Electric machine

An electric machine includes a plurality of conductive tracks defining an electric power circuit, to provide current for electronic power components, a printed circuit board which mounts the conductive tracks, an enclosure which defines a housing for the printed circuit board and has a dissipative wall, a heat-conductive filler interposed between the printed circuit board and the enclosure, an elastic device acting on the printed circuit board to press it against the dissipative wall, a conductive element held in position by at least one heat-sensitive joint and disposed to connect a first track and a second track to close the electric power circuit. The elastic device includes an elastic element pressed against the conductive element to apply a force on the heat-sensitive joint and the heat-sensitive joint is configured to break if subjected to a temperature higher than a predetermined threshold temperature to open the electric power circuit.

This application claims priority to Italian Patent Application 102018000009611 filed Oct. 19, 2018, the entirety of which is incorporated by reference herein.

This invention relates to an electric machine, with reference in particular to a system for protecting it against high internal temperatures, and to a method for making the electric machine.

In general, an electric machine, for example a brushless electric motor to which reference is made hereinafter without limiting the scope of the invention, comprises a casing having inside a stator, rigidly connected to the casing, and a rotor, for example with permanent magnets, rotatably connected to the casing.

An example of a prior art electric machine used as a reference for the present patent is described in application WO2013008180 in the name of the present Applicant.

An electronic module, connected to the stator in brushless electric motors, comprises a printed circuit board and, disposed on the printed circuit board itself, a plurality of active and passive electronic components defining a power section and a plurality of electronic signal components defining a control section.

Some of these electronic components are surface mounted devices (SMD)—for example the power MOSFETs—and have the advantage of being able to be attached to the printed circuit board in bulk using an oven brazing process.

The casing is closed by a cap to form a sealed container from which connecting pins protrude to allow powering and controlling the electronic drive circuitry. In sealed electric machines, the cap is closed hermetically in such a way as to insulate the electrical and movable components from humidity, dust and atmospheric agents.

Electric machines of this kind are used in particular in the automotive sector and in other sectors subject to stringent safety regulations which are constantly changing. In particular, overheating of these electric machines constitutes a real risk, especially if the machines are installed near internal combustion engines or other sources of heat.

The possible effects of overheating may include malfunctioning of the electric machine itself, interference or damage to the power supply system it is associated with and possible undesirable consequences for other devices connected to it and, in the worst cases, fire and explosion.

To prevent overheating and reduce its undesirable effects, several solutions have been designed and implemented. Most of these are based on the development and improvement of the convective dissipation of the heat generated inside the machine. Patent document WO2014125412, in the name of the present Applicant, describes a solution aimed at dissipating the heat generated by the electronic components of the electric machine.

Disadvantageously, these solutions are ineffective in the case of unexpected rises in the temperature of the atmosphere in which the machine is working and with which the machine is designed to exchange heat in order to dispose of it. Such temperature rises might be due, for example, to a failure of a cooling system outside the machine.

The above mentioned solutions might also be ineffective in the case of malfunctions and exceptional events regarding the electric machine and which might cause unexpected rises in temperature.

The demand from the market and from supervisory and certification bodies for increasingly safe electric machines has given rise to the need for an intrinsic safety mechanism to cut off the machine power supply in the event of overheating before the temperature can reach dangerous levels.

One possible solution known from the prior art consists in fitting a customary thermal fuse along a power supply circuit of the electronic power components inside the electric machine, so as to cut off the power supply to stop the machine in the event of overheating. This solution has several disadvantages, however.

Known thermal fuses comprise a fusible pellet used to hold down a preloaded conductive spring. When the temperature rises, the pellet melts and releases the spring, thereby breaking the circuit. Disadvantageously, the structure of known thermal fuses is such that they cannot be oven brazed together with other electrical components because the heat in the oven would trigger the thermal fuse. They must therefore be installed after the printed circuit boards have gone through the oven and they must be attached using a dedicated localized process which increases electric machine assembly time and costs.

Another disadvantage of known thermal fuses is that, on account of the fluxes they contain, they have a tendency to gradually deteriorate when subjected to constantly high temperatures, even if lower than their nominal trigger temperature. This makes them unsuitable for use in high-temperature environments such as those typical of automotive applications, often the engine compartment of the vehicles.

