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Principles of Electrical Machines by V.K. Mehta: A Book Review and Summary


Principles of Electrical Machines by V.K. Mehta




In this article, we will explore the principles of electrical machines, based on the book Principles of Electrical Machines by V.K. Mehta and Rohit Mehta. This book is an ideal text for students who look to gain a current and clear understanding of the subject as all theories and concepts are explained with lucidity and clarity. The book covers various topics such as electromechanical energy conversion, DC generators, armature reaction and commutation, DC motors, speed control of DC motors, testing of DC machines, transformers, three-phase induction motors, circle diagrams, single-phase motors, alternators, synchronous motors, and special-purpose electric machines. In this article, we will focus on some of the key concepts from the first four chapters of the book.




Principles Of Electrical Machines [v.k. Mehta].pdf



Introduction




What are electrical machines?




Electrical machines are devices that convert one form of energy into another. For example, a generator converts mechanical energy into electrical energy, while a motor converts electrical energy into mechanical energy. Electrical machines can also be used to transform electrical energy from one voltage level to another, or from one frequency to another. Electrical machines can be classified into two main categories: static and dynamic. Static machines do not have any moving parts, such as transformers and capacitors. Dynamic machines have rotating or reciprocating parts, such as generators and motors.


Why study electrical machines?




Electrical machines are essential for various applications in industry, transportation, communication, power generation, and distribution. They are also used in domestic appliances, such as fans, refrigerators, washing machines, etc. Studying electrical machines helps us to understand how they work, how to design them, how to operate them, how to maintain them, and how to improve their performance and efficiency. Studying electrical machines also helps us to appreciate the principles of electromagnetism, which are the basis of many phenomena in nature and technology.


What are the main types of electrical machines?




The main types of electrical machines are:


  • DC machines: These are machines that operate on direct current (DC) supply. They can be either generators or motors. DC generators produce DC voltage and current from mechanical input, while DC motors produce mechanical output from DC input. DC machines have commutators and brushes to change the direction of current in the armature windings.



  • AC machines: These are machines that operate on alternating current (AC) supply. They can be either generators or motors. AC generators produce AC voltage and current from mechanical input, while AC motors produce mechanical output from AC input. AC machines do not have commutators and brushes, but use slip rings and induction coils instead.



  • Synchronous machines: These are AC machines that operate at a constant speed that is proportional to the frequency of the supply. They can be either generators or motors. Synchronous generators produce AC voltage and current with a fixed phase relation with the supply, while synchronous motors follow the same phase relation with the supply. Synchronous machines have field windings on the rotor and armature windings on the stator.



  • Induction machines: These are AC machines that operate at a speed that is slightly less than the synchronous speed. They can be either generators or motors. Induction generators produce AC voltage and current by induction from the supply, while induction motors draw AC current by induction from the supply. Induction machines have only armature windings on the stator and a squirrel-cage or a wound rotor.



Electromechanical Energy Conversion




What is electromechanical energy conversion?




Electromechanical energy conversion is the process of converting mechanical energy into electrical energy, or vice versa, by means of electromagnetic forces. The basic principle of electromechanical energy conversion is that whenever a conductor carrying current is placed in a magnetic field, it experiences a force. Conversely, whenever a conductor is moved in a magnetic field, it induces an electromotive force (emf) in it. The magnitude of the force or the emf depends on the strength of the current, the strength of the magnetic field, and the angle between them.


How does a generator work?




A generator is a machine that converts mechanical energy into electrical energy. A generator consists of a coil of wire (called the armature) that rotates in a magnetic field (produced by permanent magnets or electromagnets). As the coil rotates, it cuts the magnetic flux, which induces an emf in it. The emf causes a current to flow in the coil, which can be delivered to an external circuit through slip rings and brushes. The direction of the current changes with every half rotation of the coil, resulting in an alternating current (AC) output. The frequency of the output depends on the speed of rotation and the number of poles of the magnetic field.


How does a motor work?




