The hysteresis loss in a D.C. machine least depends on

The correct answer is: D. Rate of flow of ventilating air.

Hysteresis loss is the energy loss that occurs in a magnetic material when it is subjected to a changing magnetic field. The loss is caused by the movement of magnetic domains within the material as the field changes. The rate of flow of ventilating air does not affect the hysteresis loss, as it does not affect the movement of magnetic domains.

The other options are all factors that affect the hysteresis loss.

• Frequency of magnetic reversals: The higher the frequency of magnetic reversals, the greater the hysteresis loss. This is because the magnetic domains have to move more often to follow the changing field.
• Maximum value of flux density: The higher the maximum value of flux density, the greater the hysteresis loss. This is because the magnetic domains have to be further displaced to follow the changing field.
• Volume and grade of iron: The larger the volume of iron and the higher the grade of iron, the greater the hysteresis loss. This is because the magnetic domains have more space to move and are more easily displaced in higher-grade iron.