Other disadvantages of known thermal fuses are due to their relatively high unit cost, the technical limitations they are subjected to, which limit their operating currents and temperatures and which are often in conflict with specific market requirements for the electric machine of the kind in question, and the impossibility of integrating them with other machine components and systems to make them more efficient in terms of cost, weight and overall functioning.

In this context, the technical purpose which forms the basis of this invention is to propose an electric machine and a method to make it to overcome the above-mentioned disadvantages of the prior art.

The aim of this invention is to provide an electric machine equipped with a high temperature protection system and a method for making such a machine which are more efficient in terms of production costs and components.

A further aim of this invention is to provide an electric machine equipped with a high temperature protection system and a method for making such a machine, where the high temperature protection system is resistant to prolonged use at high temperatures close to the threshold temperature at which the protection is triggered.

A yet further aim of this invention is to provide an electric machine whose possible supported operating current and temperature ranges are wider than those allowed by prior art devices.

The numeral1generically denotes an electric machine according to this invention which is hereinafter described in detail only insofar as necessary for understanding this disclosure.

The electric machine1is preferably a rotary electric machine and, in the preferred embodiment of it, it is an electric motor of the sealed type, that is to say without any opening giving access to the inside of it except possibly for pressure relief valves, which are, in any case, sealed. Express reference is made to this embodiment without thereby losing in generality.

In the embodiment illustrated, the electric machine1comprises a casing2and a cap3which closes the casing2to define, together with the casing2, an enclosure4, or container, which is closed and preferably sealed.

The cap3has a dissipative wall4aconfigured to facilitate dissipation of the heat from the components inside the enclosure4and is preferably provided with a plurality of dissipating fins disposed at the dissipative wall4aand directed towards the outside of the enclosure4.

Preferably, the electric machine1comprises a stator5, fixedly mounted in the casing2, and a rotor6associated therewith and connected to the enclosure4rotatably about an axis of rotation “R”.

Schematically, the stator5comprises: a ferromagnetic core5a; a plurality of conductive windings5bwound around it; and isolators interposed between the ferromagnetic core and the windings.

At least the ferromagnetic core5aand the isolators define a stator mount for the conductive windings5b.

Generally speaking, the term “stator mount”, as used in this disclosure, means the set of components of the electric machine1which keep the windings5bin shape and position, prevent accidental movement thereof and facilitate heat dissipation.

The isolators in the stator5, commonly known as “front pieces”, are insulants and preferably made of polymeric material.

A first front piece40of the stator front pieces is provided with a plurality of holes40aextending parallel to the axis of rotation “R”.

The holes40aallow the stator5to be keyed into the casing2using a machine which uses a plurality of bits to press directly on the ferromagnetic core through the holes40a, thus preventing damage to the front piece40.

The electric machine1also comprises power pins7aand7bwhich, in the embodiment illustrated, pass through the enclosure4and are configured for electrical connection to a direct current power source outside the electric machine1.

The electric machine1comprises an electronic module10which in turn comprises a printed circuit board11, or PCB, and a plurality of electronic power components12.

In the preferred embodiment, the electronic power components12comprise a plurality of power transistors—for example, MOSFETs12a—electrically connected to the windings5bto modulate their voltages and electric currents in order to drive and control the rotation of the rotor6.

Preferably, the electronic module10, and in particular, the printed circuit board11is mounted on a supporting element13disposed on an underside of the printed circuit board11.

The supporting element13is interposed between the electronic module and the stator and rotor5and6. The supporting element13is preferably made of self-extinguishing, electrically insulating plastics and is provided with a housing for the electronic module10.

The supporting element13has a lateral surface which is shaped to match the casing2or the cap3so as to reduce, in practice, the possibility of movement of the electronic module10inside the enclosure4.

The electronic module10comprises a plurality of conductive tracks20disposed on the printed circuit board11, preferably in relief thereon, between the power pins7aand7band the electronic power components12in such a way as to form an electric power circuit10ato power the electronic power components12. In the embodiment illustrated, the conductive tracks20are in relief by 1.8 mm on the printed circuit board11.

