Why Do Low-Voltage Generators Use 2/3 Pitch, While Medium & High Voltage Prefer 5/6 Pitch?

Wrong winding pitch selection is not a small mistake.
At best, it leads to overheating and nuisance tripping. At worst, it can cause insulation failure, severe damage, or even generator burnout.

Let’s break down the real engineering logic behind this critical design choice.

A Real Scenario from the Field

At a construction site, two diesel generator sets were being commissioned:

1. One 400V low-voltage generator
2. One 10kV high-voltage generator

They looked almost identical.

But an experienced electrician asked a simple question:

“Are their winding pitches the same?”

The answer? Absolutely not.

And that difference is crucial.

1. Low Voltage Generators (400V / 690V): Why 2/3 Pitch Is Essential

Most low-voltage generators operate in a three-phase four-wire system, which includes a neutral line (N).

This changes everything.

Recommended: 2/3 Pitch Winding

1. Suppresses Third Harmonics

In a 3-phase 4-wire system, third harmonics (and multiples of 3) have a return path through the neutral line.

• 2/3 pitch windings naturally eliminate third harmonic components
• This prevents harmonic currents from accumulating in the neutral

Result: No overheating, no cable damage, no fire risk

2. Cleaner Power Output (Low THD)

By suppressing third harmonics:

• Voltage waveform becomes closer to a pure sine wave
• Total Harmonic Distortion (THD) is significantly reduced

Result: Stable and high-quality power supply

3. Better Load Protection

Clean power means:

• Motors run cooler, quieter, and last longer
• Sensitive equipment (PLC, sensors, lab devices) operates reliably
• Stronger compatibility with non-linear loads (VFDs, rectifiers)

4. Easier Paralleling Operation

Lower harmonics = lower circulating currents between generators

Result:

• Stable synchronization
• Reduced tripping risks
• Reliable parallel operation

Why NOT Use 5/6 Pitch in Low Voltage?

Using 5/6 pitch in a 3-phase 4-wire system can be dangerous:

• Strong third harmonic currents in neutral
• Neutral cable overheating or burning
• Overload on transformers and switchgear
• High fire risk

Additionally:

• Poor waveform quality
• Higher THD
• Difficult and unstable paralleling

2. Medium & High Voltage Generators (6kV / 10kV / 13.8kV): Why 5/6 Pitch Is Better

Medium and high voltage generators usually operate in a three-phase three-wire system.

There is no neutral line (or it is not connected).

Recommended: 5/6 Pitch Winding

1. No Neutral = No Third Harmonic Circulation

Even if third harmonics exist:

• There is no return path
• No harmonic current buildup
• No overheating risk

2. Higher Fundamental Efficiency

Without needing to suppress third harmonics, 5/6 pitch offers:

• Higher winding utilization
• Greater power output
• Better voltage waveform quality

3. Better Suppression of 5th & 7th Harmonics

In high-voltage systems:

• 5th and 7th harmonics are more dangerous (affect insulation)

5/6 pitch effectively reduces these harmonics.

Result:

• Improved insulation reliability
• Longer generator lifespan

4. Optimized for Insulation & Temperature

High-voltage generators demand:

• Strong insulation systems
• Controlled electric field distribution

5/6 pitch helps:

• Reduce end-winding stress
• Improve thermal performance
• Extend insulation life

Why NOT Use 2/3 Pitch in High Voltage?

• No benefit (no neutral → no third harmonic issue)
• Lower fundamental voltage performance
• Worse waveform compared to 5/6 pitch
• Reduced overall efficiency

In short: You pay the cost, but gain nothing

3. Quick Selection Guide

4. The Core Rule (Simple but Critical)

Low voltage → Use 2/3 pitch to eliminate harmonics
High voltage → Use 5/6 pitch for efficiency and insulation performance

Final Thoughts

Winding pitch selection is not arbitrary. It is a precise engineering decision based on:

• Voltage level
• System configuration
• Harmonic behavior
• Insulation requirements
• Efficiency targets

Choose correctly:

Stable operation for 10+ years
High efficiency and reliability

Choose incorrectly:

Overheating
Frequent tripping
Severe damage or generator failure

If you are designing or selecting a diesel generator—especially for data centers, mining, or industrial applications—this detail is not optional.

It’s fundamental.

Hosem Power, as a manufacturer with 20 years of experience, we ensure every generator we export is matched with the correct technical specifications for your local grid. If you have questions about choosing the right specs for your project, contact our team today, to get the favorable pricing and technical supporting

Email: sale@hosempower.com

Whatsapp: +8613208904365