Electronically commutated electric motors are provided with a static motor stator comprising a ferromagnetic stator body and at least one stator coil, and with a permanently magnetized motor rotor being rotatable with respect to the motor stator about an axis of rotation. The motor rotor is alternately magnetized in the circumferential direction so that the rotating motor rotor generates an alternating magnetic field at a static circumferential position. The electric motor is provided with a static magnetic field sensor detecting the alternating rotor magnetic field and providing a corresponding trigger signal. The electric motor is also provided with a motor electronics energizing the stator coil, wherein the drive current is commutated based on the trigger signal provided by the magnetic field sensor. As a result, the alternately magnetized motor rotor is driven by the alternating stator magnetic field generated by the electromagnetic motor stator. Such an electronically commutated electric motor is, for example, disclosed in EP 2 450 575 Al, wherein the electric motor drives the pump wheel of a motor vehicle fluid pump. However, in particular at low rotor speeds, the motor rotor of such an electronically commutated electric motor can "toggle", i.e. the motor rotor is alternately moved forth and back by a small angle, but does not rotate. The toggle effect can occur if the motor rotor rotates so slow that the polarization of the stator magnetic field is reversed before the motor rotor has passed a critical rotational position. As a result, the polarity reversal of the stator magnetic field causes a backward rotation of the motor rotor instead of a forward rotation. If the interface between two oppositely magnetized rotor sectors passes the magnetic field sensor during this backward rotation, the magnetic field sensor triggers the motor electronics to commutate the drive current so that the polarity of the stator magnetic field is reversed again. As a result, the motor rotor is moved slightly forth and back, wherein during this toggle movement, the interface between two oppositely magnetized rotor sectors is moved forth and back in front of the magnetic field sensor so that the magnetic polarity detected by the magnetic field sensor continuously alternates and, as a result, the stator drive current is continuously commutated.

Since the motor electronics successively energizes the motor stator, significant energy is consumed during the motor rotor toggling. However, the total consumed energy during the motor rotor toggling is waste energy because the motor rotor does not rotate. In addition, the motor rotor toggling cannot be identified by the motor electronics without additional sensor elements, because the trigger signal generated by the toggling motor rotor cannot be distinguished from a trigger signal generated by a properly rotating motor rotor. As a result, the electric motor can become "trapped" in the toggling state and, as a result, does not allow a reliable drive of the motor rotor.

It is an object of the invention to provide an energy-efficient electronically commutated electric motor, which allows a reliable drive of the motor rotor for a wide rotor speed range and, in particular, for low rotor speeds.

This object is achieved with an electronically commutated electric motor with the features of claim 1. The electronically commutated electric motor according to the invention is provided with a static motor stator comprising a ferromagnetic stator body and at least one stator coil. The stator body can be either a massive monolithic ferromagnetic stator body or, alternatively, can be a laminated stator body composed of a stack of ferromagnetic metal sheets. The electronically commutated electric motor according to the invention is also provided with a permanently magnetized motor rotor being rotatable with respect to the motor stator about an axis of rotation. The motor rotor is alternately magnetized in the circumferential direction so that the rotating motor rotor generates an alternating magnetic field at a static circumferential position. The motor rotor can be realized as a single permanently magnetized rotor body, but can also be provided as a preferably ferromagnetic rotor body with separate permanent magnets.

