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. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
200 volts AC.
They must be installed at least 7 feet above the ground.
2000 watts
Current changing constantly in magnitude and periodically in direction.

2. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Reverse its rotation.
75 turns.
The capacitors should be discharged.
Inductance.

3. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Stator windings.
Squirrel-cage induction motor.
The chest should rise or inflate.

4. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
Isolation transformer.
Zero amps.
Shunted and taped

5. 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).
7 -630 volts.
1 amp.
Circuit Breaker.

6. What type motor is a wound-rotor induction motor?
75 turns.
Transformer
A.C.
A grounding resistor.

7. 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.
An electrical shock hazard may exist.
Lower current and higher voltage.
Lower current and higher voltage
5:1.

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...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Zero amps.
100 volts.
The sum of the two capacitors.

9. 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?
Lower current and higher voltage
200 volts AC.
Current changing constantly in magnitude and periodically in direction.
Shunted and taped

10. Operating coils - motors - transformers - relays and solenoids depend upon...
Short-circuited after power has been removed.
Electro-magnetism for their operation.
Potential transformer.
Reverse any two-phase leads.

11. AC voltage is raised or lowered by means of...
Transformers.
Derive a neutral for grounding purposes.
They must be installed at least 7 feet above the ground.
X.

12. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
The motor rotation is reversed.
Grounded phase and ground fault devices were not operating.
X.
Squirrel-cage rotor.

13. The property of an electric circuit that opposes any change in current is called...
Grounded phase and ground fault devices were not operating.
Inductance.
Continuous.
12 to 15 times a minute.

14. The secondary windings on a 10:1 step-down transformer will have a wire size...
Zero amps.
They must be installed at least 7 feet above the ground.
They should be made by the same manufacturer.
Larger than that found in the primary winding.

15. Define alternating current...
Capacitance.
Current changing constantly in magnitude and periodically in direction.
At the source transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.

16. Not true about AC power?
60 Hertz
It provides good speed control.
480 volts.
Draw less current.

17. High voltage is used to transmit power over long distances...
600 volts.
X.
Because less current flowing at high voltage causes less line loss per kilowatt.
The chest should rise or inflate.

18. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
Full-load - three-phase voltage.
Because less current flowing at high voltage causes less line loss per kilowatt.
Transformers.

19. The primary leads of a single-phase transformer should be connected to the...
5:1.
A grounding resistor.
Capacitance.
H terminals.

20. A burn causing a blister is classified as a...
5:1
Second-degree burn.
150 turns.
600 volts.

21. 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?
600 volts.
Transformers.
200 volts AC.
3 -979 Volts.

22. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
H terminals.
Current that changes constantly in magnitude and periodically in direction.
150 turns.

23. The combined capacitance of two capacitors in parallel is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Electro-magnetism for their operation.
Full-load - three-phase voltage.
The sum of the two capacitors.

24. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Capacitance.
Reverse its rotation.
2000 watts

25. What is a capacitor?
Circuit Breaker.
Provides single-phase power to smaller loads such as lights - heaters - etc.
An electromagnet.
A device used to store electrical energy.

26. The transformer used only to measure voltage is a...
They must be installed at least 7 feet above the ground.
Potential transformer.
Because less current flowing at high voltage causes less line loss per kilowatt.
A step-up transformer.

27. The purpose of a zig-zag transformer is to...
Isolation transformer.
Derive a neutral for grounding purposes.
Tied together
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

28. The primary function of a power transformer is to provide...
100 turns.
Lower current and higher voltage
Full-load - three-phase voltage.
Kilovolt ampere (KVA).

29. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Lower current and higher voltage
Kilovolt ampere (KVA).
Isolation transformer.

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

31. What is the danger when working around capacitors?
An electrical shock hazard may exist.
Potential transformer.
Continuous.
1 amp.

32. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
A grounding resistor.
Squirrel-cage induction motor.
1 amp.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
H terminals.
3 -979 Volts.
150 turns.

34. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
1 amp.
They should be made by the same manufacturer.
No.6 AWG.
7 -630 volts.

35. At what rate should mouth-to-mouth resuscitation be applied?
60 cycles per second (60 Hertz).
12 to 15 times a minute.
X.
Continuous.

36. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Inductance.
Two speeds
Hyperventilation.

37. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
The sum of the two capacitors.
Check to make sure that the victim is not still in contact with the electricity.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Stator windings.

38. 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...
75 turns.
Tied together
25 feet.
Transformers.

39. A solenoid coil is...
Second-degree burn.
Stator windings.
Draw less current.
An electromagnet.

40. 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?
Reverse its rotation.
75 turns.
800 volts.
Grounded phase and ground fault devices were not operating.

41. How do you change the direction of a 3-phase AC motor?
15 feet off the ground.
Reverse any two-phase leads.
5:1.
480 volts.

42. If any two-phase wires are reversed on a three-phase motor - the motor will...
Squirrel-cage induction motor.
Reverse its rotation.
Electro-magnetism for their operation.
60 cycles per second (60 Hertz).

43. The standard frequency of alternating current used at a mine is...
Allow for metering voltage and Current.
75 turns.
60 Hertz
They should be made by the same manufacturer.

44. 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?
Disconnect the frame-grounding conductor.
150 turns.
Draw less current.
H terminals.

45. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Reverse its rotation.
Clamp-on ammeter.
Four times less than that of the secondary.

46. How should clothing be removed from a burn?
An electrical shock hazard may exist.
Cut around clothing that has adhered to the burned area.
150 turns.
60 cycles per second (60 Hertz).

47. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Draw less current.
At the source transformer.
A device used to store electrical energy.
Zero amps.

48. The size of a transformer is measured in...
Stator windings.
A.C.
Potential transformer.
Kilovolt ampere (KVA).

49. 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.
Shunted and taped
Squirrel-cage rotor.
Cut around clothing that has adhered to the burned area.

50. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
The motor rotation is reversed.
15 feet off the ground.
Short-circuited after power has been removed.
Hyperventilation.