Introduction
In standard transformer construction, you’ll notice that the low voltage (LV) winding is always placed closer to the core, with the high voltage (HV) winding placed over it. This arrangement is not arbitrary but is based on sound engineering principles related to insulation, mechanical strength, and cost-effectiveness.
A typical transformer consists of:
- Magnetic Core: Usually made of laminated silicon steel to provide a low-reluctance path for magnetic flux
- Low Voltage (LV) Winding: The winding with fewer turns and thicker conductors
- High Voltage (HV) Winding: The winding with more turns and thinner conductors
- Insulation: Materials that electrically isolate the windings from each other and from the core
Reason 1: Easier Insulation
The primary reason for placing the LV winding closer to the core is insulation requirements. In a transformer, the core is typically grounded for safety reasons. This creates specific insulation challenges:
- The LV winding operates at a lower voltage relative to ground (the core)
- The HV winding operates at a much higher voltage relative to ground
To insulate the LV winding from the grounded core, only a relatively thin layer of insulation material is required. However, insulating the HV winding from the core would require a much thicker and more expensive insulation system.
By placing the LV winding directly against the core:
- The insulation requirement between LV winding and core is minimized
- The HV winding is inherently insulated from the core by the LV winding itself
- Overall insulation costs are significantly reduced
- The transformer can be made more compact
Reason 2: Mechanical Strength
The second important reason relates to the mechanical forces that occur during fault conditions:
- When a short circuit occurs, extremely high currents flow through the windings
- These currents create powerful electromagnetic forces that tend to deform the windings
- The forces are proportional to the square of the current (F ∝ I²)
In the LV-HV-core arrangement:
- The LV winding, being closer to the rigid core, has better mechanical support
- The core acts as a mechanical backing for the LV winding
- This arrangement helps the LV winding better withstand the radial forces during short circuits
- The HV winding, while still needing bracing, is positioned where it can better flex without catastrophic failure
If the HV winding were placed closer to the core, the LV winding would be sandwiched between the HV winding and the core, making it more vulnerable to mechanical damage during fault conditions.
Additional Benefits
Beyond the primary reasons, this arrangement offers additional advantages:
- The magnetic coupling between the LV winding and core is improved
- This results in better efficiency and reduced leakage inductance
- The LV winding can be directly wound onto the core former
- The HV winding can then be wound over the LV winding with appropriate insulation layers
- This sequential process simplifies manufacturing and quality control
Conclusion
The placement of the LV winding closer to the transformer core is an engineering optimization that provides:
- Significant cost savings in insulation materials
- Improved mechanical robustness during fault conditions
- Better overall transformer performance and reliability
This design principle is consistently applied across all power transformers, from small distribution transformers to large power transformers in substations, demonstrating its universal acceptance in the electrical engineering community.
