STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Downhole Motors 883
Flow
Figure 4-201. Basic positive displacement motor design principle. (Courtesy
Smith International, Inc.)
In general, the downhole positive displacement motor constructed on the
Moineau principle is composed of four sections: (1) the dump valve section, (2)
the multistage motor section, (3) the connecting rod section and (4) the thrust
and radial-bearing section. These sections are shown in Figure 4-202. Usually
the positive displacement motor has multichambers, however, the number
of chambers in a positive displacement motor is much less than the number of
stages in a turbine motor. A typical positive displacement motor has from two
to seven chambers.
The dump valve is a very important feature of the positive displacement motor.
The positive displacement motor does not permit fluid to flow through the
motor unless the motor is rotating. Therefore, a dump valve at the top of the
motor allows drilling fluid to be circulated to the annulus even if the motor
is not rotating. Most dump valve designs allow the fluid to circulate to the
annulus when the pressure is below a certain threshold, say below 50 psi or so.
Only when the surface pump is operated does the valve close to force all fluid
through the motor.
The multichambered motor section is composed of only two continuous parts,
the rotor and the stator. Although they are continuous parts, they usually
constitute several chambers. In general, the longer the motor section, the more
chambers. The stator is an elastomer tube formed to be the inside surface of a
rigid cylinder. This elastomer tube stator is of a special material and shape. The
material resists abrasion and damage from drilling muds containing cuttings and
hydrocarbons. The inside surface of the stator is of an oblong, helical shape.
The rotor is a rigid steel rod shaped as a helix. The rotor, when assembled into
the stator and its outside rigid housing, provides continuous seal at contact
points between the outside surface of the rotor and the inside surface of the
stator (see Figure 4-201). The rotor or driving shaft is made up of n, lodes. The