The electronically commutated electric motor according to the invention is also provided with a static magnetic field sensor. The magnetic field sensor is positioned at a static circumferential position and detects the alternating magnetic field generated by the rotating motor rotor. The magnetic field sensor provides a trigger signal corresponding to the detected magnetic field, wherein the trigger signal, in particular, depends on the magnetic polarity of the detected magnetic field. The magnetic field sensor can be, for example, a cost-efficient Hall sensor with an integrated evaluation unit. According to the invention, the magnetic field sensor is a latching-type magnetic field sensor, i.e. the magnetic field sensor provides a substantially digital trigger signal, wherein the trigger signal level only switches if the polarization of the detected magnetic field reverses. In particular, the trigger signal switches to a first trigger signal level if the detected magnetic field has a first polarization with an amplitude exceeding a first threshold level, and the trigger signal switches to a second trigger signal level if the detected magnetic field has an opposite second polarization with an amplitude exceeding a second threshold level. The electronically commutated electric motor according to the invention is also provided with a motor electronics for energizing the at least one stator coil of the motor stator based on the trigger signal being provided by the magnetic field sensor. In particular, the motor electronics energizes the stator coils with a substantially alternating drive current, wherein the drive current is commutated based on the trigger signal. The motor electronics typically comprises several power semiconductors for commutating the drive current. According to the invention, the motor rotor is provided with two axially adjacent axial rotor sections, a drive rotor section and a sensor rotor section. The drive rotor section is arranged within the axial extent of the motor stator, and the sensor rotor section is provided at a magnetic-field- sensor-facing axial end of the motor rotor. The drive rotor section and the sensor rotor section are provided with different circumferential magnetization patterns compared to each other, i.e. the sequence, the circumferential position and/or the circumferential extent of magnetic north poles, magnetic south poles and non-magnetic intermediate regions along the circumference of the motor rotor differ between the two rotor sections. The term "non-magnetic" means in the present context that the magnetization level is below a detection threshold level of the magnetic field sensor.

Since the drive rotor section is arranged within the axial extent of the motor stator, the circumferential magnetization of the drive rotor section interacts with a stator magnetic field generated by the electromagnetic motor stator for driving the motor rotor. In contrast, the alternating magnetic field generated by the circumferential magnetization pattern of the sensor rotor section is detected by the static magnetic field sensor, which is positioned adjacent to the sensor rotor section. As a result, the trigger signal which is provided to the motor electronics for commutating the drive current is substantially determined by the circumferential magnetization pattern of the sensor rotor section and not, or at least not significantly, by the circumferential magnetization pattern of the drive rotor section. As a result, the drive rotor section can be provided with a drive magnetization pattern which allows an energy-efficient drive of the motor rotor, whereas the sensor rotor section can be designed independently with a different sensor magnetization pattern, which ensures a reliable drive of the motor rotor and, in particular, avoids a toggling of the motor rotor. As a result, the electronically commutated electric motor according to the invention allows an energy-efficient and reliable drive of the motor rotor.

In a preferred embodiment of the invention, the drive rotor section of the motor rotor radially surrounds the motor stator, i.e. the electric motor is provided with an external rotor. The external rotor can be simply and reliably provided with two axially adjacent but magnetically independent axial rotor sections.

Preferably, the drive rotor section is provided with permanently-magnetic drive ring sectors being alternately magnetized in the circumferential rotor direction, and the sensor rotor section is provided with permanently- magnetic sensor ring sectors being alternately magnetized in the circumferential rotor direction and with non-magnetic sensor ring sectors located circumferentially between the permanently-magnetic sensor ring sectors. The non-magnetic sensor ring sectors between the permanently- magnetized sensor ring sectors avoid a continuous polarization reversal at a static circumferential position if the motor rotor is moved forth and back by a small angle. As a result, the non-magnetic sensor ring sectors avoid a continuous switching of the trigger signal and, as a result, reliably avoid a toggling of the motor rotor. In a preferred embodiment of the invention, the circumferential extent of each permanently-magnetic drive ring sector of the drive rotor section substantially corresponds to the circumferential extent of a sensor sector pair of the sensor rotor section, wherein the sensor sector pair is composed of one permanently-magnetic sensor ring sector and one non magnetic sensor ring sector. As a result, the number of permanently- magnetic drive ring sectors is equal to the number of permanently- magnetic sensor ring sectors. More preferably, the circumferential start position, i.e. the circumferential position of the ring sector edge which points into the rotational direction of the motor rotor, of each permanently-magnetic sensor ring sector is substantially equal to that of the corresponding permanently-magnetic drive ring sector. As a result, at a defined static circumferential position, the polarization of the sensor magnetic field always reverses concurrently with the polarization of the drive magnetic field. As a result, the trigger signal being provided by the magnetic field sensor is synchronous with the alternation frequency of the magnetic field of the rotating motor rotor. This allows a reliable and energy-efficient drive of the motor rotor without requiring complex compensation logics.

