How to turn a universal motor into a generator(Self excited DC generator experiment)

This is self excited DC generator experiment using a universal motor. In this experiment, electricity is generated by turn in one direction. Because the residual magnetic field is reduced by turning opposite direction, and the generator cannot create magnetic field. The polarity of the output voltage is changed in accordance with the polarity of the remanence. AC universal motor running on DC : https://www.youtube.com/watch?v=okF0p8hUHCM