Power Transformer Parameters, Selection, and Testing

The initial group of articles focused on the history of transformer cores, construction methods and materials, and various testing methods. After having discussed the parameters of transformers, this article expands on their losses.

-  Chuck Hansen

Part 5 - Losses

When a load, R₁, is connected to the secondary winding, the secondary current (Is) creates a magnetic field back-EMF that opposes the primary flux (Lenz's law) shown as Eback in Figure 7 (see audioXpress, February 2023). The primary current then increases by the amount necessary to restore the original flux density supported by the primary voltage. Because of the large primary and secondary winding inductances, the rate of current change depends on the transformer R-L time constants. As long as the steady-state condition exists, the average flux density will remain the same.

If the load current increases, the flux density in the core decreases due to the higher voltage drop across the primary winding resistance RP. The secondary impedance Z₁, composed of resistance R, and inductance X, causes a further reduction in the secondary voltage Vs. The ratio of the difference between the no-load secondary voltage (VSNL) and the rated-load secondary voltage (VSRL) for a fixed primary voltage is called the regulation factor. Sometimes the specified regulation factor is based on half-load, so always read the data sheet details.

Smaller transformers tend to have worse regulation factors. The thickness of the insulation is limited by the hi-pot voltage requirements, and the thickness of the wire insulation doesn't vary much with wire gauge, so there is less copper area with smaller gauges. The effective copper area for the space inside the core window available for the windings is also reduced.