ELECTRICAL MOTOR
Field of the invention
The present invention relates to a long electrical motor for use in a downhole
tool. Furthermore, the invention relates to a downhole tool comprising an
electrical motor according to the invention and to a downhole system comprising
a plurality of electrical motors according to the invention.
Background art
The present invention relates to electric motors and more particularly, the
present invention relates to electric motors especially suited for use in the
borehole drilling art. Motors of this type must be such as to meet the special
space requirements of a borehole, so that the outside diameter is generally very
limited, whereas such motors may be very long. The precise length depends on
the desired power of the motor. Further to the special space requirements during
operations in a downhole environment, this type of environment also represents
challenging conditions such as high pressure, high temperature and an acidic
environment.
One difficulty encountered in motors of this type is that the rotor shaft must be
tightly supported by at least bearings in each end of the motor, and typically also
bearings in intermediate positions to ensure that the rotation of the rotor is
progressing without wobbling, which would induce increased tear of the electrical
motor and decreased power output. Therefore, a very tight fit of the rotor within
the stator is essential in the design of an efficient electrical motor. Thermally
induced thrust loads on the rotor present a challenge to the design of such
electrical motors, primarily since, due to thermal expansion of the rotor, the rotor
may bend towards the stator resulting in increased friction and a wobbling
motion of the rotor.
Prior efforts to circumvent this problem have been made, such as US Patent
number 3,136,905 disclosing a hollow rotor for allowing cooling water inside the
rotor, thereby avoiding large thermal expansions of the rotor. By using this
approach, another problem is introduced in the design, as it becomes important
to provide a good seal on a rotating member, which is a difficult task under
normal up-hole operating conditions as it is, but is considerably more complicated
when working downhole with increased temperatures and increased pressure
differences.
Therefore, a need exists for providing an improved electrical motor with the
ability to maintain a tight fit of the rotor inside the stator and maintain the tight
fit during thermal expansions during downhole operations.
Summary of the invention
It is an object of the present invention to wholly or partly overcome the above
disadvantages and drawbacks of the prior art. More specifically, it is an object to
provide an improved electrical motor capable of operating in elevated ambient
temperatures without the use of complicated cooling systems.
The above objects together with numerous other objects, advantages, and
features, which will become evident from the below description, are accomplished
by a solution in accordance with the present invention by a downhole electrical
motor elongated in a longitudinal direction, comprising:
- a housing,
- a stator comprised inside the housing, and
- a rotor comprised inside the stator rotatably connected to the stator,
wherein the rotor has a locked end and a non-locked end and is locked in the
locked end to avoid movement of the rotor in the longitudinal direction, and the
rotor is able to move along the longitudinal direction in the non-locked end of the
rotor to avoid thrust loads on the rotor due to thermal expansion of the rotor
during rotor heating when the electrical motor is in operation and further
comprising a locked bearing connected to the stator for supporting the rotor in
the locked end and a non-locked bearing connected to the stator for supporting
the rotor in the non-locked end, wherein the bearings are at least partially made
from a ceramic material.
Also, the present invention relates to a downhole electrical motor elongated in a
longitudinal direction, comprising:
- a housing,
- a stator comprised inside the housing, and
- a rotor comprised inside the stator rotatably connected to the stator,
wherein the rotor has a locked end and a non-locked end and is locked in the
locked end to avoid movement of the rotor along the longitudinal direction, and
the rotor is able to move along the longitudinal direction in the non-locked end of
the rotor to avoid thrust loads on the rotor due to thermal expansion of the rotor
during rotor heating when the electrical motor is in operation.
Locked and non-locked refers to the ability to move in a longitudinal direction,
i.e. a locked end cannot move in the longitudinal direction and a non-locked end
may move in a longitudinal direction of the tool.
Also, a downhole electrical motor according to the invention may comprise a
locked bearing connected to the stator for supporting the rotor in the locked end.
Further, a downhole electrical motor according to the invention may comprise a
non-locked bearing connected to the stator for supporting the rotor in the nonlocked
end.
