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Mud motors have been used in oil and gas drilling for almost 100 years. They are robust, cheap and abundant. They are successfully used for drilling and maintenance of vertical and directional wells, solely or in combination with rotary steerable systems (RSS). Relatively low cost, abundance and maintainability drive their selection over more complicated systems like RSS or turbines. Combination of RSS and mud motors have proven to deliver higher rate of penetration (ROP) in conjunction with increased drill sting durability and lower energy consumption. A mud motor consists of three main parts: power section, transmission and bearing section (Fig.1). The power section is responsible for torque generation, i.e. it converts hydraulic energy of drilling fluid to mechanical energy of planetary rotor rotation. The transmission transforms planetary rotation into simple rotation and optionally passes it through some angle formed by the motor corpus needed to create well curvature. The bearing section takes the axial and thrust load from the rotor and transmission. The transmission and bearing section are often called mud motor lower end. The mud motor drives the drill bit either directly or through RSS. In the latter case no bend angle on transmission is set, and a straight motor is used. The power section is the most sensitive to operating conditions and generally defines the motor specification. Most mud motor failures are, in fact, power section related [1–2]. The power section itself consists of two parts: rotor and stator (Fig. 2). The mud motor rotor is usually made of stainless steel, with left-handed helical surface coated either with hard chrome or tungsten carbide to give extra protection from corrosion and abrasion. By analogy, the rotor can be considered as muti-start left-handed threaded bolt. The number of the thread starts is the number of the rotor lobes. 2D line of rotor cross-section in plane perpendicular to its axis is called rotor profile.