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. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
Transformer
Electro-magnetism for their operation.
800 volts.


2. What type of 3-phase AC motor is used most often around a mine?
Shunted and taped
Electro-magnetism for their operation.
Squirrel-cage induction motor.
5:1


3. The standard frequency of alternating current used at a mine is...
Squirrel-cage rotor.
60 Hertz
Circuit Breaker.
Lower current and higher voltage.


4. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
15 feet off the ground.
Disconnect the frame-grounding conductor.
Transformer
Shunted and taped


5. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Stator windings.
Disconnect the frame-grounding conductor.
X.


6. 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...
Second-degree burn.
The chest should rise or inflate.
Draw less current.
12 to 15 times a minute.


7. A control power transformer...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
12 to 15 times a minute.
The sum of the two capacitors.
Provides single-phase power to smaller loads such as lights - heaters - etc.


8. 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?
Capacitance.
3 -979 Volts.
They should be made by the same manufacturer.
75 turns.


9. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Nearly constant.
The motor rotation is reversed.
12 to 15 times a minute.
Zero amps.


10. The secondary windings on a 10:1 step-down transformer will have a wire size...
277 volts.
Larger than that found in the primary winding.
Inductance.
Carotid pulse - which is located in the neck next to the Adam's apple.


11. AC voltage is raised or lowered by means of...
Zero amps.
The motor rotation is reversed.
Potential transformer.
Transformers.


12. Which of the following does a squirrel cage induction motor have?
2000 watts
Larger than that found in the primary winding.
Stator windings.
800 volts.


13. The property of an electric circuit that opposes any change in current is called...
Inductance.
Current changing constantly in magnitude and periodically in direction.
15 feet off the ground.
Full-load - three-phase voltage.


14. 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.
25 feet.
5:1.
Two speeds
No.6 AWG.


15. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
60 cycles per second (60 Hertz).
Check to make sure that the victim is not still in contact with the electricity.
Potential transformer.


16. 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.
Reverse any two-phase leads.
5:1
277 volts.
An electrical shock hazard may exist.


17. Not true about AC power?
It provides good speed control.
The capacitors should be discharged.
X.
Allow for metering voltage and Current.


18. 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?
X.
800 volts.
Transformers.
Isolation transformer.


19. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Inductance.
Two speeds
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
7 -630 volts.


20. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
An electromagnet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Alternating current.
Kilovolt ampere (KVA).


21. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Inductance.
Two speeds
At least two phases


22. Alternating current may be defined as...
Squirrel-cage induction motor.
Grounded phase and ground fault devices were not operating.
Current that changes constantly in magnitude and periodically in direction.
It provides good speed control.


23. 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?
Shunted and taped
100 turns.
An electrical shock hazard may exist.
Isolation transformer.


24. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
The current will read zero.
Circuit Breaker.
A grounding resistor.
Carotid pulse - which is located in the neck next to the Adam's apple.


25. What steps up or steps down AC voltage?
Transformer
Disconnect the frame-grounding conductor.
Tied together
No.6 AWG.


26. 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?
200 volts AC.
Circuit Breaker.
Reverse any two-phase leads.
Stator windings.


27. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
An electrical shock hazard may exist.
Hyperventilation.
The chest should rise or inflate.


28. The main function of CT's and PT's on a main substation is to...
75 turns.
Potential transformer.
Nearly constant.
Allow for metering voltage and Current.


29. The purpose of a zig-zag transformer is to...
800 volts.
Nearly constant.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Derive a neutral for grounding purposes.


30. 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.
Larger than that found in the primary winding.
The chest should rise or inflate.
Reverse any two-phase leads.


31. The primary function of a power transformer is to provide...
Transformer
Full-load - three-phase voltage.
Hyperventilation.
The current will read zero.


32. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
The capacitors should be discharged.
Check to make sure that the victim is not still in contact with the electricity.
An electromagnet.


33. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
Check to make sure that the victim is not still in contact with the electricity.
Inductance.
Full-load - three-phase voltage.


34. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Capacitance.
1 amp.
At least two phases
Four times less than that of the secondary.


35. 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...
Transformer
Draw less current.
15 feet off the ground.
25 feet.


36. Compared to the secondary of a loaded - step-down transformer - the primary has...
The sum of the two capacitors.
1 amp.
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage.


37. If any two-phase wires are reversed on a three-phase motor - the motor will...
Current changing constantly in magnitude and periodically in direction.
Tied together
7 -630 volts.
Reverse its rotation.


38. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Inductance.
X.
A.C.


39. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Shunted and taped
Tied together
75 turns.


40. High voltage is used to transmit power over long distances...
Reverse any two-phase leads.
800 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
Stator windings.


41. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Derive a neutral for grounding purposes.
Alternating current.
277 volts.
Lower current and higher voltage


42. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
60 cycles per second (60 Hertz).
Draw less current.
12 to 15 times a minute.
Short-circuited after power has been removed.


43. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Capacitance.
Disconnect the frame-grounding conductor.
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.


44. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Short-circuited after power has been removed.
Tied together
It provides good speed control.


45. The property of an electrical circuit that opposes any change in voltage is called...
Isolation transformer.
Larger than that found in the primary winding.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Capacitance.


46. 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?
277 volts.
75 turns.
7 -630 volts.
150 turns.


47. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Squirrel-cage rotor.
1 amp.
Full-load - three-phase voltage.
600 volts.


48. Not a symptom of electrical shock?.
Cut around clothing that has adhered to the burned area.
Hyperventilation.
800 volts.
The current will read zero.


49. Current that changes periodically in direction is...
A step-up transformer.
The chest should rise or inflate.
Clamp-on ammeter.
Alternating current.


50. Where should grounding resistors be located?
Cut around clothing that has adhered to the burned area.
A.C.
At the source transformer.
3 -979 Volts.