What Is Synchronous Generator?

what is a synchronous generator?

A synchronous generator, also known as an alternator, is a type of electrical machine that converts mechanical energy into electrical energy.

It operates based on the principle of electromagnetic induction, much like an electric motor but in reverse. Instead of using electrical energy to produce mechanical motion, a synchronous generator uses mechanical energy to generate electricity.

Synchronous Generator Meaning: The term “synchronous” in synchronous generator refers to the fact that the generator’s rotor turns at a constant speed and in synchronization with the frequency of the electrical output.

In other words, the rotor rotates at the same speed as the frequency of the generated AC voltage, which is determined by the design of the generator.

Synchronous generators are commonly used in power plants to produce electricity and maintain the stability of electrical grids because they can be synchronized with the grid’s frequency.

They are also used in some types of renewable energy systems, such as hydroelectric and wind power plants, where the generator’s speed can be controlled to match the variable speed of the energy source.

Synchronous Generator Working Principle

Here’s how a synchronous generator works:

  1. Rotating Magnetic Field: Inside the generator, there is a rotor (usually in the form of a coil or a set of magnets) that is connected to a mechanical prime mover, such as a turbine in a power plant or an engine in a vehicle. When the rotor is spun by the mechanical source, it generates a rotating magnetic field.

  2. Stator: Surrounding the rotor is a stationary component called the stator, which contains a set of coils or windings. As the rotor’s magnetic field rotates, it induces an alternating current (AC) in the stator windings through electromagnetic induction.

  3. Output Voltage: The alternating current produced in the stator windings is the electrical output of the generator. This AC voltage is typically in the form of a sine wave and can be used for various applications, including powering electrical devices, distributing electricity in power grids, or charging batteries.

What are synchronous generator uses?

Synchronous generators, also known as alternators, are used in various applications where electrical power needs to be generated. Some common applications of synchronous generators include:

  1. Power Plants: Synchronous generators are commonly used in power plants, including coal, natural gas, and nuclear power plants, to convert mechanical energy into electrical energy. They play a crucial role in generating electricity for industrial, commercial, and residential use.

  2. Renewable Energy Sources: Synchronous generators can be used in renewable energy systems, such as hydroelectric power plants and some wind turbines. In these applications, the generator’s speed can be controlled to match the variable speed of the energy source, ensuring efficient power generation.

  3. Diesel and Gasoline Generators: Portable and standby generators powered by diesel or gasoline engines often incorporate synchronous generators. These generators provide backup power during outages and are commonly used in remote areas or construction sites.

  4. Marine Generators: Synchronous generators are used on ships and boats to generate electricity for various onboard systems, including lighting, navigation equipment, and HVAC systems.

  5. Aircraft Generators: Some aircraft use synchronous generators to produce electrical power for avionics, lighting, and other onboard systems.

  6. Prime Movers: Synchronous generators can be coupled to various types of prime movers, including steam turbines, gas turbines, diesel engines, and internal combustion engines, to generate electricity in a wide range of industrial and commercial settings.

  7. Frequency Conversion: Synchronous generators can be used in frequency conversion applications, where they generate electricity at one frequency and voltage level and then convert it to another for specific industrial processes or grid interconnection.

  8. Grid Support: In addition to generating electrical power, synchronous generators can provide grid support services, such as reactive power and voltage control. They play a role in stabilizing and regulating electrical grids.

  9. Research and Testing: Synchronous generators are used in laboratories and testing facilities for experiments, research, and development of electrical systems and equipment.

  10. Uninterruptible Power Supplies (UPS): In some high-power UPS systems, synchronous generators are used as a backup power source to provide continuous electricity during short-term grid failures.

It’s important to note that the specific design and characteristics of synchronous generators can vary depending on their intended application. These generators are versatile and can be customized to meet the requirements of different industries and settings.

why synchronous generator are used for production of electricity?

Synchronous generators are used for the production of electricity for several reasons, making them well-suited for various applications in power generation. Here are some of the key reasons why synchronous generators are used:

  1. Synchronization with Grid Frequency: Synchronous generators can operate at a constant speed that is synchronized with the frequency of the electrical grid. In many regions, the grid operates at a specific frequency, such as 60 Hz in the United States or 50 Hz in many other countries. Synchronous generators can match this frequency precisely, ensuring that the generated electricity is compatible with the existing grid infrastructure.

  2. Grid Stability and Reliability: Synchronous generators contribute to the stability and reliability of electrical grids. Their ability to maintain a constant speed and frequency helps stabilize the grid, which is essential for maintaining the quality and consistency of electrical power supplied to consumers.

  3. Base Load and Peak Load Generation: Synchronous generators are capable of providing both base load and peak load power generation. Base load power generation refers to the steady and continuous supply of electricity required to meet the minimum demand, while peak load power generation involves generating additional electricity during periods of high demand. Synchronous generators can adapt to varying load conditions and are often used for base load power generation.

