Patent Description:
A synchronous motor represents a motor in which the magnetic field of the rotor is constant, and the coils in the stator are energized so as to produce a rotating magnetic field, which is matched to the field of the rotor in frequency. A permanent magnet synchronous motor uses permanent magnets to produce the constant rotor field. A permanent magnet synchronous torque motor is optimized for high torque, at the expense of rotation speed, typically by increasing the diameter and the number of magnetic poles on the rotor. An example of a motor of the abovementioned type is known from <CIT>. The known motor comprises a stator and a rotor. The stator comprises a plurality of electrical windings, and a cylindrical armature from which a plurality of teeth extend radially inward. Each of the windings is wound around at least one tooth. In some stators, each winding is wound only around a single tooth. Such type of winding is referred to as a non-overlapping winding. In other stators, a winding may span over multiple teeth. Such winding is referred to as an overlapping winding.

The rotor is arranged within the armature and is provided with a plurality of permanent magnets. Furthermore, a housing in the form of a cooling ring is arranged around the stator and defines a plurality of cooling channels through which a coolant may flow for the purpose of cooling the stator.

In the abovementioned motor, the cooling ring has open cooling channels that can be closed by shrink fitting a cover onto the cooling ring.

For some applications, such as industrial machining, it is important that the motor produces as much torque as possible within a given volume or space, and/or for a given mass of the motor. Either of these can be referred to as torque density. For these applications, it is therefore required to reduce the volume occupied by and/or mass of the motor without sacrificing maximum achievable torque. The Applicant has found that for some applications, the abovementioned requirement cannot be met sufficiently using the abovementioned known motor. It is therefore an object of the present invention to provide a permanent magnet synchronous torque motor that, compared to the known motor, allows a reduction in the space occupied by and/or mass of the motor while preventing or limiting a reduction of maximum achievable torque.

<CIT> discloses a mechanically and electronically integrated module that is capable of improving cooling performance and suppressing an increase in pressure loss in the connection part between an inverter-side coolant channel, and a motor-side coolant channel or an external coolant flow, without necessitating an increase in the diameter of the device. A motor-side coolant channel and an inverter-side coolant channel are connected in a space configured from a frame, a stator core and an unloaded-side end frame, with a first channel conversion unit interposed between the channels on the outer-diameter side of a coil end on the unloaded side of a stator coil. The first channel conversion unit is provided with a channel for gradually changing the cross-sectional shape thereof, and changing from the cross-sectional shape of the opening of the inverter-side coolant channel to the cross-sectional shape of the opening of the motor-side coolant channel.

According to an aspect of the present invention, a motor system is provided according to claim <NUM> that is characterized in that the armature is made from non-grain-oriented steel, and in that a compressive stress inside the armature exceeds <NUM> MPa, and in that the electrical power supply is configured to provide electrical power and/or current to the plurality of windings such that the teeth and the armature are at least partially saturated.

Shrink-fitting results in a continuous compressive pressure between armature and cooling ring, reducing the thermal contact resistance and optimizing heat flow. The shrink-fitting has resulted in a residual stress in the armature that exceeds <NUM> MPa when measured under off conditions of the motor and normal ambient conditions of temperature, humidity, and pressure.

The process of shrink-fitting introduces compressive stress in the armature. In the art, such stress is considered detrimental as it reduces the magnetic performance of the armature material. More in particular, when comparing stressed and relaxed armature material, the magnetic polarization for the same applied magnetic field will be less for the stressed armature material. This is for example described in the paper entitled "<NPL>.

The Applicant has found that this apparent compressive stress related performance degradation will diminish when the material becomes saturated. More in particular, the Applicant has found that under conditions of moderate to strong saturation, the magnetic polarization may even be higher compared to the relaxed armature material. This behavior is illustrated in <FIG>.

