Wind Turbine Design

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Primary objective in wind turbine design is to maximize the aerodynamic efficiency, or power extracted from the wind. But this objective should be met by well satisfying mechanical strength criteria and economical aspects. In this video we will see impact of number of blades, blade shape, blade length and tower height on wind turbine design.

Check following article to know more on wind turbine design aspects.

Effect of Number of Blades

As the number of blades in the wind turbine increases aerodynamic efficiency increases, but in a diminishing manner. When we move from 2 blades to 3 blades design efficiency gain is about 3%. But as we move from 3 blades to 4 blades design, efficiency gain is marginal.

Fig.1 Efficiency gain as number of blades in wind turbine is increased
As we increase number of blades, cost of the system increases drastically. Along with that mechanical design of blades also becomes a difficult affair. With more number of blades, blades should be thinner to be aerodynamically efficient. But blades with thinner portion at the root may not withstand bending stress induced due to axial wind load. So generally wind turbines with 3 blades which can accommodate a thicker root cross-section are used.
Fig.2 Wind turbine blades have got thicker root to withstanad huge bending moment induced

Wind Turbine Blade Design

The next big factor which is affecting performance of wind turbine is shape and orientation of blade cross section. A moving machine experiences fluid flow at a different velocity than the actual velocity. It is called as relative or apparent velocity. Apparent velocity of flow is difference between actual flow and blade velocity. Absolute velocity of the flow is shown in first figure, while apparent velocity in the second figure. It is clear that apparent velocity of flow is vectorial difference between actual and blade velocity. The vector difference is shown in the first figure at a particular cross section. A rotating blade will experience an apparent velocity of flow.

Fig.3 Absolute & apparent velocity of wind
A close look at wind turbine blade will reveal that, it is having airfoil cross sections from root to tip. The driving force of wind turbine is, lift force generated, when wind flows over such airfoils. Lift force will be perpendicular to apparent velocity. Generally lift force increases with angle of attack. Along with that undesirable drag force also increases. While tangential component of lift force supports blade rotation, drag force opposes it. So a wind turbine can give maximum performance, when lift to drag ratio is maximum. This is called as, optimum angle of attack. Airfoil cross sections are aligned in a way to operate at this optimum angle of attack.
Fig.4 Lift and drag force induced over a wind turbine blade
Even though flow velocity is uniform along the length of the blade, blade velocity increases linearly as we move to the tip. So angle and magnitude of relative velocity (apparent velocity) of wind varies along the length of the blade. Apparent velocity becomes more aligned to chord direction as we move to the tip.
Fig.5 Change in apparent velocity along length of the blade
So there should be a continuous twist in the blade, so that at every airfoil cross section angle of attack is optimum.

Pitching of Blades

Wind condition can change at any time. So it is also possible to rotate wind turbine blades in its own axis, in order to achieve optimum angle of attack with varying wind condition. This is known as pitching of blades. A clever algorithm which uses wind condition and characteristics of wind turbine as input, governs the pitch angle for the maximum power production.

Fig.6 Schematic of algorithm which governs blade pitching

Blade Length

Next big factor affecting performance of wind turbine is length of the blade. As we discussed in first video lecture, power extracted by the wind turbine varies according to this equation. So it is clear that, a longer blade will favor the power extraction. But, with increase in blade length, deflection of blade tip due to axial wind force also increases. So blind increase in length of the blade may lead to dangerous situation of collision of blade and tower.

Fig.7 Blade bending due to wind load acting on it
With increase in blade length tip velocity increases. Noise produced by the turbine is a strong function of tip velocity. So, it is not permissible to increase blade length after a limit. Other factor which goes against long blades is requirement of huge mechanical structures which leads to heavy investment.

Determination of Tower Height

Most critical factor of wind turbine design is determination of proper tower height. Power input available for wind turbine varies as cube of wind speed. So a small change in wind speed will have huge effect on power production. A typical wind speed increase from ground level is shown in figure. So from power extraction point of view, it is desired to have tower height as high as possible. But difficulty in road transportation and structural design problems put a limit on maximum tower height possible.

Fig.8 Wind velocity increases with altitude resulting in more power extraction



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Working of Synchronous Motor

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Synchronous motors are widely used in the industry for high precision applications. This video gives illustrative and logical explanation on its working.
Detailed webpage version the video is given below.

Introduction

As the name suggests Synchronous motors are capable of running at constant speed irrespective of the load acting on them. Unlike induction motors where speed of the motor depends upon the torque acting on them, synchronous motors have got constant speed-torque characteristics.

