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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. The secondary windings on a 10:1 step-down transformer will have a wire size...
Larger than that found in the primary winding.
The motor rotation is reversed.
Full-load - three-phase voltage.
Potential transformer.

2. The standard frequency of power supplied by the power company is...
100 volts.
60 cycles per second (60 Hertz).
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Because less current flowing at high voltage causes less line loss per kilowatt.

3. What type of 3-phase AC motor is used most often around a mine?
Short-circuited after power has been removed.
60 Hertz
Isolation transformer.
Squirrel-cage induction motor.

4. 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?
The chest should rise or inflate.
Short-circuited after power has been removed.
A.C.
75 turns.

5. The main function of CT's and PT's on a main substation is to...
Allow for metering voltage and Current.
Squirrel-cage induction motor.
Stator windings.
Larger than that found in the primary winding.

6. Operating coils - motors - transformers - relays and solenoids depend upon...
15 feet off the ground.
Electro-magnetism for their operation.
Four times less than that of the secondary.
The sum of the two capacitors.

7. What is the danger when working around capacitors?
Transformers.
An electrical shock hazard may exist.
Hyperventilation.
5:1.

8. The minimum allowable size for trailing cables for mobile haulage equipment powered by alternating current is...
Lower current and higher voltage
No.6 AWG.
They must be installed at least 7 feet above the ground.
Squirrel-cage induction motor.

9. If the conductors of a balanced - three-phase circuit pass through a current transformer - how many amps will flow in the transformer?
60 cycles per second (60 Hertz).
Cut around clothing that has adhered to the burned area.
Zero amps.
Draw less current.

10. If any two-phase wires are reversed on a three-phase motor - the motor will...
Lower current and higher voltage.
Disconnect the frame-grounding conductor.
Inductance.
Reverse its rotation.

11. 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?
They should be made by the same manufacturer.
7 -630 volts.
800 volts.
Four times less than that of the secondary.

12. A control power transformer...
They should be made by the same manufacturer.
Grounded phase and ground fault devices were not operating.
480 volts.
Provides single-phase power to smaller loads such as lights - heaters - etc.

13. Should a ground fault occur - what device can be installed to limit the current in the neutral of a three-phase AC system?
Four times less than that of the secondary.
600 volts.
Current changing constantly in magnitude and periodically in direction.
A grounding resistor.

14. If a voltmeter is connected between a phase lead and the ground of a three-phase 480-volt system - the meter will read...
Draw less current.
277 volts.
5:1.
Disconnect the frame-grounding conductor.

15. The neutral ground-bed electrical power system of an underground mine should be separated from the lightning arrestor grounds by at least...
Hyperventilation.
At the source transformer.
25 feet.
Transformer

16. Which of the following is not needed when using an ohmmeter to determine if a motor is grounded?
Disconnect the frame-grounding conductor.
Alternating current.
Inductance.
1 amp.

17. 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.
Reverse its rotation.
Inductance.
Isolation transformer.

18. How do you change the direction of a 3-phase AC motor?
Provides single-phase power to smaller loads such as lights - heaters - etc.
Four times less than that of the secondary.
Reverse any two-phase leads.
Tied together

19. 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
Reverse its rotation.
Grounded phase and ground fault devices were not operating.
Because less current flowing at high voltage causes less line loss per kilowatt.

20. The primary leads of a single-phase transformer should be connected to the...
Squirrel-cage rotor.
H terminals.
A.C.
Tied together

21. How can you tell if a victim receiving mouth-to-mouth resuscitation is getting air into the lungs?
Check to make sure that the victim is not still in contact with the electricity.
Inductance.
The chest should rise or inflate.
Two speeds

22. The following motor leads are brought out of a 3-phase motor (T1 - T2 - T3 - T4 - T5 - T6). This motor is probably for...
The sum of the two capacitors.
Two speeds
It provides good speed control.
Hyperventilation.

