Long shunt DC motor speed control characteristics?
DC motor speed control does not normally use series, shunt compound or any other connection of the field to the same supply as the armature. The controller provides a separate regulated power supply for the field. The field current is maintained at a constant value unless field weakening is used to extend the operating speed above the normal base speed of the motor. There might be some other configurations used in special circumstances.

The term "variable torque" is usually applied to loads that have a torque requirement that is low at low speed and increases with speed. Fans and centrifugal pumps have that characteristic. DC motors are not used for such applications except when battery operated. In that case a permanent magnet motor would be preferable.