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. The purpose of a zig-zag transformer is to...
They should be made by the same manufacturer.
25 feet.
Full-load - three-phase voltage.
Derive a neutral for grounding purposes.

2. The secondary windings on a 10:1 step-down transformer will have a wire size...
Circuit Breaker.
The motor rotation is reversed.
Larger than that found in the primary winding.
Kilovolt ampere (KVA).

3. Operating coils - motors - transformers - relays and solenoids depend upon...
Lower current and higher voltage.
Electro-magnetism for their operation.
25 feet.
Alternating current.

4. 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.
Reverse any two-phase leads.
600 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.

5. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Reverse its rotation.
At least two phases
Grounded phase and ground fault devices were not operating.

6. The primary function of a power transformer is to provide...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The sum of the two capacitors.
Full-load - three-phase voltage.
2000 watts

7. 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.
Potential transformer.
15 feet off the ground.
It provides good speed control.

8. The standard frequency of power supplied by the power company is...
Alternating current.
60 cycles per second (60 Hertz).
Inductance.
They must be installed at least 7 feet above the ground.

9. 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.
Stator windings.
Cut around clothing that has adhered to the burned area.
At the source transformer.

10. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
They should be made by the same manufacturer.
2000 watts
Potential transformer.
Inductance.

11. 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...
75 turns.
Draw less current.
Because less current flowing at high voltage causes less line loss per kilowatt.
2000 watts

12. 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?
The capacitors should be discharged.
The chest should rise or inflate.
Grounded phase and ground fault devices were not operating.
600 volts.

13. A control power transformer...
Derive a neutral for grounding purposes.
5:1
60 Hertz
Provides single-phase power to smaller loads such as lights - heaters - etc.

14. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Stator windings.
Disconnect the frame-grounding conductor.
5:1.
Continuous.

15. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
A grounding resistor.
Second-degree burn.
X.
15 feet off the ground.

16. AC voltage is raised or lowered by means of...
25 feet.
Squirrel-cage rotor.
Clamp-on ammeter.
Transformers.

17. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Short-circuited after power has been removed.
Two speeds
The chest should rise or inflate.
Draw less current.

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

19. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Lower current and higher voltage.
Derive a neutral for grounding purposes.
Cut around clothing that has adhered to the burned area.
At least two phases

20. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Shunted and taped
60 Hertz
At the source transformer.

21. The standard frequency of alternating current used at a mine is...
600 volts.
Cut around clothing that has adhered to the burned area.
Electro-magnetism for their operation.
60 Hertz

22. Which of the following does a squirrel cage induction motor have?
277 volts.
480 volts.
The chest should rise or inflate.
Stator windings.

23. 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?
480 volts.
Current changing constantly in magnitude and periodically in direction.
Electro-magnetism for their operation.
800 volts.

24. A wound-rotor induction motor has all the following EXCEPT...
The chest should rise or inflate.
The current will read zero.
480 volts.
Squirrel-cage rotor.

25. A transformer with a 1:1 ratio is usually a (an)...
12 to 15 times a minute.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Isolation transformer.
Reverse its rotation.

26. What steps up or steps down AC voltage?
Squirrel-cage rotor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Transformer
Allow for metering voltage and Current.

27. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
Lower current and higher voltage
5:1.
Draw less current.

28. All transformers step voltage up or down by...
Current that changes constantly in magnitude and periodically in direction.
Inductance.
An electromagnet.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

29. What is the first step to be followed in aiding a victim of electrical shock?
Continuous.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Alternating current.
Check to make sure that the victim is not still in contact with the electricity.

30. 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.
A grounding resistor.
Full-load - three-phase voltage.
At least two phases

31. 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.
Provides single-phase power to smaller loads such as lights - heaters - etc.
5:1
Tied together
The sum of the two capacitors.

32. Current transformers - or CT's - are used to...
Lower current and higher voltage
Hyperventilation.
Reverse any two-phase leads.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

33. 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.
A.C.
Draw less current.
A device used to store electrical energy.

34. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Allow for metering voltage and Current.
75 turns.
Four times less than that of the secondary.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

35. The property of an electric circuit that opposes any change in current is called...
At least two phases
The motor rotation is reversed.
Inductance.
Nearly constant.

36. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Continuous.
Reverse its rotation.
Zero amps.
25 feet.

37. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Check to make sure that the victim is not still in contact with the electricity.
600 volts.
Clamp-on ammeter.
Two speeds

38. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Inductance.
277 volts.
Nearly constant.

39. Compared to the secondary of a loaded - step-down transformer - the primary has...
X.
Nearly constant.
Lower current and higher voltage.
600 volts.

40. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Clamp-on ammeter.
1 amp.
Transformers.
They should be made by the same manufacturer.

41. The combined capacitance of two capacitors in parallel is...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
An electrical shock hazard may exist.
Current changing constantly in magnitude and periodically in direction.
The sum of the two capacitors.

42. The primary leads of a single-phase transformer should be connected to the...
Zero amps.
H terminals.
7 -630 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.

43. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Kilovolt ampere (KVA).
Transformers.
They should be made by the same manufacturer.
The motor rotation is reversed.

44. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Reverse any two-phase leads.
The current will read zero.
Tied together

45. 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?
75 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1.
Squirrel-cage induction motor.

46. A transformer with 3000 turns in the primary winding has a primary voltage of 4 -800 volts and a secondary voltage of 240 volts. How many turns are in the secondary winding?
The current will read zero.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Disconnect the frame-grounding conductor.
150 turns.

47. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Potential transformer.
100 turns.
Nearly constant.
7 -630 volts.

48. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
200 volts AC.
Two speeds

49. The size of a transformer is measured in...
Current changing constantly in magnitude and periodically in direction.
60 Hertz
Kilovolt ampere (KVA).
Reverse any two-phase leads.

50. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
The capacitors should be discharged.
Short-circuited after power has been removed.
Potential transformer.