In <FIG>, the unbroken line refers to non-grain-oriented steel under compressive stress of about <NUM> MPa. The slope of this line, which is equal to the magnetic permeability of the material, is lower than that of the dashed line, which refers to the same material without compressive stress. The lower slope indicates that at relatively low magnetic fields (H<1e3 A/m), the polarization is lower, resulting in less flux. However, at relatively high magnetic fields (H>1e5 A/m), the polarization for the compressed material exceeds that of the relaxed material. This means that at relatively high magnetic fields, the effective magnetic saturation of the compressed material is lower. The applicant therefore reaches a different conclusion than, e.g., Daisuke Miyagi et al. based on substantially similar data, by judging performance by the polarization itself rather than by the slope of the polarization curve. The reason for this difference is to be found in the requirements of the application, in which torque density is of paramount importance.

The Applicant has further realized that this effect can be used to achieve the abovementioned object of the invention. Typically, the armature has a given thickness that on one hand is not too small to prevent excessive saturation and on the other hand is not too large to keep the motor as compact as possible. By using the abovementioned effect, the armature thickness can be reduced as degradation in performance can be at least partially compensated by the stress related improvement in magnetic performance.

Preferably, degradation in polarization in each of the teeth and armature observed when changing a current level through the windings from a non-saturated state to an at least partially saturated state may be in a range between <NUM> and <NUM> percent when compared to the polarization that would have been achieved in the absence of saturation, and more preferably between <NUM> and <NUM> percent.

Preferably, each winding is a non-overlapping winding, because this further improves torque density. Additionally or alternatively, the windings can be divided into a plurality of winding groups, each winding group being configured to receive electrical power corresponding to a respective phase of a multi-phase electrical power source. Preferably, the windings are divided into three winding groups allowing the motor to be operated using three-phase electrical power.

A ratio between the number of teeth and the number of permanent magnets lies in a range from <NUM> and <NUM>. Additionally, a ratio between a thickness of the armature and a width of the teeth lies in a range from <NUM> and <NUM>. A desired ratio between the thickness of the armature and the width of the teeth, denoted by b, can be found using the ratio between the number of teeth and the number of magnets, denoted by a: <MAT> wherein c<NUM> and c<NUM> are constants to indicate a range relative to the most preferred solution, which is indicated in between the square brackets. According to the invention, c<NUM> is <NUM> and c<NUM> is <NUM>.

<FIG>, <FIG> and <FIG> each show a cross section of a torque motor in a plane perpendicular to the motor axis. These figures differ in the teeth-to-magnet ratio, corresponding to factor a in equation <NUM>, that is used in the motor. In each figure, magnetic flux lines <NUM> are indicated. A high concentration thereof indicates a high magnetic flux density, which leads to partial saturation in the ferromagnetic material of the armature. Higher magnetic saturation implies more reluctance, which reduces the amount of magnetic flux that reaches the magnets, thereby reducing performance. As an engineering principle, an overall optimum of (in this case) torque density, is found when all limiting factors are substantially equal. In this case, the limiting factors are the magnetic saturation in various locations.

The Applicant has found that for motors having a ratio between the number of teeth and the number of permanent magnets close to unity, i.e. a is approximately <NUM>, it is preferable if the thickness of the armature is substantially equal to a width of the teeth, i.e. b is approximately <NUM>, wherein it is noted that all teeth have substantially the same width. This situation is illustrated in <FIG>.

<FIG> illustrates a motor having a stator comprising an armature <NUM> from which teeth <NUM> extend radially inward. In between teeth <NUM>, slots <NUM> are formed in which windings <NUM> are arranged. The winding for each tooth <NUM> is arranged in two adjacent slots <NUM>. For example, the winding for tooth <NUM>' comprises two parts <NUM>', <NUM>" that arranged in two adjacent slots <NUM>.

In <FIG>, the stator, of which only a part is illustrated, comprises <NUM> teeth. The rotor comprises <NUM> permanent magnets <NUM> that are mounted on a cylinder or ring-shaped body <NUM>. This situation corresponds to a=<NUM> in equation <NUM>. Furthermore, the thickness t of armature <NUM> is substantially equal to the width w of teeth <NUM>. More in particularly, using a=<NUM> in equation <NUM>, a value of <NUM> can be found for factor b using equation <NUM>.