Locked and non-locked bearings refer to the ability of the bearing to move in a
longitudinal direction of the tool, i.e. a locked bearing cannot move in the
longitudinal direction and a non-locked bearing may move in a longitudinal
direction of the tool.
Additionally, a downhole electrical motor according to the invention may
comprise at least an additional bearing arranged between the locked and the
non-locked bearings.
The bearings may be roller bearings, such as ball bearings, cylindrical roller
bearings, needle roller bearings, tapered roller bearings or spherical bearings.
Also, the roller bearing may comprise a roller element, such as a ball, cylinder,
needle, tapered or spherical element made from a ceramic material.
Moreover, the bearings may comprise a race element made from a ceramic
material.
In one embodiment, the bearings may be ball bearings.
In another embodiment, the ball bearings may comprise balls made from a
ceramic material.
Furthermore, a downhole electrical motor according to the invention may
comprise snap rings or circlips for locking the locked end of the rotor to avoid
movement of the rotor in the longitudinal direction.
In one embodiment, the rotor may comprise a shaft member, a rotor member,
and the shaft member and rotor member may be interlocked by a tongue and a
groove joint, the groove joint being elongated in the longitudinal direction of the
motor interacting with the tongue allowing movement of the shaft member in
relation to the rotor member in the longitudinal direction of the motor.
Moreover, the present invention relates to a downhole tool comprising an
electrical motor according to the invention.
Finally, the invention relates to a downhole system comprising a plurality of
electrical motors as described above, wherein the electrical motors may be
coupled by extendable coupling means.
Brief description of the drawings
The invention and its many advantages will be described in detail below with
reference to the accompanying schematic drawings, which for the purpose of
illustration show some non-limiting embodiments and in which
Fig. 1 shows a schematic diagram of an electrical motor,
Fig. 2 is a schematic diagram of an electrical motor comprising an intermediate
bearing,
Fig. 3 is a schematic diagram of an electrical motor comprising a plurality of
intermediate bearings, and
Fig. 4 is a schematic diagram of two serially coupled electrical motors.
All the figures are highly schematic and not necessarily to scale, and they show
only those parts which are necessary in order to elucidate the invention, other
parts being omitted or merely suggested.
Detailed description of the invention
Fig. 1 shows an electrical motor 1 elongated in a longitudinal direction 60 for
providing a rotational force downhole for driving units such as a hydraulic pump.
The electrical motor comprises a rotor 2 and a stator 3 contained in a motor
housing 4. The stator 3 is fixedly connected to the motor housing 4, and the
stator 3 comprises a stator member 31, a locking end member 32 and a non
locking end member 33. The rotor 2 comprises a shaft member 21, a rotor
member 22, force transmitting means 23 and locking grooves 24. Furthermore,
the rotor 2 has a locked end 2a and a non-locked end 2b. The rotor 2 is rotatably
connected to the stator 3 by a plurality of bearings 41, 42 such as at least two
bearings, i.e. a locked bearing 4 1 supporting the rotor 2 in the locked end 2a,
and a non-locked bearing 42 supporting the rotor 2 in the non-locked end 2b.
The terms locked and non-locked refer to the ability to move in the longitudinal
direction 60 of the electrical motor 1, and therefore the locked bearing 4 1 is not
able to move in the longitudinal direction 60, whereas the non-locked bearing 42
is able to move in the longitudinal direction 60 of the electrical motor 1. The
locked bearing 4 1 is a ball bearing comprising a ball 45 and furthermore
comprising a locked inner racer 43, a locked outer racer 44 and a plurality of
bearing balls 45 arranged between the locked inner racer 43 and the locked outer
racer 44. The non-locked bearing 42 is a ball bearing comprising a ball 45 and
furthermore comprising a non-locked inner racer 46, a non-locked outer racer 47
and a plurality of bearing balls 45. The locked outer racer 44 is fixedly connected
to the locking end member 32, and the locked inner racer 43 is fixedly connected
to the shaft member 2 1 of the rotor 2 by means of two locking rings 5 engaging
locking grooves 24 in the shaft member 2 1 on both sides of the inner racer 43 for
locking the longitudinal movement of the rotor 2 in the locked end. The nonlocked
outer racer 47 abuts the non-locking end member 33 of the stator 3,
thereby confining the movement of the non-locked outer racer 47 in a radial
direction of the electrical motor 1 but not confining the movement of the nonlocked
outer racer 47 in the longitudinal direction 60 of the electrical motor 1.
