Test your basic knowledge |

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 current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Electro-magnetism for their operation.
A grounding resistor.
1 amp.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

2. What type motor is a wound-rotor induction motor?
An electromagnet.
A.C.
600 volts.
The current will read zero.

3. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
800 volts.
Grounded phase and ground fault devices were not operating.
Inductance.
150 turns.

4. The primary function of a power transformer is to provide...
15 feet off the ground.
Lower current and higher voltage
Full-load - three-phase voltage.
Second-degree burn.

5. 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
An electromagnet.
25 feet.
Larger than that found in the primary winding.

6. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
Reverse its rotation.
Disconnect the frame-grounding conductor.
Alternating current.

7. Alternating current may be defined as...
Lower current and higher voltage
Current that changes constantly in magnitude and periodically in direction.
At least two phases
No.6 AWG.

8. 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.
3 -979 Volts.
75 turns.
25 feet.

9. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
25 feet.
Full-load - three-phase voltage.
Capacitance.
277 volts.

10. 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.
Cut around clothing that has adhered to the burned area.
The sum of the two capacitors.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
5:1

11. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
They should be made by the same manufacturer.
12 to 15 times a minute.
Current changing constantly in magnitude and periodically in direction.
X.

12. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Derive a neutral for grounding purposes.
Cut around clothing that has adhered to the burned area.
Circuit Breaker.
A step-up transformer.

13. 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?
Transformer
Allow for metering voltage and Current.
No.6 AWG.
75 turns.

14. 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?
Four times less than that of the secondary.
600 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Grounded phase and ground fault devices were not operating.

15. What type of 3-phase AC motor is used most often around a mine?
Draw less current.
Squirrel-cage induction motor.
60 cycles per second (60 Hertz).
Full-load - three-phase voltage.

16. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
277 volts.
100 turns.
Inductance.

17. 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?
60 Hertz
600 volts.
60 cycles per second (60 Hertz).
200 volts AC.

18. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Current changing constantly in magnitude and periodically in direction.
Disconnect the frame-grounding conductor.
Two speeds
A grounding resistor.

19. 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...
277 volts.
Disconnect the frame-grounding conductor.
100 volts.
150 turns.

20. If any two-phase wires are reversed on a three-phase motor - the motor will...
Carotid pulse - which is located in the neck next to the Adam's apple.
100 turns.
Reverse its rotation.
Current changing constantly in magnitude and periodically in direction.

21. Not a symptom of electrical shock?.
X.
Hyperventilation.
7 -630 volts.
Disconnect the frame-grounding conductor.

22. 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.
12 to 15 times a minute.
Derive a neutral for grounding purposes.
It provides good speed control.

23. The transformer used only to measure voltage is a...
Potential transformer.
3 -979 Volts.
It provides good speed control.
An electromagnet.

24. When not connected to a load - the secondary leads of a current transformer should be...
60 Hertz
Zero amps.
The sum of the two capacitors.
Shunted and taped

25. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
A.C.
Nearly constant.
A device used to store electrical energy.
480 volts.

26. The most reliable method to use to determine whether or not the heart has stopped is by checking the...

27. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Four times less than that of the secondary.
Lower current and higher voltage
The chest should rise or inflate.
Stator windings.

28. 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?
800 volts.
Alternating current.
It provides good speed control.
75 turns.

29. Current that changes periodically in direction is...
Inductance.
Alternating current.
A step-up transformer.
600 volts.

30. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Stator windings.
At the source transformer.
X.
Derive a neutral for grounding purposes.

31. All transformers step voltage up or down by...
2000 watts
Inductance.
Continuous.
A device used to store electrical energy.

32. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Inductance.
Lower current and higher voltage
100 volts.
7 -630 volts.

33. The primary leads of a single-phase transformer should be connected to the...
The sum of the two capacitors.
A.C.
1 amp.
H terminals.

34. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Current that changes constantly in magnitude and periodically in direction.
Tied together
The capacitors should be discharged.
Reverse its rotation.

35. Which of the following does a squirrel cage induction motor have?
The motor rotation is reversed.
Check to make sure that the victim is not still in contact with the electricity.
Derive a neutral for grounding purposes.
Stator windings.

36. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Current changing constantly in magnitude and periodically in direction.
The current will read zero.
Transformers.
Carotid pulse - which is located in the neck next to the Adam's apple.

37. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
60 Hertz
Current changing constantly in magnitude and periodically in direction.
Draw less current.
15 feet off the ground.

38. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Draw less current.
A device used to store electrical energy.
The motor rotation is reversed.
Short-circuited after power has been removed.

39. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Capacitance.
Clamp-on ammeter.
Zero amps.
Draw less current.

40. 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...
No.6 AWG.
Lower current and higher voltage.
Because less current flowing at high voltage causes less line loss per kilowatt.
Draw less current.

41. Current transformers - or CT's - are used to...
5:1.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.
15 feet off the ground.

42. What is the first step to be followed in aiding a victim of electrical shock?
12 to 15 times a minute.
It provides good speed control.
Check to make sure that the victim is not still in contact with the electricity.
Nearly constant.

43. 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.
5:1
150 turns.
100 turns.

44. 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?
Check to make sure that the victim is not still in contact with the electricity.
75 turns.
Circuit Breaker.
Disconnect the frame-grounding conductor.

45. How do you change the direction of a 3-phase AC motor?
Squirrel-cage induction motor.
The capacitors should be discharged.
Reverse any two-phase leads.
At the source transformer.

46. A control power transformer...
Circuit Breaker.
Transformers.
Provides single-phase power to smaller loads such as lights - heaters - etc.
No.6 AWG.

47. The standard frequency of alternating current used at a mine is...
75 turns.
An electromagnet.
60 Hertz
Shunted and taped

48. A transformer with a 1:1 ratio is usually a (an)...
A device used to store electrical energy.
Isolation transformer.
Current that changes constantly in magnitude and periodically in direction.
No.6 AWG.

49. What should the duty rating of a grounding resistor be?
Clamp-on ammeter.
Full-load - three-phase voltage.
At least two phases
Continuous.

50. Operating coils - motors - transformers - relays and solenoids depend upon...
Inductance.
Electro-magnetism for their operation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1