A motor is a machine that converts electrical energy into mechanical energy. A motor consists of a coil of wire (called the armature) that rotates in a magnetic field (produced by permanent magnets or electromagnets). When a current is passed through the coil, it creates a magnetic field around it, which interacts with the external magnetic field. The interaction produces a torque on the coil, which makes it rotate. The direction of rotation depends on the direction of the current and the polarity of the magnetic field. The speed of rotation depends on the magnitude of the current and the strength of the magnetic field.


DC Generators




What are the components of a DC generator?




A DC generator is a machine that produces direct current (DC) voltage and current from mechanical input. A DC generator consists of four main components:


  • Armature: This is the rotating part of the generator, which consists of a cylindrical core made of laminated iron and several coils of wire wound on it. The armature carries the induced emf and current.



  • Field: This is the stationary part of the generator, which consists of two or more poles made of electromagnets or permanent magnets. The field produces a uniform magnetic flux across the air gap between the poles and the armature.



  • Commutator: This is a cylindrical device made of copper segments insulated from each other and attached to one end of the armature shaft. The commutator connects each coil of the armature to two brushes that slide on its surface.



  • Brushes: These are two carbon or graphite blocks that press against the commutator and collect or deliver current from or to an external circuit.



What are the types of DC generators?




DC generators can be classified into three types based on their field winding connection:


  • Separately excited DC generator: This is a generator whose field winding is supplied by an independent DC source, such as a battery or another generator. The output voltage and current depend on the speed of rotation and the field excitation.



  • Self-excited DC generator: This is a generator whose field winding is connected to its own armature terminals, so that it uses some of its output to excite itself. The output voltage and current depend on the speed of rotation and the residual magnetism in the poles.



  • Compound DC generator: This is a generator whose field winding is partly connected in series with its armature (series winding) and partly connected in parallel with its terminals (shunt winding). The output voltage and current depend on the speed of rotation and the combination of series and shunt excitation.



How to calculate the output voltage and power of I'll try to continue the article. Here is the continuation of the article with HTML formatting: the short-circuited coil that helps in reversing the current.


  • Using brush shifting: This method involves shifting the brushes slightly forward or backward from the MNA to obtain a better commutation. The direction and amount of shift depend on the type of generator and the load condition.



DC Motors




What are the components of a DC motor?




A DC motor is a machine that converts electrical energy into mechanical energy. A DC motor consists of four main components:


  • Armature: This is the rotating part of the motor, which consists of a cylindrical core made of laminated iron and several coils of wire wound on it. The armature carries the input current and produces a torque.



  • Field: This is the stationary part of the motor, which consists of two or more poles made of electromagnets or permanent magnets. The field produces a uniform magnetic flux across the air gap between the poles and the armature.



  • Commutator: This is a cylindrical device made of copper segments insulated from each other and attached to one end of the armature shaft. The commutator reverses the direction of current in the armature coils as they pass under the brushes.



  • Brushes: These are two carbon or graphite blocks that press against the commutator and supply or receive current from or to an external circuit.



What are the types of DC motors?




DC motors can be classified into three types based on their field winding connection:


  • Series DC motor: This is a motor whose field winding is connected in series with its armature winding. The field current is equal to the armature current, and hence the field flux is proportional to the armature current. Series DC motors have high starting torque and variable speed characteristics. They are suitable for applications that require high torque at low speed, such as cranes, hoists, electric traction, etc.



  • Shunt DC motor: This is a motor whose field winding is connected in parallel with its armature winding. The field current is independent of the armature current, and hence the field flux is constant. Shunt DC motors have low starting torque and constant speed characteristics. They are suitable for applications that require constant speed at varying load, such as fans, blowers, pumps, etc.



  • Compound DC motor: This is a motor whose field winding is partly connected in series with its armature (series winding) and partly connected in parallel with its terminals (shunt winding). The field flux is a combination of series and shunt fluxes. Compound DC motors have intermediate starting torque and speed regulation characteristics. They are suitable for applications that require high torque at low speed and moderate torque at high speed, such as presses, shears, elevators, etc.



