The present invention relates to a permanent magnet synchronous torque motor. The present invention further relates to a motor system comprising such a motor. In addition, the present invention relates to a method for manufacturing the motor.

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 US 2019/0140519A1. 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.

According to the present invention, this object is achieved with the permanent magnet synchronous torque motor as defined in claim 1 that is characterized in that the cooling ring is shrink-fitted around the armature. Shrink-fitting results in a continuous compressive pressure between armature and cooling ring, reducing the thermal contact resistance and optimizing heat flow.

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“Influence of Compressive Stress on Magnetic Properties of Laminated Electrical Steel Sheets” by Daisuke Miyagi et al, IEEE transactions on magnetics, vol. 46, No. 2, February 2010, pp. 318-321.

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 figure 1.

In figure 1 , the unbroken line refers to non-grain-oriented steel under compressive stress of about 100 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<le3 A/m), the polarization is lower, resulting in less flux. However, at relatively high magnetic fields (H>le5 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, 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 a number of teeth and a number of permanent magnets preferably lies in a range from 0.75 and 1.5. Additionally or alternatively, a ratio between a thickness of the armature and a width of the teeth preferably lies in a range from 0.6 and 1.2. In a preferred embodiment, 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: c _± [i (-8a ² + 18a - 7)] < b < c ₂ [i (-8a ² + 18a - 7)] Eq. 1 wherein c _j and c ₂ are constants to indicate a range relative to the most preferred solution, which is indicated in between the square brackets. For example, c _j may equal 0.75 and more preferably 0.85, and c ₂ may equal 1.3 and more preferably 1.1.

Figures 2A, 2B and 2C 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 1 , that is used in the motor. In each figure, magnetic flux lines 7 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 a number of teeth and a number of permanent magnets close to unity, i.e. a is approximately 1, it is preferable if the thickness of the armature is substantially equal to a width of the teeth, i.e. b is approximately 1 , wherein it is noted that all teeth have substantially the same width. This situation is illustrated in figure 2A.

Figure 2A illustrates a motor having a stator comprising an armature 1 from which teeth 2 extend radially inward. In between teeth 2, slots 3 are formed in which windings 4 are arranged. The winding for each tooth 2 is arranged in two adjacent slots 3. For example, the winding for tooth 2’ comprises two parts 4’, 4” that arranged in two adjacent slots 3.

In figure 2A, the stator, of which only a part is illustrated, comprises 18 teeth. The rotor comprises 20 permanent magnets 5 that are mounted on a cylinder or ring-shaped body 6. This situation corresponds to a- 0.9 in equation 1. Furthermore, the thickness t of armature 1 is substantially equal to the width w of teeth 2. More in particularly, using a- 0.9 in equation 1 , a value of 0.9 can be found for factor b using equation 1.

As can be seen, the magnetic field lines 7, which are attributed to magnets 5 and windings 4, that pass through a tooth 2, pass through segments A of armature 1 to a much larger extent than through segments B of armature 1. In other words, magnetic flux is substantially shared between pairs of teeth 2; each pair shares a minimal amount of magnetic flux with the next pair. This means that the thickness of armature 1 has to be substantially identical to the width of teeth 2 as indicated by equation 1 and as shown in figure 2A. In this manner, teeth 2 and armature 1 will saturate substantially simultaneously thereby satisfying the above engineering principle.

Figures 2B and 2C illustrate motors having different ratios of the number of teeth and the number of magnets. In figure 2B, the ratio between the number of teeth 2 and the number of magnets 5 equals a - 3/2, whereas in figure 2C a=3/4. This corresponds to a factor b equaling 0.7 in both cases. In other words, the thickness of armature 1 is lower than the width of teeth 2.

In figures 2B and 2C, during operation, magnetic flux is shared not only between pairs of teeth 2, but between all teeth 2. This sharing implies that the magnetic flux exiting a tooth 2 and entering armature 1 flows both ways instead of just one. In turn, this implies that the amount of flux in armature 1 is lower than that in tooth 2. The engineering principle above still states that optimum performance is found when all limiting factors are substantially equal, meaning saturation in tooth 2 should be equal to saturation in armature 1 , implying that flux density should be equal. A lower flux in armature 1 relative to tooth 2 implies that the armature thickness should be reduced proportionally relative to the situation shown in figure 2A. Therefore, in figures 2B and 2C, the thickness of armature 1 is made smaller than the width of teeth 2, in correspondence with equation 1 which states that b should be 0.7.