As can be seen, the magnetic field lines <NUM>, which are attributed to magnets <NUM> and windings <NUM>, that pass through a tooth <NUM>, pass through segments A of armature <NUM> to a much larger extent than through segments B of armature <NUM>. In other words, magnetic flux is substantially shared between pairs of teeth <NUM>; each pair shares a minimal amount of magnetic flux with the next pair. This means that the thickness of armature <NUM> has to be substantially identical to the width of teeth <NUM> as indicated by equation <NUM> and as shown in <FIG>. In this manner, teeth <NUM> and armature <NUM> will saturate substantially simultaneously thereby satisfying the above engineering principle.

<FIG> and <FIG> illustrate motors having different ratios of the number of teeth and the number of magnets. In <FIG>, the ratio between the number of teeth <NUM> and the number of magnets <NUM> equals a = <NUM>/<NUM>, whereas in <FIG> a=<NUM>/<NUM>. This corresponds to a factor b equaling <NUM> in both cases. In other words, the thickness of armature <NUM> is lower than the width of teeth <NUM>.

In <FIG> and <FIG>, during operation, magnetic flux is shared not only between pairs of teeth <NUM>, but between all teeth <NUM>. This sharing implies that the magnetic flux exiting a tooth <NUM> and entering armature <NUM> flows both ways instead of just one. In turn, this implies that the amount of flux in armature <NUM> is lower than that in tooth <NUM>. The engineering principle above still states that optimum performance is found when all limiting factors are substantially equal, meaning saturation in tooth <NUM> should be equal to saturation in armature <NUM>, implying that flux density should be equal. A lower flux in armature <NUM> relative to tooth <NUM> implies that the armature thickness should be reduced proportionally relative to the situation shown in <FIG>. Therefore, in <FIG> and <FIG>, the thickness of armature <NUM> is made smaller than the width of teeth <NUM>, in correspondence with equation <NUM> which states that b should be <NUM>.

The number of teeth preferably lies within a range from <NUM> to <NUM>, more preferably in a range from <NUM> to <NUM>.

The cooling ring may comprise a base surface or plate that lies against the armature and cooling channel defining ridges that protrude radially outward from the surface or plate. The base surface or plate and the ridges are preferably integrally connected.

At least one ridge may comprise a recess for allowing coolant that moves in one of said cooling channels to move to an adjacent one of said channels. The cooling channels may be open channels. These channels can be transformed into closed channels by connecting a cover plate to the cooling ring. In this case, each channel among the plurality of channels is defined by the base surface or plate, the channel defining ridges, and the cover plate.

The teeth are preferably distributed uniformly in a circumferential direction along an inner wall of the armature. Additionally or alternatively, the teeth and the armature can be integrally connected. The armature can for example be laminated. For example, the teeth and armature can be formed using sheets of non-grain-oriented steel that are stacked in a direction parallel to the rotational axis, i.e. axial direction, of the motor.

The cooling ring may extend beyond the stator in an axial direction. A solidified molding compound or resin may be used to fill up this space.

Next, the invention will be described in more detail referring to the appended drawings, wherein:.

The stator of motor <NUM> comprises a cylindrical armature made from non-grain-oriented steel from which teeth <NUM> extend radially inward towards the rotor. Teeth <NUM> and armature <NUM> are integrally connected. Each tooth <NUM> is provided with a respective non-overlapping winding <NUM>.

The rotor comprises a cylindrical or ring-shaped body <NUM> made from ferromagnetic material such as non-grain-oriented steel or cobalt steel onto which permanent magnets <NUM>, preferably of the Rare Earth type are mounted. Motor <NUM> comprises <NUM> permanent magnets <NUM> and <NUM> teeth <NUM>. It should be noted that instead of permanent magnets <NUM>, electromagnets fed by a DC power source could equally be used.