  4. Voltage Regulation: Synchronous generators offer precise voltage regulation capabilities. By adjusting the excitation current, the generator’s output voltage can be controlled and maintained within a narrow voltage range, ensuring the quality of the electricity supplied to consumers.

  5. High Efficiency: Synchronous generators are known for their high efficiency when operated near their rated capacity. This makes them economically viable for large-scale power generation applications, such as in power plants.

  6. Controllable Reactive Power: Synchronous generators can provide both active (real) power and reactive power to the grid. The ability to supply reactive power is essential for voltage control and grid stability.

  7. Longevity and Durability: Synchronous generators are built to be robust and durable, with a long operational lifespan. This makes them suitable for continuous operation in power generation facilities.

  8. Versatility: Synchronous generators can be driven by various types of prime movers, including steam turbines, gas turbines, diesel engines, and internal combustion engines. This versatility allows them to be used in a wide range of power generation applications.

  9. Grid Support Services: Synchronous generators can provide additional grid support services, such as inertia, frequency control, and fault ride-through capabilities, which are essential for maintaining grid reliability in the presence of disturbances.

Overall, the characteristics of synchronous generators make them a reliable and flexible choice for the production of electricity in power plants and other large-scale electrical generation facilities.

They are integral to ensuring a stable and secure supply of electricity to homes, businesses, and industries.

Synchronous Generator Construction

synchronous generator rotor
Synchronous Generator Rotor

An alternator consists of two main parts stator and rotor.


The stator is the stationary part of the generator, It carries the winding in which the voltage is generated.

This means that the output power of the generator is taken from the stator.


The rotor is the rotating part that produces the field flux. It has windings that are connected to a DC source, The DC current in the rotor produces field flux.

The rotor rotates with the help of a mechanical prime-mover. This rotation makes the field to rotate cuts the windings of the stator and produces voltage.

In order to reduce hysteresis and eddy-current losses, the stator core is assembled with high-grade silicon content steel lamination. The rotor is the field generation part.

What are Carbon Brushes?

carbon brushes
carbon brushes

The older designs  of the synchronous generator had an issue of feeding the rotor with its DC current while it’s rotating, to produce field

Using carbon brushes to feed the rotor with DC current was the solution.

Brushes need continuous maintenance! so, the modification was to use a separate generator on the rotor shaft.

Nowadays, brushless excitation is the solution of brushes many issues. What is brushless exciter?

Brushless Exciter

A Brushless Excitation System is a system that uses no slip rings or carbon brushes to supply field current to the synchronous generator rotor.

With a Brushless Excitation System, we don’t need to supply external power for the field in order to generate electricity as a static excitation system.

Static Excitation System takes power for field excitation from an external power DC power source.

Nonetheless, Brushless Excitation Systems do not require any start-up external power supply to operate. It uses the residual magnetism to build up the field of excitation.

Are you wondering, what is residual magnetism? Let’s discuss it a little more.

What is Synchronous Generator Residual Magnetism?

Synchronous generator residual magnetism refers to the small amount of magnetic flux or magnetic field strength that remains in the rotor’s magnetic circuit when the generator is not actively generating electricity.

This residual magnetism is a critical characteristic of some synchronous generators and is essential for the initial excitation of the generator’s rotor.

Here’s how synchronous generator residual magnetism works:

  1. Initial Excitation: When a synchronous generator is initially started, there may be no external source of excitation applied to the rotor. In such cases, the generator relies on residual magnetism to create the initial magnetic field in the rotor.

  2. Creation of Residual Magnetism: During previous operation or startup, the generator’s field winding was energized with direct current (DC). This DC current magnetized the rotor, creating a residual magnetic field.

  3. Starting the Generator: When the generator is started without an external excitation source, the residual magnetism provides the initial magnetic field needed for excitation. This magnetic field allows the generator to begin producing electricity and establishes the synchronism with the grid’s frequency.

  4. Self-Excitation: As the generator starts to produce electricity, it generates an internal voltage. This voltage is then used to supply DC excitation to the rotor through an exciter or an automatic voltage regulator (AVR). Once the rotor receives this excitation current, the magnetic field strength increases, further enhancing the generator’s performance.

In summary, synchronous generator residual magnetism is the remaining magnetic field in the rotor that allows the generator to self-excite and begin generating electricity when initially started.

While it is essential for self-excitation, residual magnetism alone may not provide sufficient excitation for large synchronous generators, which often use separate excitation systems to control and regulate the magnetic field strength more precisely.

However, for smaller synchronous generators and some specific applications, relying on residual magnetism for initial excitation can be a cost-effective and practical solution.

