How to Select a Factory Generator?

Capacity, Specifications, and Key Selection Factors Explained

Industrial generators are primarily used for power backup, continuous supply, and stable factory operation. Their core purpose is to provide a reliable power source and prevent production loss caused by power outages.
When selecting a generator, the two most critical factors are the total power consumption of the facility and the inrush current generated by large motor-driven equipment during startup.
If the capacity is insufficient, it may lead to voltage instability, failure to start equipment, generator tripping, or overload conditions. In severe cases, it may even shorten the lifespan of production equipment.
In addition, most industrial facilities operate on a 380V three-phase system. Load balancing must also be considered during system design to avoid overheating, efficiency loss, or abnormal shutdowns caused by uneven phase loading.

Capacity Calculation: Equipment Classification for Accurate Sizing

To determine the required generator capacity, the first step is to classify all electrical equipment into two categories:

• Steady-load equipment： These devices operate with relatively constant power consumption and do not generate significant inrush current at startup.
  Examples include: factory lighting, surveillance systems, control panels, air conditioning units, and small welding machines.
  Power consumption can generally be calculated directly based on rated nameplate values.
• High inrush (motor-driven) equipment： These devices generate extremely high starting currents and are the key factor in generator sizing.
  Examples include: air compressors, pumps, compressors, machine tools, and conveyor systems.
  Direct-start motors may require 5–7 times their rated power at startup; with soft starters or variable frequency drives (VFDs), this can typically be reduced to around 2–3 times.

Simplified industrial sizing formula:

Required generator capacity = (Total operating load + peak starting demand of largest motor) × safety factor It is generally recommended to reserve an additional 20%–30% capacity margin to avoid long-term operation under high load conditions.

Capacity Rating: Prime vs Standby Power

Generator specifications are typically divided into two categories and must not be confused:

• Prime Power： Used for continuous long-duration operation. This rating should be used when the generator is the main power source.
• Standby Power： Designed for emergency backup only. Long-term continuous operation under full load is not recommended, as it will significantly reduce equipment lifespan.

Capacity Adjustment Based on Application Scenarios

• Backup power for factories： Recommend 20% capacity margin, maintaining 70%–80% load for optimal stability and efficiency.
• Long-term primary power supply： Recommend ~25% capacity margin, maintaining 60%–75% load to reduce wear and extend service life.
• High-frequency motor starting environments： Recommend 30% or more margin to avoid voltage instability or system tripping during startup.

Specification Selection Considerations

Beyond capacity, fuel type, electrical specifications, and environmental conditions must also be evaluated:

1. Three main power types

• Heavy Fuel Oil (HFO) Generators： Low fuel cost and suitable for large-scale industrial plants and continuous operation. Commonly used in power plants, marine applications, and base-load systems. However, they require more complex fuel handling and maintenance.
• Diesel Generators： The most common industrial choice, offering stable output, durability, and long operating capability. Ideal for backup and primary industrial power supply.
• Natural Gas Generators： Low emissions, lower noise, and stable fuel costs. Suitable for fixed installations with stable gas supply and long-term operation.

2. Electrical specifications and protection systems

• Voltage & frequency： Typically 380V three-phase systems, with 50Hz or 60Hz depending on region.
• Voltage stability： Recommended for facilities with sensitive or motor-intensive loads.
• Protection systems： Overload, short-circuit, high-temperature, and low-oil-pressure protection are essential for safe operation.

3. Special environmental conditions

• High temperature / high altitude： Require additional capacity margin due to reduced generator performance.
• Urban or residential areas： Noise-reduction (silent type) generators are recommended to meet environmental regulations.

Common Mistakes to Avoid

Most non-technical procurement teams make the following mistakes:

1. Ignoring motor inrush current and only calculating steady load.
2. Using standby power rating for long-term operation.
3. Uneven three-phase load distribution causing overheating or tripping.
4. Failing to reserve capacity for future expansion.

Expert Recommendation

Key points for factory generator selection:

First confirm total power demand, then calculate motor starting load, reserve appropriate capacity margin, and base selection on prime power rating. Also ensure proper three-phase load balance and future expansion planning.

Generator selection is not about higher capacity, but about a balanced evaluation of load demand, operating hours, fuel type, maintenance capability, budget, and environmental requirements.

It is strongly recommended to have the Zhen Yi professional team conduct a full site and load assessment before system design to ensure the most suitable power solution is selected.