Motor <NUM> further comprise a cooling ring <NUM>, made from a low density, high thermal conductivity metal, such as aluminum, that has been shrink-fitted around armature <NUM>. Cooling ring <NUM> comprises a base surface <NUM> from which ridges <NUM> extend outwardly. The ridges define open cooling channels <NUM> through which a coolant may flow. Additionally, ridges <NUM> define a pair of grooves <NUM> in which an O-ring is received (not shown). A recess <NUM> may be provided in at least one ridge <NUM> for allowing coolant that flows in a channel <NUM> to move to an adjacent channel <NUM>.

Prior to mounting cooling ring <NUM> around armature <NUM>, the inner diameter of cooling ring <NUM> was smaller than the outer diameter of armature <NUM>. Consequently, after shrink fitting cooling ring <NUM>, which has a thickness in a radial direction that is at least <NUM> percent of the thickness of armature <NUM>, a considerable amount of compressive stress will be present in armature <NUM>, typically in the order of <NUM> MPa.

As shown, cooling ring <NUM> extends in the axial direction beyond the stator. This space may be filled with a solidified molding compound or resin to protect and fixate windings <NUM>.

In <FIG>, two connectors <NUM>, <NUM> can be observed. Connector <NUM> is used for providing three-phase electrical power to windings <NUM>. Connector <NUM> comprises four contacts, one for each phase and one for the common return path.

Connector <NUM> comprises contacts through which measurements signals from inside motor <NUM> can be obtained. For example, positional sensors may be provided inside motor <NUM> for the purpose of determining the relative position between stator and rotor. This position is valuable information for the electrical power supply (not shown) that drives the motor. More in particular, the current supplied to the various windings can be in dependence of the mutual position of rotor and stator. This information can also be used when ramping up the speed of the rotor. The electrical power supply may be configured to change the frequency of the supplied power or current signal.

A cover plate (not shown) may be connected to cooling ring <NUM> to transform the open channels <NUM> into closed channels. Such cover plate may be fixedly attached to cooling ring <NUM> by various known means. O-rings provided in grooves <NUM> ensure that the coolant remains in channels <NUM> defined by cooling ring <NUM>.

Typically, cooling channels <NUM> extend in a spiraled manner around cooling ring <NUM>. An inlet and outlet (not shown) may be provided through which a coolant, e.g., a liquid, can be fed to and extract from cooling ring <NUM>, respectively.

Claim 1:
A motor system, comprising:
a permanent magnet synchronous torque motor (<NUM>), comprising:
a stator comprising a plurality of electrical windings (<NUM>), and a cylindrical armature (<NUM>) from which a plurality of teeth (<NUM>) extend radially inward, each of the windings being wound around at least one tooth;
a rotor arranged within the armature and being provided with a plurality of permanent magnets (<NUM>); and
a cooling ring (<NUM>) arranged around the stator and defining a plurality of cooling channels (<NUM>) through which a coolant may flow for the purpose of cooling the stator,
wherein the cooling ring is shrink-fitted around the armature,
wherein a ratio between the number of teeth and the number of permanent magnets lies in a range from <NUM> to <NUM>, and wherein a ratio between a thickness of the armature, in a radial direction with respect to the armature, and a width of the teeth, in a circumferential direction with respect to the armature, lies in a range from <NUM> to <NUM>,
wherein ratio between the thickness of the armature and the width of the teeth, denoted by b, can be found using the ratio between the number of teeth and the number of permanent magnets, denoted by a, using: <MAT>
the motor system further comprising an electrical power supply for providing electrical power and/or current to the plurality of windings for the purpose of generating a rotating magnetic field,
characterized in that the armature is made from non-grain-oriented steel, and in that a compressive stress inside the armature exceeds <NUM> MPa, and in that the electrical power supply is configured to provide electrical power and/or current to the plurality of windings such that the teeth and the armature are at least partially saturated.