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2024 Gas > Electrical > Flashcards

Electrical Flashcards

1
Q

What method of source of energy may be used to generate electricity?

A) Heat
B) Friction
C) Chemical reaction
D) All are correct

A

All are correct

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2
Q

What are the advantages of single-phase induction motors?

A

Simple construction, reliable, economical for small power ratings

Single-phase induction motors are suitable for applications with lower power requirements.

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3
Q

How does the electrical power factor of single-phase induction motors compare to three-phase induction motors?

A

Lower for single-phase induction motors

A low power factor indicates less efficient use of electrical power.

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4
Q

What percentage of output do single-phase induction motors develop compared to three-phase induction motors of the same size?

A

About 50%

This indicates the lower efficiency and performance of single-phase motors.

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5
Q

What is the starting torque like for asynchronous motors or single-phase induction motors?

A

Low

Low starting torque can limit their use in applications requiring high initial torque.

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6
Q

How does the efficiency of single-phase induction motors compare to three-phase induction motors?

A

Less efficient

This affects operational costs and performance in larger applications.

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7
Q

What is the definition of an electrical motor?

A

A device that converts electrical energy into mechanical energy

This is the fundamental function of all electrical motors.

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8
Q

What type of motor is most widely used in a three-phase system?

A

Induction motor

Induction motors are preferred for their robustness and efficiency.

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9
Q

What causes electrons to move in a conductor during electromagnetic induction?

A

Movement of a conductor through a magnetic field or movement of the field across a conductor

This principle is fundamental to the operation of many electrical devices.

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10
Q

How does an electrical motor work using electromagnets?

A

By creating a rotating magnetic field that induces movement in the rotor

This process is critical for the motor’s operation and efficiency.

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11
Q

What does the three-phase power curve consist of?

A

Three separate single-phase curves evenly separated by 120 electrical degrees

This configuration ensures continuous power delivery.

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12
Q

What is the benefit of a three-phase power supply?

A

Always has a phase at maximum (peak) power

This leads to more stable and reliable power output compared to single-phase systems.

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13
Q

True or False: Single-phase systems can sometimes have no power output.

A

True

This is due to the nature of single-phase power delivery.

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14
Q

What is the arrangement of coils in a three-phase power supply?

A

Three coils (windings) that are imbedded into the stator at 120 degrees apart

This configuration allows for continuous power generation as the rotor spins.

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15
Q

What are the two types of magnets that can be used in the rotor of a three-phase power supply?

A

Permanent magnets or electromagnets

These magnets create a rotating magnetic field essential for inducing voltage.

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16
Q

What is induced voltage?

A

Voltage generated when the rotor’s magnetic field cuts across the coils on the stator

This phenomenon occurs due to electromagnetic induction.

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17
Q

List the three things needed to produce voltage in a three-phase power supply.

A
  • Conductors (the coils on the stator)
  • A strong magnetic field (magnets)
  • Movement (rotating rotor)

All three elements must be present for voltage generation.

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18
Q

How does electricity generation occur in a three-phase power supply?

A

As the rotor spins, each coil on the stator produces electricity sequentially

The first coil produces electricity, followed by the second and then the third as the rotor rotates.

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19
Q

What happens to the electrical output of the coils as the rotor passes them?

A

One coil reaches maximum output while the others transition between zero and maximum output

This creates a continuous flow of electricity.

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20
Q

True or False: In a three-phase system, there is always a coil producing maximum voltage output.

A

True

This is a key advantage over single-phase systems.

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21
Q

Fill in the blank: In a three-phase power supply, the coils are arranged at _______ degrees apart.

A

120

This spacing is critical for the operation of three-phase systems.

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22
Q

What is the difference in output between a three-phase and a single-phase system?

A

Three-phase systems have continuous power output with one coil at maximum voltage, while single-phase systems have a single output wave

This results in more stable and efficient power delivery in three-phase systems.

