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. 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?
The current will read zero.
5:1
Second-degree burn.
75 turns.


2. Current that changes periodically in direction is...
100 turns.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Nearly constant.
Alternating current.


3. High voltage is used to transmit power over long distances...
Continuous.
They should be made by the same manufacturer.
An electrical shock hazard may exist.
Because less current flowing at high voltage causes less line loss per kilowatt.


4. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Full-load - three-phase voltage.
100 volts.
Short-circuited after power has been removed.
X.


5. The transformer used only to measure voltage is a...
Potential transformer.
5:1
7 -630 volts.
Nearly constant.


6. A solenoid coil is...
Lower current and higher voltage
Full-load - three-phase voltage.
12 to 15 times a minute.
An electromagnet.


7. How should clothing be removed from a burn?
Cut around clothing that has adhered to the burned area.
The current will read zero.
12 to 15 times a minute.
A device used to store electrical energy.


8. 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...
Hyperventilation.
Zero amps.
It provides good speed control.
100 volts.


9. 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?
Reverse any two-phase leads.
3 -979 Volts.
Shunted and taped
Reverse its rotation.


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


11. The property of an electrical circuit that opposes any change in voltage is called...
A device used to store electrical energy.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Capacitance.
800 volts.


12. The combined capacitance of two capacitors in parallel is...
Grounded phase and ground fault devices were not operating.
Four times less than that of the secondary.
The sum of the two capacitors.
Because less current flowing at high voltage causes less line loss per kilowatt.


13. What should the duty rating of a grounding resistor be?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Continuous.
Potential transformer.
Isolation transformer.


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.
5:1.
60 cycles per second (60 Hertz).
H terminals.
Carotid pulse - which is located in the neck next to the Adam's apple.


15. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Draw less current.
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
X.


16. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
They must be installed at least 7 feet above the ground.
Zero amps.
2000 watts
Disconnect the frame-grounding conductor.


17. All transformers step voltage up or down by...
Second-degree burn.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Tied together
Inductance.


18. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
5:1.
The capacitors should be discharged.
1 amp.
15 feet off the ground.


19. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Draw less current.
The motor rotation is reversed.


20. A burn causing a blister is classified as a...
Second-degree burn.
Reverse its rotation.
Kilovolt ampere (KVA).
Derive a neutral for grounding purposes.


21. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
800 volts.
Alternating current.
Four times less than that of the secondary.


22. Not a symptom of electrical shock?.
Hyperventilation.
Continuous.
X.
Four times less than that of the secondary.


23. AC voltage is raised or lowered by means of...
75 turns.
Larger than that found in the primary winding.
X.
Transformers.


24. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Full-load - three-phase voltage.
At least two phases
25 feet.
Short-circuited after power has been removed.


25. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Short-circuited after power has been removed.
Kilovolt ampere (KVA).
150 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.


26. The size of a transformer is measured in...
The sum of the two capacitors.
They must be installed at least 7 feet above the ground.
A grounding resistor.
Kilovolt ampere (KVA).


27. What is the danger when working around capacitors?
At the source transformer.
Current that changes constantly in magnitude and periodically in direction.
An electrical shock hazard may exist.
15 feet off the ground.


28. When not connected to a load - the secondary leads of a current transformer should be...
Isolation transformer.
Shunted and taped
150 turns.
15 feet off the ground.


29. Not true about AC power?
Hyperventilation.
Reverse its rotation.
The sum of the two capacitors.
It provides good speed control.


30. What steps up or steps down AC voltage?
5:1
Transformer
Potential transformer.
A step-up transformer.


31. 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?
5:1
480 volts.
75 turns.
Transformer


32. Current in an AC circuit may be measured by...
Clamp-on ammeter.
They must be installed at least 7 feet above the ground.
The sum of the two capacitors.
Current changing constantly in magnitude and periodically in direction.


33. 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...
800 volts.
Tied together
Clamp-on ammeter.
Two speeds


34. A wound-rotor induction motor has all the following EXCEPT...
25 feet.
Squirrel-cage rotor.
Capacitance.
Second-degree burn.


35. Current transformers - or CT's - are used to...
800 volts.
5:1
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
An electromagnet.


36. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
Inductance.
100 turns.
Check to make sure that the victim is not still in contact with the electricity.


37. The standard frequency of power supplied by the power company is...
Capacitance.
Allow for metering voltage and Current.
7 -630 volts.
60 cycles per second (60 Hertz).


38. 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
12 to 15 times a minute.
1 amp.
Electro-magnetism for their operation.


39. Define alternating current...
A device used to store electrical energy.
Reverse its rotation.
Larger than that found in the primary winding.
Current changing constantly in magnitude and periodically in direction.


40. 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?
Inductance.
Draw less current.
600 volts.
An electrical shock hazard may exist.


41. Alternating current may be defined as...
25 feet.
Reverse any two-phase leads.
Current that changes constantly in magnitude and periodically in direction.
Isolation transformer.


42. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
Reverse any two-phase leads.
Kilovolt ampere (KVA).
They should be made by the same manufacturer.


43. Where should grounding resistors be located?
The chest should rise or inflate.
At the source transformer.
Hyperventilation.
15 feet off the ground.


44. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Continuous.
Nearly constant.
Four times less than that of the secondary.


45. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
They must be installed at least 7 feet above the ground.
7 -630 volts.
Circuit Breaker.
The capacitors should be discharged.


46. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
No.6 AWG.
At the source transformer.
A grounding resistor.
The current will read zero.


47. 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?
Short-circuited after power has been removed.
Squirrel-cage induction motor.
200 volts AC.
Allow for metering voltage and Current.


48. What type of 3-phase AC motor is used most often around a mine?
A step-up transformer.
Kilovolt ampere (KVA).
Capacitance.
Squirrel-cage induction motor.


49. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Draw less current.
At least two phases
1 amp.
At the source transformer.


50. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
2000 watts
The sum of the two capacitors.
Derive a neutral for grounding purposes.
3 -979 Volts.