IEEE 389-1996 pdf download IEEE Recommended Practice for Testing Electronics Transformers and Inductors
It should be kept in mind that the test itself is destructive in nature and cannot be applied to a sound coil foran extended period. Therefore. the test duration should be carefully controlled to limit it to the minimumtime required to obtain a meaningful indication.
The test consists of applying a controlled train of current pulses to the coil. The leading edge of the pulsesmay have a moderate slope to allow the current in the coll to rise to a predetermined level. Once this level isreached, the curent is disconnected from the coil, and the resultant voltage rise, due to the fast collapse ofthe current through the inductance of the coil, is observed on an oscilloscope or by means of a comparator toestablish the peak value reached (E = L
The coil should be unloaded during this observation so that the voltage is not limited by some unspecifiedloading. A high-impedance input oscilloscope coupled to the circuit with a frequency-compensated highimpedance voltage divider may be used. Loading should be of the order of 1-10 M.
It is not necessary to have a horizontal sweep operating on the oscilloscope. Prior to the application of thetest, the spot on the oscilloscope can be conveniently centered horizontally and can rest on some base levelnear the bottom of the screen. During the test the excursion of the spot is observed against the calibration ofthe screen, which acts as a peak reading voltmeter A 0.5-1.0 s time interval at a 120 Hz rate is sufficient toallow an operator to visually detect any drop or instability in the image caused by internal short circuits thatload the coil.
A more sophisticated method uses a comparator consisting of a dc-biased diode bucked against the risingwaveform as a detecting device. With the dc bias set to the desired test voltage level, the peak currentthrough the coil is adjusted until the peak exceeds the dc bias by some predetermined amount (10-20 V).The top of the pulse train is clipped and fed to a counter. Pass or fail can be determined by comparing thecount with the total number of pulses. If the clipped pulse count is less than the total number of pulsesapplied by more than, for example, two or three counts, a failure indication is obtained.
In order to ensure the successful generation of high-voltage pulses without the use of excessive current, thetest coil should be placed over an iron core during the test. Best results are obtained from cores made up ofthin nickel-iron laminations. The core should be grounded for operator protection.
As a precaution against unwarranted damage to the coil, the test current should always be increased fromzero until the observation yields the desired voltage level. A test coil may be used for calibration and saved ifproduction of an item is apt to be repeated. The proper current level set on this coil can then be used on theproduction coils without further adjustment.
Some suggestions as to the determination of the desired voltage level to be used in a specific coil follow.
Assume a coil consisting of 10 000 turns of 36 AWG magnet wire with an insulation rated at 400 V/mil.wound on a form with 250 turns per layer, and therefore having 40 layers. The insulation thickness of thewire is 0.0005 in: therefore two wires laid tightly side by side, having 2 x 0.0005 in separation (0.001 in)should withstand 400 V for an extended time.
IEEE 389-1996 pdf download
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