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23
Q

What role does the rotor play in a three-phase power supply?

A

The rotor creates a rotating magnetic field that induces voltage in the stator coils

The rotor’s motion is essential for electricity generation.

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24
Q

What is the significance of the sine wave forms for single and three-phase systems?

A

They illustrate the differences in voltage output over time between the two systems

Sine waves for three-phase systems show overlapping outputs, enhancing power stability.

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25
Q

What are the two main components of a three-phase induction motor?

A

Stator and rotor

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26
Q

What materials are used to make the stator in a three-phase induction motor?

A

Stacks of laminated, stamped sheets of high-grade silicon steel

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27
Q

Why are the laminations in the stator insulated from each other?

A

To cut down losses due to heat (eddy currents and hysteresis)

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28
Q

What is the arrangement of the insulated three-phase windings in the stator?

A

Placed at 120 degrees around the stator

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29
Q

What does the application of a three-phase supply to the windings create?

A

A rotating magnetic field

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30
Q

Fill in the blank: The significant parts of a three-phase induction motor include the stator, rotor, ________, bearings, shaft, and end bells.

A

enclosure

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31
Q

True or False: The rotor is the most important component of a three-phase induction motor.

A

False

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32
Q

What is the appearance of the rotor in a three-phase motor commonly referred to as?

A

Squirrel cage

The term ‘squirrel cage’ refers to the rotor’s cylindrical shape and design.

33
Q

What material is the rotor of a three-phase motor primarily made of?

A

Laminated high grade silicon steel

This material choice helps reduce losses due to heat.

34
Q

What types of losses are minimized by using laminated high grade silicon steel in the rotor?

A
  • Eddy current losses
  • Hysteresis losses

These losses contribute to inefficiency in motor operation.

35
Q

What are the conductors on the rotor made of?

A

Solid copper rods

Copper is used for its excellent electrical conductivity.

36
Q

How are the copper rods on the rotor connected?

A

Short circuited by copper end rings

This configuration contributes to the ‘cage’ appearance of the rotor.

37
Q

What is the purpose of skewing the rotor’s cage?

A

To prevent the rotor’s magnetic field from lining up perfectly with the stator’s magnetic field

This allows the rotor’s magnetic field to chase the stator’s magnetic field.

38
Q

What problem does skewing the rotor help avoid?

A

Locking the rotor in one position (lock rotor)

Locking would prevent the rotor from turning.

39
Q

What is a three-phase motor considered?

A

Self-starting

A three-phase motor can start on its own without external help.

40
Q

What happens when three-phase power is applied to the stator winding?

A

A rotating magnetic field is created

This field induces a magnetic field in the rotor.

41
Q

What is the term for the speed of the rotating magnetic field in the stator?

A

Synchronous speed

This speed is essential for understanding motor operation.

42
Q

What do we call the revolutions per minute of the rotor?

A

Rated speed

This is the actual speed at which the rotor operates.

43
Q

What is the difference between synchronous speed and rated speed referred to as?

A

Slip

Slip is a key factor in motor performance.

44
Q

How is slip measured?

A

As a percentage difference between synchronous and rated speed

Slip indicates the amount of torque produced by the motor.

45
Q

What is the relationship between slip percentage and torque rating?

A

Greater slip percentage results in greater torque rating

This is crucial for understanding motor efficiency.

46
Q

What are Variable Frequency Drives (VFDs) also known as?

A
  • Adjustable speed drives
  • Adjustable frequency drives
  • AC drives
  • Inverters
  • Drives
  • Divers

VFDs are important for controlling motor speed.

47
Q

What is the advantage of using VFDs for controlling pump and fan speed?

A

More efficient and accurate than throttling and dampening

This method improves system performance.

48
Q

What does a Variable Frequency Drive control?

A

The speed of an electric motor

VFDs vary frequency and voltage to adjust motor speed.

49
Q

What is the frequency range that a VFD can vary between to increase speed?

