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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. To protect against over-current in a three-phase circuit over-current protection must be provided for...
At least two phases
75 turns.
A step-up transformer.
An electromagnet.

2. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
2000 watts
1 amp.
H terminals.
Short-circuited after power has been removed.

3. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Lower current and higher voltage
Capacitance.
The capacitors should be discharged.
A.C.

4. How should clothing be removed from a burn?
7 -630 volts.
Squirrel-cage induction motor.
Draw less current.
Cut around clothing that has adhered to the burned area.

5. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
The sum of the two capacitors.
Transformers.
5:1

6. Not an acceptable method for the installations of surface transformers?
12 to 15 times a minute.
Draw less current.
Because less current flowing at high voltage causes less line loss per kilowatt.
They must be installed at least 7 feet above the ground.

7. At what rate should mouth-to-mouth resuscitation be applied?
An electromagnet.
Alternating current.
12 to 15 times a minute.
The current will read zero.

8. All transformers step voltage up or down by...
5:1.
Alternating current.
Inductance.
Short-circuited after power has been removed.

9. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
At the source transformer.
The chest should rise or inflate.
Four times less than that of the secondary.
800 volts.

10. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
X.
Isolation transformer.
Transformer
Circuit Breaker.

11. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Continuous.
Full-load - three-phase voltage.
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

12. 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?
Derive a neutral for grounding purposes.
Isolation transformer.
Squirrel-cage induction motor.
75 turns.

13. 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...
Nearly constant.
Draw less current.
The motor rotation is reversed.
Zero amps.

14. What is a capacitor?
600 volts.
A device used to store electrical energy.
Circuit Breaker.
25 feet.

15. Alternating current may be defined as...
Lower current and higher voltage.
Current that changes constantly in magnitude and periodically in direction.
Four times less than that of the secondary.
Shunted and taped

16. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Lower current and higher voltage.
At the source transformer.
480 volts.
Inductance.

17. 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.
Continuous.
480 volts.
Inductance.

18. The size of a transformer is measured in...
Kilovolt ampere (KVA).
A grounding resistor.
Clamp-on ammeter.
800 volts.

19. Current in an AC circuit may be measured by...
A device used to store electrical energy.
Clamp-on ammeter.
Lower current and higher voltage
At least two phases

20. Where should grounding resistors be located?
They must be installed at least 7 feet above the ground.
Provides single-phase power to smaller loads such as lights - heaters - etc.
At the source transformer.
A step-up transformer.

21. The secondary windings on a 10:1 step-down transformer will have a wire size...
Check to make sure that the victim is not still in contact with the electricity.
Larger than that found in the primary winding.
Inductance.
150 turns.

22. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
1 amp.
Short-circuited after power has been removed.
They must be installed at least 7 feet above the ground.
600 volts.

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?
800 volts.
Check to make sure that the victim is not still in contact with the electricity.
3 -979 Volts.
Lower current and higher voltage.

24. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Lower current and higher voltage.
Disconnect the frame-grounding conductor.
An electrical shock hazard may exist.
A grounding resistor.

25. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
100 volts.
A grounding resistor.
Hyperventilation.
200 volts AC.

26. 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.
60 Hertz
The motor rotation is reversed.
Continuous.

27. What steps up or steps down AC voltage?
A device used to store electrical energy.
150 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Transformer

28. 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.
7 -630 volts.
Current changing constantly in magnitude and periodically in direction.
3 -979 Volts.

29. What is the danger when working around capacitors?
An electrical shock hazard may exist.
Circuit Breaker.
Check to make sure that the victim is not still in contact with the electricity.
1 amp.

30. Which of the following does a squirrel cage induction motor have?
100 turns.
Stator windings.
Inductance.
Nearly constant.

31. 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?
Transformer
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
600 volts.
It provides good speed control.

32. How do you change the direction of a 3-phase AC motor?
A step-up transformer.
5:1.
Reverse any two-phase leads.
Carotid pulse - which is located in the neck next to the Adam's apple.

33. 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?
No.6 AWG.
3 -979 Volts.
A device used to store electrical energy.
Inductance.

34. A control power transformer...
Provides single-phase power to smaller loads such as lights - heaters - etc.
Capacitance.
60 Hertz
Hyperventilation.

35. 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?
150 turns.
Stator windings.
Squirrel-cage induction motor.
No.6 AWG.

36. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Check to make sure that the victim is not still in contact with the electricity.
Reverse any two-phase leads.
The motor rotation is reversed.
15 feet off the ground.

37. Compared to the secondary of a loaded - step-down transformer - the primary has...
No.6 AWG.
5:1.
The chest should rise or inflate.
Lower current and higher voltage

38. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Clamp-on ammeter.
7 -630 volts.
Derive a neutral for grounding purposes.
15 feet off the ground.

39. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
200 volts AC.
Kilovolt ampere (KVA).
A step-up transformer.

40. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
277 volts.
Inductance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Four times less than that of the secondary.

41. The standard frequency of power supplied by the power company is...
A.C.
60 cycles per second (60 Hertz).
X.
Inductance.

42. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Continuous.
277 volts.
75 turns.
Potential transformer.

43. The main function of CT's and PT's on a main substation is to...
Derive a neutral for grounding purposes.
Larger than that found in the primary winding.
Allow for metering voltage and Current.
Stator windings.

44. A solenoid coil is...
An electromagnet.
A step-up transformer.
H terminals.
15 feet off the ground.

45. What type of 3-phase AC motor is used most often around a mine?
Because less current flowing at high voltage causes less line loss per kilowatt.
100 volts.
Inductance.
Squirrel-cage induction motor.

46. When not connected to a load - the secondary leads of a current transformer should be...
Electro-magnetism for their operation.
Because less current flowing at high voltage causes less line loss per kilowatt.
Shunted and taped
An electromagnet.

47. AC voltage is raised or lowered by means of...
Disconnect the frame-grounding conductor.
Transformers.
Squirrel-cage rotor.
Capacitance.

48. Operating coils - motors - transformers - relays and solenoids depend upon...
At the source transformer.
Short-circuited after power has been removed.
Electro-magnetism for their operation.
The motor rotation is reversed.

49. A burn causing a blister is classified as a...
A device used to store electrical energy.
X.
Second-degree burn.
Clamp-on ammeter.

50. Not true about AC power?
It provides good speed control.
75 turns.
The capacitors should be discharged.
At least two phases