INTRODUCTION TO AC MACHINE:
AC machines refer to electric machines that operate using alternating current (AC) as their power
source. They are widely used in various applications, including power generation, industrial
machinery, electric vehicles, and more.
Classification of AC machine
AC machines can be classified into different types based on various factors such as their
construction, operating principle, and application. Here are the common classifications of AC
machines:
1. Synchronous Machines:
a) Synchronous Generators: These machines convert mechanical energy into electrical
energy by generating a synchronized AC voltage. They are commonly used in power
plants for electricity generation.
b) Synchronous Motors: These machines operate as motors and convert electrical energy
into mechanical energy. They are used in applications requiring constant speed and
precise control, such as industrial machinery and synchronous motor-driven loads.
2. Induction Machines:
a. Induction Motors: Also known as asynchronous motors, these machines are the most
common type of AC motors. They work on the principle of electromagnetic induction
and are widely used in various industrial, commercial, and residential applications.
b. Single-Phase Induction Motors: These motors are designed for single-phase AC power
supply and are commonly used in household appliances, small tools, and fans.
c. Three-Phase Induction Motors: These motors are designed for three-phase AC power
supply and are widely used in industrial applications, pumps, compressors, and more.
d. Induction Generators: These machines can operate as generators when mechanically
driven. They are commonly used in renewable energy systems such as wind turbines.
3. Wound Rotor Machines:
A. Wound Rotor Induction Motors: These motors have a rotor with three-phase windings
externally connected to resistors or an external circuit. They offer improved starting
torque and speed control capabilities and are used in applications requiring high torque
at startup or adjustable speed control.
4. Doubly Fed Induction Generators (DFIG):
A. DFIGs are special types of induction generators commonly used in wind power
systems. They have rotor windings connected to a power converter that allows variable
speed operation and improved control of power generation.
5. Permanent Magnet Machines:
1. Permanent Magnet Synchronous Motors (PMSM): These motors use permanent
magnets embedded in the rotor for field excitation, offering high efficiency, high torque
density, and precise control. They are widely used in applications such as electric
vehicles, robotics, and high-performance industrial machinery.
2. Permanent Magnet Synchronous Generators (PMSG): These generators use
permanent magnets in the rotor to generate electrical energy. They are commonly used
in renewable energy systems, including wind turbines.
The Two significant types of AC machines are the synchronous machine and the asynchronous
(induction) machine.
1.
Asynchronous (Induction) Machine: An asynchronous machine, commonly known as an
induction machine, operates at a speed lower than the rotating magnetic field in the stator.
Unlike synchronous machines, induction machines do not require a separate DC power
supply for rotor excitation. They are simpler in construction, more robust, and widely
used in various applications.
Key features of asynchronous machines:
a) Induction Principle: Induction machines work on the principle of electromagnetic
induction. The rotating magnetic field in the stator induces currents in the rotor,
creating a torque that drives the rotor in the opposite direction.
b) Self-Starting: Asynchronous machines are self-starting since the rotating magnetic
field induces currents in the rotor even at a standstill.
c) Slip: The difference between the synchronous speed of the rotating magnetic field
and the rotor speed is known as slip. Slip determines the torque and speed
characteristics of the machine.
d) Variable Speed: Asynchronous machines can operate at different speeds by varying
the frequency or the number of stator poles.
Applications of asynchronous machines:
1. Motor Applications: Induction motors are widely used in industries, appliances,
and transportation systems due to their ruggedness, simplicity, and costeffectiveness.
2. Generators: Asynchronous generators are used in small-scale power generation,
such as wind turbines and hydroelectric plants.
2. Synchronous Machine: A synchronous machine is an AC machine in which the rotor rotates
at the same speed as the rotating magnetic field in the stator. The stator is the stationary part,
and the rotor is the rotating part of the machine. It operates in synchronization with the
frequency and phase of the AC power supply. Synchronous machines are primarily used for
power generation in large power plants and as synchronous motors in certain industrial
applications.
Key features of synchronous machines:
a. Synchronization: The rotor speed of a synchronous machine is synchronized with the
frequency of the AC power supply.
b. Field Excitation: Synchronous machines require a separate DC power supply to energize
the rotor field windings, creating a magnetic field that interacts with the stator magnetic
field.
c. Power Factor Control: Synchronous machines can actively control the power factor by
adjusting the excitation field. They can operate at leading, unity, or lagging power factors,
making them useful for power factor correction and reactive power compensation.
d. Constant Speed: Synchronous machines operate at a constant speed, making them suitable
for applications where precise speed control is required.
Applications of synchronous machines:
1) Power Generation: Synchronous generators are commonly used in power plants to
convert mechanical energy into electrical energy.
2) Motor Applications: Synchronous motors are used in industrial machinery, such as
pumps, compressors, and high-precision equipment that requires constant speed
operation.
Principles of Operation of a Synchronous Machine
The principle of operation of a synchronous machine is based on the interaction between the
rotating magnetic field in the stator and the magnetic field produced by the direct current (DC)
excitation in the rotor. Here's a step-by-step explanation of the operating principle:
a) Rotating Magnetic Field: The stator of a synchronous machine consists of a three-phase
winding that is supplied with an AC power source. As the AC voltage is applied to the
stator windings, a rotating magnetic field is generated. The direction of this magnetic
field rotates at the synchronous speed determined by the frequency of the AC power
supply and the number of poles in the machine.
b) Rotor Field Excitation: The rotor of a synchronous machine has field windings that are
connected to a DC power source. When the DC power is applied to the rotor field
windings, it creates a magnetic field. The polarity of the field depends on the direction
of the current flow in the rotor windings.
c) Magnetic Field Interaction: The rotor's magnetic field interacts with the rotating
magnetic field produced by the stator. The rotor's magnetic field attempts to align with
the rotating magnetic field of the stator.
d) Synchronization: If the rotor is initially at rest, it will start rotating in an effort to align
its magnetic field with the rotating magnetic field of the stator. The rotor accelerates
until its speed matches the synchronous speed of the stator's rotating magnetic field. At
synchronous speed, the rotor field is in perfect synchronization with the stator field.
e) Torque Generation: Once the rotor is synchronized, the interaction between the stator
and rotor magnetic fields produces torque. The torque is developed due to the
magnetic attraction and repulsion forces between the stator and rotor fields. This
torque allows the synchronous machine to perform mechanical work, such as driving
a generator or rotating a load.
f) Power Factor Control: One of the unique features of synchronous machines is the
ability to control the power factor. By adjusting the DC excitation current in the rotor
field windings, the power factor of the machine can be varied. The power factor control
allows synchronous machines to operate at leading, unity, or lagging power factors,
making them useful for power factor correction and reactive power compensation in
electrical systems.
In summary, the principle of operation of a synchronous machine involves the interaction
between the rotating magnetic field produced by the stator and the magnetic field generated by
the DC-excited rotor. This interaction leads to torque generation and the ability to control the
power factor, making synchronous machines suitable for various applications in power
generation and industrial systems.