The conductive tracks20are disposed at a position facing the dissipative wall4aand are preferably made of a copper alloy that is a good conductor of both electricity and heat. Further, the conductive tracks20are sized in such a way as to define heat pipes between the electronic power components12and the dissipative wall4a. In particular, this thermal sizing of the conductive tracks20involves oversizing them for the purposes of electrical conductivity, which does not have unwanted effects and minimizes electrical resistance.

A heat-conductive filler8, comprising, for example, a heat-conductive paste, is interposed between the conductive tracks20and the dissipative wall4ato fill in gaps between the two parts and facilitate heat conduction between them, as illustrated inFIGS. 2, 3 and 6, where the heat-conductive filler8bis represented by cross-hatching.

The heat-conductive filler8is also interposed between at least some of the electronic power components12and the dissipative wall4ato facilitate direct dissipation of the heat generated by them.

The conductive tracks20have respective surfaces for contact with the MOSFETs12aand with the dissipative wall4a(through the heat-conductive filler8), and which are sized in such a way as to create, in conjunction with the size of the conductive tracks20themselves, a preferential path for dissipating the heat generated by the MOSFETs12a.

This path is preferably configured to keep the temperature of the MOSFETs12abelow 160° C., preferably at 150° C. when the electric machine1operates in an atmosphere at 120° C., as for example the engine compartment of a vehicle.

The electric machine1comprises a plurality of elastic means30acting on the electronic module10in such a way as to press it against the cap3.

In the example illustrated, the elastic means30comprise metal springs30a, which might be replaced, for example, by elastomers and the like.

The heat-conductive filler8, which is at least partly compressed, facilitates heat conduction at least between the conductive tracks20and the dissipative wall4aand between the electronic components and the dissipative wall4a, allowing the heat produced by the electric machine1to be dissipated regularly over time.

In the preferred embodiment, at least some of the springs30aare inserted in the holes40ain the front piece40and have one end abutted against the ferromagnetic core and the other end abutted against the supporting element13to press the entire module10away from the stator5and towards the dissipative wall4a. That way, amongst other things, the conductive tracks20and the heat-conductive filler8above it are forced against the dissipative wall4a.

The plurality of conductive tracks20preferably comprises a first conductive track21and a second conductive track22forming an integral part of the electric power circuit10aso that the power supply current of the electronic power components12passes through them.

In the embodiment illustrated by way of example, the first track21and the second track22are disposed on the top of the printed circuit board11, preferably in relief thereon, in proximity to the power pins7aand7b. More specifically, the first track21comprises one of the power pins7aand7b, whilst the second track22is connected to the rest of the electronic module10.

The electronic module10comprises a conductive element25—for example, a metal plate or bridge—which is electrically connected to the first and the second track21and22to close the electric power circuit10a.

In the example illustrated, the conductive element25allows the currents between the power pins7aand7bto power the plurality of electronic power components12and to drive the rotor6in rotation.

The conductive element25preferably has a thickness which is greater than or equal to 1.8 mm; that is to say of the same order of size as the tracks so as not to create a resistive element of significant impact on the electric power circuit10a.

The conductive element25is fixed to the first and second tracks21and22by respective heat-sensitive joints26which, in the embodiment illustrated, are welds comprising a brazing alloy, preferably an alloy of tin and silver.

In the preferred embodiment, the conductive element25is disposed to partly overlap the first and the second track21and22to form a bridge therebetween.

Advantageously, the elastic means30comprise an elastic element31, preferably a metal spring, to which reference is expressly made without thereby losing in generality, acting on the conductive element25in such a way as to apply a thrust force thereon directed away from the first and second conductive tracks21and22.

This thrust force is transmitted to the electronic module10through the heat-sensitive joints26in such a way as to contribute to pushing it towards the dissipative wall4a.

Preferably, a first end of the elastic element31is disposed in abutment against the ferromagnetic core5aof the stator5.

The elastic element31passes through one of the holes40aon the front piece which has a specific cup-like housing to come directly into abutment against the ferromagnetic core5a.

