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. Alternating current may be defined as...
A step-up transformer.
They must be installed at least 7 feet above the ground.
Full-load - three-phase voltage.
Current that changes constantly in magnitude and periodically in direction.


2. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
At the source transformer.
3 -979 Volts.
Tied together
They should be made by the same manufacturer.


3. All transformers step voltage up or down by...
It provides good speed control.
Inductance.
At the source transformer.
Second-degree burn.


4. The speed of an alternating current - squirrel-cage induction motor is...
Circuit Breaker.
Nearly constant.
Derive a neutral for grounding purposes.
Shunted and taped


5. When not connected to a load - the secondary leads of a current transformer should be...
Shunted and taped
Reverse its rotation.
Squirrel-cage rotor.
It provides good speed control.


6. Not true about AC power?
Derive a neutral for grounding purposes.
A step-up transformer.
They must be installed at least 7 feet above the ground.
It provides good speed control.


7. Current that changes periodically in direction is...
Alternating current.
The capacitors should be discharged.
150 turns.
A grounding resistor.


8. 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
Provides single-phase power to smaller loads such as lights - heaters - etc.
Transformers.
A grounding resistor.


9. 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...
7 -630 volts.
The motor rotation is reversed.
60 Hertz
Draw less current.


10. 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.
The motor rotation is reversed.
277 volts.
Cut around clothing that has adhered to the burned area.


11. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Circuit Breaker.
Shunted and taped
5:1.


12. What type motor is a wound-rotor induction motor?
Kilovolt ampere (KVA).
1 amp.
Grounded phase and ground fault devices were not operating.
A.C.


13. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Shunted and taped
Grounded phase and ground fault devices were not operating.
The motor rotation is reversed.
200 volts AC.


14. 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...
60 cycles per second (60 Hertz).
Lower current and higher voltage
Tied together
150 turns.


15. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
X.
Current that changes constantly in magnitude and periodically in direction.
100 volts.


16. What is the first step to be followed in aiding a victim of electrical shock?
Check to make sure that the victim is not still in contact with the electricity.
No.6 AWG.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
3 -979 Volts.


17. What type of 3-phase AC motor is used most often around a mine?
277 volts.
Squirrel-cage induction motor.
15 feet off the ground.
5:1


18. AC voltage is raised or lowered by means of...
60 cycles per second (60 Hertz).
At the source transformer.
Four times less than that of the secondary.
Transformers.


19. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
277 volts.
Electro-magnetism for their operation.
480 volts.
Continuous.


20. Not an acceptable method for the installations of surface transformers?
No.6 AWG.
They must be installed at least 7 feet above the ground.
Nearly constant.
Transformer


21. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
12 to 15 times a minute.
7 -630 volts.
480 volts.


22. 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.
Larger than that found in the primary winding.
5:1
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.


23. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
12 to 15 times a minute.
The capacitors should be discharged.
600 volts.
Cut around clothing that has adhered to the burned area.


24. The standard frequency of alternating current used at a mine is...
Continuous.
60 Hertz
5:1
3 -979 Volts.


25. 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?
75 turns.
3 -979 Volts.
75 turns.
Short-circuited after power has been removed.


26. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
25 feet.
Lower current and higher voltage.
12 to 15 times a minute.


27. 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...
Cut around clothing that has adhered to the burned area.
100 volts.
Four times less than that of the secondary.
Current that changes constantly in magnitude and periodically in direction.


28. The primary leads of a single-phase transformer should be connected to the...
H terminals.
100 turns.
12 to 15 times a minute.
5:1


29. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
Inductance.
Electro-magnetism for their operation.
Squirrel-cage rotor.


30. How should clothing be removed from a burn?
Transformers.
75 turns.
Cut around clothing that has adhered to the burned area.
At least two phases


31. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Derive a neutral for grounding purposes.
Shunted and taped
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Allow for metering voltage and Current.


32. 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.
100 volts.
Shunted and taped
No.6 AWG.


33. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
No.6 AWG.
3 -979 Volts.
Zero amps.
480 volts.


34. 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?
Hyperventilation.
800 volts.
Lower current and higher voltage.
5:1


35. A transformer with a 1:1 ratio is usually a (an)...
Squirrel-cage induction motor.
Isolation transformer.
H terminals.
100 turns.


36. What should the duty rating of a grounding resistor be?
The sum of the two capacitors.
Continuous.
75 turns.
Kilovolt ampere (KVA).


37. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
No.6 AWG.
The chest should rise or inflate.
Reverse its rotation.
A.C.


38. Define alternating current...
Reverse any two-phase leads.
At least two phases
Current changing constantly in magnitude and periodically in direction.
Zero amps.


39. 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.
Squirrel-cage induction motor.
5:1
An electrical shock hazard may exist.
75 turns.


40. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
No.6 AWG.
Grounded phase and ground fault devices were not operating.
5:1


41. Compared to the secondary of a loaded - step-down transformer - the primary has...
Squirrel-cage rotor.
Lower current and higher voltage.
5:1.
Isolation transformer.


42. 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.
Draw less current.
X.
Kilovolt ampere (KVA).


43. How do you change the direction of a 3-phase AC motor?
150 turns.
Reverse any two-phase leads.
Disconnect the frame-grounding conductor.
75 turns.


44. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Grounded phase and ground fault devices were not operating.
Inductance.
Hyperventilation.


45. Where should grounding resistors be located?
60 cycles per second (60 Hertz).
277 volts.
At the source transformer.
Transformer


46. The size of a transformer is measured in...
12 to 15 times a minute.
A grounding resistor.
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.


47. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
At the source transformer.
Full-load - three-phase voltage.
Two speeds
They should be made by the same manufacturer.


48. A wound-rotor induction motor has all the following EXCEPT...
Capacitance.
Squirrel-cage rotor.
Cut around clothing that has adhered to the burned area.
Kilovolt ampere (KVA).


49. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
150 turns.
A step-up transformer.
7 -630 volts.
It provides good speed control.


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...
Draw less current.
Derive a neutral for grounding purposes.
At the source transformer.
Electro-magnetism for their operation.