Synchronous vs Asynchronous Generators: What It Means for RECD for DG Set Applications

Diesel generator sets are widely used as backup and prime power sources across factories, hospitals, commercial buildings, telecom facilities, and other critical installations. While discussions about generators often focus on kVA capacity, fuel consumption and emissions, the type of alternator used inside the genset is equally important.

The two major categories are synchronous and asynchronous generators. Both can convert mechanical energy into electrical energy, but they operate differently and are suited to different applications. Understanding this difference can help generator users make better technical decisions, particularly when electrical performance and emission-control requirements need to be considered together.

Understanding Synchronous Generators

A synchronous generator operates at a speed that remains synchronised with the frequency of the electrical output. In a conventional 50 Hz, four-pole alternator, for example, the synchronous speed is 1500 RPM.

The rotor creates the magnetic field while the stator contains the windings in which electrical power is generated. Modern synchronous alternators generally use an excitation system and an Automatic Voltage Regulator (AVR) to maintain the required output voltage when the connected load changes.

This ability to regulate voltage independently makes synchronous alternators particularly suitable for standalone generator applications.

For a detailed explanation of the construction, excitation system, operating speed and applications of these machines, the Aceget article Synchronous vs Asynchronous Generators: Key Differences provides a useful technical comparison.

Understanding Asynchronous Generators

An asynchronous generator is also known as an induction generator. Unlike a synchronous generator, its rotor does not operate exactly at synchronous speed. During generation, the rotor normally turns slightly faster than synchronous speed.

Another major difference is excitation. An asynchronous generator does not normally have a separate DC excitation system on its rotor. Instead, it requires magnetising or reactive power from an external source, such as the electrical grid or a suitably designed capacitor arrangement.

Because of this operating principle, asynchronous generators are commonly associated with applications such as wind energy, small hydro installations and certain grid-connected generation systems.

They are much less common in conventional standalone diesel generator sets because a backup genset needs to establish its own electrical supply without depending on an already energised grid.

Synchronous vs Asynchronous Generators: Key Differences

This comparison explains why synchronous alternators are the normal choice for conventional standalone diesel generator sets.

Why Diesel Gensets Usually Use Synchronous Alternators

A diesel generator is expected to provide power when the utility supply is unavailable. Therefore, it needs to establish voltage and frequency independently and supply connected equipment without depending on another electrical source.

A synchronous alternator is well suited to this requirement. Its excitation system and AVR allow it to respond to changing electrical loads and maintain relatively stable output conditions.

This becomes especially important when a generator supplies equipment such as motors, pumps, compressors, industrial machinery or sensitive electronic systems.

Generator equipment is also covered by Indian technical standards. The Bureau of Indian Standards provides information on standards for generators and diesel generators through its Know Your Standard portal, where users can search applicable standards and related documents.

Does Alternator Type Determine RECD Selection?

This is where generator users need to keep two different technical systems separate.

The alternator is responsible for generating electrical power, whereas a Retrofit Emission Control Device is designed to control pollutants from the diesel engine's exhaust.

Therefore, selecting an RECD for DG Set should not be based simply on whether the alternator is synchronous or asynchronous.

The more relevant factors include the diesel engine characteristics, exhaust configuration, operating conditions, emission requirements and compatibility of the retrofit system with the engine.

The Central Pollution Control Board's official RECD emission-compliance procedure covers emission-compliance testing of retrofit emission-control devices for diesel power-generating-set engines. This shows why the engine and emission-control system need to be evaluated separately from the electrical characteristics of the alternator.

Why This Difference Matters During Generator Upgrades

Consider an industrial facility experiencing voltage fluctuations while also planning an emission-control upgrade.

The voltage issue could involve the AVR, excitation system, governor, load changes or alternator performance. The emissions issue, on the other hand, requires attention to the diesel engine, exhaust system and emission-control technology.

These are connected because they are parts of the same generator, but they are not the same technical problem.

Keeping the two areas separate helps facility managers avoid selecting an emission-control system based only on the generator's electrical rating.

What Should Buyers and Operators Check?

Before selecting or upgrading a diesel generator, users should review several electrical parameters, including:

• Rated kVA and kW

• Voltage and frequency

• Single-phase or three-phase configuration

• Standby or prime-power duty

• Load characteristics

• Alternator type

• AVR arrangement

• Motor-starting requirements

If an emission-control retrofit is also required, the assessment should be carried out separately. The engine model, engine family, exhaust arrangement, operating load and applicable RECD certification should be reviewed before choosing the retrofit system.

This approach is particularly useful when an older generator is being upgraded for continued operation. The electrical side of the machine and the emissions side should each be evaluated according to their respective technical requirements.

The Role of Synchronous Alternators in Modern Generator Systems

Synchronous alternators continue to be widely used because they provide several characteristics required by standalone power systems. They can establish their own electrical output, support voltage regulation through excitation control and operate effectively with automatic control systems.

They are also suitable for applications involving generator paralleling, load sharing and synchronisation, where precise control of frequency, voltage and phase relationships becomes important.

Asynchronous generators have their own advantages, particularly in applications where grid support is available and simple, robust induction-machine construction is beneficial. However, their operating requirements make them fundamentally different from the alternators normally installed in standalone diesel gensets.

Final Takeaway

Synchronous and asynchronous generators may both generate electricity, but their operating principles and practical applications are significantly different.

Synchronous generators are particularly suitable for standalone diesel generator sets because they can operate independently and provide controlled voltage and frequency. Asynchronous generators are more commonly found in applications such as wind, small hydro and grid-connected generation.

For diesel generator users, the important point is that alternator technology and emission-control technology perform different functions. When evaluating an RECD for DG Set, the focus should remain on the diesel engine, exhaust system, operating conditions and applicable certification requirements rather than treating the alternator type as the deciding factor.