The non-locked inner racer 46 is fixed to the shaft member 2 1 of the rotor 2 by
means of a locking ring 5 engaging a locking groove 24 in the shaft member 2 1
on a side of the inner non-locked inner racer 46 opposite to the rotor member 22,
thereby allowing longitudinal movement of the rotor 2 due to thermal expansion
of the rotor 2.
The locking and non-locking end members 32, 33 may advantageously be
produced by spark eroding, spark cutting, burning and similar techniques where a
very thin cut may be made in a very thick material by removing material with a
spark. This enables the user to produce the locking and non-locking end
members 32, 33 in their entire thickness which optimises the heat transfer and
rigidity of the motor. Alternatively, the locking and non-locking end members 32,
33 are produced more conventionally by techniques such as punching a sheet of
metal and then building the locking and non-locking end members 32, 33 by a
plurality of such sheets.
Several different types of losses lead to heating in an electrical motor. The main
source of heating typically originates from electrical heating of the current
leading windings in the motor, which is also commonly referred to as copper
losses due to the fact that the windings in an electrical motor are typically made
from copper wire. Sometimes the copper losses are subdivided into primary and
secondary losses, referring to primary losses in the rotor windings and secondary
losses in the stator windings. Further losses in the electrical motor may occur due
to dissipation of magnetic energies in the stator and other types of less
significance in terms of size, stray losses stemming from leakages, generation of
harmonic energies, etc. Also, mechanical losses such as frictional losses in the
bearings may lead to heating in the electrical motor. The problems arising as a
consequence of heating in an electrical motor become increasingly important
when working in a downhole environment. Heat generating equipment in
downhole equipment faces the problem of elevated ambient temperatures,
temperatures, which, in deep wells, may exceed several hundred degrees. The
elevated ambient temperature combined with equipment, which, due to spacial
restrictions and pressure differences in the borehole, typically has a very "tight"
construction, lead to big challenges in expelling heat from the equipment.
Therefore, solutions to circumvent problems with heating have great technical as
well as commercial value in the design of downhole equipment.
During operation of the electrical motor 1, the rotor 2 rotates with respect to the
stator 3, and heat is generated primarily in the rotor 2. Therefore, the rotor 2 is
the part of the electrical motor experiencing the highest temperature during
operation. The stator 3 and motor housing 4 also increase in temperature as heat
dissipates from the rotor 2 towards the surroundings of the electrical motor 1.
The rotor 2 only has solid-solid connections with the surroundings through the
bearings 41, 42 to avoid frictional losses, and therefore the temperature gradient
is very large between rotor and stator. Furthermore, given the fact that typical
bearing balls of the bearings 41, 42 are made from materials of relatively low
thermal conductance such as stainless steel types, the heat transmission through
the bearings is very low. Other types of bearing balls with higher thermal
conductivity such as more conventional chrome steel balls may also be utilised.
An intervening space 6 between the stator 3 and the rotor 2 is filled with oil to
avoid ingress of borehole fluid into the motor 1. Oil is also a relatively poor
thermal conductor and the temperature gradient through the oil in the
intervening space 6 is therefore also very high. To sum up, the rotor 2 has
difficulties in expelling the heat generated during operation.
