What Makes a Premium High Voltage Coil?

We have been manufacturing coils for over 30 years and our skilled engineers are experts in high voltage coils and their applications. Our guide to what makes a premium high voltage coil gives further information on what to look out for when choosing a supplier.

Insulation System

The insulation system of a HV motor or alternator is critical in terms of its performance, longevity and value as an investment to the asset owner. Selecting the appropriate insulation requires consideration of application, circumstance, atmospheric condition and geographic location.

Houghton International have developed a range of systems that cover the broad scope of requirements faced by our expanding customer base.

Corona Discharge Protection

Corona discharge is an electrical phenomenon created by the ionisation of air around a conductor as a side effect of an electric field with a high potential gradient or strength.

In high voltage rotating machines, Corona activity is present at 6600V and above, increasing in intensity and severity as the machine’s potential gradient rises. The protection against Corona discharge and its adverse debilitating effects on a winding’s insulation is what governs the design concept and features of a premium HV coil.

Every HV coil should posses a conductive outer layer which grounds the coil in the slot. It should also feature a stress grading system which protects the coil as it leaves the slot, where the potential gradient is at its greatest and Corona activity is therefore most intense.

Tan δ / Tip Up

Tan δ and Tip Up is the accepted measure of a premium HV coil. What is measured is the AC power factor involved in the passage of AC leakage current through the insulation wall.

This measures the air gaps or void content in a cured HV coil. This measure is taken using a Schering bridge rectifier, which quantifies the capacitive content of insulation through the slot portion of the coil.

Most standards require only a sample percentage of coils out of an entire coil set to be tested. However, we advise testing the entire coil set in order to ensure dielectric integrity and machine longevity.


When describing fit we refer to how closely together the coil fits into the stator slot. Any area of the slot section that is not ground leaves an air gap, meaning Corona Discharge activity will occur above 6600V.

Variances in fit promote Corona activity, hence coils should be manufactured to at least 0.25mm across the length of the coil and similarly 0.5mm for skew slot coils from any given slot width.
Hot pressed resin rich slot sections guarantee such a fit, where VPI systems rely on sufficient resin penetration to ensure the main wall is adequately ground.

Shape Consistency

Shape consistency is a critical feature because the more fitting the shape of the coil, the less mechanical stresses are placed on it during the insertion process.

Shape consistency is critical because across a significant number of coils, if the shape is out, the engineer can lose the space available, which creates many problems, not least placing further adverse stresses that can increase the likelihood of failure.

Houghton International’s tried and tested methods of controlling shape consistency have underlined our value to the people who work with our products.

Dielectric Integrity

Routine electrical tests are carried out according to BS EN 60034, BS EN 50209 and IEEE 286.

Typical routine testing regimes:

  • Surge comparison or turn to turn test (2 UN) + 1000
  • Tan δ / Tip Up
  • Hi-pot (AC and DC flash) test (2 UN + 1000)1.2
  • Lamination test
  • Partial Discharge test
  • Voltage Endurance test

Our guide to recording accurate coil data

When placing an order for HV coils, in order to ensure an accurate fit and specification it is critical to ensure that key data points are measured correctly. See our guide to find out our recommended best practice.

View Guide
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