This paper outlines three design modifications of the conventional power section of PDM operating on air. Theoretically, an increase in power output can be achieved by using motor power section with variable cross sectional area. The paper discusses the energy relationships for the various design modifications. Also the paper offers insights to the advantages and disadvantages of various designs.


Positive displacement motors (PDM) are used extensively for both conventional and underbalanced drilling. However, the performance of positive displacement motor (PDM) operating on compressible fluids drops down as compared to the similar operation with slightly compressible drilling fluids. This results in an increase in the operating cost and reduction of potential benefits associated with underbalanced drilling.

Due to the highly compressible nature of air, the conventional power section requires modification. The primary objective of this paper is to analyse possible power section modifications that would be suitable for highly compressible fluids. In general, an increase in the motor power output can be achieved by one of the following:

  • Increasing the intake pressure,

  • Decreasing the delivery pressure,

  • Increasing the volume of the intake.

Higher pressure at the top of the power requires higher supply pressure. This increases the capacity of the compressor and the cost of operation. Decreased pressure at the exit and the increased chamber volume can be accomplished by having a variable cross sectional area in the cavity dimensions. This causes the volume of the air-trapped between the cavities to expand inside the powersection rather than at the exit. It is anticipated this not only should increase the motor horsepower but also reduce vibrations due to elimination of expansion at the exit.

Variable cross-sectional Area:

It is vital to perform a detailed evaluation of the design based on the variable cross sectional area. Theoretically, any one or combinations of the following can increase the chamber volume:

  • increasing pitch of the housing/shaft,

  • increasing housing diameter,

  • decreasing shaft diameter.

Diameter of the housing

The diameter of the housing can be decreased or increased independently along the power section for a constant motor outside diameter. Increase in diameter of the housing alone results in a constant volume cavity but will result in a thin elastomer on the larger diameter side of the power section. This may result in an uneven wear of the housing elastomer.

Diameter of the Shaft

The diameter of the shaft also can be decreased or increased along the powersection. Decrease in the diameter may cause an increased slippage and more interference between the housing and shaft. Diameter of shaft/housing combinations should form conjugate profiles in order to have a continuous contact between the shaft and housing. Otherwise, it might result in an uneven incomplete rotation.

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