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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. Three-phase AC underground cables must be provided with overcurrent protection using a...
An electrical shock hazard may exist.
The current will read zero.
5:1.
Circuit Breaker.

2. The purpose of a zig-zag transformer is to...
Transformer
200 volts AC.
Derive a neutral for grounding purposes.
Shunted and taped

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

4. The primary leads of a single-phase transformer should be connected to the...
An electrical shock hazard may exist.
H terminals.
A grounding resistor.
5:1.

5. 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.
Draw less current.
H terminals.
The chest should rise or inflate.

6. A control power transformer...
5:1.
Because less current flowing at high voltage causes less line loss per kilowatt.
Potential transformer.
Provides single-phase power to smaller loads such as lights - heaters - etc.

7. To protect against over-current in a three-phase circuit over-current protection must be provided for...
1 amp.
The capacitors should be discharged.
Draw less current.
At least two phases

8. 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
A grounding resistor.
Draw less current.
Current that changes constantly in magnitude and periodically in direction.

9. What type motor is a wound-rotor induction motor?
A.C.
No.6 AWG.
They must be installed at least 7 feet above the ground.
Transformer

10. All transformers step voltage up or down by...
Inductance.
A grounding resistor.
Squirrel-cage induction motor.
Current that changes constantly in magnitude and periodically in direction.

11. Current that changes periodically in direction is...
60 Hertz
200 volts AC.
A device used to store electrical energy.
Alternating current.

12. A wound-rotor induction motor has all the following EXCEPT...
Current changing constantly in magnitude and periodically in direction.
Squirrel-cage rotor.
The current will read zero.
Stator windings.

13. 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?
480 volts.
A step-up transformer.
They should be made by the same manufacturer.
75 turns.

14. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Squirrel-cage induction motor.
No.6 AWG.
7 -630 volts.
Alternating current.

15. The speed of an alternating current - squirrel-cage induction motor is...
200 volts AC.
Inductance.
It provides good speed control.
Nearly constant.

16. A burn causing a blister is classified as a...
The chest should rise or inflate.
7 -630 volts.
Carotid pulse - which is located in the neck next to the Adam's apple.
Second-degree burn.

17. 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.
3 -979 Volts.
5:1
Lower current and higher voltage.
75 turns.

18. 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...
12 to 15 times a minute.
A grounding resistor.
100 volts.
150 turns.

19. The transformer used only to measure voltage is a...
Potential transformer.
200 volts AC.
The capacitors should be discharged.
No.6 AWG.

20. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Kilovolt ampere (KVA).
Shunted and taped
X.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

21. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
2000 watts
600 volts.
480 volts.
H terminals.

22. 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?
At least two phases
Transformer
75 turns.
3 -979 Volts.

23. The primary function of a power transformer is to provide...
Because less current flowing at high voltage causes less line loss per kilowatt.
Full-load - three-phase voltage.
They must be installed at least 7 feet above the ground.
5:1.

24. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
The motor rotation is reversed.
Transformers.
2000 watts

25. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Derive a neutral for grounding purposes.
At least two phases
An electrical shock hazard may exist.

26. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Inductance.
The sum of the two capacitors.
Current changing constantly in magnitude and periodically in direction.
The capacitors should be discharged.

27. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
60 cycles per second (60 Hertz).
The current will read zero.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
100 volts.

28. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
7 -630 volts.
150 turns.
60 cycles per second (60 Hertz).

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

30. The main function of CT's and PT's on a main substation is to...
75 turns.
12 to 15 times a minute.
Allow for metering voltage and Current.
60 cycles per second (60 Hertz).

31. Compared to the secondary of a loaded - step-down transformer - the primary has...
Lower current and higher voltage.
It provides good speed control.
Inductance.
Lower current and higher voltage

32. How do you change the direction of a 3-phase AC motor?
A device used to store electrical energy.
Cut around clothing that has adhered to the burned area.
Nearly constant.
Reverse any two-phase leads.

33. What is the danger when working around capacitors?
Capacitance.
The sum of the two capacitors.
Inductance.
An electrical shock hazard may exist.

34. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
A device used to store electrical energy.
They should be made by the same manufacturer.
The capacitors should be discharged.
Draw less current.

35. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
Capacitance.
No.6 AWG.
Grounded phase and ground fault devices were not operating.
Cut around clothing that has adhered to the burned area.

36. Current in an AC circuit may be measured by...
Clamp-on ammeter.
A device used to store electrical energy.
An electromagnet.
A step-up transformer.

37. 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?
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
150 turns.
Short-circuited after power has been removed.
Capacitance.

38. The property of an electric circuit that opposes any change in current is called...
100 volts.
Larger than that found in the primary winding.
Inductance.
Draw less current.

39. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
At least two phases
5:1.
60 Hertz

40. 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.
At least two phases
Second-degree burn.
5:1.

41. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
The motor rotation is reversed.
Circuit Breaker.
60 Hertz

42. What should the duty rating of a grounding resistor be?
5:1.
Continuous.
Hyperventilation.
277 volts.

43. If any two-phase wires are reversed on a three-phase motor - the motor will...
Four times less than that of the secondary.
Reverse its rotation.
Lower current and higher voltage
150 turns.

44. What type of 3-phase AC motor is used most often around a mine?
Lower current and higher voltage
A step-up transformer.
Squirrel-cage induction motor.
Shunted and taped

45. When not connected to a load - the secondary leads of a current transformer should be...
Larger than that found in the primary winding.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
H terminals.
Shunted and taped

46. Not a symptom of electrical shock?.
Hyperventilation.
277 volts.
Lower current and higher voltage.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

47. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
A.C.
75 turns.
Four times less than that of the secondary.
7 -630 volts.

48. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Lower current and higher voltage
A device used to store electrical energy.
2000 watts
Current that changes constantly in magnitude and periodically in direction.

49. A transformer with a 1:1 ratio is usually a (an)...
480 volts.
Continuous.
Isolation transformer.
A step-up transformer.

50. What is a capacitor?
2000 watts
Allow for metering voltage and Current.
1 amp.
A device used to store electrical energy.