The second end of the spring31, on the other hand, is operatively disposed in abutment against the conductive element25.

In practice, in the embodiment illustrated by way of example, the spring31has a line of action which is parallel to the axis of rotation of the machine1.

A cover or spacer32made of electrically insulating material is disposed between the end of the spring31and the conductive element25to prevent short circuits between the latter and the ferromagnetic core5a.

Advantageously, the thrust force applied by the elastic element31on the conductive element25creates strain in the heat-sensitive joints26and the latter are configured to resist it mechanically when their temperature is below a predetermined threshold temperature, specifically a temperature at which the brazing alloy melts, preferably between 180° C. and 250° C., and still more preferably, between 200° C. and 230° C.

If the temperature of the heat-sensitive joints26is greater than the predetermined threshold temperature, the joints mechanically yield to the strain and break, for example because the brazing alloy melts, causing the conductive element25to move away from the first and/or the second conductive track21and22so as to break the electrical connection between them.

This event opens the power supply circuit of the electric power circuit10aand instantaneously cuts the power supply to the electronic power components12, causing the electric machine1to stop. Advantageously, these features constitute a high temperature protection for the electric machine1.

Preferably, the conductive element25is configured to allow electric currents greater than 80 amps, preferably greater than 100 amps, to pass between the first track21and the second track22. More specifically, the interface surfaces between the conductive element25and the first and second tracks21and22are sized to allow such currents to pass without causing the heat-sensitive joints26to break or electrical resistance to be generated at the ends of the conductive element25in an order of magnitude comparable with that of the dissipative components of the electric machine1.

In an alternative embodiment, not illustrated, the conductive element25is fixed to only one between the first and the second track21and22by a heat-sensitive joint26and is permanently connected electrically to the other so that if the temperature rises above the threshold temperature, only the heat-sensitive joint26breaks under the pressure of the elastic element31, causing the electric power circuit10ato open.

The printed circuit board11and the supporting element13are shaped to allow positioning the spring31between the ferromagnetic core5aand the conductive element25.

Preferably, the printed circuit board11has a recess, i.e. an absence of material, at the spring31.

In the embodiment illustrated, the absence of material or recess in the printed circuit board11is defined by a portion formed on its outer perimeter delimited by a concave end edge14.

Preferably, the supporting element13has a through hole13aunder and substantially at the concave portion.

Preferably, the first and second tracks21and22have a first and a second end portion21aand22a, respectively, facing each other and protruding from the end edge14towards each other to form cantilevered conductive portions extending over the recess of the printed circuit board11.

Preferably, the conductive element25is welded to respective areas of the first and second end portion21aand22awhere the first and second tracks are thinner than they are in the rest of their structure. These thin areas are preferably shaped to match an outer profile of the conductive element25to define a housing for containing or positioning the conductive element.

In other words, the end portions21a,22aof the tracks21,22are shaped in such a way as to define the housing to contain the conductive element25.

More specifically, the conductive element25is welded to the first and the second end portion21aand22ain such a way as to be suspended above the recess in the printed circuit board11, thanks also to the configuration of the containment housing.

Preferably, the printed circuit board11and the conductive element25are configured and mutually positioned in such a way that the vertical projection of the conductive element onto the positioning plane of the printed circuit board11is entirely inside the recess and at least 1 mm—preferably 2 mm—away from the end edge14.

Advantageously, this feature prevents melting other accidental damage to the printed circuit board11from causing a short circuit with the conductive element25by closing the electric power circuit10aafter it is opened when the heat-sensitive joint26breaks.

Preferably, the first and second end portions21aand22aare spaced apart by a mutual distance D of between 5 mm and 10 mm, preferably between 6.5 mm and 8.5 mm and still more preferably, between 7 mm and 8 mm. In the embodiment illustrated, the distance D is 7.5 mm and determines the maximum voltage across the first and the second track21and22within which the components described can guarantee that the electric power circuit10aremains open after the conductive element25has been triggered, preventing an electric arc from being generated across the first and the second track21and22.

With reference in particular toFIG. 5, the part of the enclosure4above the conductive element25defines a housing space “V” adapted to enable the conductive element25to move away from the first and second tracks21and22.

