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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. 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...
Draw less current.
Alternating current.
100 volts.
5:1.

2. Which of the following does a squirrel cage induction motor have?
Stator windings.
No.6 AWG.
Lower current and higher voltage.
Capacitance.

3. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
They must be installed at least 7 feet above the ground.
Reverse any two-phase leads.
1 amp.
Two speeds

4. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
5:1
Nearly constant.
Disconnect the frame-grounding conductor.
Short-circuited after power has been removed.

5. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
100 volts.
The sum of the two capacitors.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
5:1.

6. 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...
150 turns.
60 Hertz
Draw less current.
It provides good speed control.

7. 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...
Draw less current.
Inductance.
The sum of the two capacitors.
75 turns.

8. High voltage is used to transmit power over long distances...
Because less current flowing at high voltage causes less line loss per kilowatt.
800 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Alternating current.

9. 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.
Because less current flowing at high voltage causes less line loss per kilowatt.
100 volts.
Current changing constantly in magnitude and periodically in direction.
5:1

10. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
The motor rotation is reversed.
Zero amps.
25 feet.
Cut around clothing that has adhered to the burned area.

11. To protect against over-current in a three-phase circuit over-current protection must be provided for...
60 Hertz
Transformer
277 volts.
At least two phases

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?
600 volts.
60 Hertz
Zero amps.
Electro-magnetism for their operation.

13. What is the danger when working around capacitors?
2000 watts
480 volts.
An electrical shock hazard may exist.
Provides single-phase power to smaller loads such as lights - heaters - etc.

14. The transformer used only to measure voltage is a...
H terminals.
Check to make sure that the victim is not still in contact with the electricity.
Potential transformer.
Squirrel-cage induction motor.

15. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
X.
Transformers.
Zero amps.
Current changing constantly in magnitude and periodically in direction.

16. Compared to the secondary of a loaded - step-down transformer - the primary has...
No.6 AWG.
Full-load - three-phase voltage.
Lower current and higher voltage.
Continuous.

17. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
60 Hertz
They must be installed at least 7 feet above the ground.
They should be made by the same manufacturer.

18. The property of an electric circuit that opposes any change in current is called...
Reverse its rotation.
Inductance.
The current will read zero.
A.C.

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

20. At what rate should mouth-to-mouth resuscitation be applied?
Reverse any two-phase leads.
7 -630 volts.
12 to 15 times a minute.
A device used to store electrical energy.

21. Alternating current may be defined as...
100 volts.
Four times less than that of the secondary.
Current that changes constantly in magnitude and periodically in direction.
Squirrel-cage rotor.

22. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
No.6 AWG.
Short-circuited after power has been removed.
12 to 15 times a minute.
The current will read zero.

23. What is the first step to be followed in aiding a victim of electrical shock?
Current changing constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
Check to make sure that the victim is not still in contact with the electricity.
It provides good speed control.

24. The primary function of a power transformer is to provide...
A device used to store electrical energy.
At the source transformer.
12 to 15 times a minute.
Full-load - three-phase voltage.

25. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
A step-up transformer.
3 -979 Volts.
At least two phases

26. A wound-rotor induction motor has all the following EXCEPT...
Transformers.
A device used to store electrical energy.
Squirrel-cage rotor.
The current will read zero.

27. The main function of CT's and PT's on a main substation is to...
Zero amps.
Allow for metering voltage and Current.
A grounding resistor.
Squirrel-cage induction motor.

28. Current that changes periodically in direction is...
Squirrel-cage induction motor.
Alternating current.
Squirrel-cage rotor.
Electro-magnetism for their operation.

29. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
277 volts.
A grounding resistor.
The chest should rise or inflate.
Carotid pulse - which is located in the neck next to the Adam's apple.

30. Three-phase AC underground cables must be provided with overcurrent protection using a...
Squirrel-cage rotor.
Kilovolt ampere (KVA).
Circuit Breaker.
Carotid pulse - which is located in the neck next to the Adam's apple.

31. The standard frequency of alternating current used at a mine is...
Squirrel-cage rotor.
Clamp-on ammeter.
3 -979 Volts.
60 Hertz

32. What type motor is a wound-rotor induction motor?
Short-circuited after power has been removed.
200 volts AC.
They must be installed at least 7 feet above the ground.
A.C.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
An electromagnet.
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage
7 -630 volts.

34. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
3 -979 Volts.
100 volts.

35. 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?
H terminals.
100 turns.
The sum of the two capacitors.
They should be made by the same manufacturer.

36. 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.
A grounding resistor.
1 amp.
A.C.

37. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Shunted and taped
1 amp.
Check to make sure that the victim is not still in contact with the electricity.

38. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
25 feet.
Tied together
12 to 15 times a minute.
75 turns.

39. The secondary windings on a 10:1 step-down transformer will have a wire size...
X.
The capacitors should be discharged.
It provides good speed control.
Larger than that found in the primary winding.

40. 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...
A grounding resistor.
Full-load - three-phase voltage.
Tied together
Transformers.

41. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Continuous.
The motor rotation is reversed.
600 volts.

42. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Transformer
Derive a neutral for grounding purposes.
Two speeds
The chest should rise or inflate.

43. Where should grounding resistors be located?
At the source transformer.
75 turns.
Four times less than that of the secondary.
Lower current and higher voltage

44. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
12 to 15 times a minute.
Reverse any two-phase leads.
Inductance.
The current will read zero.

45. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
800 volts.
Current that changes constantly in magnitude and periodically in direction.
Disconnect the frame-grounding conductor.

46. When not connected to a load - the secondary leads of a current transformer should be...
Second-degree burn.
Shunted and taped
Tied together
Larger than that found in the primary winding.

47. 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.
Isolation transformer.
75 turns.
7 -630 volts.
5:1.

48. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Squirrel-cage rotor.
Disconnect the frame-grounding conductor.
They should be made by the same manufacturer.
A.C.

49. Current in an AC circuit may be measured by...
X.
Clamp-on ammeter.
Nearly constant.
60 Hertz

50. 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?
Reverse its rotation.
5:1.
150 turns.
Allow for metering voltage and Current.