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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. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
A step-up transformer.
Inductance.
Second-degree burn.
Cut around clothing that has adhered to the burned area.

2. What type motor is a wound-rotor induction motor?
The current will read zero.
Shunted and taped
Grounded phase and ground fault devices were not operating.
A.C.

3. 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?
An electrical shock hazard may exist.
Carotid pulse - which is located in the neck next to the Adam's apple.
Squirrel-cage rotor.
800 volts.

4. If 13 -200 volts are measured from phase-to-phase across a wye-connected secondary - What is the phase-to-ground voltage?
7 -630 volts.
60 cycles per second (60 Hertz).
Tied together
No.6 AWG.

5. Current transformers - or CT's - are used to...
Lower current and higher voltage
The sum of the two capacitors.
The chest should rise or inflate.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

6. The primary leads of a single-phase transformer should be connected to the...
Squirrel-cage induction motor.
5:1.
H terminals.
At least two phases

7. Define alternating current...
Current changing constantly in magnitude and periodically in direction.
75 turns.
100 volts.
480 volts.

8. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
7 -630 volts.
150 turns.
480 volts.
Reverse any two-phase leads.

9. 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?
Kilovolt ampere (KVA).
3 -979 Volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
A.C.

10. AC voltage is raised or lowered by means of...
Stator windings.
Derive a neutral for grounding purposes.
Shunted and taped
Transformers.

11. The speed of an alternating current - squirrel-cage induction motor is...
Clamp-on ammeter.
Alternating current.
Nearly constant.
800 volts.

12. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Shunted and taped
Two speeds
Inductance.
A grounding resistor.

13. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
75 turns.
200 volts AC.
25 feet.
Transformer

14. Compared to the secondary of a loaded - step-down transformer - the primary has...
A.C.
Lower current and higher voltage
A device used to store electrical energy.
Zero amps.

15. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Second-degree burn.
2000 watts
75 turns.
No.6 AWG.

16. 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.
Current that changes constantly in magnitude and periodically in direction.
5:1.
60 Hertz
Hyperventilation.

17. 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...
Lower current and higher voltage
Tied together
Reverse its rotation.
A.C.

18. The property of an electric circuit that opposes any change in current is called...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Inductance.
Current changing constantly in magnitude and periodically in direction.
The capacitors should be discharged.

19. 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?
25 feet.
Second-degree burn.
75 turns.
Electro-magnetism for their operation.

20. The main function of CT's and PT's on a main substation is to...
Draw less current.
Two speeds
Allow for metering voltage and Current.
Clamp-on ammeter.

21. Where should grounding resistors be located?
Squirrel-cage rotor.
Lower current and higher voltage
Allow for metering voltage and Current.
At the source transformer.

22. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Nearly constant.
Potential transformer.
Zero amps.
Disconnect the frame-grounding conductor.

23. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
The capacitors should be discharged.
Cut around clothing that has adhered to the burned area.
Clamp-on ammeter.
Zero amps.

24. Using a 10:1 ratio transformer - the terminals on the secondary side or lower voltage side would be marked...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
7 -630 volts.
Full-load - three-phase voltage.
X.

25. At what rate should mouth-to-mouth resuscitation be applied?
Two speeds
Cut around clothing that has adhered to the burned area.
Draw less current.
12 to 15 times a minute.

26. Why is 440-volt - three-phase power more commonly used in mining than 220-volt - three-phase power?
25 feet.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Squirrel-cage rotor.
Provides single-phase power to smaller loads such as lights - heaters - etc.

27. 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?
Transformers.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
600 volts.
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.

28. The standard frequency of power supplied by the power company is...
A.C.
Transformers.
Capacitance.
60 cycles per second (60 Hertz).

29. What is the first step to be followed in aiding a victim of electrical shock?
Kilovolt ampere (KVA).
Current that changes constantly in magnitude and periodically in direction.
Check to make sure that the victim is not still in contact with the electricity.
It provides good speed control.

30. The load to which the secondary winding of a current transformer was connected has opened. The secondary leads should be...
Draw less current.
Inductance.
Short-circuited after power has been removed.
12 to 15 times a minute.

31. 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...
480 volts.
100 volts.
Transformers.
Carotid pulse - which is located in the neck next to the Adam's apple.

32. Alternating current may be defined as...
Tied together
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.
H terminals.

33. 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.
600 volts.
25 feet.
Circuit Breaker.

34. The secondary windings on a 10:1 step-down transformer will have a wire size...
Current that changes constantly in magnitude and periodically in direction.
Lower current and higher voltage
Larger than that found in the primary winding.
600 volts.

35. What happens to a motor that is connected to a three-phase cable when two of the cable's leads are reversed?
Potential transformer.
The sum of the two capacitors.
600 volts.
The motor rotation is reversed.

36. Current that changes periodically in direction is...
Larger than that found in the primary winding.
The motor rotation is reversed.
Full-load - three-phase voltage.
Alternating current.

37. Current in an AC circuit may be measured by...
Clamp-on ammeter.
Derive a neutral for grounding purposes.
Grounded phase and ground fault devices were not operating.
They should be made by the same manufacturer.

38. 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...
Inductance.
Draw less current.
Second-degree burn.
200 volts AC.

39. What type of 3-phase AC motor is used most often around a mine?
Squirrel-cage induction motor.
5:1.
A.C.
A step-up transformer.

40. A transformer with a 1:1 ratio is usually a (an)...
It provides good speed control.
Transformer
2000 watts
Isolation transformer.

41. The property of an electrical circuit that opposes any change in voltage is called...
Capacitance.
Allow for metering voltage and Current.
Electro-magnetism for their operation.
3 -979 Volts.

42. Not an acceptable method for the installations of surface transformers?
They must be installed at least 7 feet above the ground.
25 feet.
The motor rotation is reversed.
Provides single-phase power to smaller loads such as lights - heaters - etc.

43. When troubleshooting a circuit with a bank of capacitors - what precautions must be taken?
Second-degree burn.
Kilovolt ampere (KVA).
Lower current and higher voltage
The capacitors should be discharged.

44. Not true about AC power?
Because less current flowing at high voltage causes less line loss per kilowatt.
It provides good speed control.
Derive a neutral for grounding purposes.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.

45. If you use a clamp-on ammeter or tong tester around all three-phase wires of an energized three- phase circuit simultaneously...
277 volts.
Zero amps.
The current will read zero.
No.6 AWG.

46. If any two-phase wires are reversed on a three-phase motor - the motor will...
75 turns.
Reverse its rotation.
Short-circuited after power has been removed.
Two speeds

47. The purpose of a zig-zag transformer is to...
600 volts.
At least two phases
Derive a neutral for grounding purposes.
277 volts.

48. How do you change the direction of a 3-phase AC motor?
Reverse any two-phase leads.
Tied together
Shunted and taped
Check to make sure that the victim is not still in contact with the electricity.

49. If 2000 watts is read in the primary of a single-phase transformer - What is the wattage in the secondary? (Assuming no losses.)
A step-up transformer.
2000 watts
Capacitance.
A grounding resistor.

50. The primary function of a power transformer is to provide...
Reverse its rotation.
Because less current flowing at high voltage causes less line loss per kilowatt.
Full-load - three-phase voltage.
2000 watts