What is Turbine?

 What is Turbine? 

A turbine is a device that harnesses the kinetic energy of some fluid - such as water, steam, air, or combustion gases - and turns this into the rotational motion of the device itself. These devices are generally used in electrical generation, engines, and propulsion systems and are classified as a type of engine. They are classified as such because engines are simply technologies that take an input and generate an output. A simple turbine is composed of a series of blades - currently steel is one of the most common materials used - and allows the fluid to enter the turbine, pushing the blades. These blades then spin and eject the fluid which now has less energy it did than when it entered the turbine. Some of the energy is captured by the turbine and used.

Turbines are used in many different areas, and each type of turbine has a slightly different construction to perform its job properly. Turbines are used in wind power, hydropower, in heat engines, and for propulsion. Turbines are extremely important because of the fact that nearly all electricity is generated by them.

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Principles of Electromechanical Energy Conversion

Principles of Electromechanical Energy Conversion:-

• Electromechanical Energy Conversion :- Conversion of other forms of energy in electrical form have many advantages like easy control, utilise, reliable, efficient etc. An electromechanical energy conversion device is one which converts electrical energy into mechanical energy and vice- versa.

Categories of various electromechanical energy conversion: -

(i) First category:- involves small motion, processes only low energy signals from electrical to mechanical or vice-versa. Example : telephone receivers, loud-speakers, microphone.

(ii) Second category:- consists of force or torque-producing devices with limited mechanical motion. Example: electromagnets, relays, moving-iron instruments.

(iii) Third category:- consists of continuous energy conversion devices. Example: generators and motors.

State electromechanical energy conversion. Also explain its significance:-

"Energy can neither be created nor be destroyed". One can only change its forms using appropriate energy conversion processes Energy conversion takes place between well known pairs of forms of energy.

1. Electrical- Chemical

2. Electrical -Thermal

3. Electrical- Optical

4. Electrical - Sound

5. Electrical- Mechanical

Electromechanical energy conversion is a process in which electrical energy is converted into mechanical energy or mechanical energy into electrical energy. The main advantage of the conversion is that energy in electrical form can be transmitted, utilized and controlled more reliably, easily and efficiently. Energy conversion derives are required at path ends of an electrical system, since energy is neither available and nor required in electrical form. Electromechanical energy conversion finds application in following categories of system:

(a) Transducers: Devices for obtaining signal for measurement/control.

(b) Force-producing devices : Solenoid-actuators, relays, electromagnets.

(c) Devices for continuous-energy conversion : Motor/generator.

Principle of Electromechanical Energy Conversion in rotating machines : - 

When energy is converted from one form to another, the principle of conversion of energy can be evoked. According to this principle, energy can neither be created nor destroyed, it can merely be converted from one form to another. In an energy conversion device, out of the total input energy, some energy is converted into the required form, some energy is stored and the rest is dissipated. In view of this, the energy balance equation must include these energy terms, and for a motor, it is:

(Total Electrical Energy Input) = (Mechanical Energy Output) + (Total Energy Stored) + (Total Energy Dissipated)

For generator action,

(Total Mechanical Energy Input) = (Electrical Energy Output) + (Total Energy Stored) + (Total Energy Dissipated)

So, the principle of en rgy conversion is based on energy balance. For a rotating machine

W elec. = W mech. - W fld.

Where,

W elect. → net electrical energy input

Wmech→ energy converted into mechanical form

Wfld.→ stored energy + energy losses (change in magnetic stored energy).

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Synchronous Motors: Applications, Starting Methods & Working Principle:-

                Electrical motors are an Electro-mechanical device that converts electrical energy to mechanical energy. Based on the type of input we have classified it into single phase and 3 phase motors.

The most common type of 3 phase motors are synchronous motors and induction motors. When three-phase electric conductors are placed in certain geometrical positions (i.e. in a certain angle from one another) – an electrical field is generated. The rotating magnetic field rotates at a certain speed known as the synchronous speed.

If an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with the same speed of rotating field.

This is where the term synchronous motor comes from, as the speed of the rotor of the motor is the same as the rotating magnetic field.

It is a fixed speed motor because it has only one speed, which is synchronous speed. This speed is synchronized with the supply frequency. The synchronous speed is given by:

Where:

• N= The Synchronous Speed (in RPM – i.e. Rotations Per Minute)
• f = The Supply Frequency (in Hz)
• p = The number of Poles

Construction of Synchronous Motor:-

Usually, its construction is almost similar to that of a 3 phase induction motor, except the fact that here we supply DC to the rotor, the reason of which we shall explain later. Now, let us first go through the basic construction of this type of motor. From the above picture, it is clear that how do we design this type of machine. We apply three phase supply to the stator and DC supply to the rotor.

Main Features of Synchronous Motors:-

1. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition
3. This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

Principle of Operation Synchronous Motor:-

Synchronous motors are a doubly excited machine, i.e., two electrical inputs are provided to it. Its stator winding which consists of a We provide three-phase supply to three-phase stator winding, and DC to the rotor winding.

