High Voltage Pulsed Energy Surge Testing of Magnetic Components

Vitrek’s new V815 Impulse Coil Winding tester tests the electrical characteristics of magnetic coil winding components without damaging the sample. The detection is carried out when the same electric impulse by capacitor discharge is applied to the DUT (Device-Under-Test) where the resulting waveform is compared to the stored waveform of a standard known sample. The decaying voltage waveform that is generated in response to the impulse, is related to the Q- factor and inductance (impedance) of the coil. In this sense, the tester can detect turn & layer shorts, the differences in the number of turns and the material of the core. When a DUT’s coil layer has a short circuit, the short circuit area is reflected as an increase of energy loss. When a high impulse voltage is applied, poor insulation can appear as a corona or layer discharge. Selectable high voltage pulses between 100V and 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. TheVitrek Impulse Winding Tester V815 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.

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 V815 uses 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.

Corona Discharge

An electrical corona discharge is a condition where the electric field strength or potential gradient about a conductor is excessively strong bringing about an electrical discharge by ionizing external elements, e.g., air, surrounding the conductor yet conditions are insufficient to cause complete breakdown or arcing. At the point of ionization, the surrounding medium element becomes plasmatized or ionized gas and becomes conductive resulting in an additional energy loss.

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. 

Magnetic Coil Decay Waveform Area Comparison

Area Size Comparison

Area comparisons are a means to judge energy loss or efficiency of magnetic coiled designs. Using pre-selected area parameters, the V815 compares the area of the DUT to that of the standard waveform’s area. Percent differences in area may be programmed into the instrument to detect under-performing samples on a Pass/Fail basis.

Corona Number Comparison

Corona amount comparisons is a means to judge energy loss or to evaluate inductive properties and efficiency of magnetic coiled designs. The V815 compares the area of the DUT to that of the standard waveform’s area. Percent differences in area may be programmed into the instrument to detect under-performing samples. This test method is a rapid means of determining the inductance, L, of the DUT, as it differs from the standard. Briefly, Statistics are formulated using Laplacian probability techniques as a method in digital filter processing for detecting the edge intensity of an image. In the application using the Corona Number, the 2nd derivative of the image is used to find and extract the partial discharge. The discontinuity of the value hidden in the wave data (noise) is digitized, resulting in the detection of the partial discharge.

 

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.A newly produced magnetic component such as, transformer, coil, choke, motor, relay, etc. may be tested by comparing its reaction to a pulse of energy to that reaction experienced by a known properly fabricated unit. The waveform decay, as illustrated above, 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.
TheVitrek Impulse Winding Tester V815 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.

PC Linkage for Waveform |Analysis

To make analysis easier, the V815 offers USB 2.0 interface for connecting with PC for user to upload or download the parameters of waveforms.

V815 is Packed with Features

The V815 provides waveform area, waveform comparison, differential area, flutter values and corona values. It has full statistics for each winding stored in memory.  Its built-in High Voltage calibration and testing keep this Impulse tester durable.  A key lock to lock the unit in test mode is a safety feature that prevents any unexpected contact of the keypad.            

The V815 is loaded with a reserved RS-232, RS-485, and GPIB port; with a separate printer port and remote port.  The large test screen has pass/fail indicators and an indication of the last test result.  The navigation keys can be used to move the cursor or select different fields on the screen display.

Test voltage ranges are 200V to 5kV in 100V steps with a measurement time of 50ms.  The display screen has a 320 x 240 dot LCD display with CFL back lighting, includes an Internal speaker and HV LED indicators.