When the rotor 2 heats up during operation, the dimensions of the rotor will
increase as a consequence of thermal expansion. Due to the elongated nature of
the electrical motor 1, which stems from the spacial restrictions of working in a
borehole, the thermal expansion is, in particular, a problem in the elongated
direction. Since electrical motors may be up to several metres long while only few
centimetres wide, the thermal expansion in the elongate direction may result in
significant changes in the length of the rotor 2. Conventionally, problems of
thermal expansion have been dealt with by actively or passively cooling the rotor
2 to avoid large temperature gradients between the stator 3 and the rotor 2. The
problem is that since the stator 3 and rotor 2 are conventionally coupled in both
ends, a large thermal expansion of the rotor 2 simultaneous to a smaller thermal
expansion of the stator 3 and motor housing 4 results in a thrust load on the
rotor 2 causing the rotor 2 to deflect. This type of deflection may cause a
wobbling motion during rotation of the rotor 2, which may reduce the efficiency
of the electrical motor 1, and, in the worst-case scenario, destroy the electrical
motor 1.
The bearings 41, 42, 50 may preferably be roller bearings, such as ball bearings,
cylindrical roller bearings, needle roller bearings, tapered roller bearings or
spherical bearings. And the bearings may at least partially be made from a
ceramic material to provide a bearing with improved durability and performance
during elevated working temperatures. The roller element 45 such as a ball,
cylinder, needle, tapered or spherical element may be made from a ceramic
material. Also, the race elements 43, 44, 46, 47 may be made from ceramic
material to improve the performance of the bearings.
The locking rings 5 locking the bearings 41, 42 may advantageously be a snap
ring 5 or circlips 5 engaging the locking grooves 24.
In the electrical motor shown in Fig. 1, the rotor 2 may, during operation,
thermally expand towards the non-locked bearing 42 without deflecting. The nonlocked
bearing 42 abuts the non-locking end member 33 and is therefore
confined in the radial direction, but the non-locked bearing 42 may move in the
longitudinal direction 60, thereby being capable of compensating for the thermal
expansion of the rotor again, thus avoiding deflection of the rotor 2.
Fig. 2 shows another electrical motor 1 further comprising an intermediate nonlocked
bearing 50 in an intermediate position between the non-locked bearing 42
and the locked bearing 41. The purpose of arranging the intermediate non-locked
bearing 50 between the non-locked bearing 42 and the locked bearing 4 1 is to
increase stability of the rotor 2, especially if the rotor 2 is very long. It is
essential that intermediate bearings 50 must be non-locked so that parts of the
rotor 2 may be locked while other parts are not. If the intermediate bearing 50 is
non-locked, the rotor 2 is free to expand throughout the entire length of the rotor
2. Adding an intermediate non-locked bearing 50 halfway between the nonlocked
bearing 42 and the locked bearing 4 1 will reduce by half a length of the
rotor unsupported by bearings. As shown in Fig. 3, a plurality of intermediate
non-locked bearings 50 may be introduced in case of very long motors or in case
of high requirements of stability of the rotor 2.
Fig. 4 shows two coupled electrical motors 1 connected in series. Each motor 1
comprises a non-locked bearing 42 and a locked bearing 4 1 suspending a rotor
shaft 21, thereby allowing each of the rotor shafts 2 1 to move freely according to
the invention. The shafts 2 1 are coupled between the motors 1 by means of
extendable coupling means 51. The extendable coupling means 5 1 comprises a
locked coupling end 51a and a non-locked coupling end 51b, due to the fact that
the extendable coupling means 51 couples two shafts 2 1 between a locked
bearing 4 1 and non-locked bearing 42. Therefore, the extendable coupling means
5 1 must have at least two capabilities, namely to be able to transfer the
rotational force from one shaft 2 1 to another and to accommodate the length
change of the shaft 2 1 coupled to the non-locked coupling end 51b by being
extendable. One embodiment of extendable coupling means 51 having these two
capabilities is an extendable universal joint, but even simpler extendable coupling
means having at least the capability of transferring rotation and accommodating
a length change, such as simple tongue and groove joints, may be used. The
electrical motor housings 4 in Fig. 4 have been coupled by motor housing
connection means 52.
