Stator and rotor profile for improved power section performance and reliability

A progressive cavity pump or a positive displacement motor comprising:

• an external member comprising three or more lobes;
• and
• an internal member extending through the external member and comprising one less lobe than the external member,
• wherein one of the internal member and the external member rotates with respect to the other,
• wherein a curvature of a profile of each of the internal member and external member is finite at all points,
• wherein a ratio of a lobe volume of the external member to a valley volume of the external member enclosed between a minor external member diameter and a major external member diameter is between 0.9 and 1.2, and
• wherein a lobe height of the external member is related to a ratio of the minor diameter of the external member to one less than the number of lobes of the external member.