# Electrical Machines by Ashfaq 31: Concepts, Examples, and Problems

## Electrical Machines by Ashfaq 31: A Comprehensive Guide

Electrical machines are devices that convert electrical energy into mechanical energy or vice versa. They are essential for many applications in industry, transportation, power generation, and domestic appliances. Electrical machines can be classified into two main categories: direct current (DC) machines and alternating current (AC) machines. DC machines operate on a constant voltage and current source, while AC machines operate on a sinusoidal voltage and current source. Both types of machines have advantages and disadvantages depending on the application.

## electrical machines by ashfaq 31

If you want to learn more about electrical machines, one of the best books you can read is "Electrical Machines by Ashfaq 31". This book is written by Ashfaq Hussain Zaidi, a professor of electrical engineering at Aligarh Muslim University in India. He has more than 40 years of teaching and research experience in the field of electrical machines. He has also authored several other books on electrical engineering topics.

"Electrical Machines by Ashfaq 31" is a comprehensive guide that covers all the aspects of electrical machines in a clear and concise manner. The book is divided into four parts: types of electrical machines, analysis of electrical machines, control of electrical machines, and special topics in electrical machines. Each part consists of several chapters that explain the principles, construction, operation, and applications of different types of electrical machines. The book also includes numerous examples, problems, diagrams, tables, and figures to illustrate the concepts and enhance the understanding of the readers.

## Types of Electrical Machines

In this part of the book, you will learn about the basic types of electrical machines: DC machines, AC machines, and transformers. You will also learn about their characteristics, advantages, disadvantages, and applications.

### DC Machines

DC machines are electrical machines that operate on a direct current (DC) source. They can be classified into two types: DC generators and DC motors. DC generators convert mechanical energy into electrical energy by using electromagnetic induction. DC motors convert electrical energy into mechanical energy by using electromagnetic force.

The main components of a DC machine are:

The armature: The rotating part that carries the conductors where the induced voltage or current is generated.

The field system: The stationary part that produces the magnetic field required for the operation of the machine.

The commutator: The device that connects the armature conductors to the external circuit and reverses the direction of the current in the armature every half cycle.

The brushes: The devices that make contact with the commutator and transfer the current between the armature and the external circuit.

The operation of a DC machine depends on the type and arrangement of the field system. There are four types of field systems:

Separately excited: The field system is supplied by a separate DC source.

Shunt: The field system is connected in parallel with the armature.

Series: The field system is connected in series with the armature.

Compound: The field system is a combination of shunt and series.

The applications of DC machines include:

DC generators: Battery charging, welding, lighting, traction, and power supply.

DC motors: Electric vehicles, cranes, hoists, elevators, fans, blowers, pumps, and machine tools.

### AC Machines

AC machines are electrical machines that operate on an alternating current (AC) source. They can be classified into two types: AC generators and AC motors. AC generators convert mechanical energy into electrical energy by using electromagnetic induction. AC motors convert electrical energy into mechanical energy by using electromagnetic force.

The main components of an AC machine are:

The stator: The stationary part that carries the windings where the induced voltage or current is generated.

The rotor: The rotating part that interacts with the magnetic field produced by the stator.

The slip rings: The devices that connect the rotor windings to the external circuit and allow the current to flow in the same direction in the rotor.

The brushes: The devices that make contact with the slip rings and transfer the current between the rotor and the external circuit.

The operation of an AC machine depends on the type and arrangement of the stator and rotor windings. There are two types of stator windings:

Single-phase: The stator winding has only one phase that produces a pulsating magnetic field.

Three-phase: The stator winding has three phases that produce a rotating magnetic field.

There are two types of rotor windings:

Squirrel-cage: The rotor winding consists of short-circuited bars that form a cage-like structure.

Wound: The rotor winding consists of coils that are connected to slip rings.

The applications of AC machines include:

AC generators: Power generation, transmission, and distribution.

AC motors: Industrial drives, pumps, compressors, fans, blowers, conveyors, mixers, and air conditioners.

### Transformers

Transformers are electrical machines that transfer electrical energy from one circuit to another by using electromagnetic induction. They can be classified into two types: step-up transformers and step-down transformers. Step-up transformers increase the voltage and decrease the current in the secondary circuit. Step-down transformers decrease the voltage and increase the current in the secondary circuit.

The main components of a transformer are:

The primary winding: The winding that is connected to the input voltage source.

The secondary winding: The winding that is connected to the output load.

The core: The magnetic material that links the primary and secondary windings.