The armature is preferably made from non-grain-oriented steel although other materials are not excluded such as cobalt steel. The shrink-fitting has preferably resulted in a residual stress in the armature that exceeds 100 MPa when measured under off conditions of the motor and normal ambient conditions of temperature, humidity, and pressure.

A number of teeth preferably lies within a range from 15 to 90, more preferably in a range from 24 to 48.

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.

According to a second aspect, the present invention provides a motor system that comprises the motor as defined above and 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. 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. For example, 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 8 and 40 percent when compared to the polarization that would have been achieved in the absence of saturation, and more preferably between 15 and 25 percent.

According to a third aspect, the present invention provides a method for manufacturing the motor as defined above. This method comprises the step of providing a stator having a plurality of electrical windings, and a cylindrical armature from which a plurality of teeth extend radially inward, wherein each of the windings being wound around at least one tooth. The method further comprises the step of providing a rotor being provided with a plurality of magnets, preferably permanent magnets, and providing a cooling ring that defines a plurality of cooling channels through which a coolant may flow for the purpose of cooling the stator. The cooling ring has an inner diameter that, at least at room temperature, is smaller than an outer diameter of the armature at that temperature.

According to the invention, the method is characterized in that the method comprises the step of shrink fitting the cooling ring around the armature. This shrink fitting may comprise the steps of increasing the inner diameter of the cooling ring to become larger than the outer diameter of the armature by heating the cooling ring, arranging the cooling ring around the armature, and allowing the cooling ring to cool down for obtaining a shrink-fit of the cooling ring around the armature. Additionally or alternatively, the method may comprise arranging the rotor within the armature. Typically, the rotor is fixed in position relative to the stator prior to mounting the motor in the final application to prevent the rotor and stator to engage each other as both are made from or comprise magnetic material.

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

Figure 1 illustrates the polarization as function of the magnetic field for non-grain-oriented steel; Figures 2A-2C illustrate combinations of armature thickness and teeth width for different teeth-to-magnet ratios;

Figure 3 illustrates a perspective view of a motor in accordance with the present invention; and

Figure 4 illustrates a cross sectional view of the motor taken perpendicular to the axial direction of the motor.

Figure 3 illustrates a perspective view of a motor 100 in accordance with the present invention and figure 4 illustrates a cross sectional view of this motor taken perpendicular to the axial direction of the motor.

The stator of motor 100 comprises a cylindrical armature made from non -grain-oriented steel from which teeth 2 extend radially inward towards the rotor. Teeth 2 and armature 1 are integrally connected. Each tooth 2 is provided with a respective non-overlapping winding 4.

The rotor comprises a cylindrical or ring-shaped body 6 made from ferromagnetic material such as non-grain-oriented steel or cobalt steel onto which permanent magnets 5, preferably of the Rare Earth type are mounted. Motor 100 comprises 38 permanent magnets 5 and 36 teeth 2. It should be noted that instead of permanent magnets 5, electromagnets fed by a DC power source could equally be used.

Motor 100 further comprise a cooling ring 90, made from a low density, high thermal conductivity metal, such as aluminum, that has been shrink-fitted around armature 1. Cooling ring 90 comprises a base surface 95 from which ridges 92 extend outwardly. The ridges define open cooling channels 93 through which a coolant may flow. Additionally, ridges 92 define a pair of grooves 91 in which an O-ring is received (not shown). A recess 94 may be provided in at least one ridge 92 for allowing coolant that flows in a channel 93 to move to an adjacent channel 93.

Prior to mounting cooling ring 90 around armature 1 , the inner diameter of cooling ring 90 was smaller than the outer diameter of armature 1. Consequently, after shrink fitting cooling ring 90, which has a thickness in a radial direction that is at least 60 percent of the thickness of armature 1 , a considerable amount of compressive stress will be present in armature 1 , typically in the order of 100 MPa.

As shown, cooling ring 90 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 4.

In figure 3, two connectors 80, 81 can be observed. Connector 80 is used for providing three-phase electrical power to windings 4. Connector 80 comprises four contacts, one for each phase and one for the common return path.

Connector 81 comprises contacts through which measurements signals from inside motor 100 can be obtained. For example, positional sensors may be provided inside motor 100 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 90 to transform the open channels 93 into closed channels. Such cover plate may be fixedly attached to cooling ring 90 by various known means. O-rings provided in grooves 91 ensure that the coolant remains in channels 93 defined by cooling ring 90.

Typically, cooling channels 93 extend in a spiraled manner around cooling ring 90. 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 90, respectively.

In the above, the present invention has been explained using detailed embodiments thereof. It should be appreciated by the skilled person that various modifications can be made to these embodiments without departing from the scope of the invention that is defined by the appended claims.