electro-magnetism for their operation.
Reverse its rotation.
The sum of the two capacitors.

14. Compared to the secondary of a loaded - step-down transformer - the primary has...
A device used to store electrical energy.
100 turns.
Lower current and higher voltage
Draw less current.

15. Define alternating current...
5:1
Clamp-on ammeter.
Current changing constantly in magnitude and periodically in direction.
Short-circuited after power has been removed.

16. Compared to the secondary of a loaded - step-down transformer - the primary has...
It provides good speed control.
200 volts AC.
Lower current and higher voltage.
Potential transformer.

17. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Grounded phase and ground fault devices were not operating.
75 turns.
Current changing constantly in magnitude and periodically in direction.
The chest should rise or inflate.

18. 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?
60 cycles per second (60 Hertz).
A device used to store electrical energy.
3 -979 Volts.
At the source transformer.

19. The transformer used only to measure voltage is a...
200 volts AC.
Potential transformer.
60 cycles per second (60 Hertz).
Draw less current.

20. 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...
Kilovolt ampere (KVA).
5:1.
Tied together
Provides single-phase power to smaller loads such as lights - heaters - etc.

21. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Electro-magnetism for their operation.
X.
1 amp.
At least two phases

22. AC voltage is raised or lowered by means of...
Transformers.
5:1
Circuit Breaker.
100 volts.

23. 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.
Reverse any two-phase leads.
Inductance.
Continuous.

24. What should the duty rating of a grounding resistor be?
Continuous.
Check to make sure that the victim is not still in contact with the electricity.
Inductance.
Clamp-on ammeter.

25. A solenoid coil is...
25 feet.
At the source transformer.
An electromagnet.
Draw less current.

26. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Stator windings.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Nearly constant.
5:1.

27. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Inductance.
1 amp.
Draw less current.
They should be made by the same manufacturer.

28. The size of a transformer is measured in...
Kilovolt ampere (KVA).
They must be installed at least 7 feet above the ground.
Nearly constant.
Inductance.

29. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Lower current and higher voltage.
Draw less current.
A grounding resistor.
Electro-magnetism for their operation.

30. What type motor is a wound-rotor induction motor?
A.C.
Squirrel-cage rotor.
They should be made by the same manufacturer.
Electro-magnetism for their operation.

31. 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?
Transformer
800 volts.
The current will read zero.
Full-load - three-phase voltage.

32. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Draw less current.
2000 watts

33. The main function of CT's and PT's on a main substation is to...
Kilovolt ampere (KVA).
Allow for metering voltage and Current.
Reverse any two-phase leads.
Squirrel-cage induction motor.

34. The secondary windings on a 10:1 step-down transformer will have a wire size...
A grounding resistor.
75 turns.
Allow for metering voltage and Current.
Larger than that found in the primary winding.

35. Current in an AC circuit may be measured by...
Allow for metering voltage and Current.
5:1
X.
Clamp-on ammeter.

36. The property of an electric circuit that opposes any change in current is called...
Inductance.
Zero amps.
Isolation transformer.
200 volts AC.

37. 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?
The sum of the two capacitors.
200 volts AC.
100 volts.
Kilovolt ampere (KVA).

38. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
Hyperventilation.
Electro-magnetism for their operation.
They should be made by the same manufacturer.

39. 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?
600 volts.
A step-up transformer.
Kilovolt ampere (KVA).
Transformer

40. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Current that changes constantly in magnitude and periodically in direction.
Shunted and taped
25 feet.
H terminals.

41. Which of the following does a squirrel cage induction motor have?
Stator windings.
Allow for metering voltage and Current.
100 turns.
Alternating current.

42. If any two-phase wires are reversed on a three-phase motor - the motor will...
Isolation transformer.
3 -979 Volts.
Full-load - three-phase voltage.
Reverse its rotation.

43. Not a symptom of electrical shock?.
Hyperventilation.
Short-circuited after power has been removed.
Isolation transformer.
Reverse any two-phase leads.

44. 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?
100 turns.
Electro-magnetism for their operation.
Capacitance.
Lower current and higher voltage

45. What is the first step to be followed in aiding a victim of electrical shock?
Inductance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Isolation transformer.
Check to make sure that the victim is not still in contact with the electricity.

46. 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...
Circuit Breaker.
100 volts.
Isolation transformer.
Tied together

47. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
12 to 15 times a minute.
Derive a neutral for grounding purposes.
2000 watts
At the source transformer.

48. 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.
5:1.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Kilovolt ampere (KVA).

49. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Isolation transformer.
Disconnect the frame-grounding conductor.
Current that changes constantly in magnitude and periodically in direction.
Circuit Breaker.

50. Three-phase AC underground cables must be provided with overcurrent protection using a...
25 feet.
150 turns.
Circuit Breaker.
Stator windings.