Preferably, the circumferential extent of the non-magnetic sensor ring sectors is larger than that of the permanently-magnetic sensor ring sectors so that the circumferential distance between the permanent- magnetic sensor ring sectors is relatively large. As a result, the motor rotor can be moved by a relatively large angle without changing the polarization of the magnetic field detected by the magnetic field sensor. This reliably avoids a toggling of the motor rotor and, as a result, provides an energy-efficient and reliable drive of the motor rotor. In a preferred embodiment of the invention, the motor rotor with the two axially adjacent rotor sections is a monolithic body which, preferably, has a homogenous material composition. The entire rotor body is magnetized in a single process, wherein the drive rotor section and the sensor rotor section are provided with different circumferential magnetization patterns. This provides a compact and robust motor rotor not requiring any assembly of separate rotor parts.

Alternatively, the drive rotor section and the sensor rotor section are defined by separate rotor bodies being fixed to each other. The separate rotor bodies can be separately magnetized so that they can be provided with different circumferential magnetization patterns in a simple way. The separate rotor bodies can be fixed to each other, for example, adhesively.

In a preferred embodiment of the invention, the non-magnetic sensor ring sectors are provided by axial rotor recesses. This allows a simple and reliable realization of the non-magnetic sensor ring sectors.

Preferably, the magnetic field sensor and the sensor rotor section of the motor rotor are provided in that way, that for every magnetic rotor rest position, i.e. the rotor positions which the motor rotor moves into if not being driven, a permanently-magnetic sensor ring sector is located within the detection range of the magnetic field sensor. This ensures a reliable and efficient startup of the motor rotor rotation.

In a preferred embodiment of the invention, the magnetic field sensor is arranged radially inside of and radially adjacent to the sensor rotor section. This provides a reliable detection of the sensor magnetic field and, as a result, allows an efficient drive of the motor rotor.

An embodiment of the invention is described with reference to the enclosed drawings, wherein figure 1 shows a detail view of a longitudinally sectioned electronically commutated electric motor according to the invention, wherein the electric motor is part of a motor vehicle pump,

figure 2 shows a transverse section of a drive rotor section of the electric motor of figure 1, and

figure 3 shows a transverse section of a sensor rotor section of the electric motor of figure 1.

Figure 1 shows a motor vehicle fluid pump 8 with an electronically commutated electric motor 10 according to the invention. The motor vehicle fluid pump 8 can be, for example, a coolant pump of a motor vehicle coolant system.

The electric motor 10 is provided with a static motor stator 12 comprising a laminated stator body 14 and several stator coils 16. The electric motor 10 is also provided with a pot-shaped motor rotor 18 radially surrounding the motor stator 12 and being rotatable with respect to the motor stator 12 about an axis of rotation R. As a result, in the present embodiment of the invention, the electric motor 10 is a so-called external rotor motor.

The motor rotor 18 is co-rotatably connected with a pump wheel 20, which, in the present embodiment of the invention, is provided integrally with the motor rotor 18. The motor rotor 18 is provided with two axially adjacent axial rotor sections, a drive rotor section 22 and a sensor rotor section 24. The drive rotor section 22 is axially located within the axial extent A of the motor stator 12, and the sensor rotor section 24 is located at a pump-wheel-remote axial end 26 of the motor rotor 18. In the present embodiment of the invention, the motor rotor 18 is a monolithic body with a homogenous material composition. Alternatively, the motor rotor 18 can be composed of separate rotor bodies being fixed to each other, wherein the drive rotor section 22 and the sensor rotor section 24 are defined by separate rotor bodies. In this case, the sensor rotor section 24 is, preferably, provided as a separate rotor body being attached to a transversal end surface 27 of the drive rotor section 22, for example, by an adhesive bond.

