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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. 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
75 turns.
Squirrel-cage rotor.
Stator windings.

2. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
12 to 15 times a minute.
A.C.
The current will read zero.
X.

3. 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.
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage
15 feet off the ground.

4. Operating coils - motors - transformers - relays and solenoids depend upon...
Electro-magnetism for their operation.
Squirrel-cage rotor.
An electrical shock hazard may exist.
Check to make sure that the victim is not still in contact with the electricity.

5. 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.
A device used to store electrical energy.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
At the source transformer.
5:1

6. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
A step-up transformer.
Carotid pulse - which is located in the neck next to the Adam's apple.
Lower current and higher voltage

7. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Kilovolt ampere (KVA).
100 volts.
Short-circuited after power has been removed.

8. 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.
The current will read zero.
No.6 AWG.
Current changing constantly in magnitude and periodically in direction.

9. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
The capacitors should be discharged.
Disconnect the frame-grounding conductor.
Squirrel-cage rotor.
Cut around clothing that has adhered to the burned area.

10. 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...
X.
The current will read zero.
Tied together
Current changing constantly in magnitude and periodically in direction.

11. Compared to the secondary of a loaded - step-down transformer - the primary has...
Potential transformer.
The capacitors should be discharged.
Cut around clothing that has adhered to the burned area.
Lower current and higher voltage.

12. The size of a transformer is measured in...
25 feet.
Nearly constant.
Kilovolt ampere (KVA).
Zero amps.

13. 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?
Electro-magnetism for their operation.
Short-circuited after power has been removed.
200 volts AC.
Potential transformer.

14. A solenoid coil is...
Current that changes constantly in magnitude and periodically in direction.
Allow for metering voltage and Current.
Current changing constantly in magnitude and periodically in direction.
An electromagnet.

15. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
Two speeds
200 volts AC.
Cut around clothing that has adhered to the burned area.
Shunted and taped

16. Current that changes periodically in direction is...
Carotid pulse - which is located in the neck next to the Adam's apple.
75 turns.
Alternating current.
At the source transformer.

17. Not a symptom of electrical shock?.
At the source transformer.
Potential transformer.
Hyperventilation.
Isolation transformer.

18. All transformers step voltage up or down by...
A device used to store electrical energy.
Derive a neutral for grounding purposes.
2000 watts
Inductance.

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...
Squirrel-cage induction motor.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Cut around clothing that has adhered to the burned area.
Draw less current.

20. The property of an electric circuit that opposes any change in current is called...
The chest should rise or inflate.
7 -630 volts.
277 volts.
Inductance.

21. What is the first step to be followed in aiding a victim of electrical shock?
The capacitors should be discharged.
Check to make sure that the victim is not still in contact with the electricity.
Squirrel-cage induction motor.
Alternating current.

22. What type of 3-phase AC motor is used most often around a mine?
Four times less than that of the secondary.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Squirrel-cage induction motor.
100 volts.

23. Current transformers - or CT's - are used to...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
60 Hertz
Isolation transformer.
12 to 15 times a minute.

24. The main function of CT's and PT's on a main substation is to...
Stator windings.
An electromagnet.
Allow for metering voltage and Current.
3 -979 Volts.

25. At what rate should mouth-to-mouth resuscitation be applied?
Zero amps.
Current that changes constantly in magnitude and periodically in direction.
Alternating current.
12 to 15 times a minute.

26. The secondary windings on a 10:1 step-down transformer will have a wire size...
Allow for metering voltage and Current.
60 Hertz
Larger than that found in the primary winding.
Squirrel-cage rotor.

27. To protect against over-current in a three-phase circuit over-current protection must be provided for...
The capacitors should be discharged.
Capacitance.
A.C.
At least two phases

28. The transformer used only to measure voltage is a...
Potential transformer.
Zero amps.
The motor rotation is reversed.
Current that changes constantly in magnitude and periodically in direction.

29. A current transformer has a ratio of 350:5. If the phase current is 70 amperes - what would be the secondary current?
Check to make sure that the victim is not still in contact with the electricity.
1 amp.
The chest should rise or inflate.
It provides good speed control.

30. Current in an AC circuit may be measured by...
100 volts.
Zero amps.
Clamp-on ammeter.
480 volts.

31. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Short-circuited after power has been removed.
Lower current and higher voltage.
1 amp.
480 volts.

32. What type motor is a wound-rotor induction motor?
X.
Grounded phase and ground fault devices were not operating.
A.C.
5:1.

33. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
Full-load - three-phase voltage.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
The capacitors should be discharged.
1 amp.

34. The standard frequency of alternating current used at a mine is...
At the source transformer.
60 Hertz
Check to make sure that the victim is not still in contact with the electricity.
Shunted and taped

35. 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.
Lower current and higher voltage.
They must be installed at least 7 feet above the ground.
Grounded phase and ground fault devices were not operating.

36. What should the duty rating of a grounding resistor be?
Stator windings.
Capacitance.
Continuous.
Disconnect the frame-grounding conductor.

37. The standard frequency of power supplied by the power company is...
Potential transformer.
Current changing constantly in magnitude and periodically in direction.
Full-load - three-phase voltage.
60 cycles per second (60 Hertz).

38. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Squirrel-cage rotor.
The chest should rise or inflate.
25 feet.

39. The combined capacitance of two capacitors in parallel is...
Squirrel-cage rotor.
Transformer
At the source transformer.
The sum of the two capacitors.

40. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Shunted and taped
Alternating current.
The motor rotation is reversed.

41. 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?
Check to make sure that the victim is not still in contact with the electricity.
100 turns.
Provides single-phase power to smaller loads such as lights - heaters - etc.
H terminals.

42. 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.
60 cycles per second (60 Hertz).
An electromagnet.
60 Hertz

43. Not true about AC power?
It provides good speed control.
Draw less current.
Kilovolt ampere (KVA).
No.6 AWG.

44. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
A.C.
It provides good speed control.
The motor rotation is reversed.
Derive a neutral for grounding purposes.

45. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Zero amps.
Check to make sure that the victim is not still in contact with the electricity.
No.6 AWG.
Disconnect the frame-grounding conductor.

46. When not connected to a load - the secondary leads of a current transformer should be...
3 -979 Volts.
Stator windings.
Shunted and taped
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

47. 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?
800 volts.
Squirrel-cage induction motor.
600 volts.
75 turns.

48. 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?
Two speeds
Because less current flowing at high voltage causes less line loss per kilowatt.
150 turns.
75 turns.

49. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
Kilovolt ampere (KVA).
Transformers.
Squirrel-cage induction motor.
They should be made by the same manufacturer.

50. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
12 to 15 times a minute.
The current will read zero.
Lower current and higher voltage.
Short-circuited after power has been removed.