The rotor comprises a shaft member 2 1 and a rotor member 22. The shaft
member 2 1 and rotor member 22 may be interlocked by a tongue 6 1 and groove
joint 62 as shown in Fig. 2. The groove joint 62 is elongated in the longitudinal
direction 60 of the motor 1 interacting with the tongue 6 1 allowing movement of
the shaft member 2 1 in relation to the rotor member 22 in the longitudinal
direction 60 of the motor 1. The groove joint 62 may be arranged in the rotor
member 22 and the tongue 6 1 in the shaft member 2 1 or vice versa.
The bearing balls 45 are preferably made from a highly endurable material such
as a ceramic material or a high grade steel. The bearing ball material is
preferably very hard, endurable, and resistant to high temperatures and can be
manufactured with great dimensional precision and low tolerances. The entire
bearing 41, 42 including bearing balls 45 and racers 43, 44, 46, 47 may be made
from the same material, or alternatively the so-called hybrid bearings may be
utilised combining e.g. ceramic bearing balls 45 with steel racers 44. The choice
of material allows the user to improve characteristics of the electrical motor 1
such as to decrease friction in the bearings 41, 42, to increase the lifetime of the
bearings 41, 42 or to increase the heat transfer through the bearings 41, 42 from
the rotor 2 towards the stator 3 and motor housing 4.
Although the invention has been described in the above in connection with
preferred embodiments of the invention, it will be evident for a person skilled in
the art that several modifications are conceivable without departing from the
invention as defined by the following claims.
Claims
1. A downhole electrical motor (1) elongated in a longitudinal direction (60),
comprising:
- a housing (4),
- a stator (3) comprised inside the housing,
- a rotor (2) comprised inside the stator rotatably connected to the stator,
wherein the rotor has a locked end (2a) and a non-locked end (2b) and is locked
in the locked end to avoid movement of the rotor in the longitudinal direction,
and the rotor is able to move along the longitudinal direction in the non-locked
end of the rotor to avoid thrust loads on the rotor due to thermal expansion of
the rotor during rotor heating when the electrical motor is in operation and
further comprising a locked bearing (41) connected to the stator for supporting
the rotor in the locked end and a non-locked bearing (42) connected to the stator
for supporting the rotor in the non-locked end, wherein the bearings (41, 42) are
at least partially made from a ceramic material.
2. A downhole electrical motor (1) according to claim 1, further comprising at
least an additional non-locked bearing (50) arranged between the locked (41)
bearing and the non-locked bearing (42).
3. A downhole electrical motor (1) according to claim 1 or 2, wherein the
bearings (41, 42, 50) are roller bearings, such as ball bearings, cylindrical roller
bearings, needle roller bearings, tapered roller bearings or spherical bearings.
4. A downhole electrical motor (1) according to claim 3, wherein the roller
bearing (41, 42, 50) comprises a roller element (45), such as a ball, cylinder,
needle, tapered or spherical element made from a ceramic material.
5. A downhole electrical motor (1) according to any of claims 1-4, wherein the
bearings (41, 42, 50) comprise a race element (43, 44, 46, 47) made from a
ceramic material.
6. A downhole electrical motor (1) according to any of the preceding claims,
further comprising snap rings (5) or circlips (5) for locking the locked end of the
rotor to avoid movement of the rotor along the longitudinal direction.
7. A downhole electrical motor (1) according to any of the preceding claims,
wherein the rotor comprises a shaft member (21), a rotor member (22), and the
shaft member and rotor member are interlocked by a tongue (61) and a groove
joint (62), the groove joint being elongated in the longitudinal direction of the
motor interacting with the tongue allowing movement of the shaft member in
relation to the rotor member in the longitudinal direction of the motor.
8. A downhole tool comprising an electrical motor (1) according to any one of
claims 1-7.
9. A downhole system comprising a plurality of electrical motors according to
any one of claims 1-8, wherein the electrical motors are coupled by extendable
coupling means (51).