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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. Not true about AC power?
It provides good speed control.
75 turns.
Continuous.
Clamp-on ammeter.

2. The standard frequency of alternating current used at a mine is...
Because less current flowing at high voltage causes less line loss per kilowatt.
75 turns.
60 Hertz
Larger than that found in the primary winding.

3. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
No.6 AWG.
It provides good speed control.
Alternating current.
Lower current and higher voltage

4. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
277 volts.
Check to make sure that the victim is not still in contact with the electricity.
800 volts.

5. Which of the following does a squirrel cage induction motor have?
60 cycles per second (60 Hertz).
277 volts.
200 volts AC.
Stator windings.

6. Surface transformer installations - unless constructed to eliminate shock hazards - must be installed at least...
5:1.
15 feet off the ground.
Stator windings.
Tied together

7. 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.
The sum of the two capacitors.
Lower current and higher voltage.
5:1

8. The combined capacitance of two capacitors in parallel is...
Lower current and higher voltage
The sum of the two capacitors.
The capacitors should be discharged.
A grounding resistor.

9. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
200 volts AC.
Capacitance.
Two speeds
5:1.

10. To protect against over-current in a three-phase circuit over-current protection must be provided for...
Grounded phase and ground fault devices were not operating.
3 -979 Volts.
480 volts.
At least two phases

11. Compared to the secondary of a loaded - step-down transformer - the primary has...
Squirrel-cage induction motor.
480 volts.
The current will read zero.
Lower current and higher voltage.

12. 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.
200 volts AC.
The capacitors should be discharged.
1 amp.

13. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
Inductance.
The sum of the two capacitors.
75 turns.

14. 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?
X.
800 volts.
Tied together
Two speeds

15. A solenoid coil is...
An electromagnet.
At the source transformer.
7 -630 volts.
A.C.

16. 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
Full-load - three-phase voltage.
At the source transformer.
An electrical shock hazard may exist.

17. The purpose of a zig-zag transformer is to...
Derive a neutral for grounding purposes.
2000 watts
Zero amps.
Alternating current.

18. Current in an AC circuit may be measured by...
An electrical shock hazard may exist.
Alternating current.
Clamp-on ammeter.
Stator windings.

19. 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.
Current that changes constantly in magnitude and periodically in direction.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

20. 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.
Circuit Breaker.
Nearly constant.
A device used to store electrical energy.

21. 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?
600 volts.
Electro-magnetism for their operation.
Transformers.
480 volts.

22. 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
Derive a neutral for grounding purposes.
They should be made by the same manufacturer.
2000 watts

23. 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.
The chest should rise or inflate.
Full-load - three-phase voltage.
Second-degree burn.

24. 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...
Disconnect the frame-grounding conductor.
Squirrel-cage rotor.
100 volts.
Stator windings.

25. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Hyperventilation.
The capacitors should be discharged.
Lower current and higher voltage
15 feet off the ground.

26. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
It provides good speed control.
Lower current and higher voltage
Short-circuited after power has been removed.

27. If current were flowing through a grounding resistor in a 440-volt - resistance-grounded system - this would probably be because...
X.
800 volts.
Grounded phase and ground fault devices were not operating.
60 cycles per second (60 Hertz).

28. 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?
2000 watts
Inductance.
150 turns.
75 turns.

29. What steps up or steps down AC voltage?
150 turns.
Transformer
It provides good speed control.
60 cycles per second (60 Hertz).

30. What is the first step to be followed in aiding a victim of electrical shock?
Nearly constant.
Check to make sure that the victim is not still in contact with the electricity.
Derive a neutral for grounding purposes.
100 volts.

31. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
The current will read zero.
Current changing constantly in magnitude and periodically in direction.
Inductance.
Capacitance.

32. 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.
The motor rotation is reversed.
Kilovolt ampere (KVA).
277 volts.

33. 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.
Second-degree burn.
Inductance.
Transformer

34. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
Zero amps.
Shunted and taped
A device used to store electrical energy.
Grounded phase and ground fault devices were not operating.

35. The speed of an alternating current - squirrel-cage induction motor is...
3 -979 Volts.
Nearly constant.
Current changing constantly in magnitude and periodically in direction.
An electrical shock hazard may exist.

36. A burn causing a blister is classified as a...
Zero amps.
Second-degree burn.
75 turns.
X.

37. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
The chest should rise or inflate.
Transformer
480 volts.
The capacitors should be discharged.

38. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
Because less current flowing at high voltage causes less line loss per kilowatt.
7 -630 volts.
Four times less than that of the secondary.
X.

39. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
A.C.
Grounded phase and ground fault devices were not operating.
Short-circuited after power has been removed.
Disconnect the frame-grounding conductor.

40. 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...
An electrical shock hazard may exist.
Draw less current.
Short-circuited after power has been removed.
X.

41. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
Disconnect the frame-grounding conductor.
Draw less current.
Reverse its rotation.

42. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Squirrel-cage induction motor.
A device used to store electrical energy.
75 turns.
277 volts.

43. Three-phase AC underground cables must be provided with overcurrent protection using a...
Circuit Breaker.
Squirrel-cage induction motor.
Alternating current.
Shunted and taped

44. 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.
5:1.
A grounding resistor.
7 -630 volts.
It provides good speed control.

45. The current on the primary side of a step-down transformer with a 4:1 ratio will be...
Circuit Breaker.
800 volts.
Four times less than that of the secondary.
480 volts.

46. What type motor is a wound-rotor induction motor?
Potential transformer.
A.C.
Current changing constantly in magnitude and periodically in direction.
Check to make sure that the victim is not still in contact with the electricity.

47. The primary leads of a single-phase transformer should be connected to the...
Grounded phase and ground fault devices were not operating.
5:1
H terminals.
60 Hertz

48. Where should grounding resistors be located?
A.C.
Continuous.
At the source transformer.
The sum of the two capacitors.

49. The transformer used only to measure voltage is a...
Potential transformer.
The current will read zero.
Capacitance.
Kilovolt ampere (KVA).

50. A transformer with a 1:1 ratio is usually a (an)...
Inductance.
Isolation transformer.
Lower current and higher voltage
The motor rotation is reversed.