IEEE 295 (Complete Document)
Revision / Edition: 69 Chg: W/ REAF Date: December 5, 2007
STANDARD FOR ELECTRONICS POWER TRANSFORMERS
The Electronics Transformer Technical Committee for several years has been working towards the generation of a Standard for electronics purpose power transformers even while under AIEE auspices. Earlier attempts seemed to parallel the distribution and power transformer Standards too closely, and hence missed the point of departure of how electronics transformers needed, at least in part, a different, treatment than power distribution transformer Standards. After the completion of the Wide-Band Standards Nos. 111 and 264 by the committee, a pattern was established that could be followed in the Power Transformer Standard to achieve the desired results. This proposed Standard can be well considered to be the product of the merger of AIEE and IRE people into the new blend of IEEE as it represents concessions oil the part of each to the other at various points.
Some interesting problems had to be resolved in the process. For example, in the use of solid-state rectifiers it is necessary to provide fast and sensitive protection for the easily destroyed diodes in the event of faulting. This, in turn, led to a new definition of transformer inrush current, which we found was not previously defined satisfactorily. Also, the extensive use of combined alternating and direct voltages and currents in electronics transformers made it necessary to resolve certain insulation testing problems that have plagued the industry for years. Yet to say that these problems are resolved is an oversimplification of the situation. Nevertheless, a good start has been made in this regard.
We have attempted to give guidance and direction in the sensitive area of corona testing without foreclosing on the adoption of new Standards and understandings on the subject.
For all sizes of electronics purpose power transformers, it is expected that this Standard will supersede the use of USA C57 Standards.
The character and applications of transformers used in electronic circuits are enough different from other categories so that none of the existing Standards does a satisfactory job. Much confusion and conflict has obtained in the past several years because of this lack, with different people having a wide variety of opinion relative to many aspects of specification and testing.
This Standard is intended to control by exception in the areas where conflict and disagreement have been most noticeable. Other well-known Standards are listed that should be used in areas where general agreement exists.
This Standard pertains to power transformers and inductors that are used in electronic equipments and supplied by power lines or generators of essentially sine wave or polyphase voltage. Guides to application and test procedures are included. Appendices contain certain precautions, recommended practices, and guidelines for typical values. Provision is made for relating the characteristics of transformers to the associated rectifiers and circuits.
Certain pertinent definitions relating to transformers and transformer applications, which have not been found elsewhere, are included with appropriate discussion. Attempts are made to alert the industry and profession to factors that are commonly overlooked.
This Standard includes, but is not limited to, the following specific transformers and inductors.
Rectifier supply transformers for either high- or low-voltage supplies.
Filament and cathode heater transformers.
Transformers for alternating current resonant charging circuits.
Inductors used in rectifier filters.
Autotransformers with fixed taps.