IEEE 389-1990 pdf free.IEEE Recommended Practice for Testing Electronic Transformers and Inductors.
IEEE 389 should be unloaded during this ob-servation so that the voltage is not limited bysome unspecified loading. A high-impedanceinput oscilloscope coupled to the circuit with afrequency-compensated high-impedance volt-age divider may be used. Loading should be ofthe order of 1 to 10 MS2.
It is not necessary to have a horizontal sweepoperating on the oscilloscope. Prior to the application of the test, the spot on the oscilloscopecan be conveniently centered horizontally andcan rest on some base level near the bottom ofthe screen. During the test the excursion of thespot is observed against the calibration of thescreen，which acts as a peak reading voltmeter.A 0.5 to 1 s time interval at a 120 Hz rateis sufficient to allow an operator to visuallydetect any drop or instability in the imagecaused by internal short circuits that load thecoil.
A more sophisticated method uses acomparator consisting of a dc-biased diodebucked against the rising waveform as a detecting device. With the dc bias set to the de-sired test voltage level，the peak currentthrough the coil is adjusted until the peak exceeds the dc bias by some predeterminedamount (10 to 20 V).The top of the pulse train isclipped and fed to a counter. Pass or fail can bedetermined by comparing the count with thetotal number of pulses.If the clipped pulsecount is less than the total number of pulses applied by more than, for example, two or threecounts, a failure indication is obtained.
In order to assure the successfulgeneration of highvoltage pulses，without theuse of excessive current, the test coil should beplaced over an iron core during the test. Bestresults are obtained from cores made up of thinnickel-iron laminations.The core should begrounded for operator protection.
Suppose our test criterion is that a coil wherethe end turn drops two layers should beacceptable.The voltage between these turnsthen can be 1/o of the total across the coil. If400 v is a passing level, then 400 times 10,or4000 v, should be the peak voltage generated inthe coil.
It can be seen from the preceding text thatlong coils with fewer layers are more apt tofail a surge test than short coils of manylayers.In the preceding example，it now becomesobvious that the crossover lead insulationshould withstand at least 4000 v peak.Transformers normally driven by asine-wave orsquare-wave source shouldwithstand，across the primary,an inducedvoltage of twice the highest normal operatingvoltage at a frequency of twice the normal operating frequency.
In case of repeated induced voltagetesting, since the application of test potentialsmay impair the strength of the transformerinsulation,any test carried out in accordancewith 4.3.1 or 4.3.2 should, if repeated, be madeat not more than 90% of the specified test poten-tial for the same time interval.IEEE 389 pdf download.