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. Define alternating current...
The chest should rise or inflate.
Current changing constantly in magnitude and periodically in direction.
A grounding resistor.
They must be installed at least 7 feet above the ground.


2. AC voltage is raised or lowered by means of...
Inductance.
The chest should rise or inflate.
5:1.
Transformers.


3. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
The current will read zero.
Current that changes constantly in magnitude and periodically in direction.
Draw less current.
Zero amps.


4. What should the duty rating of a grounding resistor be?
Two speeds
Continuous.
12 to 15 times a minute.
Clamp-on ammeter.


5. Which of the following does a squirrel cage induction motor have?
Stator windings.
Carotid pulse - which is located in the neck next to the Adam's apple.
600 volts.
They should be made by the same manufacturer.


6. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
Derive a neutral for grounding purposes.
Tied together
Current changing constantly in magnitude and periodically in direction.


7. To protect against over-current in a three-phase circuit over-current protection must be provided for...
A device used to store electrical energy.
The current will read zero.
The sum of the two capacitors.
At least two phases


8. A transformer with a 1:1 ratio is usually a (an)...
Second-degree burn.
Two speeds
Shunted and taped
Isolation transformer.


9. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Hyperventilation.
The motor rotation is reversed.
75 turns.


10. What steps up or steps down AC voltage?
5:1.
15 feet off the ground.
Transformer
Transformers.


11. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Carotid pulse - which is located in the neck next to the Adam's apple.
Alternating current.
The sum of the two capacitors.


12. 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.
Kilovolt ampere (KVA).
5:1.
Grounded phase and ground fault devices were not operating.
Check to make sure that the victim is not still in contact with the electricity.


13. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
Current that changes constantly in magnitude and periodically in direction.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Tied together


14. A solenoid coil is...
Current changing constantly in magnitude and periodically in direction.
A step-up transformer.
An electromagnet.
Derive a neutral for grounding purposes.


15. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
100 volts.
75 turns.
Two speeds
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.


16. If any two-phase wires are reversed on a three-phase motor - the motor will...
No.6 AWG.
X.
Reverse its rotation.
Four times less than that of the secondary.


17. 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?
Current that changes constantly in magnitude and periodically in direction.
75 turns.
1 amp.
Continuous.


18. 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...
100 turns.
5:1
Reverse any two-phase leads.
Draw less current.


19. A control power transformer...
12 to 15 times a minute.
Provides single-phase power to smaller loads such as lights - heaters - etc.
800 volts.
277 volts.


20. 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?
It provides good speed control.
3 -979 Volts.
The sum of the two capacitors.
Squirrel-cage induction motor.


21. How should clothing be removed from a burn?
A grounding resistor.
Current that changes constantly in magnitude and periodically in direction.
1 amp.
Cut around clothing that has adhered to the burned area.


22. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
3 -979 Volts.
15 feet off the ground.
Alternating current.


23. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Check to make sure that the victim is not still in contact with the electricity.
No.6 AWG.
25 feet.
7 -630 volts.


24. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Inductance.
480 volts.
Check to make sure that the victim is not still in contact with the electricity.
Carotid pulse - which is located in the neck next to the Adam's apple.


25. 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.
Current that changes constantly in magnitude and periodically in direction.
A step-up transformer.
Squirrel-cage rotor.


26. Not true about AC power?
They should be made by the same manufacturer.
Draw less current.
480 volts.
It provides good speed control.


27. A burn causing a blister is classified as a...
Lower current and higher voltage
Current that changes constantly in magnitude and periodically in direction.
Second-degree burn.
No.6 AWG.


28. The secondary windings on a 10:1 step-down transformer will have a wire size...
A step-up transformer.
Larger than that found in the primary winding.
15 feet off the ground.
Circuit Breaker.


29. The size of a transformer is measured in...
The capacitors should be discharged.
A grounding resistor.
Continuous.
Kilovolt ampere (KVA).


30. What is the first step to be followed in aiding a victim of electrical shock?
The capacitors should be discharged.
3 -979 Volts.
It provides good speed control.
Check to make sure that the victim is not still in contact with the electricity.


31. What is a capacitor?
7 -630 volts.
Larger than that found in the primary winding.
Grounded phase and ground fault devices were not operating.
A device used to store electrical energy.


32. 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?
Grounded phase and ground fault devices were not operating.
600 volts.
At the source transformer.
Full-load - three-phase voltage.


33. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
100 volts.
5:1
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electromagnet.


34. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Circuit Breaker.
277 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.
A device used to store electrical energy.


35. The primary function of a power transformer is to provide...
Lower current and higher voltage
Kilovolt ampere (KVA).
The motor rotation is reversed.
Full-load - three-phase voltage.


36. 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?
Kilovolt ampere (KVA).
Current changing constantly in magnitude and periodically in direction.
200 volts AC.
Derive a neutral for grounding purposes.


37. What is the danger when working around capacitors?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Draw less current.
An electrical shock hazard may exist.
2000 watts


38. Where should grounding resistors be located?
The chest should rise or inflate.
At the source transformer.
12 to 15 times a minute.
Derive a neutral for grounding purposes.


39. 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?
75 turns.
Alternating current.
25 feet.
Because less current flowing at high voltage causes less line loss per kilowatt.


40. 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.
The capacitors should be discharged.
Nearly constant.
7 -630 volts.
5:1


41. Current transformers - or CT's - are used to...
277 volts.
The capacitors should be discharged.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Inductance.


42. 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...
Draw less current.
75 turns.
The chest should rise or inflate.
200 volts AC.


43. What type of 3-phase AC motor is used most often around a mine?
H terminals.
Potential transformer.
600 volts.
Squirrel-cage induction motor.


44. The property of an electric circuit that opposes any change in current is called...
The motor rotation is reversed.
Second-degree burn.
Inductance.
Provides single-phase power to smaller loads such as lights - heaters - etc.


45. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
The capacitors should be discharged.
2000 watts
Inductance.
At the source transformer.


46. 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.
Stator windings.
Capacitance.
Potential transformer.


47. Not an acceptable method for the installations of surface transformers?
Disconnect the frame-grounding conductor.
Draw less current.
The chest should rise or inflate.
They must be installed at least 7 feet above the ground.


48. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Tied together
X.
No.6 AWG.
Reverse any two-phase leads.


49. The standard frequency of alternating current used at a mine is...
Continuous.
Transformer
Nearly constant.
60 Hertz


50. 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...
60 Hertz
100 volts.
480 volts.
Stator windings.