A

60Hz to 120Hz

This range allows for significant speed adjustments.

50
Q

What is the primary challenge when adjusting the speed of AC motors compared to DC motors?

A

AC motors are harder to adjust than DC motors

However, AC motors are more available and easier to transform.

51
Q

What is the process for converting AC power to DC in a VFD?

A

AC power is sent into a full wave bridge rectifier

This conversion is necessary for the VFD operation.

52
Q

What type of output does the full wave bridge rectifier produce?

A

Non-filtered DC voltage

This output is then sent to a DC bus/filter.

53
Q

What is the role of an inverter in a motor control system?

A

The inverter converts DC to AC by rapidly opening and closing switches.

54
Q

What type of power is produced by an inverter?

A

AC output.

55
Q

How can the speed of a motor be controlled?

A

By controlling the frequency and voltage output of the inverter.

56
Q

What is the purpose of a rectifier in a power system?

A

To convert AC to DC.

57
Q

What does a Variable Frequency Controller (VFC) do?

A

It controls the frequency and voltage to manage motor speed.

58
Q

What does ‘VFD’ stand for?

A

Variable Frequency Drive.

59
Q

Fill in the blank: The electrical consumption of the system can be reduced by using _______ motors.

A

higher efficiency

60
Q

What components benefit from maintaining proper airflow (CFM)?

A

Evaporator coils, heat exchangers, condenser coils.

61
Q

True or False: Higher efficiency motors contribute to improved overall system efficiency.

A

True.

62
Q

What is the significance of a DC Bus/Filter in motor control?

A

It helps in managing and smoothing the DC power supply.

63
Q

What type of power is ‘Filtered DC’?

A

DC power that has been smoothed and regulated.

64
Q

What type of power is ‘Modified AC’?

A

AC power that has been altered from its pure form, often for specific applications.

65
Q

What is a characteristic of ‘3-Phase AC’ power?

A

It provides a more stable and efficient power supply for motors.

66
Q

What is the primary power supply type for electronically commutated motors (ECM)?

A

AC power supply

The AC supply is converted to DC for better speed control.

67
Q

What are the main components of an electronically commutated motor (ECM)?

A

Brushless DC motor and microprocessor

The microprocessor enables the combination of AC and DC motor features.

68
Q

What is a limitation of AC motors regarding speed?

A

Limited speed range

AC motors operate efficiently only at the peak of their performance curve.

69
Q

What device can be used to control motor speed in AC motors?

A

M-D (Motor Drive)

M-D increases or decreases the frequency of the AC supply.

70
Q

How do DC motors differ from AC motors in terms of speed control?

A

Better speed control

DC motors allow for finer adjustments in speed.

71
Q

What mechanical components do DC motors use that can lead to wear?

A

Carbon brushes and commutator rings

These components contribute to noisier operation.

72
Q

What is a challenge associated with using a DC power supply?

A

Harder to come by

DC power supply is less commonly available than AC.

73
Q

What does an ECM use to create a rotating magnetic field?

A

Electronics

ECMs do not use commutators and slip rings like DC motors.

74
Q

What is the function of the onboard microprocessor in an ECM?

A

Converts AC to DC and controls motor operation

It directs current for speed and direction control.

75
Q

List some benefits of using an electronically commutated motor (ECM).

A
  • Uses less energy
  • Wider range of speeds
  • Runs cooler
  • Programmable

These advantages make ECMs efficient for various applications.

76
Q

In which systems can ECM motors be used?

A

HVAC systems, refrigeration, evaporators, condensers

ECMs are versatile and applicable in many fields.

77
Q

True or False: ECMs are less efficient than traditional AC or DC motors.

A

False

ECMs are more efficient due to their electronic control.

78
Q

Fill in the blank: The microprocessor in an ECM has an integrated _______ to manage current direction.

A

controller

This function enhances the motor’s performance.

2024 Gas (3 decks)

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