The electric machine1also comprises a stop element35, which is located near the conductive element25, along a direction of thrust of the elastic element31, in order to determine a stop position of the conductive element25driven by the elastic element31when the heat-sensitive joint26breaks.

The stop element35is preferably at least partly coupled to the supporting element13.

More specifically, the stop element35is disposed in the housing space “V”, preferably fastened to the printed circuit board11and/or to the supporting element13by a reversible clip, in such a way that the conductive element25is stopped at a position where the elastic element31is still loaded and presses the conductive element25against the stop element35to prevent further movements which might create a short circuit between one of the conductive tracks20and the enclosure4, connected to earth, or between other components of the electronic module10and the electric power circuit10a.

This stop position determines a distance of at least 2 mm—preferably at least 3 mm—between the conductive element25and each of both the first and the second track21and22.

Preferably, the elastic element31is configured to apply a residual pressure on the conductive element25at the stop position so as to lock it at that position in the presence of accelerations on the conductive element up to 20 g—preferably up to 50 g—which can occur on the electric machine1in situations created by high impact or resonance in the vehicle engine compartment where it is essential that the electric power circuit10aremains open.

In the preferred embodiment, the stop element35has a concave shape and is disposed in such a way as to fully enclose the conductive element25at the stop position. Further, the stop element35and the supporting element13are configured to define a protective cage around the conductive element25to prevent access by particles or extraneous objects large enough to close the electric power circuit10awhen the conductive element25is at the stop position.

More specifically, the stop element35defines an upper portion of the cage and a portion of the supporting element13around the through hole13afor the passage of the elastic element31is configured to close the upper portion at the bottom.

Preferably, both the stop element35and the supporting element13are made of self-extinguishing, electrically insulating materials with a melting point at least 80° C. higher than the predetermined threshold temperature, so as to keep the electric power circuit10ain the open condition in the presence of exceptionally high temperatures inside the enclosure4and other faults or exceptional events.

In a preferred embodiment, the electronic module10comprises a protection circuit50disposed along the electric power circuit10acomprising at least one MOSFET12a(preferably two), and having a twofold function.

One function of the protection circuit50is to provide an active protection against inversion of polarity at the terminals of the electric power circuit10a; should such an event occur, the protection circuit50opens the power circuit10ato prevent damage to the electronic module10.

To minimize the possibility of a short circuit remaining in the electronic module10after the electric power circuit10ahas been opened by the conductive element25, the latter is disposed in proximity to the power pin7a.

In the embodiment described by way of example, the two components (conductive element/power pin are approximately 30 mm apart.

The power pin7ais preferably one end21bof the first track21. To allow the aforementioned weld to be made, the mass of the first track21must be able to provide a heat capacity greater than 2.5 J/K, preferably greater than 3 J/K, which means approximately 7.5 grams of copper based alloy. Advantageously, this heat capacity allows the first track21to absorb the heat transient connected with the making of the aforementioned weld before the material the heat-sensitive joint26is made of melts.

In a further embodiment, not illustrated, components similar to those described are positioned along the electric power circuit10aand replicated both in proximity to the power pin7aand in proximity to the power pin7b. Advantageously, this system allows the electric machine1to be protected not only against problems involving a short circuit between the negative pin and other components of the electric machine1(for example, the cap3), where opening the electric power circuit10aat the positive pin would be useless, but also against problems in a short circuiting between the positive pin and other components.

This invention also has for an object a method for making an electric machine1of the type described above, comprising a step of preparing an electronic module10of the type described above.

The description of the method is limited to the steps necessary for the understanding of this invention. The method comprises a step of fastening a conductive element25to the electric power circuit10aby means of at least one heat-sensitive joint26, preferably one for fastening the conductive element25to the first track21and one for fastening the conductive element25to the second track22, in such a way as to close the electric power circuit10a.

