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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. How do you change the direction of a 3-phase AC motor?
Lower current and higher voltage
Reverse any two-phase leads.
Grounded phase and ground fault devices were not operating.
Full-load - three-phase voltage.

2. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
2000 watts
Potential transformer.
1 amp.
600 volts.

3. The main function of CT's and PT's on a main substation is to...
Four times less than that of the secondary.
Allow for metering voltage and Current.
Squirrel-cage rotor.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

4. 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?
100 volts.
800 volts.
Cut around clothing that has adhered to the burned area.
200 volts AC.

5. The transformer used only to measure voltage is a...
An electromagnet.
Because less current flowing at high voltage causes less line loss per kilowatt.
Potential transformer.
A grounding resistor.

6. Not true about AC power?
Allow for metering voltage and Current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
It provides good speed control.
Hyperventilation.

7. 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...
Check to make sure that the victim is not still in contact with the electricity.
277 volts.
480 volts.
100 volts.

8. 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.
A grounding resistor.
60 Hertz
They must be installed at least 7 feet above the ground.

9. The primary leads of a single-phase transformer should be connected to the...
The motor rotation is reversed.
Current changing constantly in magnitude and periodically in direction.
H terminals.
12 to 15 times a minute.

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

11. Which of the following does a squirrel cage induction motor have?
Inductance.
They must be installed at least 7 feet above the ground.
Stator windings.
The sum of the two capacitors.

12. The combined capacitance of two capacitors in parallel is...
The capacitors should be discharged.
Nearly constant.
The sum of the two capacitors.
100 turns.

13. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
H terminals.
Continuous.
The capacitors should be discharged.
3 -979 Volts.

14. Current that changes periodically in direction is...
Alternating current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
H terminals.
5:1.

15. All transformers step voltage up or down by...
Zero amps.
480 volts.
Inductance.
15 feet off the ground.

16. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
At least two phases
7 -630 volts.
Two speeds
They must be installed at least 7 feet above the ground.

17. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
Lower current and higher voltage.
Circuit Breaker.
X.
The current will read zero.

18. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
60 Hertz
60 cycles per second (60 Hertz).
Grounded phase and ground fault devices were not operating.
200 volts AC.

19. The purpose of a zig-zag transformer is to...
Nearly constant.
Derive a neutral for grounding purposes.
2000 watts
Electro-magnetism for their operation.

20. A burn causing a blister is classified as a...
480 volts.
Nearly constant.
800 volts.
Second-degree burn.

21. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
Larger than that found in the primary winding.
480 volts.
They must be installed at least 7 feet above the ground.
5:1.

22. Current transformers - or CT's - are used to...
A grounding resistor.
480 volts.
Allow for metering voltage and Current.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

23. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
25 feet.
X.
Potential transformer.
At the source transformer.

24. 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...
At least two phases
The current will read zero.
Draw less current.
2000 watts

25. Compared to the secondary of a loaded - step-down transformer - the primary has...
7 -630 volts.
Lower current and higher voltage
The capacitors should be discharged.
Two speeds

26. Compared to the secondary of a loaded - step-down transformer - the primary has...
The chest should rise or inflate.
Lower current and higher voltage.
5:1.
They should be made by the same manufacturer.

27. The secondary windings on a 10:1 step-down transformer will have a wire size...
Alternating current.
A grounding resistor.
Larger than that found in the primary winding.
600 volts.

28. Current in an AC circuit may be measured by...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
12 to 15 times a minute.
Clamp-on ammeter.
They must be installed at least 7 feet above the ground.

29. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
600 volts.
Continuous.
Short-circuited after power has been removed.
60 cycles per second (60 Hertz).

30. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Potential transformer.
A step-up transformer.
277 volts.
They should be made by the same manufacturer.

31. Define alternating current...
An electromagnet.
Current changing constantly in magnitude and periodically in direction.
Draw less current.
Kilovolt ampere (KVA).

32. 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.
60 cycles per second (60 Hertz).
5:1.
Because less current flowing at high voltage causes less line loss per kilowatt.
Nearly constant.

33. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Current changing constantly in magnitude and periodically in direction.
Disconnect the frame-grounding conductor.
The capacitors should be discharged.
800 volts.

34. Not a symptom of electrical shock?.
An electromagnet.
Because less current flowing at high voltage causes less line loss per kilowatt.
Hyperventilation.
Isolation transformer.

35. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Inductance.
Larger than that found in the primary winding.
Capacitance.
A grounding resistor.

36. AC voltage is raised or lowered by means of...
They should be made by the same manufacturer.
1 amp.
Transformers.
480 volts.

37. High voltage is used to transmit power over long distances...
75 turns.
Squirrel-cage rotor.
Inductance.
Because less current flowing at high voltage causes less line loss per kilowatt.

38. A control power transformer...
25 feet.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Zero amps.
Kilovolt ampere (KVA).

39. What type motor is a wound-rotor induction motor?
Squirrel-cage induction motor.
Because less current flowing at high voltage causes less line loss per kilowatt.
A.C.
480 volts.

40. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Alternating current.
An electromagnet.
Second-degree burn.

41. 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?
The current will read zero.
Cut around clothing that has adhered to the burned area.
75 turns.
2000 watts

42. 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.
Transformer
Grounded phase and ground fault devices were not operating.
Alternating current.

43. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
5:1
Squirrel-cage rotor.
Zero amps.
100 turns.

44. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Check to make sure that the victim is not still in contact with the electricity.
H terminals.
Clamp-on ammeter.
Four times less than that of the secondary.

45. Not an acceptable method for the installations of surface transformers?
Continuous.
They must be installed at least 7 feet above the ground.
Check to make sure that the victim is not still in contact with the electricity.
Reverse its rotation.

46. At what rate should mouth-to-mouth resuscitation be applied?
A.C.
A step-up transformer.
Reverse its rotation.
12 to 15 times a minute.

47. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
A step-up transformer.
Larger than that found in the primary winding.
1 amp.

48. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Alternating current.
600 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Tied together

49. What type of 3-phase AC motor is used most often around a mine?
150 turns.
75 turns.
The current will read zero.
Squirrel-cage induction motor.

50. 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?
Carotid pulse - which is located in the neck next to the Adam's apple.
Inductance.
3 -979 Volts.
The sum of the two capacitors.