AU755665B2

AU755665B2 - Electric motor cooled by a fluid and having high specific power

Info

Publication number: AU755665B2
Application number: AU18566/00A
Authority: AU
Inventors: Thorsten Siess
Original assignee: Impella Cardiosystems GmbH; Impella Cardiotechnik AG
Current assignee: Abiomed Europe GmbH
Priority date: 1998-12-02
Filing date: 1999-11-26
Publication date: 2002-12-19
Anticipated expiration: 2019-11-26
Also published as: PL348519A1; EP1142084B1; DE29821564U1; KR20010086066A; ZA200104258B; WO2000033446A8; WO2000033446A1; IL143165A0; JP5093869B2; CZ20011928A3; BR9915843A; CA2352234A1; RU2001117822A; DE19982578D2; NO20012498D0; DE59912438D1; AU1856600A; ATE302495T1; EP1142084A1; CN1329770A

Abstract

Disclosed is a micromotor with a motor housing (20) formed of polymer material. Said motor housing (20) surrounds the stator (24). The motor drives a pump part (12), which pushes fluid along the motor housing (20). The polymer material of the motor housing (20) contains an electrically isolating filler having caloric conductibility, especially Al2O3. Thereby, the temperatures arising in the motor housing are reduced. Furthermore, the hollow spaces of the coils (24a) of the stator can be filled up with a polymer material which contains a filler with caloric conductibility. The internal temperature of the motor is reduced as a result of the heat carried away to the fluid flowing around the motor. The micromotor is particularly useful for an intravascular blood pump.

Description

Electric motor cooled by a fluid and havinq high specific power The invention refers to a fluid-cooled electric motor with high specific power comprising a motor housing injection molded from a polymer material and enclosing a stator, and in particular to a micromotor that can be introduced into the vascular system of a body to drive a blood pump situated within the body.

WO 98/44619 describes a micromotor that is part of a blood pump to be introduced into the body of a patient. The micromotor has a motor housing injection molded from polymer material and having an outer diameter no larger than 8.0 mm. The motor is part of a blood pump pumping blood in the vascular system of a patient, the blood flowing along the motor housing, thereby cooling the motor. A relatively high temperature is generated in the motor housing, since polymer material has poor thermal conductivity. Due to the thermos bottle effect, temperatures of 60' C occur. Although the motor housing is cooled by the blood flowing along the same at a temperature of 37' C, the shaft extending from the motor housing has a higher temperature. There is a risk that the temperature-critical medium blood is damaged by too high a shaft temperature. Seals sealing a shaft having a high temperature are subject to greater wear, thereby reducing the service life of the micromotor. Further, consideration must be given to the fact that the efficiency decreases at a high inner temperature within the motor. With micromotors in particular, which often rely on battery power for operation, a high efficiency is important. An excessive inner temperature in the motor and the accompanying deterioration of efficiency, the mechanically available power is also reduced.

1A Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

It is a preferred feature of the invention to provide a fluid-cooled micromotor with improved thermal dissipation.

According to a first aspect, the present invention is a fluid-cooled electric motor with a motor housing made of polymer material and enclosing a stator, the length of the motor housing being at least twice the outer diameter of the motor housing.

In the present micromotor, the polymer material of the motor housing includes a thermally conductive electrically insulating filler. This filler *o causes the motor housing to have a high conductivity so that the heat occurring inside the housing can be transferred better into the surrounding blood.

Preferably, the filler is a ceramic material, in particular A1 2 0 3 Due to this material, given a corresponding filling amount, the thermal conductivity of thermoplastics or duroplastics can be increased significantly from a typical 0.05 W/mK to up to 2 W/mK without the electric conductivity increasing. It has been found that at a fluid temperature of 37 C, the internal temperature of the motor can be reduced from an initial 60* C to between 40* C and 45 C. Within the motor, no thermal build-up occurs, since the thermally conductive motor housing provides for a better transfer of the heat to the blood. Therefore, the components of the electric motor are subjected to a lesser thermal stress and, thus, also to a lesser wear. Moreover, the motor can be operated with improved efficiency.