The operation of a transformer depends on the principle of mutual induction. When an alternating voltage is applied to the primary winding, it produces an alternating magnetic flux in the core. This flux induces an alternating voltage in the secondary winding according to Faraday's law of electromagnetic induction. The ratio of the primary and secondary voltages is equal to the ratio of the primary and secondary turns according to Lenz's law of electromagnetic induction. The power transferred from the primary to the secondary circuit is equal to the product of the voltage and current in each circuit according to Joule's law of electric power.

The applications of transformers include:

Step-up transformers: Power transmission and distribution, high-voltage testing, X-ray machines, and microwave ovens.

Step-down transformers: Power supply, lighting, heating, welding, and electronic devices.

## Analysis of Electrical Machines

## Analysis of Electrical Machines

In this part of the book, you will learn about how to analyze electrical machines using different methods and tools. You will also learn about how to measure and evaluate their performance parameters such as efficiency, power factor, torque, speed, and losses.

### Circuit Models

Circuit models are simplified representations of electrical machines that use equivalent circuits and phasor diagrams. Equivalent circuits are circuits that have the same voltage and current characteristics as the actual machine. Phasor diagrams are graphical representations of alternating voltages and currents using vectors. Circuit models help to understand the behavior of electrical machines under different operating conditions and to calculate their performance parameters.

The circuit models of electrical machines are based on the following assumptions:

The magnetic flux is sinusoidal and distributed uniformly in the air gap.

The magnetic circuit is linear and has negligible reluctance.

The iron losses and stray losses are negligible.

The armature reaction is negligible.

The circuit models of different types of electrical machines are:

DC machines: The equivalent circuit consists of a voltage source that represents the induced voltage in the armature, a resistor that represents the armature resistance, and a variable resistor that represents the load. The phasor diagram shows the relationship between the induced voltage, the terminal voltage, the armature current, and the load angle.

AC machines: The equivalent circuit consists of a voltage source that represents the induced voltage in the stator, a resistor and an inductor that represent the stator resistance and leakage reactance, a transformer that represents the mutual induction between the stator and rotor, and a resistor and an inductor that represent the rotor resistance and leakage reactance. The phasor diagram shows the relationship between the induced voltage, the terminal voltage, the stator current, the rotor current, the power factor angle, and the slip angle.

Transformers: The equivalent circuit consists of a voltage source that represents the induced voltage in the primary, a resistor and an inductor that represent the primary resistance and leakage reactance, a transformer that represents the mutual induction between the primary and secondary, and a resistor and an inductor that represent the secondary resistance and leakage reactance. The phasor diagram shows the relationship between the induced voltage, the terminal voltage, the primary current, the secondary current, and the phase angle.

### Performance Parameters

Performance parameters are quantities that measure how well an electrical machine performs its function. They include efficiency, power factor, torque, speed, and losses. Performance parameters can be calculated using circuit models or measured using testing methods.

The performance parameters of different types of electrical machines are:

DC machines: Efficiency is the ratio of output power to input power. Power factor is not applicable for DC machines. Torque is the rotational force produced by the machine. Speed is the rotational speed of the machine. Losses are the power dissipated in various parts of the machine such as copper loss, iron loss, mechanical loss, and stray loss.

AC machines: Efficiency is the ratio of output power to input power. Power factor is the ratio of real power to apparent power. Torque is the rotational force produced by the machine. Speed is the rotational speed of the machine. Losses are similar to DC machines but also include core loss and slip loss.

Transformers: Efficiency is similar to AC machines but output power is replaced by transferred power. Power factor is similar to AC machines but real power is replaced by active power. Torque and speed are not applicable for transformers. Losses are similar to AC machines but also include magnetizing loss and hysteresis loss.

### Testing Methods

Testing methods are procedures that are used to measure or verify the performance parameters of electrical machines using instruments such as voltmeters, ammeters, wattmeters, tachometers, dynamometers, etc. Testing methods can be classified into four types: open-circuit test, short-circuit test, load test, and no-load test.

The testing methods of different types of electrical machines are:

DC machines: Open-circuit test is used to measure the induced voltage in the armature at different speeds. Short-circuit test is used to measure the armature resistance and calculate the copper loss at different currents. Load test is used to measure the terminal voltage, current, power, torque, speed, and efficiency at different loads. No-load test is not applicable for DC machines.

AC machines: Open-circuit test is used to measure the induced voltage in the stator at different frequencies. Short-circuit test is used to measure the stator resistance and leakage reactance and calculate the copper loss at different currents. Load test is similar to DC machines but also measures the power factor and slip. No-load test is used to measure the no-load current and calculate the core loss and magnetizing reactance.

Transformers: Open-circuit test is similar to AC machines but measures the primary voltage and current. Short-circuit test is similar to AC machines but measures the secondary voltage and current. Load test is similar to AC machines but measures the transferred power and efficiency. No-load test is similar to AC machines but measures the primary current and power.

