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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. Where should grounding resistors be located?
At the source transformer.
Nearly constant.
Cut around clothing that has adhered to the burned area.
Derive a neutral for grounding purposes.

2. What is a capacitor?
A step-up transformer.
Squirrel-cage induction motor.
A device used to store electrical energy.
600 volts.

3. Current transformers - or CT's - are used to...
12 to 15 times a minute.
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.

4. A wound-rotor induction motor has all the following EXCEPT...
277 volts.
150 turns.
Because less current flowing at high voltage causes less line loss per kilowatt.
Squirrel-cage rotor.

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?
No.6 AWG.
75 turns.
They should be made by the same manufacturer.
25 feet.

6. 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?
Provides single-phase power to smaller loads such as lights - heaters - etc.
75 turns.
The motor rotation is reversed.
Circuit Breaker.

7. The property of an electrical circuit that opposes any change in voltage is called...
7 -630 volts.
Electro-magnetism for their operation.
Capacitance.
An electrical shock hazard may exist.

8. The purpose of a zig-zag transformer is to...
At the source transformer.
200 volts AC.
7 -630 volts.
Derive a neutral for grounding purposes.

9. All transformers step voltage up or down by...
Check to make sure that the victim is not still in contact with the electricity.
Inductance.
Current changing constantly in magnitude and periodically in direction.
H terminals.

10. A solenoid coil is...
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Two speeds
An electromagnet.

11. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Capacitance.
Larger than that found in the primary winding.
Transformer

12. What steps up or steps down AC voltage?
A step-up transformer.
Tied together
It provides good speed control.
Transformer

13. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
Kilovolt ampere (KVA).
15 feet off the ground.
The capacitors should be discharged.
Derive a neutral for grounding purposes.

14. 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...
Reverse its rotation.
Provides single-phase power to smaller loads such as lights - heaters - etc.
Draw less current.
Clamp-on ammeter.

15. 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.
Squirrel-cage rotor.
5:1.
Inductance.

16. What should the duty rating of a grounding resistor be?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Continuous.
Inductance.
Current that changes constantly in magnitude and periodically in direction.

17. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
1 amp.
A grounding resistor.
Cut around clothing that has adhered to the burned area.
Disconnect the frame-grounding conductor.

18. Which of the following does a squirrel cage induction motor have?
Lower current and higher voltage.
277 volts.
Stator windings.
Lower current and higher voltage

19. 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...
Provides single-phase power to smaller loads such as lights - heaters - etc.
75 turns.
Draw less current.
Potential transformer.

20. The standard frequency of power supplied by the power company is...
Transformer
At least two phases
60 cycles per second (60 Hertz).
Short-circuited after power has been removed.

21. What is the danger when working around capacitors?
An electrical shock hazard may exist.
600 volts.
25 feet.
1 amp.

22. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
A device used to store electrical energy.
Reverse its rotation.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Zero amps.

23. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Zero amps.
Provides single-phase power to smaller loads such as lights - heaters - etc.
150 turns.
Short-circuited after power has been removed.

24. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Carotid pulse - which is located in the neck next to the Adam's apple.
Disconnect the frame-grounding conductor.
X.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

25. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Lower current and higher voltage
5:1
Squirrel-cage rotor.
Grounded phase and ground fault devices were not operating.

26. Alternating current may be defined as...
Disconnect the frame-grounding conductor.
5:1
Current that changes constantly in magnitude and periodically in direction.
Draw less current.

27. Current in an AC circuit may be measured by...
Clamp-on ammeter.
An electrical shock hazard may exist.
12 to 15 times a minute.
Circuit Breaker.

28. At what rate should mouth-to-mouth resuscitation be applied?
A device used to store electrical energy.
12 to 15 times a minute.
Reverse its rotation.
Cut around clothing that has adhered to the burned area.

29. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
600 volts.
150 turns.
Squirrel-cage induction motor.

30. The combined capacitance of two capacitors in parallel is...
Clamp-on ammeter.
Squirrel-cage induction motor.
Nearly constant.
The sum of the two capacitors.

31. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Current that changes constantly in magnitude and periodically in direction.
100 volts.
A device used to store electrical energy.
The capacitors should be discharged.

32. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
277 volts.
Kilovolt ampere (KVA).
Nearly constant.
Full-load - three-phase voltage.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
Cut around clothing that has adhered to the burned area.
A grounding resistor.
A.C.

34. 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...
Tied together
Stator windings.
75 turns.
60 Hertz

35. Define alternating current...
A step-up transformer.
Current changing constantly in magnitude and periodically in direction.
800 volts.
Lower current and higher voltage.

36. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
3 -979 Volts.
200 volts AC.
Larger than that found in the primary winding.

37. The size of a transformer is measured in...
Kilovolt ampere (KVA).
Draw less current.
600 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.

38. 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?
An electrical shock hazard may exist.
100 turns.
Stator windings.
An electromagnet.

39. The secondary windings on a 10:1 step-down transformer will have a wire size...
A grounding resistor.
Allow for metering voltage and Current.
Larger than that found in the primary winding.
Clamp-on ammeter.

40. 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?
Larger than that found in the primary winding.
200 volts AC.
277 volts.
They should be made by the same manufacturer.

41. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Four times less than that of the secondary.
1 amp.
It provides good speed control.
An electromagnet.

42. The main function of CT's and PT's on a main substation is to...
Reverse its rotation.
A device used to store electrical energy.
15 feet off the ground.
Allow for metering voltage and Current.

43. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Full-load - three-phase voltage.
The capacitors should be discharged.
Current changing constantly in magnitude and periodically in direction.

44. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
Four times less than that of the secondary.
100 volts.
Short-circuited after power has been removed.

45. 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.
5:1
Four times less than that of the secondary.
277 volts.

46. The speed of an alternating current - squirrel-cage induction motor is...
Nearly constant.
5:1
Draw less current.
Kilovolt ampere (KVA).

47. 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...
An electromagnet.
A grounding resistor.
100 volts.
Hyperventilation.

48. AC voltage is raised or lowered by means of...
A.C.
Transformers.
Full-load - three-phase voltage.
Shunted and taped

49. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
A step-up transformer.
25 feet.
Squirrel-cage induction motor.
Draw less current.

50. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
The capacitors should be discharged.
100 volts.
A.C.