The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering the 50 Hz power frequency, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolutions in 1 min or 50 revolutions in 1 sec.

                At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next instant it will be N-S causing attractive force. But due to the inertia of the rotor, it is unable to rotate in any direction due to that attractive or repulsive forces, and the rotor remains in standstill condition. Hence a synchronous motor is not self-starting.

Here we use some mechanical means which initially rotates the rotor in the same direction as the magnetic field to speed very close to synchronous speed. On achieving synchronous speed, magnetic locking occurs, and the synchronous motor continues to rotate even after removal of external mechanical means.

But due to the inertia of the rotor, it is unable to rotate in any direction due to that attractive or repulsive forces, and the rotor remains in standstill condition. Hence a synchronous motor is not self-starting.

Here we use some mechanical means which initially rotates the rotor in the same direction as the magnetic field to speed very close to synchronous speed. On achieving synchronous speed, magnetic locking occurs, and the synchronous motor continues to rotate even after removal of external mechanical means.

1.  Motor starting with an external prime Mover:     Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor. Here, we do not apply DC excitation initially. It rotates at speed very close to its synchronous speed, and then we give the DC excitation. After some time when magnetic locking takes place supply to the external motor is cut off.
2. Damper winding      In this case, the synchronous motor is of salient pole type, additional winding is placed in rotor pole face. Initially, when the rotor is not rotating, the relative speed between damper winding and rotating air gap flux is large and an emf is induced in it which produces the required starting torque. As speed approaches synchronous speed, emf and torque are reduced and finally when magnetic locking takes place; torque also reduces to zero. Hence in this case synchronous motor first runs as three phase induction motor using additional winding and finally it is synchronized with the frequency.

Application of Synchronous Motors:-

1. Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive.
2. Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc.

DC Motor or Direct Current Motor:-

What is DC Motor ?

The electric motor operated by dc is called dc motor. This is a device that converts DC electrical energy into a mechanical energy

Principle of DC Motor

When a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. In other words, when a magnetic field and an electric field interact, a mechanical force is produced. The DC motor or direct current motor works on that principal. This is known as motoring action.

The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger, and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of current, then the thumb represents the direction in which force is experienced by the shaft of the DC motor.

Structurally and construction wise a direct current motor is exactly similar to a DC generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port.Here in a DC motor, the supply voltage E and current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed ω from the mechanical port or output port.

the parameter K relates the input and output port variables of the direct current motor.

 

So from the picture above, we can well understand that motor is just the opposite phenomena of a DC generator, and we can derive both motoring and generating operation from the same machine by simply reversing the ports. 

Advantages of DC Motor:

1. Provide excellent speed control for acceleration and deceleration
2. Easy to understand design

Simple, cheap drive design

Types of DC Motors

Direct motors are named according to the connection o the field winding with the armature. There are 3 types:

1. Shunt wound DC motor
2. Series wound DC motor
3. Compound wound DC motor

Detailed Description of a DC Motor:-

To understand the DC motor in details lets consider the diagram below,

The circle in the center represents the direct current motor. On the circle, we draw the brushes. On the brushes, we connect the external terminals, through which we give the supply voltage. On the mechanical terminal, we have a shaft coming out from the center of the armature, and the shaft couples to the mechanical load. On the supply terminals, we represent the armature resistance Ra in series.

Now, let the input voltage E, is applied across the brushes. Electric current which flows through the rotor armature via brushes, in presence of the magnetic field, produces a torque Tg. Due to this torque Tg the dc motor armature rotates. As the armature conductors are carrying currents and the armature rotates inside the stator magnetic field, it also produces an emf Eb in the manner very similar to that of a generator. The generated Emf Eb is directed opposite to the supplied voltage and is known as the back Emf, as it counters the forward voltage.
The back emf like in case of a generator is represented by

Where, P = no of poles
φ = flux per pole
Z= No. of conductors
A = No. of parallel paths
and N is the speed of the DC Motor.
So, from the above equation, we can see Eb is proportional to speed ‘N.’ That is whenever a direct current motor rotates; it results in the generation of back Emf. Now let’s represent the rotor speed by ω in rad/sec. So Eb is proportional to ω.
So, when the application of load reduces the speed of the motor, Eb decreases. Thus the voltage difference between supply voltage and back emf increases that means E − Eb increases. Due to this increased voltage difference, the armature current will increase and therefore torque and hence speed increases. Thus a DC Motor is capable of maintaining the same speed under variable load.

Now armature current Ia is represented by

Now at starting,speed ω = 0 so at starting Eb = 0.

Now since the armature winding electrical resistance Ra is small, this motor has a very high starting current in the absence of back Emf. As a result we need to use a starter for starting a DC Motor.
Now as the motor continues to rotate, the back emf starts being generated and gradually the current decreases as the motor picks up speed.