Does A generator Lose Its Residual Magnetism With Time?

Yes, a generator can lose its residual magnetism over time if it is not used or operated for an extended period.

Residual magnetism is the term used to describe the small amount of magnetic field strength that remains in the generator’s rotor when it is not actively generating electricity.

This residual magnetism is crucial for self-excitation in some types of generators, including some synchronous generators and self-excited induction generators.

Here’s how the loss of residual magnetism can occur and its implications:

  1. Inactivity: When a generator is not in use or has been sitting idle for an extended period, the magnetic field in the rotor can gradually weaken due to natural factors such as hysteresis and eddy current losses.

  2. Decay: Over time, the magnetic materials in the rotor may demagnetize partially, reducing the residual magnetic field strength.

  3. Moisture and Environmental Factors: Environmental conditions, such as high humidity, can contribute to the degradation of the magnetic materials in the rotor, leading to a decrease in residual magnetism.

The loss of residual magnetism can be problematic because some generators rely on this initial magnetic field to start generating electricity. For example, self-excited induction generators and certain types of small synchronous generators need this residual magnetism to initiate the excitation process and produce electrical voltage.

To address the issue of lost residual magnetism, generators can be “flashed” or “remagnetized.” This process involves applying a direct current (DC) voltage to the generator’s field winding or rotor while it is disconnected from the load. The DC voltage helps to restore the magnetic field strength in the rotor. Once the residual magnetism is reestablished, the generator can be started and operated as usual.

It’s essential to periodically check and, if necessary, reestablish the residual magnetism in generators that rely on it, especially if they have been inactive for an extended period. This maintenance practice ensures that the generator can start and generate electricity reliably when needed.


What is Generator Excitation Field Flashing?

Flashing the exciter field is the process of restoring the residual magnetism of the generator exciter. It is necessary to conduct field flashing on a generator when the voltage does not build up.

The problem is usually caused due to insufficient residual magnetism in the exciter and generator fields in the system.

In some circumstances, a generator that has been out of service for a long time will lose its residual magnetism and need flashing.

If there isn’t enough residual magnetism after the field flash, then an injection of current can be made into the rotor from another source to create full voltage.

We use a battery for this flashing process, or a housing unit may provide direct current or alternating current might be rectified.

This type of current is required for a very short time, and it is referred to as field flashing. Even small, portable generator sets may need to undergo field flashing in the course of routine operation.

Exciter flashing steps

  • Disconnect the exciter leads F+ and F-, and connect F+ to the battery-positive terminal.
  • Touch F- to the battery negative terminal for about 5 seconds.
  • Reconnecting the exciter leads to the voltage regulator again.
  • Start the generator. If it’s OK, it will build up its voltage.
  • You may need to repeat the process of flashing the exciter field if the generator fails to build voltage.

Can Synchronous Generator Work Without Excitation?

No, a synchronous generator cannot work without excitation. Excitation is a critical aspect of the operation of a synchronous generator, and it is necessary to establish and maintain the magnetic field in the rotor for the generator to function properly.

Here’s why excitation is essential in a synchronous generator:

  1. Magnetic Field Generation: In a synchronous generator, the rotor creates a rotating magnetic field. This magnetic field is crucial for inducing voltage in the stator windings through electromagnetic induction. Without a magnetic field in the rotor, there would be no electromagnetic induction, and therefore, no generation of electrical voltage.

  2. Synchronization: The rotor of a synchronous generator must rotate at a specific speed that is synchronized with the frequency of the generated AC voltage. This speed is known as the synchronous speed and is determined by the generator’s design and the grid’s frequency (e.g., 60 Hz in the United States). Excitation is necessary to maintain the rotor’s magnetic field strength, which in turn ensures that the generator operates at the correct synchronous speed.

  3. Voltage Regulation: The excitation system also allows for the control of the generator’s output voltage. By adjusting the excitation current, the voltage output can be controlled and regulated to match the desired electrical requirements. This voltage regulation is crucial for maintaining the stability of the electrical grid.

There are different methods of excitation used in synchronous generators, including:

  • Brushless excitation systems: These systems use solid-state devices and electronic control to provide the necessary field excitation to the rotor.

  • Static excitation systems: These systems use a combination of transformers, rectifiers, and voltage regulators to provide the excitation current to the rotor.

  • Direct current (DC) excitation: In older generators, a separate DC excitation system with a DC generator or an exciter was used to provide the required magnetic field.

In summary, excitation is an integral part of a synchronous generator’s operation, and without it, the generator cannot generate electrical power or maintain the necessary synchronization and voltage regulation.

What Is The Function Of The Diode Bridge In A Generator Exciter?

synchronous generator excitation system
synchronous generator excitation system

The diode bridge job is to rectify ِcurrent from the exciter rotor, i.e. convert it from AC to DC.