Synchronous motors have got higher efficiency (electrical to mechanical power conversion ratio) than its counterparts. Its efficiency ranges from 90 – 92%.

Fig.1 Synchronous motors are high efficiency and high accuracy machines

The Working Principle: RMF – Constant Magnetic field interaction

The constant speed characteristic is achieved by interaction between a constant and rotating magnetic field. Rotor of synchronous motor produces a constant magnetic field and Stator produces a Rotating magnetic field.

Fig.2 Interaction between a revolving and constant magnetic field helps in achieving constant speed characteristic

Stator: Revolving Magnetic Field

The field coil of stator is excited by a 3 phase AC supply. This will produce a revolving magnetic field (RMF), which rotates at synchronous speed. The way RMF is produced with 3 phase AC excitation is explained in a separate article. RMF produced in a synchronous motor and its direction is marked in Fig.2

Rotor: Constant Magnetic field

Rotor is excited by a D.C power supply, magnetic field produced around the rotor coil by DC excitation is shown below. It is clear that the rotor acts like a permanent magnet due to such magnetic field. Alternatively rotor can also be made of permanent magnet.

Interaction of Rotor and RMF is interesting. Assume you are giving an initial rotation to the rotor, with same direction of RMF. You can see that opposite poles of RMF and Rotor will attract each other and they will get locked magnetically. This means that rotor will rotate at the same speed of RMF, or rotor will rotate at synchronous speed.
Fig.3 In first figure opposite poles of RMF and Rotor pole get attracted, rotor already rotating: In second figure poles are magnetically locked

Synchronous Speed

Speed at which RMF rotates or Synchronous speed can easily be derived as follows.

It is clear from the relationship that speed of synchronous motor,Ns(rpm) is directly proportional to frequency of the electricity,f(Hz).P represents number of poles of the rotor. This means that if one has got control over frequency of the electricity, speed of synchronous motor can be very accurately controlled. This is the reason why they are suitable for high precision applications.

Why Synchronous motors are not self starting ?

But if the rotor has got no initial rotation, situation is quite different. North Pole of the Rotor will obviously get attracted by South Pole of RMF, and will start to move in the same direction. But since the rotor has got some inertia, this starting speed will be very low. By this time South pole of RMF will be replaced by a North pole. So it will give repulsive force. This will make the rotor move backward. As a net effect the rotor won’t be able to start.

Fig.4 In first figure opposite poles of RMF and rotor get attracted, when the rotor has no initial rotation: In 2nd figure this becomes a repulsive force
So it can be summarized that synchronous motors are not inherently self starting.

Making Synchronous Motor Self Start – Use of Damper winding

To make synchronous motor self start, a squirrel cage arrangement is cleverly fitted through pole tips. They are also called as damper windings.

Fig.5 Damper winding (squirrel cage) is fitted through poles of the rotor
At the starting rotor field coils are not energized. So with revolving magnetic field, electricity is induced in squirrel cage bars and rotor starts rotating just like an induction motor starts.
Fig.7 Damper winding helps synchronous motor start just like an induction motor starts
When the rotor has achieved its maximum speed, rotor field coils are energized. So as discussed earlier poles of rotor gets locked with poles of RMF and will start rotating at synchronous speed. When rotor rotates at synchronous speed, relative motion between squirrel cage and RMF is zero. This means zero current and force on squirrel cage bars, thus it will not affect synchronized operation of the motor.

Synchronous motor out of Synchronism

Synchronous motors will produce constant speed irrespective of motor load only if the load is within the capability of motor. If external torque load is more than torque produced by the motor, it will slip out of synchronism and will come to rest. Low supply voltage and excitation voltage are other reasons of going out of synchronism. It is interesting to note that synchronous motor has got the same constructional features of an alternator.

Synchronous Condenser

Synchronous motors can also be used to improve overall power factor of the system. When the sole purpose of application is power factor improvement synchronous motors are referred as synchronous condenser. In such situation shaft of the motor is not connected to any mechanical load and it spins freely.



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Working of Refrigerator & Refrigeration Principle

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Refrigeration technology is commonly used in domestic and industrial applications. This video gives a detailed and logical introduction to the workings of refrigerators using the vapor compression cycle.

An elaborated webpage version of the video is given below.



The Basic Principle

The basic principle of refrigeration is simple. You simply pass a colder liquid continuously around the object that is to be cooled. This will take heat from the object. In the example shown, a cold liquid is passed over an apple, which is to be cooled. Due to the temperature difference, the apple loses heat to the refrigerant liquid. The refrigerant in turn is heated due to heat absorption from the apple.

