How to choose a suitable alternator for a diesel generator of Grupel? -Part 1

In order to achieve reliable and efficient operation of the power system, choosing the right alternator for a diesel generator plays a key role. Whether it is a backup power supply for a hospital, an industrial plant or a site without access to the power grid, the compatibility between the diesel engine and the alternator determines the quality of the electricity generated, the stability under load and the durability of the equipment.

1. What is an alternator?

An alternator is an electromechanical device that converts the mechanical energy generated by the diesel engine into alternating current (AC) electrical energy. Its main components are:

Rotor (rotating part) – it is connected to the engine and rotates, creating a magnetic field.

Stator (fixed part) – surrounds the rotor and contains windings in which voltage is induced.

Voltage regulator (AVR) – controls the level of the generated voltage.

Excitation system – provides starting current to create the magnetic field.

2. Why is choosing the right alternator so important?

The alternator determines:

The power and stability of the output voltage.

Compatibility with electrical loads – especially important for sensitive electronics.

Load efficiency – the selected alternator must maintain a stable voltage during sudden changes in load.

Generator life – an incorrectly sized alternator can overload the engine or wear out prematurely.

3. Types of alternators for diesel generators

There are different types of alternators according to their excitation method and design:

3.1 Brushless alternators

Most popular with modern generators. Low maintenance – no brushes. Reliable and long-lasting.

3.2 With brushes and slip rings

Used in older or specific designs. They are subject to more frequent maintenance – cleaning, brush replacement.

3.3 Synchronous alternators

Generate alternating current with a constant frequency. Extremely suitable for applications requiring a stable frequency (50/60 Hz).

3.4 Asynchronous alternators

Simpler and cheaper, but less efficient. Suitable for basic or low-priority applications.

4. Main technical characteristics of the alternator

4.1 Power (kVA/kW)

Must match the power of the diesel engine and the load requirements.

Good practice: the alternator should be sized 10–20% higher than the maximum load.

4.2 Voltage and frequency

Standard: 230V / 400V and 50Hz in Europe.

For export: 60Hz or other voltage (e.g. 110V, 480V) may be required.

4.3 Number of phases

Single-phase alternator – up to 10–15 kVA, used in domestic conditions.

Three-phase alternator – above 10–15 kVA, for industrial loads.

4.4 Insulation class

Classified as F, H, etc.

A higher insulation class allows operation at higher temperatures.

4.5 Efficiency and cosφ (power factor)

Typically cosφ = 0.8 for most generators.

It affects the actual power that the alternator can deliver.

The choice of alternator should not be underestimated – it is the heart of every diesel generator. The appropriate model must meet the load, operating conditions and the specifics of the consumers. Always consult with manufacturers and specialists and remember that reliability often lies in the details.