This direct current is sent straight to the primary rotor field coils, which require DC current to excite the main rotor. The automated voltage regulator adjusts the field current to keep the AC output voltage constant as the load varies.

Because the magnetic field requires DC electricity, this procedure is needed for a generator’s alternator to function.

Before the exciter’s AC output can be utilized to create electrical energy, it must be converted to DC power. This occurs within a diode rectifier.

Why Does a Synchronous Generator Need a Battery?

A synchronous generator itself doesn’t typically require a battery to operate. Synchronous generators are primarily mechanical-electrical devices that convert mechanical energy into electrical energy by electromagnetic induction.

They do not store electrical energy like batteries do. However, there can be situations where a battery is associated with a synchronous generator or used in conjunction with it for specific purposes:

  1. Engine Starting: In applications where a synchronous generator is coupled to an internal combustion engine (e.g., diesel or gas engine), a battery is often used to provide the initial starting power for the engine. The battery powers the starter motor, which cranks the engine until it starts running. Once the engine is running, it can drive the synchronous generator to generate electricity.

  2. Backup Power: In some cases, a battery backup system may be integrated with a synchronous generator to provide uninterrupted power during the brief transition period when the generator starts up and reaches its operating speed. This ensures a continuous supply of power during grid outages or when switching to generator power.

  3. Control and Monitoring: Batteries can be used in control and monitoring systems associated with synchronous generators. For example, batteries may power control and protection circuits, sensors, and monitoring equipment that ensure the generator operates safely and efficiently.

  4. Excitation System: Some synchronous generators use an exciter, which is a smaller generator that supplies the excitation current to the main generator’s rotor. The exciter may include a small battery or capacitors to provide initial field excitation when the generator starts up. Once the generator is operational, it can self-excite and maintain the required field current.

In summary, while synchronous generators themselves do not rely on batteries for their core operation, batteries may be employed in conjunction with them for purposes such as engine starting, backup power, control, monitoring, and excitation.

These applications enhance the overall functionality and reliability of power generation systems that include synchronous generators.

Read also my detailed article : Electrical generator batteries, 6 important Answers.

Synchronous Generator vs. Induction Generator

There are some differences between synchronous and induction generators, here are the main of them.

  • The frequency of the synchronous Gen. is equal to f = N*P/120 HZ, this is because the rotor rotates with the same synchronous speed, While in the case of the induction generator, the rotor speed is not the same as the synchronous speed, so its frequency is lower than the value of the formula of the frequency.
  • The construction of the induction generator is less complicated because it needs no brushes or DC source for the excitation. The induction generator takes its excitation from the reactive power of the power system.

Below is a table of comparison between the two generators.

  Synchronous Induction
Frequency f = N * P / 120 Hz Lower than the calculated value of the formula, f = N * P / 120 Hz
Excitation A separate DC excitation source is required For field excitation, it takes reactive power from the power system
Construction Needs brushes or a small generator on the rotor the generator is less complicated

Is Synchronous Generator Self-Starting?

Synchronous generators are not typically self-starting. Unlike some other types of generators, such as induction generators or self-excited induction generators, synchronous generators require an external source of mechanical power to initiate their operation. Here’s why synchronous generators are not self-starting:

  1. Initial Magnetic Field: Synchronous generators rely on the presence of a magnetic field in their rotor to induce electrical voltage in the stator windings. This magnetic field is created by the excitation system, which can be in the form of a DC power supply, an exciter generator, or other means. Before the generator can produce electricity, this initial magnetic field must be established.

  2. Synchronization with Grid: Synchronous generators are designed to operate at a specific speed that is synchronized with the frequency of the electrical grid or the desired frequency for the application. Achieving this synchronization requires precise control of the generator’s speed and excitation. This synchronization cannot occur without an external force to initially set the rotor in motion.

To start a synchronous generator, an external mechanical prime mover is used. Common prime movers include:

  • Steam Turbines: In power plants, steam turbines are often used as prime movers for synchronous generators. The high-pressure steam from a boiler is directed onto the turbine blades, causing the rotor to start rotating.

  • Gas Turbines: Gas turbines are another common prime mover, especially in combined cycle power plants. The combustion of natural gas or other fuels drives the turbine, which is connected to the generator.

  • Diesel Engines: In backup generators and smaller power generation systems, diesel engines can be used to start and drive synchronous generators.

  • Hydro Turbines: In hydroelectric power plants, the flow of water through a turbine is used as the prime mover to start and rotate the synchronous generator.

In summary, synchronous generators are not self-starting and require an external mechanical force, such as a prime mover, to initiate their operation.

Once the generator is spinning at the correct speed and the magnetic field is established through the excitation system, it can generate electrical power and be synchronized with the electrical grid or the desired frequency.

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