Fig.1 Basic principle of refrigeration is illustrated in this figure
It is clear that, if we can produce cold liquid refrigerant continuously, we can achieve continuous refrigeration. This simple fact forms the core of the refrigeration technology. We will next see how this is achieved.

Components of Refrigerator & Working

An inside view of a refrigerator is shown.

Fig.2 An inside view of a refrigerator
It has 4 main components: compressor, condenser, evaporator, and throttling device. Of these components, the throttling device is the one that is responsible for the production of the cold liquid. So we will first analyze the throttling device in a detailed way and move on to the other components.

Throttling Device

The throttling device obstructs the flow of liquid; cold liquid is produced with the help of this device. In this case, the throttling device is a capillary tube. The capillary tube has an approximate length of 2 m and an inside diameter of around 0.6 mm, so it offers considerable resistance to the flow.

Fig.3 A Capillary tube: This results in sudden drop in pressure and temperature
For effective throttling at the inlet, the refrigerant should be a high-pressure liquid. The throttling device restricts the flow, which causes a tremendous pressure drop. Due to the drop in pressure, the boiling point of the refrigerant is lowered, and it starts to evaporate. The heat required for evaporation comes from the refrigerant itself, so it loses heat, and its temperature drops. If you check the temperature across the throttling device, you will notice this drop.

It is wrong to say that the throttling is a process. We know only the end points of throttling, that is, the states before and after throttling. We don’t know the states in between, since this is a highly irreversible change. So it would be correct to call throttling a phenomenon rather than a process.

Evaporator - Heat Absorption Process

The next phase is simple: this cold liquid is passed over the body that has to be cooled. As a result, the refrigerant absorbs the heat. During the heat absorption process, the refrigerant further evaporates and transforms into pure vapor. A proper heat exchanger is required to carry the cold refrigerant over the body. This heat exchanger is known as an evaporator.

Fig.4 Cold liquid is passed through a heat exchanger know as evaporator for absorbing heat from the refrigerator
So we have produced the required refrigeration effect. If we can return this low-pressure vapor refrigerant to the state before the throttling process (that is the high-pressure liquid state), we will be able to repeat this process. So first step, let’s raise the pressure.

Compressor

A compressor is introduced for this purpose. The compressor will raise the pressure back to its initial level. But since it is compressing gas, along with pressure, temperature will also be increased. This is unavoidable.

Fig.5 A compressor is used to raise pressure of the refrigerant
Now the refrigerant is a high-pressure vapor. To convert it to the liquid state, we must introduce another heat exchanger.

Condenser

This heat exchanger is fitted outside the refrigerator, and the refrigerant temperature is higher than atmospheric temperature. So heat will dissipate to the surroundings. The vapor will be condensed to liquid, and the temperature will return to a normal level.

Fig.6 Condenser heat exchanger is fitted outside the refrigerator so it will reject heat to the surroundings

So the refrigerant is back to its initial state again: a high-pressure liquid. We can repeat this cycle over and over for continuous refrigeration. This cycle is known as the vapor compression cycle. Refrigeration technology based on the vapor compression cycle is the most commonly used one in domestic and industrial applications.

Refrigeration Accessories

You can find more details on refrigerator components here. Evaporators and condensers have fins attached to them. The fins increase the surface area available for convective heat transfer and thus will significantly enhance heat transfer.

Fig.7 Fins attached to the condenser and evaporator
Since the evaporator is cooling the surrounding air, it is common that water will condense on it, forming frost. The frost will act as an insulator between the evaporator heat exchanger and the surrounding air. Thus it will reduce the effectiveness of the heat removal process. Frequent removal of frost is required to enhance the heat transfer. An automatic defrosting mechanism is employed in all modern refrigerators.

More on Compressor

Apart from raising the pressure, the compressor also helps maintain the flow in the refrigerant circuit. Usually, a hermetically sealed reciprocating type compressor is used for this purpose. You might have noticed that, your household refrigerator consumes a lots of electricity compared to the other devices. In a vapor compression cycle, we have to compress the gas; compressing the gas and raising pressure is a highly energy intensive affair. This is the reason why the refrigerator based on the vapor compression refrigeration technology consumes a lot of electricity.

Coefficient of Performance

The heat and power transfer happening in a vapor compression refrigeration circuit is shown below.

Fig.8 Energy interaction happening in a refrigeration system
A simple energy balance of the system yields the following relationship.
It is often required to evaluate performance of a refrigerator or compare between different refrigeration technologies. A term called Coefficient of Performance (C.O.P) helps in doing this. To understand this term completely, we need to know what is the input and output of a refrigeration system. What we need from a refrigerator is the cooling effect. Or QABSORBED is the output of a refrigeration cycle. Input to the refrigerator is the power given to the compressor. So the term C.O.P can easily be defined as output by input and is expressed as follows.



