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Wind Energy Technology_FA24 > [L7]_ELECTRICITY & GENERATORS > Flashcards

[L7]_ELECTRICITY & GENERATORS Flashcards

1
Q

ENGY-688

Who observed a magnet move due to an electric current in 1820?

[L7]

A

Hans Christian Oersted.

[L7]

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2
Q

ENGY-688

Why is induction in transformers important in electrical systems?

[L7]

A

It’s used to increase or decrease the voltage of alternating current in a system.

[L7]

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3
Q

ENGY-688

What determines the rotor’s alignment with the stator in a synchronous generator under zero load?

[L7]

A

The poles of the stator’s magnetic field align radially with the rotor’s poles.

[L7]

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4
Q

ENGY-688

Even when no power is delivered to the grid by the generator, what is the grid still supplying to the stator of a synchronous generator?

[L7]

A

Current to energize the magnetic field of the stator.

[L7]

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5
Q

ENGY-688

In newer designs of synchronous generators, how is the placement of the magnet and armature changed?

[L7]

A

The magnet is on the stator and is coupled to the hub, while the armature is on the rotor and is stationary.

[L7]

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6
Q

ENGY-688

Why are variable speed generators more efficient for capturing wind energy?

[L7]

A

They can operate at peak power for all wind speeds below the rated wind speed.

[L7]

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7
Q

ENGY-688

Why do DDSG generators produce more torque than a 1500 rpm induction generator when comparing at the same power?

[L7]

A

The rotation speed of DDSG turbines is much lower.

[L7]

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8
Q

ENGY-688

In asynchronous generators, what is the significance of the air gap in the core?

[L7]

A

It allows the magnetic field to travel between the stator and the rotor.

[L7]

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9
Q

ENGY-688

What happens in an asynchronous generator when the rotor speed and stator are in sync?

[L7]

A

The slip is zero.

[L7]

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10
Q

ENGY-688

Why are soft starters used in induction generators?

[L7]

A

To control the flow of current during startup, reducing cost and damage to the equipment.

[L7]

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11
Q

ENGY-688

Why are capacitor banks deployed in asynchronous generators?

[L7]

A

To reduce the need for reactive power from the grid and thus reduce costs.

[L7]

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12
Q

ENGY-688

What challenges arise when trying to make traditional induction generators variable speed?

[L7]

A

They operate with a very small range of slip, meaning their rotor speed is almost constant.

[L7]

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13
Q

ENGY-688

How do asynchronous generators differ from synchronous ones regarding the source of their magnetic fields?

[L7]

A

Asynchronous generators’ magnetic fields come from the stator connected to the grid, not from an internal DC source.

[L7]

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14
Q

ENGY-688

Why are DDSG generators’ rotors large?

[L7]

A

To accommodate the large number of poles.

[L7]

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15
Q

ENGY-688

What is the function of a rectifier in a variable speed synchronous generator system?

[L7]

A

Convert variable AC to DC.

[L7]

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16
Q

ENGY-688

In a synchronous generator, what determines the angular speed of the stator’s magnetic field?

[L7]

A

The grid.

[L7]

17
Q

ENGY-688

What is a key difference between AC and DC current in terms of flow direction?

[L7]

A

AC changes direction periodically, whereas DC flows in one direction.

[L7]

18
Q

ENGY-688

What occurs when you place a rare earth magnet inside a wire coil and change the direction of the current?

[L7]

A

The magnet will move back and forth, known as a solenoid.

[L7]

19
Q

ENGY-688

How can you increase the strength of the magnetic field generated by a looped wire?

[L7]

A

Increase the number of loops in the coil.

[L7]

20
Q

ENGY-688

What’s another name for a wind turbine that can adjust its rotor speed based on wind conditions?

[L7]

A

Variable speed turbine.

[L7]

21
Q

ENGY-688

What is the primary method employed to make the output from these generators compatible with a constant frequency and voltage grid?

[L7]

A

The voltage and frequency from variable speed synchronous generators are first converted from variable AC to DC and then from DC back to constant frequency and voltage AC using a rectifier and an inverter, respectively.

[L7]

22
Q

ENGY-688

What role does the breakdown point play in the operating characteristics of an asynchronous generator, specifically concerning slip?

[L7]

A

The breakdown point is where the torque reaches its maximum. Beyond this point, the torque decreases even if the slip continues to increase.

[L7]

23
Q

ENGY-688

How does the angular speed ω of a stator’s magnetic field compare to the angular speed of rotation of the rotor when the opposite poles of the stator and rotor are in radial alignment?

[L7]

A

They are the same.

[L7]

24
Q

ENGY-688

Describe the consequence of a solenoid’s action when a rare earth magnet is placed inside and the current direction is continuously changed.

[L7]

A

The magnet will oscillate or move back and forth according to the changing current direction.

[L7]

25
Q

ENGY-688

For asynchronous generators, define ‘slip’ and its significance in the generator’s operational state.

[L7]

A

Slip is the difference between the synchronous speed of the magnetic field and the rotor speed. It determines the generator’s mode of operation, with negative slip indicating generation mode.

[L7]

26
Q

ENGY-688

In an induction generator, what specific component configuration gives it the name “squirrel cage”?

[L7]

A

The rotor, which has shorted bars (often aluminum) that form a cage-like structure.

[L7]

27
Q

ENGY-688

Given the formula for determining the frequency of a synchronous generator
[f = (n/120) ​ p], what happens to the frequency if the number of poles (p) is doubled while keeping the speed (n) constant?

[L7]

A

The frequency doubles.

[L7]

28
Q

ENGY-688

In the context of electromagnetic induction in a transformer, what critical principle defines the ratio of voltage change between the primary and secondary coils?

[L7]

A

The turns ratio: the voltage across the primary coil to the voltage across the secondary coil is directly proportional to the number of turns in the primary to the number of turns in the secondary.

[L7]

29
Q

ENGY-688

Can you identify another key figure from the same era as Oersted who made significant contributions to electromagnetism, and specify their most well-known experiment?

[L7]

A
  • Michael Faraday for the Faraday’s Law of Electromagnetic Induction

[L7]

30
Q

ENGY-688

When comparing DDSG generators with traditional generators, what physical characteristic change leads to the significant increase in torque for DDSG generators?

[L7]

A

The larger radius thus allowing the greater number of poles.

[L7]

Wind Energy Technology_FA24 (9 decks)

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