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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. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
277 volts.
An electromagnet.
Capacitance.

2. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
They should be made by the same manufacturer.
Four times less than that of the secondary.
800 volts.

3. What type motor is a wound-rotor induction motor?
A.C.
Check to make sure that the victim is not still in contact with the electricity.
Shunted and taped
An electromagnet.

4. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
The motor rotation is reversed.
Current changing constantly in magnitude and periodically in direction.
60 cycles per second (60 Hertz).

5. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Reverse its rotation.
It provides good speed control.
Short-circuited after power has been removed.
Potential transformer.

6. Current transformers - or CT's - are used to...
1 amp.
A grounding resistor.
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

7. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage rotor.
150 turns.
Squirrel-cage induction motor.
Reverse any two-phase leads.

8. What is the first step to be followed in aiding a victim of electrical shock?
The motor rotation is reversed.
Continuous.
Alternating current.
Check to make sure that the victim is not still in contact with the electricity.

9. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
800 volts.
Potential transformer.
Grounded phase and ground fault devices were not operating.
100 volts.

10. The main function of CT's and PT's on a main substation is to...
Grounded phase and ground fault devices were not operating.
Squirrel-cage rotor.
Allow for metering voltage and Current.
Inductance.

11. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Cut around clothing that has adhered to the burned area.
X.
Provides single-phase power to smaller loads such as lights - heaters - etc.
At least two phases

12. Not a symptom of electrical shock?.
Hyperventilation.
It provides good speed control.
Carotid pulse - which is located in the neck next to the Adam's apple.
H terminals.

13. The speed of an alternating current - squirrel-cage induction motor is...
800 volts.
Nearly constant.
Short-circuited after power has been removed.
At the source transformer.

14. 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...
Transformers.
100 turns.
Tied together
At least two phases

15. 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.
Alternating current.
Zero amps.
100 turns.

16. The purpose of a zig-zag transformer is to...
Capacitance.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Derive a neutral for grounding purposes.
Zero amps.

17. 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...
Four times less than that of the secondary.
Draw less current.
Short-circuited after power has been removed.
60 cycles per second (60 Hertz).

18. The combined capacitance of two capacitors in parallel is...
The sum of the two capacitors.
25 feet.
Transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

19. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
A device used to store electrical energy.
It provides good speed control.
100 volts.

20. To protect against over-current in a three-phase circuit over-current protection must be provided for...
A grounding resistor.
At least two phases
7 -630 volts.
480 volts.

21. How do you change the direction of a 3-phase AC motor?
A grounding resistor.
Reverse any two-phase leads.
Second-degree burn.
A.C.

22. A control power transformer...
Transformers.
Provides single-phase power to smaller loads such as lights - heaters - etc.
The chest should rise or inflate.
Nearly constant.

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

24. Operating coils - motors - transformers - relays and solenoids depend upon...
100 turns.
Full-load - three-phase voltage.
5:1
Electro-magnetism for their operation.

25. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
3 -979 Volts.
7 -630 volts.
Transformer
A.C.

26. 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?
75 turns.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Shunted and taped
A step-up transformer.

27. Current in an AC circuit may be measured by...
7 -630 volts.
Clamp-on ammeter.
No.6 AWG.
Transformers.

28. Where should grounding resistors be located?
At the source transformer.
Derive a neutral for grounding purposes.
Transformer
H terminals.

29. A transformer with a 1:1 ratio is usually a (an)...
Isolation transformer.
The motor rotation is reversed.
The chest should rise or inflate.
5:1.

30. A transformer has 1 -600 turns in the primary and 400 turns in the secondary. The primary voltage is 2 -400 volts. What is the secondary voltage?
Disconnect the frame-grounding conductor.
Inductance.
600 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.

31. 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...
Draw less current.
60 Hertz
100 volts.
The capacitors should be discharged.

32. 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?
800 volts.
Clamp-on ammeter.
Because less current flowing at high voltage causes less line loss per kilowatt.
100 turns.

33. What should the duty rating of a grounding resistor be?
The capacitors should be discharged.
480 volts.
3 -979 Volts.
Continuous.

34. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Clamp-on ammeter.
Provides single-phase power to smaller loads such as lights - heaters - etc.
75 turns.
The motor rotation is reversed.

35. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
A grounding resistor.
Circuit Breaker.
Draw less current.
Transformer

36. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
Reverse its rotation.
Transformers.
The chest should rise or inflate.
2000 watts

37. Current that changes periodically in direction is...
800 volts.
Alternating current.
Hyperventilation.
15 feet off the ground.

38. Which of the following does a squirrel cage induction motor have?
Disconnect the frame-grounding conductor.
Stator windings.
It provides good speed control.
Short-circuited after power has been removed.

39. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
5:1
Zero amps.
15 feet off the ground.
Inductance.

40. When connecting 3 single-phase transformers into a three-phase system - which of the following is NOT required for the single-phase transformers.
2000 watts
They should be made by the same manufacturer.
Nearly constant.
At least two phases

41. Compared to the secondary of a loaded - step-down transformer - the primary has...
Inductance.
Derive a neutral for grounding purposes.
Lower current and higher voltage
Alternating current.

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?
Inductance.
Isolation transformer.
150 turns.
3 -979 Volts.

43. All transformers step voltage up or down by...
1 amp.
Lower current and higher voltage.
Inductance.
Carotid pulse - which is located in the neck next to the Adam's apple.

44. A solenoid coil is...
An electromagnet.
15 feet off the ground.
480 volts.
The sum of the two capacitors.

45. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Four times less than that of the secondary.
A step-up transformer.
Inductance.
Cut around clothing that has adhered to the burned area.

46. The primary function of a power transformer is to provide...
200 volts AC.
480 volts.
Full-load - three-phase voltage.
Zero amps.

47. If any two-phase wires are reversed on a three-phase motor - the motor will...
Reverse its rotation.
Second-degree burn.
A step-up transformer.
At the source transformer.

48. 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.
Capacitance.
5:1.
An electrical shock hazard may exist.
Continuous.

49. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
150 turns.
Squirrel-cage rotor.
2000 watts
Disconnect the frame-grounding conductor.

50. A wound-rotor induction motor has all the following EXCEPT...
Shunted and taped
Squirrel-cage rotor.
480 volts.
Stator windings.