Explanation:
When a three brush generator operates at low speed, the generated voltage is less than the battery voltage. This can be explained by considering the working principle of a three brush generator and the effect of speed on its performance.

Working Principle of a Three Brush Generator:
A three brush generator is a type of DC generator that uses three brushes instead of the usual two brushes. The three brushes are known as the positive brush, the negative brush, and the equalizing brush. The positive brush is connected to the positive terminal of the generator, the negative brush is connected to the negative terminal, and the equalizing brush is connected to the field winding.

When the generator is in operation, current flows from the positive brush through the load, and then back to the negative brush. The equalizing brush is used to control the output voltage of the generator by adjusting the amount of current flowing through the field winding.

The Effect of Speed on Generator Performance:
The speed of a generator has a significant impact on its performance. The faster the generator rotates, the higher the generated voltage. This is because the rate of change of magnetic flux in the generator coils is directly proportional to the speed of rotation. Therefore, at higher speeds, the generated voltage increases.

Explanation of the Correct Answer:
At low speeds, the rate of change of magnetic flux in the generator coils is relatively low. This means that the generated voltage is also low. On the other hand, the battery voltage remains constant regardless of the generator speed.

Therefore, when a three brush generator operates at low speed, the generated voltage is less than the battery voltage. This is because the generator is not able to produce enough voltage to match the battery voltage due to the low speed of rotation.

Conclusion:
In summary, when a three brush generator operates at low speed, the generated voltage is less than the battery voltage. This is because the rate of change of magnetic flux in the generator coils is low at low speeds, resulting in a lower generated voltage.