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 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.
Disconnect the frame-grounding conductor.
60 cycles per second (60 Hertz).
Circuit Breaker.

2. Not true about AC power?
Current changing constantly in magnitude and periodically in direction.
A grounding resistor.
60 Hertz
It provides good speed control.

3. Current transformers - or CT's - are used to...
Transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The sum of the two capacitors.
Tied together

4. Compared to the secondary of a loaded - step-down transformer - the primary has...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Lower current and higher voltage
A.C.
1 amp.

5. 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.
It provides good speed control.
Kilovolt ampere (KVA).

6. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
75 turns.
75 turns.
It provides good speed control.

7. The primary function of a power transformer is to provide...
Full-load - three-phase voltage.
Reverse any two-phase leads.
A device used to store electrical energy.
5:1

8. The standard frequency of power supplied by the power company is...
200 volts AC.
Hyperventilation.
60 cycles per second (60 Hertz).
5:1.

9. The standard frequency of alternating current used at a mine is...
800 volts.
60 Hertz
100 turns.
Inductance.

10. 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.
Capacitance.
The chest should rise or inflate.
7 -630 volts.

11. Current in an AC circuit may be measured by...
100 volts.
Clamp-on ammeter.
Capacitance.
Full-load - three-phase voltage.

12. AC voltage is raised or lowered by means of...
The current will read zero.
It provides good speed control.
Grounded phase and ground fault devices were not operating.
Transformers.

13. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Inductance.
The chest should rise or inflate.
15 feet off the ground.
Current that changes constantly in magnitude and periodically in direction.

14. 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.
277 volts.
The motor rotation is reversed.
5:1
Nearly constant.

15. The secondary windings on a 10:1 step-down transformer will have a wire size...
Allow for metering voltage and Current.
Larger than that found in the primary winding.
Reverse its rotation.
Check to make sure that the victim is not still in contact with the electricity.

16. 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...
An electrical shock hazard may exist.
An electromagnet.
100 volts.
Lower current and higher voltage

17. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Inductance.
Nearly constant.
The chest should rise or inflate.
A grounding resistor.

18. Define alternating current...
Full-load - three-phase voltage.
Current changing constantly in magnitude and periodically in direction.
Two speeds
Current that changes constantly in magnitude and periodically in direction.

19. The size of a transformer is measured in...
60 cycles per second (60 Hertz).
An electromagnet.
H terminals.
Kilovolt ampere (KVA).

20. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
60 cycles per second (60 Hertz).
The sum of the two capacitors.
800 volts.
1 amp.

21. 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.
Kilovolt ampere (KVA).
Zero amps.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

22. Which of the following does a squirrel cage induction motor have?
Transformers.
Current that changes constantly in magnitude and periodically in direction.
Reverse its rotation.
Stator windings.

23. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Inductance.
The motor rotation is reversed.
75 turns.
600 volts.

24. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
150 turns.
No.6 AWG.
480 volts.
The motor rotation is reversed.

25. 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.
Disconnect the frame-grounding conductor.
100 volts.
Draw less current.

26. 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?
Capacitance.
Isolation transformer.
Reverse any two-phase leads.
3 -979 Volts.

27. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Larger than that found in the primary winding.
75 turns.
Nearly constant.

28. The combined capacitance of two capacitors in parallel is...
Grounded phase and ground fault devices were not operating.
Isolation transformer.
The sum of the two capacitors.
Lower current and higher voltage.

29. What is the first step to be followed in aiding a victim of electrical shock?
1 amp.
Alternating current.
75 turns.
Check to make sure that the victim is not still in contact with the electricity.

30. Current that changes periodically in direction is...
Current that changes constantly in magnitude and periodically in direction.
Alternating current.
Lower current and higher voltage.
Second-degree burn.

31. How should clothing be removed from a burn?
Circuit Breaker.
Continuous.
Capacitance.
Cut around clothing that has adhered to the burned area.

32. The property of an electrical circuit that opposes any change in voltage is called...
Electro-magnetism for their operation.
3 -979 Volts.
Capacitance.
An electrical shock hazard may exist.

33. 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?
They should be made by the same manufacturer.
200 volts AC.
150 turns.
60 Hertz

34. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
800 volts.
A step-up transformer.
A device used to store electrical energy.
100 volts.

35. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
480 volts.
150 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
277 volts.

36. What type motor is a wound-rotor induction motor?
A.C.
They must be installed at least 7 feet above the ground.
Second-degree burn.
Lower current and higher voltage.

37. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
600 volts.
Clamp-on ammeter.
Grounded phase and ground fault devices were not operating.
Zero amps.

38. 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.
Stator windings.
No.6 AWG.
Current that changes constantly in magnitude and periodically in direction.

39. A wound-rotor induction motor has all the following EXCEPT...
The chest should rise or inflate.
2000 watts
200 volts AC.
Squirrel-cage rotor.

40. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
Larger than that found in the primary winding.
A device used to store electrical energy.
Because less current flowing at high voltage causes less line loss per kilowatt.

41. A solenoid coil is...
An electromagnet.
They must be installed at least 7 feet above the ground.
Hyperventilation.
X.

42. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Squirrel-cage induction motor.
Short-circuited after power has been removed.
Allow for metering voltage and Current.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

43. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Draw less current.
The chest should rise or inflate.
Disconnect the frame-grounding conductor.
Inductance.

44. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Squirrel-cage rotor.
480 volts.
Reverse its rotation.
Draw less current.

45. How do you change the direction of a 3-phase AC motor?
Lower current and higher voltage.
A grounding resistor.
Because less current flowing at high voltage causes less line loss per kilowatt.
Reverse any two-phase leads.

46. 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...
Continuous.
They should be made by the same manufacturer.
Tied together
Inductance.

47. The transformer used only to measure voltage is a...
Transformers.
Four times less than that of the secondary.
Potential transformer.
75 turns.

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

49. The speed of an alternating current - squirrel-cage induction motor is...
Current that changes constantly in magnitude and periodically in direction.
1 amp.
Nearly constant.
The sum of the two capacitors.

50. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
The motor rotation is reversed.
25 feet.
Continuous.
Larger than that found in the primary winding.