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AC Theory

Instructions:

Answer 50 questions in 20 minutes. 1 minute extra for reading the instructions.
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. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
277 volts.
200 volts AC.
An electrical shock hazard may exist.
The capacitors should be discharged.

2. What should the duty rating of a grounding resistor be?
No.6 AWG.
Circuit Breaker.
Continuous.
At the source transformer.

3. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
The motor rotation is reversed.
It provides good speed control.
Because less current flowing at high voltage causes less line loss per kilowatt.

4. 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?
Draw less current.
They must be installed at least 7 feet above the ground.
200 volts AC.
Larger than that found in the primary winding.

5. 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?
A.C.
Isolation transformer.
Check to make sure that the victim is not still in contact with the electricity.
75 turns.

6. 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?
No.6 AWG.
An electromagnet.
H terminals.
150 turns.

7. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Two speeds
Clamp-on ammeter.
An electrical shock hazard may exist.

8. The standard frequency of power supplied by the power company is...
800 volts.
60 cycles per second (60 Hertz).
The chest should rise or inflate.
Capacitance.

9. Not true about AC power?
Draw less current.
It provides good speed control.
A step-up transformer.
Current that changes constantly in magnitude and periodically in direction.

10. 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?
75 turns.
No.6 AWG.
200 volts AC.
100 turns.

11. Where should grounding resistors be located?
Nearly constant.
At the source transformer.
At least two phases
The motor rotation is reversed.

12. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
7 -630 volts.
Disconnect the frame-grounding conductor.
A step-up transformer.
Potential transformer.

13. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
75 turns.
Shunted and taped
H terminals.
X.

14. Not a symptom of electrical shock?.
Potential transformer.
The capacitors should be discharged.
Current that changes constantly in magnitude and periodically in direction.
Hyperventilation.

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.
The sum of the two capacitors.
H terminals.
5:1
Nearly constant.

16. At what rate should mouth-to-mouth resuscitation be applied?
12 to 15 times a minute.
The current will read zero.
The capacitors should be discharged.
25 feet.

17. When not connected to a load - the secondary leads of a current transformer should be...
Transformers.
No.6 AWG.
Because less current flowing at high voltage causes less line loss per kilowatt.
Shunted and taped

18. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
277 volts.
Reverse any two-phase leads.
Two speeds

19. 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.
7 -630 volts.
5:1.
Continuous.
Two speeds

20. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
60 cycles per second (60 Hertz).
Isolation transformer.
The chest should rise or inflate.
800 volts.

21. Not an acceptable method for the installations of surface transformers?
480 volts.
Kilovolt ampere (KVA).
They must be installed at least 7 feet above the ground.
277 volts.

22. 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 device used to store electrical energy.
An electrical shock hazard may exist.
The motor rotation is reversed.

23. Current in an AC circuit may be measured by...
Clamp-on ammeter.
5:1
800 volts.
Continuous.

24. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
X.
Kilovolt ampere (KVA).
15 feet off the ground.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

25. Which of the following does a squirrel cage induction motor have?
Stator windings.
Transformer
1 amp.
Inductance.

26. A solenoid coil is...
Shunted and taped
60 cycles per second (60 Hertz).
An electromagnet.
Alternating current.

27. 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...
It provides good speed control.
Inductance.
Tied together
5:1

28. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
60 Hertz
X.
Short-circuited after power has been removed.
1 amp.

29. How do you change the direction of a 3-phase AC motor?
60 cycles per second (60 Hertz).
Cut around clothing that has adhered to the burned area.
Reverse any two-phase leads.
H terminals.

30. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
200 volts AC.
A step-up transformer.
A grounding resistor.

31. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
A device used to store electrical energy.
Four times less than that of the secondary.
A grounding resistor.
75 turns.

32. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Circuit Breaker.
Zero amps.
7 -630 volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

33. Current that changes periodically in direction is...
Current changing constantly in magnitude and periodically in direction.
600 volts.
Two speeds
Alternating current.

34. A burn causing a blister is classified as a...
Alternating current.
Second-degree burn.
277 volts.
Inductance.

35. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
An electromagnet.
Derive a neutral for grounding purposes.
60 Hertz

36. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
277 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
The capacitors should be discharged.

37. What type motor is a wound-rotor induction motor?
A.C.
No.6 AWG.
Because less current flowing at high voltage causes less line loss per kilowatt.
An electromagnet.

38. Alternating current may be defined as...
An electrical shock hazard may exist.
25 feet.
Current that changes constantly in magnitude and periodically in direction.
Four times less than that of the secondary.

39. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
2000 watts
Hyperventilation.
Circuit Breaker.
X.

40. What type of 3-phase AC motor is used most often around a mine?
At the source transformer.
Squirrel-cage induction motor.
Inductance.
An electrical shock hazard may exist.

41. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
75 turns.
5:1
Circuit Breaker.
1 amp.

42. 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?
3 -979 Volts.
Disconnect the frame-grounding conductor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The chest should rise or inflate.

43. Current transformers - or CT's - are used to...
Isolation transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Full-load - three-phase voltage.
They should be made by the same manufacturer.

44. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Zero amps.
Current changing constantly in magnitude and periodically in direction.
A device used to store electrical energy.

45. What is a capacitor?
Because less current flowing at high voltage causes less line loss per kilowatt.
A device used to store electrical energy.
The capacitors should be discharged.
75 turns.

46. What is the first step to be followed in aiding a victim of electrical shock?
Check to make sure that the victim is not still in contact with the electricity.
X.
75 turns.
Reverse its rotation.

47. AC voltage is raised or lowered by means of...
Transformers.
Because less current flowing at high voltage causes less line loss per kilowatt.
An electrical shock hazard may exist.
Draw less current.

48. The speed of an alternating current - squirrel-cage induction motor is...
It provides good speed control.
Nearly constant.
A step-up transformer.
Stator windings.

49. Compared to the secondary of a loaded - step-down transformer - the primary has...
An electromagnet.
600 volts.
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage.

50. The purpose of a zig-zag transformer is to...
Transformer
Derive a neutral for grounding purposes.
Tied together
1 amp.