Capacitors, Magnetic Circuits, and Transformers is a free introductory textbook on the physics of capacitors, coils, and transformers. See the editorial for more information.... 
Home The Transformer Transformer Losses and Efficiency  
Search the VIAS Library  Index  
Transformer Losses and Efficiency
The losses in a transformer are the core loss due to hysteresis and eddy currents in the core, as explained in Chapter 5; and the copper losses in the windings and stray losses due to eddy currents induced by the leakage fluxes in the tank and other parts of the structure. The sum of the copper losses and the stray losses is known as load losses and is determined from the shortcircuit test, being I^{2}R_{eq} The core loss is determined from the opencircuit test. The power efficiency of a device is the ratio of the useful power output to the power input, the power input being equal to the useful power output plus the power losses. Thus
The rated load efficiency of transformers is generally quite high (as high as 90 percent for transformers, as small as 1 kva) and since the losses can be determined readily, Eq. 659 is preferred to Eq. 658 for calculations of efficiency for reasons of accuracy. Methods of testing transformers and of calculating their performance have been developed to a high degree and are specified in the ASA Standards. These include corrections for temperature and other refinements that are not within the scope of this text.
For instance, to compute the energy efficiency for a 24hr period, or the allday efficiency, it is necessary to integrate the core loss and the load losses over a 24hr period. The core losses are practically constant since the voltage of a power transformer is nearly constant. The energy in kilowatt hours associated with the core loss is merely the product of the core loss in kilowatts and the time in hours during which the transformer is energized. It is more difficult to compute the energy loss due to the load losses (I^{2}R) when the load is fluctuating. However, if the daily load schedule is known and minor fluctuations of the load can be neglected, the load curve can be approximated by a step curve, and the energy losses can be calculated for a 24hr period as shown in Example 65 below.


Home The Transformer Transformer Losses and Efficiency 