Measuring the exciting current of a Transformer.

The exciting current of a transformer refers to the current drawn in order to build the voltage in the secondary.  The current is also known as the magnetizing current.  This current drawn lags behind the applied voltage by 90 degrees.  The exciting current is in phase with the magnetic flux.

The exciting current of a transformer can be measured by measuring the no-load current of the primary when the secondary is kept open.  Any abnormality in the exciting current may indicate shorted turns or defects in the core.