Which DC motor type provides the best speed regulation under varying load conditions?

A DC shunt motor is running at its rated speed. If a rheostat connected in the field circuit is adjusted such that the resistance of the field winding circuit increases, what will be the effect on the motor speed, assuming the supply voltage and load torque remain constant?

In large DC machines, how is the demagnetizing and cross-magnetizing effect of armature reaction effectively neutralized?