How to calculate the speed and torque of a DC motor?




The speed and torque of a DC motor depend on its terminal voltage, armature resistance, field flux, and load. The following equations can be used to calculate them:


The back emf equation: $$E_b = V - I_a R_a$$


The speed equation: $$N = \fracE_bk \phi$$


The torque equation: $$T = k \phi I_a$$


In these equations,


  • $E_b$ is the back emf in volts



  • $V$ is the terminal voltage in volts



  • $I_a$ is I'll try to continue the article. Here is the continuation of the article with HTML formatting: the armature current in amperes



  • $R_a$ is the armature resistance in ohms



  • $k$ is a constant that depends on the motor construction



  • $\phi$ is the field flux in webers



  • $N$ is the speed in revolutions per minute (rpm)



  • $T$ is the torque in newton-meters (N-m)



What are the applications of DC motors?




DC motors are widely used in various fields that require speed control, high starting torque, or constant speed. Some of the common applications of DC motors are:


  • Elevators: DC series motors are used in elevators because they provide high starting torque and smooth speed control. The speed of the motor can be varied by changing the field current or the armature voltage.



  • Steel mills: DC shunt or compound motors are used in steel mills because they provide constant speed and good speed regulation. The speed of the motor can be adjusted by changing the field current.



  • Rolling mills: DC compound motors are used in rolling mills because they provide high torque at low speed and moderate torque at high speed. The speed of the motor can be controlled by changing the field current or the armature voltage.



  • Locomotives: DC series motors are used in locomotives because they provide high starting torque and variable speed characteristics. The speed of the motor can be controlled by changing the armature voltage.



  • Excavators: DC series motors are used in excavators because they provide high starting torque and smooth speed control. The speed of the motor can be controlled by changing the field current or the armature voltage.



Conclusion




In this article, we have learned about the principles of electrical machines, based on the book Principles of Electrical Machines by V.K. Mehta and Rohit Mehta. We have covered some of the key concepts from the first four chapters of the book, such as electromechanical energy conversion, DC generators, armature reaction and commutation, and DC motors. We have also seen how to calculate the output voltage, power, speed, and torque of a DC generator or motor, and what are some of the applications of DC machines. We hope this article has been informative and helpful for you.


FAQs




Here are some frequently asked questions about electrical machines:



  • What is the difference between a generator and a motor?



A generator is a machine that converts mechanical energy into electrical energy, while a motor is a machine that converts electrical energy into mechanical energy.


  • What is the difference between a DC machine and an AC machine?



A DC machine is a machine that operates on direct current (DC) supply, while an AC machine is a machine that operates on alternating current (AC) supply.


  • What is the difference between a brushed DC motor and a brushless DC motor?



A brushed DC motor is a motor that uses a commutator and brushes to reverse the direction of current in the armature coils, while a brushless DC motor is a motor that uses electronics to switch I'll try to continue writing the FAQs. Here are some more frequently asked questions about electrical machines:


  • What is the difference between a series DC motor and a shunt DC motor?



A series DC motor is a motor whose field winding is connected in series with its armature winding, while a shunt DC motor is a motor whose field winding is connected in parallel with its armature winding.


  • What is the difference between a synchronous AC motor and an induction AC motor?



A synchronous AC motor is a motor that operates at a constant speed that is proportional to the frequency of the supply, while an induction AC motor is a motor that operates at a speed that is slightly less than the synchronous speed.


  • What are some of the advantages and disadvantages of DC machines?



Some of the advantages of DC machines are:


  • They have simple and easy speed control.



  • They have high starting torque and good speed regulation.



  • They can operate on both AC and DC supply with a rectifier.



Some of the disadvantages of DC machines are:


  • They require commutator and brushes, which cause sparking and wear.



  • They have low efficiency and high maintenance cost.



  • They have limited speed range and output power.






This is the end of the article. Thank you for reading. I hope you have learned something new and useful about electrical machines. If you have any questions or feedback, please feel free to leave a comment below. 71b2f0854b


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