The length of the motor housing is at least twice the outer diameter of this housing. Thus. it is an elongate motor housing with a small diameter. Such motor housing has a large surface relative to the volume. The thermally conductive filler contained in the housing wall, in combination with the large housing surface, allows for an effective thermal transferto a medium flowing along the housing. Preferably, the length of the motor housing is at least times, in particular at least three times, the outer diameter.

The filler is also advantageous in the manufacture of the micromotor by injection molding or in vacuum molding, since a large amount of filler reduces the curing time of a duromer because of the lesser duromer thicknesses and the improved thermal conductivity. Thus, the heat curing time in an oven is significantly reduced.

The amount of filler should be at least 40 percent by weight to achieve a significant increase in the thermal conductivity of the matrix material. The filler, initially present as a fine powder, is mixed into the liquid polymer material, whereupon the forming is done by injection molding, drenching, and the like.

An important insulator within a micromotor is formed by the wire coils. Cavities filled with air exist between the windings, which form an ideal heat insulator.

Moreover, the individual wires are coated with insulating sheaths that also present good thermal insulators. According to a preferred embodiment of the invention, which is of independent importance, the cavities of the stator coils are filled with a polymer material including a thermally conductive filler.

Thereby, the coils become good thermal conductors, where the current-induced heat generated in the coils is dissipated to the yoke and/or the motor housing with little thermal resistance. The polymer material filling the cavities causes an improved heat transfer to the iron yoke sheet arrangement which in turn passes the heat on to the motor housing. Introducing the filler-loaded polymer material into the cavities of the coils is suitably done by drenching or by casting under a combination of vacuum and pressure, where all cavities are intentionally filled with thermally conductive polymer material.

The following is a detailed description of an embodiment of the invention taken in conjunction with the drawings.

In the Figures: Fig. 1 is a longitudinal section of the micromotor and the pump connected thereto, and Fig. 2 is an enlarged representation of the detail II of Fig. 1.

Fig. 1 described below already forms subject matter of WO 98/44619.

'i4L -c Fig. 1 illustrates an intravascular blood pump 10, i.e. a blood pump that can be pushed through the blood vessels of a patient so as to arrive in the heart.

In general, the outer diameter of such a blood pump is nowhere larger than 8mm.

The pump 10 has a drive portion 11 and a pump portion 12 rigidly connected thereto. The drive portion 11 includes an electric micromotor 21 with an elongate cylindrical housing 20. At the rear end, the housing 20 is closed by an end wall adjoined in a sealing manner by a flexible catheter 14. Electric wires 23 for power supply to and control of the electric motor 21 extend through the catheter 14, together with further wires 23a connected to sensors of the blood pump In a conventional manner, the stator 24 of the motor has a plurality of circumferentially distributed coils 24a, as well as a magnetic yoke 24b made of metal sheets and extending longitudinally. It is extrusion-coated with the motor housing 20. The stator 24 encloses the rotor 26 connected to the motor shaft 25 and consisting of permanent magnets magnetized in the direction of action. A bearing 27 supports the rear end of the motor shaft in the motor housing or the end wall 22. The shaft extends along the entire length of the motor housing 20 and projects forward therefrom.

The front end of the motor housing is formed by a stationary hub member that is an integral part of the housing 20. The outer diameter of the hub member tapers towards the front end where a bearing 33 for supporting the motor shaft 25 is provided. This bearing is simultaneously designed as a shaft seal.

The motor shaft 25 protrudes forward from the hub member 30 and carries an impeller wheel 34 with a hub 35 sitting on the shaft end and blades 36 or pump vanes projecting therefrom and being inclined with respect to the axis of the impeller wheel 34. The impeller wheel 34 is contained in a cylindrical pump housing 32 that is connected to a ring 38 on the hub member 30, the connection being established through three circumferentially distributed webs 39. It is visible that the motor housing 20 and the pump housing 32 are rigidly connected and have the same outer diameter, and that the diameter of the pump 10 is nowhere larger than this outer diameter.