23. The size of a transformer is measured in...
At least two phases
Kilovolt ampere (KVA).
5:1
Two speeds

24. How should clothing be removed from a burn?
Potential transformer.
Alternating current.
480 volts.
Cut around clothing that has adhered to the burned area.

25. When not connected to a load - the secondary leads of a current transformer should be...
The capacitors should be discharged.
60 cycles per second (60 Hertz).
200 volts AC.
Shunted and taped

26. The purpose of a zig-zag transformer is to...
Larger than that found in the primary winding.
15 feet off the ground.
The motor rotation is reversed.
Derive a neutral for grounding purposes.

27. 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?
The capacitors should be discharged.
12 to 15 times a minute.
75 turns.
Two speeds

28. High voltage is used to transmit power over long distances...
The sum of the two capacitors.
Carotid pulse - which is located in the neck next to the Adam's apple.
Tied together
Because less current flowing at high voltage causes less line loss per kilowatt.

29. 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...
Derive a neutral for grounding purposes.
Tied together
Current that changes constantly in magnitude and periodically in direction.
Draw less current.

30. What type motor is a wound-rotor induction motor?
Zero amps.
A.C.
Four times less than that of the secondary.
Nearly constant.

31. If a voltmeter were placed between two phase leads of a three-phase 480-volt system - the meter would read...
480 volts.
A step-up transformer.
Check to make sure that the victim is not still in contact with the electricity.
They should be made by the same manufacturer.

32. Compared to the secondary of a loaded - step-down transformer - the primary has...
Transformer
X.
Lower current and higher voltage.
Inductance.

33. Compared to the secondary of a loaded - step-down transformer - the primary has...
Because 440-volt power draws less current so cables and control circuit wiring can be smaller.
Reverse any two-phase leads.
Draw less current.
Lower current and higher voltage

34. 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.
Full-load - three-phase voltage.
5:1.
Because less current flowing at high voltage causes less line loss per kilowatt.
150 turns.

35. Current in an AC circuit may be measured by...
No.6 AWG.
Because less current flowing at high voltage causes less line loss per kilowatt.
Clamp-on ammeter.
At least two phases

36. The standard frequency of alternating current used at a mine is...
Capacitance.
Draw less current.
Inductance.
60 Hertz

37. 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...
480 volts.
X.
Four times less than that of the secondary.
Draw less current.

38. What is the first step to be followed in aiding a victim of electrical shock?
At least two phases
Check to make sure that the victim is not still in contact with the electricity.
The chest should rise or inflate.
Provides single-phase power to smaller loads such as lights - heaters - etc.

39. The transformer used only to measure voltage is a...
Potential transformer.
Full-load - three-phase voltage.
1 amp.
Inductance.

40. 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 Hertz
The sum of the two capacitors.
A step-up transformer.

41. Define alternating current...
Reverse its rotation.
The chest should rise or inflate.
H terminals.
Current changing constantly in magnitude and periodically in direction.

42. A transformer with a 1:1 ratio is usually a (an)...
Alternating current.
The motor rotation is reversed.
Draw less current.
Isolation transformer.

43. Alternating current may be defined as...
Current that changes constantly in magnitude and periodically in direction.
Hyperventilation.
100 turns.
Isolation transformer.

44. A transformer with 10 turns on the primary coil and 100 turns on the secondary is classified as...
Draw less current.
150 turns.
A step-up transformer.
Electro-magnetism for their operation.

45. 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?
7 -630 volts.
3 -979 Volts.
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Stator windings.

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

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47. A solenoid coil is...
Reduce high AC current to lower values and provide isolation between high voltage circuits and metering.
Shunted and taped
Electro-magnetism for their operation.
An electromagnet.

48. What steps up or steps down AC voltage?
Electro-magnetism for their operation.
Transformer
X.
Kilovolt ampere (KVA).

49. The property of an electrical circuit that opposes any change in voltage is called...
Continuous.
Clamp-on ammeter.
Lower current and higher voltage
Capacitance.

50. Not true about AC power?
No.6 AWG.
Inductance.
It provides good speed control.
The sum of the two capacitors.