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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. Current transformers - or CT's - are used to...
An electrical shock hazard may exist.
Transformer
Potential transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

2. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Transformers.
5:1
Four times less than that of the secondary.
Disconnect the frame-grounding conductor.

3. The property of an electrical circuit that opposes any change in voltage is called...
25 feet.
The chest should rise or inflate.
Lower current and higher voltage
Capacitance.

4. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Continuous.
No.6 AWG.
Stator windings.
X.

5. 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.
Electro-magnetism for their operation.
An electromagnet.
2000 watts
5:1.

6. The speed of an alternating current - squirrel-cage induction motor is...
150 turns.
600 volts.
Nearly constant.
A step-up transformer.

7. The transformer used only to measure voltage is a...
Continuous.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Potential transformer.
Lower current and higher voltage.

8. What is a capacitor?
Four times less than that of the secondary.
Current changing constantly in magnitude and periodically in direction.
100 turns.
A device used to store electrical energy.

9. Not true about AC power?
Alternating current.
Lower current and higher voltage
Because less current flowing at high voltage causes less line loss per kilowatt.
It provides good speed control.

10. To protect against over-current in a three-phase circuit over-current protection must be provided for...
480 volts.
Stator windings.
Check to make sure that the victim is not still in contact with the electricity.
At least two phases

11. 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?
A device used to store electrical energy.
Second-degree burn.
100 volts.
600 volts.

12. 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...
100 volts.
12 to 15 times a minute.
A device used to store electrical energy.
Current changing constantly in magnitude and periodically in direction.

13. How do you change the direction of a 3-phase AC motor?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
5:1.
Reverse any two-phase leads.
An electromagnet.

14. AC voltage is raised or lowered by means of...
Transformers.
277 volts.
An electrical shock hazard may exist.
Squirrel-cage induction motor.

15. 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.
Inductance.
5:1.
25 feet.
5:1

16. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
75 turns.
A step-up transformer.
No.6 AWG.
277 volts.

17. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
Nearly constant.
60 cycles per second (60 Hertz).
They should be made by the same manufacturer.

18. Compared to the secondary of a loaded - step-down transformer - the primary has...
60 Hertz
Because less current flowing at high voltage causes less line loss per kilowatt.
Lower current and higher voltage.
Lower current and higher voltage

19. 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...
Check to make sure that the victim is not still in contact with the electricity.
Tied together
Electro-magnetism for their operation.
The current will read zero.

20. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Stator windings.
Provides single-phase power to smaller loads such as lights - heaters - etc.
No.6 AWG.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

21. 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?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Two speeds
75 turns.
The current will read zero.

22. 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...
600 volts.
Draw less current.
Inductance.
100 volts.

23. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Lower current and higher voltage
Larger than that found in the primary winding.

24. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
An electromagnet.
Inductance.
The chest should rise or inflate.
Draw less current.

25. The purpose of a zig-zag transformer is to...
600 volts.
Derive a neutral for grounding purposes.
Squirrel-cage induction motor.
25 feet.

26. 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?
Tied together
Allow for metering voltage and Current.
800 volts.
Capacitance.

27. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
60 Hertz
3 -979 Volts.
No.6 AWG.

28. All transformers step voltage up or down by...
5:1
Inductance.
Allow for metering voltage and Current.
480 volts.

29. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Lower current and higher voltage
Short-circuited after power has been removed.
Two speeds
A.C.

30. The standard frequency of alternating current used at a mine is...
Lower current and higher voltage
5:1
60 Hertz
Full-load - three-phase voltage.

31. The main function of CT's and PT's on a main substation is to...
At the source transformer.
Allow for metering voltage and Current.
5:1.
Squirrel-cage rotor.

32. Compared to the secondary of a loaded - step-down transformer - the primary has...
Tied together
Lower current and higher voltage
75 turns.
A grounding resistor.

33. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Allow for metering voltage and Current.
Grounded phase and ground fault devices were not operating.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
No.6 AWG.

34. What is the first step to be followed in aiding a victim of electrical shock?
A device used to store electrical energy.
A step-up transformer.
They should be made by the same manufacturer.
Check to make sure that the victim is not still in contact with the electricity.

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.
Draw less current.
No.6 AWG.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

36. How should clothing be removed from a burn?
Carotid pulse - which is located in the neck next to the Adam's apple.
800 volts.
A step-up transformer.
Cut around clothing that has adhered to the burned area.

37. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Nearly constant.
Provides single-phase power to smaller loads such as lights - heaters - etc.
5:1
The motor rotation is reversed.

38. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Allow for metering voltage and Current.
Provides single-phase power to smaller loads such as lights - heaters - etc.
25 feet.
Zero amps.

39. What type of 3-phase AC motor is used most often around a mine?
Transformers.
75 turns.
Squirrel-cage induction motor.
Current that changes constantly in magnitude and periodically in direction.

40. 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?
Draw less current.
12 to 15 times a minute.
75 turns.
Shunted and taped

41. The standard frequency of power supplied by the power company is...
Tied together
60 cycles per second (60 Hertz).
Continuous.
Clamp-on ammeter.

42. What is the danger when working around capacitors?
An electrical shock hazard may exist.
5:1.
Isolation transformer.
Continuous.

43. Not an acceptable method for the installations of surface transformers?
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage.
Isolation transformer.
They must be installed at least 7 feet above the ground.

44. Which of the following does a squirrel cage induction motor have?
Provides single-phase power to smaller loads such as lights - heaters - etc.
277 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
Stator windings.

45. Alternating current may be defined as...
Kilovolt ampere (KVA).
H terminals.
An electromagnet.
Current that changes constantly in magnitude and periodically in direction.

46. 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.
600 volts.
An electromagnet.
Provides single-phase power to smaller loads such as lights - heaters - etc.

47. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
75 turns.
75 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Carotid pulse - which is located in the neck next to the Adam's apple.

48. What type motor is a wound-rotor induction motor?
A.C.
An electromagnet.
Inductance.
Reverse any two-phase leads.

49. The property of an electric circuit that opposes any change in current is called...
Inductance.
5:1.
Stator windings.
Check to make sure that the victim is not still in contact with the electricity.

50. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
X.
A.C.
A grounding resistor.
Zero amps.