High Voltage Impulse Testing Exposes Hidden-Subtle Defects

Vitrek’s new V700 / V710 / V720 Series  Impulse Winding Testers provides a non-destructive way to test windings using Hipot high voltage pulses or surges to compare the resulting waveform of the sample under test to that of a know unit’s waveform. By comparing the decay waveforms with a standard unit, winding deviations, deviation in core material, number of turns, shorted turns, and insulation breakdown can be identified. Selectable high voltage pulses between 100Vand 5000V featured by this hipot impulse surge tester are utilized to stress and break down any existing marginal insulation quality between windings attributed to nicked, frayed or excessively thin or weak insulation coatings.

High voltage non-destructive analysis of transformers, motors, generators, armatures, relays, solenoids, inductive chokes, and other wound magnetic products is a sure method of detecting manufacturing faults that cannot be exposed by simple LCR Meter measurements. Fault types such as improper materials, shorted turns, nicked insulation, and damaged windings are typical manufacturing faults that can be determined by exciting the magnetic properties with a stimulating voltage and comparing decaying wave forms to that of a waveform of a known proper product sample. LCR meter analysis simply is not capable of detecting and defining these fault types. The Vitrek 700 Series use a high voltage impulse to interrogate and expose these otherwise undetectable faults without destroying the product. The impulse creates a signature pattern that is an overall precisely reproducible form from good product to good product to register a matching profile waveform. Waveform comparisons can result in differences in impulse waveforms between known standard samples and untested unproven production samples. These differences expose faults which would otherwise go undetected and could result in increased product infant mortality.

Why Use Impulse Testing?

Impulse testing characterizes a winding in a beneficial way not possible using a conventional LCR meter. Comparing the waveform from a good device to that of the device under test will show differences in the number of turns, changes in the transformer core material, shorted turns and corona discharge (damage to the winding). The differences are shown as a waveform that decays at a different rate, which would be indicated by the area under the curve. A waveform that is out of phase with the standard waveform would be indicated by differential area size, or a combination of both indicated by waveform comparison.
Corona discharge, indicated by a spike on the waveform, is normally present when a high voltage charge is applied across a wire (winding) that has some damage to the insulation.

The short high voltage pulse will cause no damage to the device under test.

Intelligence Gained from Waveform Analysis

The peak of the initial waveform represents the peak impulse voltage applied to the test sample or the Device Under Test (DUT). When the impulse voltage is stopped or terminated, a voltage oscillation decaying waveform is initiated.  The voltage decay curve of the impulse wave form is indicative of the Q-factor of the wound magnetic product and mainly depends on the inductance and the stray capacitance of the product’s winding. These two properties determine the frequency of the decaying waveform. Deviations from this ‘standard’ waveform represent fault potentials which may result from improper wire gage, improper lamination gage or count, shorted turns, turns deviations from design, arcing due to insulation nicks or frayed wires, etc. This impulse testing technique insures that the product under examination is merely tested and not damaged as a result of the testing, but rather is exposed as having a manufacturing deficiency. 

Early Rewards through Impulse Testing

Impulse testing is the one unique test method to detect potentially early fatal damaging failures of wound magnetic products in the field that could otherwise slip under the ‘radar’ because the products survive normal testing and QC scrutiny. Deficiencies as cited above represent product infant mortality resulting in costly field product failures which could be greatly reduced with proper testing.

High Voltage Impulse Testing Theory

When a short high voltage pulse is applied to a perfect unloaded inductor a sine wave would be seen that continued to infinity without changing amplitude. However, the perfect inductor does not exist and making a measurement will always apply a small load to the device under test. So what is actually seen is a sine wave with decaying amplitude.

The waveform decay is related to the Q-factor of the coil: the higher the Q, the slower the rate of decay. The frequency of the waveform is related to the inductance and stray capacitance of the winding.
The Impulse Winding Tester V700 / V710 allows the user to store up to 200 ‘good’ waveforms. The instrument will then compare these waveforms against the waveform(s) of the device(s) under test.

Production Test Software

Vitrek’s 700 Series HV Impulse Testers are fully programmable through its RS232 port.  Specialized production test software is available as an option.  Component waveforms can be stored against serial numbers to ensure thorough and rigorous testing comparisons for newly manufactured components.