As shown in Figure 2, the drive rotor section 22 is provided with permanently-magnetic drive ring sectors 28. The permanently-magnetic drive ring sectors 28 are alternately magnetized along the circumference of drive rotor section 22. Magnetic north poles are labeled with the letter N and magnetic south poles are labeled with the letter S. The permanently-magnetic drive ring sectors 28 cover substantially the entire circumference of the drive rotor section 22. However, caused by the manufacturing process, there can be insignificant non-magnetic regions between the permanently-magnetic drive ring sectors 28. Since the drive rotor section 22 is axially located within the axial extent A of the motor stator 12, the permanently-magnetic drive ring sectors 28 interact with the magnetic field generated by the electromagnetic motor stator 12 for driving the motor rotor 18. As shown in Figure 3, the sensor rotor section 24 is provided with permanently-magnetic sensor ring sectors 30 and with non-magnetic sensor ring sectors 32, wherein the non-magnetic sensor ring sectors 32 are located circumferentially between the permanently-magnetic sensor ring sectors 30. In the present embodiment of the invention, the non- magnetic sensor ring sectors 32 are realized by axial recesses within the sensor rotor section 24. Alternatively, the non-magnetic sensor ring sectors 32 can be provided by massive material, wherein the material is provided with a magnetization level being below a detection threshold level of a magnetic field sensor 38. Since the sensor rotor section 24 is axially located outside of the axial extent of the motor stator 12, the non- magnetic sensor ring sectors 32 do not significantly affect the drive of the motor rotor 18 by the motor stator 12.

The drive rotor section 22 and the sensor rotor section 24 are provided in that way, that the circumferential extent of each permanently-magnetic drive ring sector 28 of the drive rotor section 22 corresponds to the circumferential extent of a sensor sector pair 34 being composed of one permanently-magnetic sensor ring sector 30 and one non-magnetic sensor ring sector 32. The magnetic polarization of each permanently-magnetic drive ring sector 28 is equal to the magnetic polarization of the corresponding permanently-magnetic sensor ring sector 30. The circumferential start position of each permanently-magnetic drive ring sector 28 is equal to that of the corresponding permanently-magnetic sensor ring sector 30.

The electric motor 10 is also provided with a motor electronics 36 and with the magnetic field sensor 38. The motor electronics 36 is electrically connected with the stator coils 16 for energizing the stator coils with an electric drive current. The motor electronics 36 commutates the electric drive current based on a trigger signal being provided by the magnetic field sensor 38.

The magnetic field sensor 38 is a latching-type Hall sensor and is arranged radially inside of and radially adjacent to the sensor rotor section 24. As a result, the magnetic field sensor 38 detects a sensor magnetic field generated by the permanently-magnetic sensor ring sectors 30 of the sensor rotor section 24. The magnetic field sensor 38 provides a substantially digital trigger signal, wherein the trigger signal has a first signal level if a magnetic north pole N is detected and wherein the trigger signal has a second trigger level if a magnetic south pole S is detected. As a result, the trigger signal can only switch for a transition from a non- magnetic sensor ring sector 32 to a permanently-magnetic sensor ring sector 30 and cannot switch for any transition from a permanently- magnetic sensor ring sector 30 to a non-magnetic sensor ring sector 32. As a result, if the motor rotor 18 is slightly moved back and forth, the trigger signal can only switch once and does not continuously switch between the two trigger signal levels. This avoids a toggling of the motor rotor 18 and, as a result, allows a reliable drive of the motor rotor 18. The magnetic field sensor 38 and the sensor rotor section 24 are provided in that way, that for every magnetic rotor rest position of the motor rotor 18 a permanently-magnetic sensor ring sector 30 is located within the detection range of the magnetic field sensor 38.

Reference list

8 fluid pump

10 electric motor

12 motor stator

14 stator body

16 stator coils

18 motor rotor

20 pump wheel

22 drive rotor section

24 sensor rotor section

26 axial motor rotor end

27 drive rotor section end surface

28 permanently-magnetic drive ring sectors 30 permanently-magnetic sensor ring sectors

32 non-magnetic sensor ring sectors

34 sensor sector pair

36 motor electronics

38 magnetic field sensor

A axial extent

N magnetic north pole

R axis of rotation

S magnetic south pole