During the rotation of the impeller wheel 34, blood is taken in through the take-in port 37 of the pump housing 32 and driven to the rear in the axial direction within the pump housing 32. Through the annular gap between the pump housing 32 and the motor housing 20, the blood flows outward along the hub member 30 to flow further along the motor housing 20. This guarantees the dissipation of the heat generated in the drive, without any damage to the blood caused by excessive surface temperatures (above 41" C) on the motor housing It is also possible to operate the pump portion 12 with the supply direction reversed, where the blood is drawn along the motor housing and exits axially from the front end opening 37.

A pressure sensor 68 is embedded in the circumferential wall of the motor housing 20, which communicates with a line 23a. This line 23a is encapsulated in the motor housing 20 and extends through the front wall 22 into the catheter 14. At the proximal end of the catheter, the line 23a and the wire 23 may be connected to an extracorporeal control device that controls the operation of the pump The micromotor 21 is manufactured by injection molding using an injection mold into which a mandrel is inserted that carries the stator components 24a, 24b. Polymer material is then injected into the mold. Thereafter, the injection mold and the mandrel are removed from the motor housing 20 and the rotor 26 is finally mounted through the opening 31 of the hub member The polymer material used is preferably a liquid epoxy resin including a filler percentage of at least 40% by weight. The filler used is a fine powder of AL 2 0 3 The current-induced heat generated in the coils 24a of the stator 24 is first transferred to the yoke arrangement 24b and, through heat transfer, from there to the motor housing 20. Since the motor housing 20 has a good heat conductivity, there is a low temperature gradient to the blood flowing around the motor housing at a temperature of 37 C.

Fig. 2 illustrates the wire windings 40 of a coil 24a, where each wire is provided with an insulating sheath 41. The spaces between the wire windings or their insulations are filled with polymer material 42 containing A1 2 0 3 as a filler. Again the filler amouniint is at least 40% byhv weight. The nnlvmpr material 42 filling the cavities is in full surface contact with the yoke sheet arrangement 24b, which is in full surface contact with the motor housing Due to the improved cooling, a high mechanical power output of 5 W can be achieved at u 30,000 rpm. Because of the substantially improved thermal conductivity in all motor components and the substantially improved heat coupling at the transition between coil, yoke and motor housing, the temperature inside the motor can be reduced significantly.

Thus, in the structure described, the transfer of 10 W of heat from within the motor to the outside requires a mere driving temperature difference of A6 5 8 K opposed to A5 18 25 K when using a non-filled polymer.

The complete filling of all winding cavities with polymer material 42 further allows for a smooth and insulating coil coating 43 on the inner surface of the 7 coil. When the motor is designed as a brush-less synchronous machine, it is possible to drive the motor filled with liquid on the inside, provided the coils are fully encapsulated.

Claims

1. A fluid-cooled electric motor with a motor housing made of polymer material and enclosing a stator, the length of the motor housing being at least twice the outer diameter of the motor housing, characterised in that the polymer material of the motor housing contains a thermally conductive, electrically insulating filler in an amount of at least 40% by weight.

2. The electric motor of claim 1, wherein the filler comprises A1 2 0 3

13. The electric motor of claim 1 or 2, wherein the stator comprises coils whose cavities are filled with a polymer material including a thermally conductive filler. 4. The electric motor of claim 3, wherein the polymer material filling the cavities is in thermally conductive contact with a surrounding yoke sheet 20 arrangement. The electric motor of any one of claims 1 to 4, wherein a pump portion driven by the motor also includes filler. 25 6. The electric motor of claim 3 or 4, wherein the coil has a smooth insulating inner surface made of said polymer material. Dated this 21 st day of October 2002 IMPELLA CARDIOTECHNIK AG Patent Attorneys for the Applicant: F B RICE CO