A pump and, in particular, the armature of the stator of the electrical drive motor of such pumps are usually designed for providing a certain head H0 at zero flow rate with maximum efficiency. Normally, the maximum efficiency is reached by a design that allows providing the required head H0 at zero flow rate in a non-filed weakening mode at a maximum modulation index close to 1.
EP 2 133 991 describes a speed controlled pump which makes use of field weakening to optimise electrical power consumption at low flow rates.