Starting an Electric Motor from a Diesel Generator: How a Frequency Converter Saves Your Generator

Imagine a facility equipped with a diesel generator and a powerful asynchronous motor (pump, fan, or conveyor). When the motor is started directly, the generator "chokes," a deep voltage drop occurs, the circuit breaker or emergency protection trips, and in the worst case, the generator fails. The reason is the high starting currents, which are 6–8 times the rated values and overload the "soft" power source.

The solution is simple and accessible – installing a frequency converter (FC). In this article, we will explore the physics of the process, show how the FC solves the problem, and provide practical advice on selecting and setting up the equipment.

1. Detailed Analysis of the Problem: Why Don’t the Generator and Motor "Get Along"?

1.1 Nature of the Starting Current

An asynchronous motor at the start moment, by its electrical characteristics, resembles a short circuit to the network. The rated current of a 10 kW motor is about 20 A, but during direct start it can briefly reach 120–160 A.

Rated current of 10 kW ≈ 20 A.

Starting current during DOL-start – 6–8× rated current.

Deep voltage drop on the generator busbars.

1.2 Limitations of the Diesel Generator

The diesel generator is a soft power source that cannot instantly deliver the starting current of an asynchronous motor. When attempting to quickly provide a high current, the following occurs:

Deep voltage drop – network lighting "flickers" or turns off, sensitive equipment shuts down.

DG protection activation – the generator automatically switches off to prevent overheating of the windings.

Mechanical and electrical damage – in the worst case, winding failure of the generator.

2. The Frequency Converter as an "Intermediary"

2.1 How Does the FC Solve the Problem?

The frequency converter controls the voltage and frequency supplied to the asynchronous motor, providing:

Smooth acceleration (Ramp-up): The FC supplies voltage from zero and gradually increases it to the rated value, reducing the starting current.

Current limitation: A maximum current is programmed, for example, 1.5× rated, that doesn't overload the generator.

Comparison of starting:

2.2 Additional Advantages of the FC

Motor protection from overheating, overload, and phase loss.

Speed regulation according to technological process needs.

Reduction of mechanical loads – absence of jolts and shocks during start.

3. Practical Guide: Selection and Setup

3.1 Correct Power Selection

A common mistake is choosing a generator "exactly" matching the motor power. For proper operation with an FC, a margin is needed:

FC power ≈ motor rated power + 10% (for example, 11 kW for a 10 kW motor).

Generator (kVA) – 1.5–2 times the FC power (11 kW × 1.5 = 16.5 kVA; optimally 20–22 kVA).

This margin accounts for the FC input capacitors and peak loads during starting.

3.2 Key FC Settings for Operation with DG

Acceleration time (Ramp-up time): 15–30 s (replace the standard 5–10 s) for a smoother start.

Current limit: 110–130% of the rated current.

Control mode: scalar (V/f) – sufficient for pumps and fans.

3.3 Is an Input Choke Needed?

The FC generates harmonics that can harm the AVR of the diesel generator. An input choke smooths distortions and protects the generator voltage regulator. When working from a DG, installing a choke is strongly recommended.

4. Common Mistakes

Generator and FC of the same power (e.g., 10 kW and 10 kW) – the motor will not start.

No current limitation set in the FC settings.

Too short acceleration time – peak loads on the DG.

Connecting sensitive equipment to the same generator without filters.

5. Conclusion and Call to Action

The frequency converter is not just an "improvement," but a mandatory component for reliable and safe starting of an electric motor from a diesel generator. You protect the generator from overloads, reduce repair costs, and ensure stable operation of the technological process.

Not sure which frequency converter suits your system? Contact our engineers – we provide a free calculation and select the optimal model. Contacts | Frequency Converter Catalog