Synchronous Machines and Alternators

Introduction

Synchronous machines are electromechanical devices that convert mechanical energy to electrical energy or vice versa with the key feature of maintaining a constant relationship between the rotor speed and the frequency of the electrical grid. The most common type of synchronous machine is the alternator, which is widely used in power generation systems around the world.

Synchronous machines are called "synchronous" because they run at a speed exactly proportional to the system frequency. This speed-frequency relationship is expressed by the equation: N = 120f/P, where N is the rotational speed in revolutions per minute (RPM), f is the frequency in Hertz, and P is the number of poles.

Construction of Synchronous Machines

A synchronous machine consists of two main components:

• Stator: The stationary part of the machine that houses the armature windings. In alternators, the stator contains the three-phase windings where the generated voltage is induced.
• Rotor: The rotating part that carries the field winding. The current flowing through the rotor winding creates the magnetic field necessary for energy conversion.

There are two main types of rotor construction:

• Salient Pole Rotor: These have projecting poles with concentrated windings. They are typically used in low-speed applications and can have up to 70 or more poles.
• Cylindrical/Round Rotor: These have a smooth cylindrical construction with distributed windings. They are used in high-speed applications and usually have 2 or 4 poles.

Operating Principles

Synchronous machines operate on the principle of electromagnetic induction discovered by Michael Faraday. When the rotor, with its dc magnetic field, rotates within the stator, the magnetic flux from the rotor cuts across the armature conductors in the stator windings, inducing a voltage according to Faraday's law of electromagnetic induction.

The magnitude of the induced voltage depends on:

• The strength of the rotor magnetic field
• The speed of rotation
• The number of stator conductors
• The configuration of the stator windings

Types of Synchronous Machines

Synchronous machines can be classified based on various criteria:

• By Rotor Type: Salient pole type and Cylindrical rotor type
• By Method of Excitation: Separately excited, Self-excited
• By Speed: Low speed (hydrogenerators), High speed (turboalternators)
• By Application: Motors, Generators (Alternators), Condensers

Alternators in Power Generation

Alternators are synchronous machines specifically designed for generating alternating current. They are the heart of modern power generation systems, converting mechanical energy from turbines driven by various energy sources into electrical energy.

Most power plants around the world use large alternators driven by:

• Steam Turbines: In thermal and nuclear power plants
• Gas Turbines: In natural gas-fired power plants
• Hydraulic Turbines: In hydroelectric power plants
• Wind Turbines: In wind farms (though many use doubly-fed induction generators)
• Internal Combustion Engines: In smaller standby or emergency power systems

Excitation Systems

The field winding of a synchronous machine requires DC current to produce the magnetic field. Various excitation systems provide this current:

• DC Exciter: A small DC generator mounted on the same shaft as the synchronous machine
• Static Exciter: Uses a rectifier fed from the machine's own output through a transformer
• Brushless Exciter: An AC generator with rotating rectifiers mounted on the main shaft, eliminating brushes and slip rings
• Permanent Magnet Excitation: Uses permanent magnets for smaller machines

Voltage Regulation

Voltage regulation in synchronous machines is achieved by controlling the field current through an automatic voltage regulator (AVR). The AVR monitors the terminal voltage and adjusts the field current to maintain the desired voltage level despite load changes.

The mathematical relationship between generated voltage, terminal voltage, and load current is given by the phasor equation: Es = Vt + Ia (Ra + jXa), where Es is the generated voltage, Vt is the terminal voltage, Ia is the armature current, Ra is the armature resistance, and Xa is the synchronous reactance.

Parallel Operation of Alternators

Alternators are often operated in parallel to share loads and improve system reliability. For successful parallel operation, the following conditions must be satisfied:

• Equal voltages
• Equal frequency
• Phase sequence must be identical
• Phase voltages must be in phase

Once synchronized, the machines share the load according to their droop characteristics. The active power sharing is controlled by adjusting the mechanical input to the prime movers, while reactive power sharing is controlled by adjusting the excitation.

Synchronous Motors

When supplied with AC power, a synchronous machine can operate as a motor. Its unique characteristic is that it runs at a constant speed regardless of load changes, provided the load does not exceed the maximum (breakdown) torque.

Synchronous motors have two main applications:

• Power Factor Correction: When operated with over-excitation (excited beyond the level required for normal operation), synchronous motors draw leading current, improving the power factor of the electrical system.
• Constant Speed Applications: They are used in applications requiring precise speed control such as large air compressors, pumps, and fans.

Synchronous Condensers

A synchronous condenser is a synchronous motor operating without mechanical load. Its sole purpose is to provide reactive power compensation in power systems. By controlling its excitation, a synchronous condenser can either absorb or generate reactive power as needed:

• Over-excited: Acts as a capacitor, supplying reactive power to the system
• Under-excited: Acts as an inductor, absorbing reactive power from the system

Performance Characteristics

Understanding the performance characteristics of synchronous machines is crucial for their proper application in power systems:

• Voltage Regulation: The change in terminal voltage from no-load to full-load while the field current and speed remain constant.
• Efficiency: The ratio of output power to input power, typically ranging from 85% to 98% for large synchronous machines.
• Power Factor: The ratio of real power to apparent power, which can be controlled by adjusting the field excitation in synchronous motors.
• Sudden Short Circuit Characteristics: The behavior of the machine when subjected to short circuits, characterized by direct-axis and quadrature-axis subtransient, transient, and steady-state reactances.

Modern Developments

Recent technological advancements have improved synchronous machines in several ways:

• Use of Superconductors: High-temperature superconductors are being tested for field windings to increase efficiency and reduce machine size.
• Advanced Materials: New magnetic materials with better magnetic properties are being developed for stator and rotor cores.
• Digital Control Systems: Advanced digital control techniques improve the dynamic performance of synchronous machines in power systems.
• Partial Pitch Windings: Optimized winding configurations that reduce harmonics and improve efficiency.

Conclusion

Synchronous machines, particularly alternators, form the backbone of modern power generation systems worldwide. Their ability to operate at constant speed proportional to system frequency, controllable power factor, and high efficiency make them indispensable in both generation and specific motor applications.

Understanding the principles, construction, and operating characteristics of synchronous machines is essential for electrical engineers involved in power systems design and operation. As power systems continue to evolve with increasing renewable energy integration, synchronous machines will continue to play a crucial role in maintaining system stability and reliability.