## Control of Electrical Machines

In this part of the book, you will learn about how to control electrical machines using different methods and devices. You will also learn about how to regulate their speed, voltage, and power according to the requirements of the application.

### Speed Control

Speed control is the process of adjusting the speed of an electrical machine to a desired value or range. Speed control can be achieved by varying the input voltage, frequency, or resistance of the machine or by using external devices such as rheostats, choppers, inverters, etc.

The speed control methods of different types of electrical machines are:

DC machines: The speed of a DC machine can be controlled by varying the armature voltage or the field current. The armature voltage can be varied by using a rheostat in series with the armature or a chopper in parallel with the armature. The field current can be varied by using a rheostat in series with the field or a chopper in parallel with the field.

AC machines: The speed of an AC machine can be controlled by varying the supply frequency or the slip. The supply frequency can be varied by using an inverter that converts DC to AC with variable frequency. The slip can be varied by using a rheostat in series with the rotor or a slip ring induction motor that has an external resistance connected to its slip rings.

### Voltage Control

Voltage control is the process of adjusting the voltage of an electrical machine to a desired value or range. Voltage control can be achieved by varying the input voltage, frequency, or resistance of the machine or by using external devices such as transformers, regulators, rectifiers, etc.

The voltage control methods of different types of electrical machines are:

DC machines: The voltage of a DC machine can be controlled by varying the armature voltage or the field current. The armature voltage can be varied by using a transformer that steps up or down the input voltage or a rectifier that converts AC to DC with variable voltage. The field current can be varied by using a regulator that maintains a constant field current regardless of load variations.

AC machines: The voltage of an AC machine can be controlled by varying the supply voltage or frequency. The supply voltage can be varied by using a transformer that steps up or down the input voltage or a regulator that maintains a constant output voltage regardless of load variations. The supply frequency can be varied by using an inverter that converts DC to AC with variable frequency.

Transformers: The voltage of a transformer can be controlled by varying the turns ratio or the tap position. The turns ratio can be varied by using a variable transformer that has movable contacts on its windings. The tap position can be varied by using a tap changer that switches between different taps on its windings.

### Power Control

Power control is the process of adjusting the power output or input of an electrical machine to a desired value or range. Power control can be achieved by varying the input voltage, frequency, resistance, or power factor of the machine or by using external devices such as capacitors, reactors, filters, etc.

The power control methods of different types of electrical machines are:

DC machines: The power output of a DC machine can be controlled by varying the armature current or torque. The armature current can be varied by using a rheostat in series with the armature or a chopper in parallel with the armature. The torque can be varied by using a dynamometer that applies a counter torque on the machine.

### Power Control

Power control is the process of adjusting the power output or input of an electrical machine to a desired value or range. Power control can be achieved by varying the input voltage, frequency, resistance, or power factor of the machine or by using external devices such as capacitors, reactors, filters, etc.

The power control methods of different types of electrical machines are:

DC machines: The power output of a DC machine can be controlled by varying the armature current or torque. The armature current can be varied by using a rheostat in series with the armature or a chopper in parallel with the armature. The torque can be varied by using a dynamometer that applies a counter torque on the machine.

AC machines: The power output of an AC machine can be controlled by varying the stator current or torque. The stator current can be varied by using a rheostat in series with the stator or a chopper in parallel with the stator. The torque can be varied by using a dynamometer that applies a counter torque on the machine.

Transformers: The power input of a transformer can be controlled by varying the primary voltage or current. The primary voltage can be varied by using a transformer that steps up or down the input voltage or a regulator that maintains a constant output voltage regardless of load variations. The primary current can be varied by using a capacitor or a reactor that changes the power factor of the input circuit.

## Special Topics in Electrical Machines

In this part of the book, you will learn about some special topics in electrical machines that are not covered in the previous parts. These topics include synchronous machines, induction machines, and single-phase machines. You will also learn about their characteristics, advantages, disadvantages, and applications.

### Synchronous Machines

Synchronous machines are AC machines that operate at a constant speed that is equal to the synchronous speed. The synchronous speed is determined by the supply frequency and the number of poles of the machine. Synchronous machines can be classified into two types: synchronous generators and synchronous motors. Synchronous generators convert mechanical energy into electrical energy by using electromagnetic induction. Synchronous motors convert electrical energy into mechanical energy by using electromagnetic force.

The main components of a synchronous machine are:

The stator: The stationary part that carries the three-phase windings where the induced voltage or current is generated.

The rotor: The rotating part that carries the field windings where the excitation voltage or current is supplied.

The slip rings: The devices that connect the rotor field windings to the external circuit and allow the current to flow in the same direction in the rotor.

The brushes: The devices that make contact with the slip rings and transfer the current between the rotor and the external circuit.

The operation of a synchronous machine depends on