To calculate and design a Distribution Transformer is relatively simple following theoretical calculations given at the school. To optimize its calculation and design in order to produce it with great quality (low losses and maintenance free) and economically competitive requires skills, experience, and the help of new available software of calculation and optimization. Greater skills and experience is then even required for the Power transformers for Transmission where high voltages and quality materials play an important role.
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.
Experimental transformers using superconducting windings achieve efficiencies of 99.85%,while the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses are significant.
The losses vary with load current, and may be expressed as "no-load" or "full-load" loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, which encourages development of low-loss transformers.