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How does an Alternator work ?

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Alternators are the workhorse of the power generation industry.It is capable to generate AC power at a specified frequency. They are also referred as Synchronous generators. This video gives a detailed and illustrative introduction on working of alternators.
Detailed webpage version of of the video is given below.



The Basic Principe

Electricity is produced in alternators by electromagnetic induction. To generate electricity in a coil either the coil should rotate with respect to a magnetic field or a magnetic field should rotate with respect to the coil.

Fig.1 Two methods to produce electricity: Rotating coil and Rotating magnetic field concept
In the case of alternators the latter approach is used. The reason behind rotating magnetic filed approach will be discussed in coming sessions.

Main Parts and Working

Rotor and Armature coils are the 2 main parts of an alternator. Rotor produces a rotating magnetic field. Armature coils are stationary and rotating magnetic flux associated with the rotor induces electricity in the armature coils.

Fig.2 Rotor and Armature coils are the 2 main parts of an alternature
The kind of rotor shown here is known as Salient pole rotor. For gaining better insight of its working let’s consider a rotor with just 4 poles. Rotor coils are excited with a DC power source. Magnetic field produced around it would be as shown.
Fig.3 A 4 pole salient pole rotor and magnetic filed produced around it when excited by a D.C power supply
The rotor is made to rotate by a prime mover. This makes the rotor flux also rotate along with it, at the same speed.

Such revolving magnetic flux now intersects the armature coils, which is fitted around the rotor. This will generate an alternating E.M.F across the winding.

Fig.4 When rotor is made to rotate electricity gets induced in armature coils

Frequency of Induced E.M.F

Since 4 pole rotor has got 2 pairs of N-S pole, when the rotor turns a half revolution the induced E.M.F takes one complete cycle. So it is clear that frequency of the induced E.M.F is directly proportional to the number of poles and rotor speed. It can be easily established that frequency of induced E.M.F f(Hz), rotor speed N(rpm) and number of poles P are connected through the following relationship.

It is clear from this relationship that, frequency of electricity produced is synchronized with mechanical rotational speed.

Production of 3 Phase Electricity

For producing 3 phase A.C current, 2 more such armature coils which are in 120 degree phase difference with the first is put in the stator winding.

Fig.5 For producing 3 phase electricity 2 more armature wingdings which are 120 degree apart from the first is introduced
Generally one end of these 3 coils are star connected and 3 phase electricity is drawn from the other ends. Neutral cable can be drawn from the star connected end.

When to use a Salient pole rotor ?

It is clear from the equation above that in order to produce 60 Hz electricity a 4 pole rotor should run at following a speed of 1800 RPM. Such huge RPM will induce a tremendous centrifugal force on poles of the rotor and it may fail mechanically overtime.

Fig.6 Rotors with less number of poles require high RPM, this in turn induces huge centrifugal force on poles of the rotor
So salient pole rotors are generally having 10-40 poles; which demands lower rpm. Or salient pole rotors are used when the prime mover rotates at relatively lower speed (120 - 400 RPM), such as water turbines and I.C engines.

Pole core & Stator core

Pole core is used to effectively transfer magnetic flux and they are made with fairly thick steel lamina. Such insulated lamina reduces energy loss due to eddy current formation. At the stator side also core lamina are used to enhance the magnetic flux transfer.

Fig.7 Pole and stator core enhances magnetic flux transfer and they are made of laminated steel lamina

Self Excited Generator

DC current is supplied to rotor via a pair of slip rings. This is the reason why rotating magnetic field approach is used in alternator. If rotating coil method were used, slip rings have to fitted along with the armature coils in order to collect electricity. But transferring such high voltage electricity via slip ring is rather impractical. It is quite possible to transfer low voltage DC excitation current via slip rings.

This DC current is supplied either from an external source or from a small DC generator which is fitted on the same prime mover. Such alternators are called self excited.

Fig.8 Slip rings are used to supply DC current to the rotor coil; this DC current could come from an inbuilt DC generator
With variation of load generator terminal output voltage will vary. It is desired to keep the terminal voltage in a specified limit. An automatic voltage regulator helps in achieving this. Voltage regulation can be easily achieved by controlling the field current. If terminal voltage is below the desired limit AVR increases the field current, thus the field strength. This will result in increase in terminal voltage. If terminal voltage is below the specified limit the reverse is done.



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