Preferably, the step of fastening comprises oven brazing, performed when the printed circuit board11is passed through an oven, where a plurality of electronic power components12, including the aforementioned MOSFETs12a, are attached to the conductive tracks20by braze welding. More specifically, the heat-sensitive joints26are made by interposing the above described brazing alloy between the conductive element25and the first and/or the second track21and22and melting the same during oven braze welding. Preferably, the first and second tracks21and22are pre tinned to facilitate braze welding and before being braze welded in the oven, a cut is made in the tin layer to prevent capillary spreading of the brazing alloy on the surface.

The electronic module10is then preferably placed in the supporting element13.

In an embodiment of it, this method comprises a step of fastening the stop element35at the conductive element25along its direction of thrust as described above with reference to the electric machine1.

The stop element35is preferably at least partly coupled to the supporting element13.

Once the stator5and the rotor6have been placed in the casing2, the springs30a,31are inserted into the respective holes40ain the front piece40and the supporting element13/electronic module10assembly is also placed in the casing, above the stator5.

Next, after placing the heat-conductive filler8at least on the conductive tracks20and on the electronic power components12, the method comprises a step of closing the casing2with the cap3.

The step of closing comprises a step of compressing the elastic means30to press the electronic module10against the dissipative wall4aof the cap3and compressing also the heat-conductive filler8.

During this step of compressing, the elastic element31is compressed between the ferromagnetic core5aand the conductive element25to create a force on the heat-sensitive joints26and to obtain the embodiment described above.

Preferably, the step of compressing the elastic means30is accomplished by moving the casing2and the cap3towards each other in such a way that the elastic element31comes into abutment with, and is compressed between, the stator5, specifically the ferromagnetic core5athereof, and the conductive element25.

After the casing2and the cap3have been moved towards each other, they are connected to each other to define and close the internally sealed enclosure4.

The method of the invention also comprises a step of electrically connecting a power cable, not illustrated, to the power pins7aand7band then sealing the connection zone.

This step is carried out in such a way as to heat the end21bfor a welding time which is less than the time necessary for the heat-sensitive joints26to reach a predetermined threshold temperature.

In some embodiments, configured to provide the electric machine1with protection both against malfunctioning involving a short-circuit between the power pin7aand other components of the electric machine1and against malfunctioning involving a short-circuit between the power pin7band other components, some of the steps of the method for making the electric machine1are specifically adapted to replicate the components which cause the electric power circuit10ato open in proximity to the power pin7b.

More specifically, when the printed circuit board11is run through the oven, two conductive elements25, similar to the ones described above, are welded to conductive tracks facing and separated from each other, similar to the first and the second track21and22described above and replicated both at the power pin7aand at the power pin7b. The subsequent steps are carried out to compress two elastic elements31between the conductive elements25and the stator mount5a, similarly to what is described above.

This invention achieves the proposed aim of overcoming the disadvantages of the prior art. In effect, connecting the conductive element to the conductive tracks using a layer of heat-sensitive material subjected to the force of the elastic element constitutes an effective system of protection against high temperature and high current, capable of cutting the electrical power supply to the electrical components of the electric machine.

Advantageously, in the above described method for making the electric machine, the conductive element is attached to the conductive tracks before being subjected to the force of the elastic element, which is added at a later stage. This allows attaching the elastic element to the conductive tracks in the same run through the oven in which the other SMD components are braze welded, allowing the high temperature protection system to be implemented without the need for additional, time consuming operations specifically for attaching the elastic element. Another advantage of the above method is that the only step that distinguishes the electric machine described from an electric machine not provided with high temperature protection is that of placing the elastic element. That means both machines can be made on the same production line, simply omitting the step of placing the elastic element if the electric machine being made is without high temperature protection.

A further advantage of the machine and production method described lies in the resistance to prolonged use at temperatures which, although high, are lower than the protection trigger temperature. This resistance is due to the fact that the fluxes normally used in the majority of prior art thermal fuses are not used.

The system described herein also has the advantage of being dimensionally scalable as required and can therefore be designed to resist currents which are substantially higher than those resisted by prior art thermal fuses.

Lastly, in the electric machine described herein, the elastic element which pushes the conductive element has the twofold purpose of providing the above described protection against high temperature and current and pressing the electronic module against the dissipative wall to facilitate dissipation of heat. Advantageously, this leads to high production efficiency and lower production costs.