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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. The speed of an alternating current - squirrel-cage induction motor is...
600 volts.
Nearly constant.
Full-load - three-phase voltage.
Squirrel-cage rotor.

2. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
277 volts.
600 volts.
12 to 15 times a minute.

3. Three-phase AC underground cables must be provided with overcurrent protection using a...
7 -630 volts.
Because less current flowing at high voltage causes less line loss per kilowatt.
Circuit Breaker.
75 turns.

4. 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?
100 turns.
The motor rotation is reversed.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
60 Hertz

5. A solenoid coil is...
Current that changes constantly in magnitude and periodically in direction.
12 to 15 times a minute.
Draw less current.
An electromagnet.

6. Define alternating current...
H terminals.
Current changing constantly in magnitude and periodically in direction.
480 volts.
75 turns.

7. 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.
2000 watts
Circuit Breaker.
5:1.
Provides single-phase power to smaller loads such as lights - heaters - etc.

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?
Full-load - three-phase voltage.
Check to make sure that the victim is not still in contact with the electricity.
Larger than that found in the primary winding.
75 turns.

9. 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...
Draw less current.
Kilovolt ampere (KVA).
75 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

10. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Because less current flowing at high voltage causes less line loss per kilowatt.
Stator windings.
200 volts AC.
At least two phases

11. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Two speeds
Reverse any two-phase leads.
7 -630 volts.
A step-up transformer.

12. 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.
Continuous.
1 amp.
Allow for metering voltage and Current.

13. If any two-phase wires are reversed on a three-phase motor - the motor will...
Alternating current.
Shunted and taped
A device used to store electrical energy.
Reverse its rotation.

14. 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?
Reverse its rotation.
60 Hertz
480 volts.
200 volts AC.

15. 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.
5:1
Capacitance.
Stator windings.

16. Where should grounding resistors be located?
At the source transformer.
Second-degree burn.
Four times less than that of the secondary.
A.C.

17. Current in an AC circuit may be measured by...
A device used to store electrical energy.
Larger than that found in the primary winding.
Clamp-on ammeter.
The motor rotation is reversed.

18. The property of an electrical circuit that opposes any change in voltage is called...
277 volts.
Hyperventilation.
Capacitance.
Draw less current.

19. A transformer with a 1:1 ratio is usually a (an)...
Larger than that found in the primary winding.
Allow for metering voltage and Current.
Isolation transformer.
Reverse its rotation.

20. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
3 -979 Volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
25 feet.
480 volts.

21. A burn causing a blister is classified as a...
The sum of the two capacitors.
Second-degree burn.
Draw less current.
7 -630 volts.

22. What steps up or steps down AC voltage?
They must be installed at least 7 feet above the ground.
Cut around clothing that has adhered to the burned area.
12 to 15 times a minute.
Transformer

23. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
3 -979 Volts.
Inductance.
480 volts.
A device used to store electrical energy.

24. The main function of CT's and PT's on a main substation is to...
They should be made by the same manufacturer.
Isolation transformer.
Clamp-on ammeter.
Allow for metering voltage and Current.

25. The property of an electric circuit that opposes any change in current is called...
Inductance.
600 volts.
A step-up transformer.
A device used to store electrical energy.

26. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Potential transformer.
Short-circuited after power has been removed.
They should be made by the same manufacturer.
X.

27. Not a symptom of electrical shock?.
Circuit Breaker.
75 turns.
15 feet off the ground.
Hyperventilation.

28. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
The sum of the two capacitors.
Derive a neutral for grounding purposes.
At the source transformer.

29. Which of the following does a squirrel cage induction motor have?
Lower current and higher voltage
Current that changes constantly in magnitude and periodically in direction.
Isolation transformer.
Stator windings.

30. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
150 turns.
The chest should rise or inflate.
No.6 AWG.
Reverse any two-phase leads.

31. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage
Shunted and taped
600 volts.
Check to make sure that the victim is not still in contact with the electricity.

32. AC voltage is raised or lowered by means of...
277 volts.
Inductance.
X.
Transformers.

33. What is a capacitor?
A device used to store electrical energy.
An electromagnet.
X.
Potential transformer.

34. 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?
Reverse its rotation.
At least two phases
800 volts.
A device used to store electrical energy.

35. 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?
75 turns.
Potential transformer.
Larger than that found in the primary winding.
Short-circuited after power has been removed.

36. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Allow for metering voltage and Current.
Lower current and higher voltage
Stator windings.

37. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Inductance.
12 to 15 times a minute.

38. A control power transformer...
Current changing constantly in magnitude and periodically in direction.
Provides single-phase power to smaller loads such as lights - heaters - etc.
At least two phases
H terminals.

39. The transformer used only to measure voltage is a...
60 cycles per second (60 Hertz).
Potential transformer.
X.
Hyperventilation.

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...
An electrical shock hazard may exist.
200 volts AC.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Tied together

41. The secondary windings on a 10:1 step-down transformer will have a wire size...
The chest should rise or inflate.
Stator windings.
Larger than that found in the primary winding.
An electromagnet.

42. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
X.
Circuit Breaker.
5:1.
2000 watts

43. Alternating current may be defined as...
A grounding resistor.
Reverse its rotation.
480 volts.
Current that changes constantly in magnitude and periodically in direction.

44. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
277 volts.
150 turns.
The motor rotation is reversed.
60 cycles per second (60 Hertz).

45. A wound-rotor induction motor has all the following EXCEPT...
Squirrel-cage rotor.
Lower current and higher voltage
H terminals.
Reverse any two-phase leads.

46. Compared to the secondary of a loaded - step-down transformer - the primary has...
Electro-magnetism for their operation.
Nearly constant.
Lower current and higher voltage.
A step-up transformer.

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...
100 volts.
Transformers.
Tied together
Inductance.

48. Not true about AC power?
It provides good speed control.
Potential transformer.
5:1
Alternating current.

49. 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...
75 turns.
277 volts.
They should be made by the same manufacturer.
Draw less current.

50. 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.
Carotid pulse - which is located in the neck next to the Adam's apple.
Transformer
Because less current flowing at high voltage causes less line loss per kilowatt.