- The primary winding of a transformer draws an exciting current that establishes flux in the core. This flux induces an emf in both the primary and secondary windings. - The maximum flux in a transformer's core is determined by the voltage-to-frequency ratio of the supply. An example calculates the maximum flux for a transformer excited by a 60Hz, 200V source and when the frequency is reduced to 50Hz. - The exciting current on no-load has two components - a magnetizing current in phase with flux and a core-loss current in phase with the induced emf. The total current lags the induced emf by a small angle.