[L7]_ELECTRICITY & GENERATORS

Question 1

ENGY-688

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

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Answer 1

Hans Christian Oersted.

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Question 2

ENGY-688

Why is induction in transformers important in electrical systems?

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Answer 2

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

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Question 3

ENGY-688

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

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Answer 3

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

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Question 4

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?

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Answer 4

Current to energize the magnetic field of the stator.

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Question 5

ENGY-688

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

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Answer 5

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

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Question 6

ENGY-688

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

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Answer 6

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

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Question 7

ENGY-688

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

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Answer 7

The rotation speed of DDSG turbines is much lower.

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Question 8

ENGY-688

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

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Answer 8

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

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Question 9

ENGY-688

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

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Answer 9

The slip is zero.

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Question 10

ENGY-688

Why are soft starters used in induction generators?

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Answer 10

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

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Question 11

ENGY-688

Why are capacitor banks deployed in asynchronous generators?

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Answer 11

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

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Question 12

ENGY-688

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

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Answer 12

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

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Question 13

ENGY-688

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

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Answer 13

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

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Question 14

ENGY-688

Why are DDSG generators’ rotors large?

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Answer 14

To accommodate the large number of poles.

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Question 15

ENGY-688

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

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Answer 15

Convert variable AC to DC.

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Question 16

ENGY-688

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

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Answer 16

The grid.

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Question 17

ENGY-688

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

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Answer 17

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

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Question 18

ENGY-688

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

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Answer 18

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

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Question 19

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

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Answer 19

Increase the number of loops in the coil.

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Question 20

ENGY-688

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

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Answer 20

Variable speed turbine.

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Question 21

ENGY-688

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

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Answer 21

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.

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Question 22

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

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Answer 22

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

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Question 23

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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?

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Answer 23

They are the same.

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Question 24

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

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Answer 24

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

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Question 25

# ENGY-688 For asynchronous generators, define ***'slip'*** and its significance in the generator's operational state. ## Footnote [L7]

Answer 25

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. ## Footnote [L7]

Question 26

# ENGY-688 In an induction generator, what specific component configuration gives it the name ***"squirrel cage"***? ## Footnote [L7]

Answer 26

The rotor, which has shorted bars (often aluminum) that form a cage-like structure. ## Footnote [L7]

Question 27

# 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**? ## Footnote [L7]

Answer 27

The frequency doubles. ## Footnote [L7]

Question 28

# 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**? ## Footnote [L7]

Answer 28

**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. ## Footnote [L7]

Question 29

# 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? ## Footnote [L7]

Answer 29

* Michael Faraday for the Faraday's Law of Electromagnetic Induction ## Footnote [L7]

Question 30

# ENGY-688 When comparing ***DDSG generators*** with traditional generators, what ***physical characteristic*** change leads to the significant ***increase*** in torque for DDSG generators? ## Footnote [L7]

Answer 30

The larger radius thus allowing the greater number of poles. ## Footnote [L7]