How does inrush current affect diesel generator sizing?

Inrush current is a critical factor to consider when sizing a diesel generator, particularly when powering equipment with motors or inductive loads. Failing to account for inrush current¹ can lead to generator overloads, voltage dips, and equipment failure.

Understanding inrush current helps ensure that the generator can handle startup power surges without compromising its performance.

Here’s how inrush current affects diesel generator sizing² and how you can manage it effectively.

What is inrush current, and why is it important when sizing a diesel generator?

Inrush current is the initial surge of current drawn by electric motors or other inductive loads when they start up. This current can be several times higher than the running current and typically lasts for a short period (milliseconds to seconds).

Inrush current is crucial because it directly impacts the generator’s ability to handle load surges during equipment startup.

Key points about inrush current:

1. Duration and Magnitude:

   • Inrush current can be 6–10 times the rated current of motors and equipment.
   • It typically lasts for a few milliseconds but can last longer in some cases (e.g., for large motors or compressors).

2. Voltage Dip and Overload Risks:

   • If a generator is undersized and unable to handle the inrush current, it can cause voltage dips and overloading, leading to generator tripping or equipment damage.

3. Impact on Generator Sizing:

   • The generator must be sized not only to handle the steady-state power (running current) but also to accommodate the initial surge when equipment starts.

Why it matters:

Ignoring inrush current can cause sudden generator overloads, especially when multiple motors or inductive loads start simultaneously. This is particularly problematic in facilities with heavy machinery, HVAC systems, or pumps.

How can you calculate the inrush current of inductive loads to select the right generator size?

Calculating the inrush current is vital for selecting the correct generator size, ensuring that the generator can handle peak loads during startup without causing voltage dips or overloads.

The inrush current for inductive loads can be calculated based on the motor’s rated power and typical inrush multiplier.

Inrush Current Calculation for Motors:

[
I{\text{inrush}} = I{\text{rated}} \times \text{Inrush Multiplier}
]

Where:

• ( I_{\text{inrush}} ) = Inrush current (amps)
• ( I_{\text{rated}} ) = Rated current (amps)
• Inrush Multiplier typically ranges from 6 to 10 depending on the motor type and size. Larger motors tend to have higher inrush values.

For example, if you have a 5 HP motor with a rated current of 10 amps, and the inrush multiplier is 7:

[
I_{\text{inrush}} = 10 \, \text{amps} \times 7 = 70 \, \text{amps}
]

Inrush Power Calculation:

The inrush power is calculated similarly to the rated power but is scaled by the inrush current:

[
P{\text{inrush}} = \frac{I{\text{inrush}} \times V \times \text{Power Factor}}{1000}
]

Where:

• ( P_{\text{inrush}} ) = Inrush power (kVA)
• ( I_{\text{inrush}} ) = Inrush current (amps)
• ( V ) = Voltage (V)
• Power Factor (PF) is typically 0.8 for motor loads.

This gives you the additional kVA required to handle the inrush current. Make sure to account for multiple inrushes if multiple motors start simultaneously.

Example Calculation for Multiple Loads:

Let’s assume you have three motors starting up simultaneously, each rated at 10 HP with a 6x inrush multiplier:

• Rated current per motor = 15 amps (assuming typical values for a 10 HP motor).
• Inrush current per motor = 15 amps × 6 = 90 amps.

For three motors, the total inrush current would be:
[
I_{\text{total inrush}} = 90 \, \text{amps} \times 3 = 270 \, \text{amps}
]

Now, calculate the generator size based on the total inrush power.

What are the best practices for managing inrush current when starting large equipment with a diesel generator?

Properly managing inrush current ensures that your generator can handle the initial surge without tripping or damaging equipment. Here are the best practices for managing inrush current:

1. Use Soft Starters or Variable Frequency Drives (VFDs):
Soft starters and VFDs help gradually ramp up motor speed and reduce inrush current by limiting the voltage and current during startup. This can reduce the surge by 40–50% and prevent overloads.

2. Proper Generator Sizing:
Ensure that the generator is sized not only for the running load but also for the peak inrush current. It’s recommended to select a generator with a capacity that can handle 1.5–2 times the rated load, especially for motor-driven equipment.

For example, if the total running load is 50 kVA, choose a generator that provides at least 75–100 kVA to handle the inrush current.

3. Stagger Equipment Start-up:
Avoid starting multiple large inductive loads (such as motors or compressors) simultaneously. Instead, stagger their start-ups to spread out the inrush currents. Use sequencers or automatic transfer switches (ATS) to control and manage the timing of equipment start-ups.

4. Consider Using Multiple Smaller Generators:
For large systems with substantial inrush current, using multiple smaller generators can help distribute the load. This allows the system to handle the surge more efficiently without overloading a single generator.

5. Implement Energy Storage or Capacitor Banks:
In some cases, capacitor banks or energy storage systems (like batteries) can be used to supply short bursts of power during inrush, reducing the strain on the generator.

6. Inrush Calculation and Monitoring:
Ensure that the inrush current is calculated properly and periodically monitor the performance of the generator. Use smart controllers or load banks to simulate startup conditions and verify that the generator can handle the load without issues.

Example:

If you have a total load of 50 kW with 15 HP motors (6x inrush multiplier), you may need a generator rated at 60–80 kVA to manage both running power and inrush. Staggering the start-up of these motors or using soft starters³ would help reduce the inrush load on the generator.

Conclusion

Inrush current can significantly affect diesel generator sizing, especially when starting motor-driven equipment. Properly calculating inrush current, using soft starters or VFDs, and sizing the generator appropriately for both running and peak loads ensure reliable operation. Staggering equipment start-up and using multiple generators or capacitor banks are additional best practices to manage i