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The most common purpose of a power electronic transformer is to convert alternating current (A.C) power from one A.C. voltage (or current) to another A.C. voltage (or current). Another common purpose is to provide electrical isolation between electrical circuits. Power is the product of voltage times current. Power transformers do not change power levels except for parasitic losses. Input power minus parasitic power losses equals output power. Ideal power transformers have no losses, hence output power equals input power. Increasing the output voltage will decrease the output current. Electric utilities prefer to transmit electricity at low current values to reduce resistive losses in the power transmission lines. Lower currents also permit smaller size transmission cables. A power transformer is used between the generating equipment and the power line(s) to step-up (increase) the transmission voltage (to high voltage) and decrease the transmission current. Distribution transformers, which are power transformers, are used to step-down (decrease) the voltage to voltage levels needed for industrial and household use.
Power electronic transformers may be classified by their power ratings (fractional VA to mega-VA), their type of construction, and/or by their intended application. The same basic power transformer may be suitable for multiple applications hence the same power transformer may be classified under several overlapping category types. The common person associates power transformers with the electric utilities, hence they think of pole transformer and distribution transformers. The power transformers used inside their appliances and electronic devices do not readily come to mind. The two broadest categories of power transformers are the electric utility power transformers and electronic power transformers (1 & 3 phase). Utility transformers are almost entirely A.C. sine wave transformers. An electronic power transformer is essentially any electronic transformer supplying power to electronic circuits. There are many sub-categories: pulse, inverting, switching (flyback, forward converter), toroidal, square wave, isolation, and others. Instrument transformers (example current transformers) are not considered to be power transformers. They measure voltage or current instead of supplying power.
Electronic transformers / power transformers range in size from a cubic centimeter to multiple cubic meters. The weight can range from a fraction of an ounce to multiple tons. The size and weight of a power transformer is dependent on several factors. A non-exhaustive list includes: desired power rating, maximum ambient temperature, allowable temperature rise, cooling method (air or liquid cooled, natural convection or forced), transformer shape, voltage dielectric requirements, required voltage regulation, operating frequency, operating waveform, and core material. Of these, the two most limiting parameters are allowed temperature rise and required voltage regulation. Operating frequency is a major parameter in selecting core material. Low frequency applications usually utilize either tape wound or laminated silicon steel cores. Moderate frequency applications utilize tape wound or laminated nickel iron cores. High frequency applications usually use ferrite cores.
Power transformers are produced in a variety of shapes. Toroidal power transformers are the high performers. They offer the smallest size (by volume and weight), less leakage inductance, and lower electromagnetic interference
(EMI). Their windings cool better because of the proportionally larger surface area. Bobbin or tube wound transformers are usually more economical to build. Long thin cores are more suitable for low frequency high Q transformers. Some shapes, pot cores for example, are self shielding (reduces EMI).