In a conventional residual magnetic flux measurement apparatus, a magnetic flux waveform is obtained by measuring a voltage waveform from a transformer and by performing integral calculation thereof; then, a residual magnetic flux is given by subtracting the mean value of the maximum value and the minimum value of the magnetic flux waveform as a correction term of direct-current component (for example, refer to Patent Document 1).
In addition, in another conventional residual magnetic flux measurement apparatus, a magnetic flux waveform is obtained by measuring a voltage waveform from a transformer and by performing integral calculation thereof; at this time, by obtaining the difference between the final value of the magnetic flux waveform and the center value of sinusoidal oscillation before interrupting an electric power source, a residual magnetic flux is given (for example, refer to Patent Document 2).    [Patent Document 1] Japanese Laid-Open Patent Publication No. H02-179220    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2000-275311