Downhole turbine tachometer

A method, comprising:

• conveying a drilling fluid through a drill string, the drill string having a drill bit arranged at a distal end thereof;
• introducing the drilling fluid into a mud motor arranged on the drill string, the mud motor having a turbine shaft operatively coupled to the drill bit and a plurality of turbine stages axially arranged along a portion of the turbine shaft, each turbine stage comprising a plurality of stator blades and a plurality of rotor blades;
• rotating the plurality of rotor blades with the drilling fluid and thereby generating pressure pulses with the mud motor as the plurality of rotor blades rotate past a corresponding plurality of stator blades and repeatedly blocks and unblocks a plurality of flow passages defined between angularly adjacent stator blades;
• detecting the pressure pulses with a pressure sensor arranged within a measure-while-drilling (MWD) tool coupled to the mud motor;
• generating data signals corresponding to the pressure pulses with the pressure sensor;
• receiving and processing the data signals with an electronics module and thereby calculating a rotational speed of the mud motor;
• and
• transmitting signals indicative of the rotational speed of the mud motor to a surface location with a communications module communicably coupled to the electronics module.

A drilling system, comprising:

• a drill bit arranged at a distal end of a drill string configured to convey a drilling fluid to the drill bit;
• a mud motor arranged on the drill string and having a turbine shaft operatively coupled to the drill bit;
• a plurality of turbine stages axially arranged along a portion of the turbine shaft, each turbine stage comprising a plurality of stator blades and a plurality of rotor blades configured to rotate with respect to the plurality of stator blades in response to movement of the drilling fluid, wherein at least two turbine stages are angularly aligned with each other on the turbine shaft;
• a pressure transducer arranged at a surface location and configured to detect pressure pulses generated by the mud motor and generate data signals corresponding to the pressure pulses;
• and
• a computer system communicably coupled to the pressure transducer and configured to process the data signals and thereby calculate a rotational speed of the mud motor.