Core Losses

Author: N.H. Crowhurst

Applied voltage is opposed by back emf

Real transformers are not the same as ideal ones, however. Some current flows in the primary even when no secondary current flows. This current induces a voltage (back emf) in the primary that opposes the applied voltage. This opposing voltage prevents the flow of excessive current in the low-resistance primary and, since the magnetization of the core depends on the current in the windings, it limits core magnetization as well. It is because the back emf does not completely cancel the applied voltage that primary current flows in a real transformer. In a properly designed transformer, this current is quite small, but it represents loss that can be visualized as a resistor connected across the primary that acts in addition to the resistance referred from the secondary load.