Editing Transformer (section)

===Effect of frequency===
The EMF of a transformer at a given flux increases with frequency.<ref name="Say1983"/> By operating at higher frequencies, transformers can be physically more compact because a given core is able to transfer more power without reaching saturation and fewer turns are needed to achieve the same impedance. However, properties such as core loss and conductor [[skin effect]] also increase with frequency. Aircraft and military equipment employ 400&nbsp;Hz power supplies which reduce core and winding weight.<ref>{{cite web | title = 400 Hz Electrical Systems | work = Aerospaceweb.org |url=http://www.aerospaceweb.org/question/electronics/q0219.shtml | access-date = May 21, 2007}}</ref> Conversely, frequencies used for some [[railway electrification system]]s were much lower (e.g. 16.7&nbsp;Hz and 25&nbsp;Hz) than normal utility frequencies (50–60&nbsp;Hz) for historical reasons concerned mainly with the limitations of early [[traction motor|electric traction motors]]. Consequently, the transformers used to step-down the high overhead line voltages were much larger and heavier for the same power rating than those required for the higher frequencies.

[[File:Power Transformer Over-Excitation.gif|thumb|Power transformer overexcitation condition caused by decreased frequency; flux (green), iron core's magnetic characteristics (red) and magnetizing current (blue).]]
Operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetizing current. At a lower frequency, the magnetizing current will increase. Operation of a large transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. Transformers may require [[protective relay]]s to protect the transformer from overvoltage at higher than rated frequency.

One example is in traction transformers used for [[electric multiple unit]] and [[High-speed rail|high-speed]] train service operating across regions with different electrical standards. The converter equipment and traction transformers have to accommodate different input frequencies and voltage (ranging from as high as 50&nbsp;Hz down to 16.7&nbsp;Hz and rated up to 25&nbsp;kV).

At much higher frequencies the transformer core size required drops dramatically: a physically small transformer can handle power levels that would require a massive iron core at mains frequency. The development of switching power semiconductor devices made [[switch mode power supply|switch-mode power supplies]] viable, to generate a high frequency, then change the voltage level with a small transformer.

Transformers for higher frequency applications such as [[Switched-mode power supply|SMPS]] typically use core materials with much lower hysteresis and eddy-current losses than those for 50/60&nbsp;Hz. Primary examples are iron-powder and ferrite cores. The lower frequency-dependant losses of these cores often is at the expense of flux density at saturation. For instance, [[Ferrite core|ferrite]] saturation occurs at a substantially lower flux density than laminated iron.

Large power transformers are vulnerable to insulation failure due to transient voltages with high-frequency components, such as caused in switching or by lightning.