METHOD AND CONTROL DEVICE FOR THERMAL MONITORING OF AT LEAST A TWO-PHASE BRUSHLESS MOTOR

Description

[0001] The invention relates to a method for thermal monitoring of a brushless motor with at least two phases according to claim 1 and a control device for controlling a brushless motor with at least two phases according to claim 5.

[0002] Electric drives include synchronous motors, which are either single-phase or three-phase synchronous machines. The rotor has a constant magnetization, which, during operation, is driven synchronously by a rotating magnetic field in the stator. This results in the rotor's rotational movement within the stator. In other words, the stator's three-phase winding, typically consisting of three strands, is controlled by a suitable circuit such that a moving magnetic field is generated by the stator coils. This field pulls the rotor along, causing it to rotate. The rotational movement of the rotor in a running synchronous motor is synchronous with the alternating voltage of the stator windings. The rotor's rotational speed is related to the frequency of the alternating voltage via the number of pole pairs in the stator windings.On the other hand, the rotor's magnetic field can be generated by permanent magnets, a process known as self-excitation. In this case, electrical contact between the stator and rotor is unnecessary. This avoids the aforementioned disadvantages and reduces the overall weight, size, and cost of such permanent magnet synchronous motors. These permanent magnet synchronous motors with switching electronics are therefore commonly referred to as brushless motors (BLDC motors). The stator coils can be controlled via a four-quadrant controller. The electronics for controlling the bridge are a variable frequency drive. Brushless motors are typically three-phase, as this offers the advantage of a defined rotor position with high dynamics, high torque, and high efficiency.

[0003] A characteristic feature of brushless motors is their commutation, which, for example, in a three-phase stator, consists of six blocks or sectors per rotational field cycle, i.e., per motor revolution, each of which differs from the switching state of the bridge circuit. Only two push-pull stages of the bridge are active at any given time, while the third push-pull stage is in the "floating" state. The voltage at this bridge point is defined by the circuit network according to the star equivalent circuit. The bridge control ensures that the motor phase that is currently changing polarity—given a trapezoidal reverse voltage—is always in the "floating" state.

[0004] Since the bridge control switches automatically, the stator field is always in the block with the optimal magnetic flux change, i.e., with the maximum generator voltage. The brushless motor thus spins up until its generator voltage matches the supply voltage. At that point, the maximum speed, or operating speed, is reached and maintained constant. The rotor speed can therefore be controlled or changed by adjusting the supply voltage.

[0005] The commutation is controlled depending on the current rotor position, which can be detected via at least one position sensor, for example as a Hall sensor, but this leads to a corresponding additional effort or cost and requires installation space.

[0006] Alternatively, sensorless commutation can be implemented, eliminating the need for position sensors by indirectly determining the current rotor position through the measurement of electrical parameters at the coils. For example, the electromotive force (EMF) generated in the stator coils can be detected and evaluated by the electronic control circuit, although this is only possible above a certain minimum rotational speed.

[0007] In other words, a brushless motor (BLDC motor) is, from a structural point of view, a rotating field machine that does not require a mechanical commutator. The rotor contains one or more permanent magnets. The interaction between the stator field and the rotor field generates an electric torque that can lead to rotation.

[0008] A motor inverter is typically required to operate a brushless motor. The motor inverter usually includes a controller and power semiconductors for controlling the motor phases. It also typically contains a DC link from which the brushless motor is powered.

[0009] Unlike permanent magnet synchronous machines (PMSM) or permanent magnet synchronous machines (PMAC), in a brushless motor, only two of the three phases are energized at any one time, while the third phase remains de-energized. The selection of which phases are energized depends on the rotor position. The transition from one sector or block to the next is also called "electronic commutation"; hence the terms "brushless" or "electronically commutated." The information about the rotor position required for the commutation of a brushless motor is typically measured by a position sensor such as a Hall sensor, resolver, or incremental encoder.

[0010] Several solutions are known in the art that allow this commutation without a position sensor. The most widespread approach to sensorless control is based on determining the rotor position and commutation times by measuring the induced voltage in the inactive phase. The voltage potentials of the motor phase and the motor star point are compared using voltage dividers, and the zero crossing is taken as the reference time for commutation.

[0011] US 5 473 240 A discloses a method for controlling a brushless permanent magnet motor with stator phase windings with respect to which the motor rotates and in which a back EMF is induced which has a fundamental component and a third harmonic component, wherein the motor is controlled by an inverter bridge operationally connected to it.

[0012] DE 10 2017 121829 A1 discloses a method for operating an electronically commutated multiphase electric motor, in which the commutation times are determined sensorlessly using a comparator from the zero crossings of the counter-electromotive force, wherein the determined commutation times are corrected depending on a hysteresis of the comparator.

[0013] US 2020 / 007056 A1 discloses a motor drive device and motor drive system for improving control stability despite an imbalance in zero-crossing points in a sensorless motor whose rotor position is estimated by an electromotive counterforce.

[0014] EP 2 826 602 A1 discloses an electric tool comprising a sensor unit for detecting the rotational position of a brushless motor, a driver circuit for supplying power to the brushless motor, and a control unit for controlling the driver circuit. If the current position information, based on the sensor's position signal, is determined to be abnormal, the control unit executes an abnormality stop to halt the drive power to the motor. If the brushless motor is selected to be driven after the abnormality stop has been executed, the control unit reactivates the brushless motor by controlling the switching elements of the drive circuit with a duty cycle lower than the duty cycle before the drive power was halted.

[0015] US 2019 / 334465 A1 discloses a motor drive with a limiting circuit arrangement that controls a motor based on a command torque and calculates a limiting rate that limits the command torque. The command torque is limited based on the limiting rate calculated by the limiting circuit arrangement. The motor drive also includes a control device that outputs electrical power to drive the motor based on the limited command torque.

[0016] US 2020 / 382028 A1 discloses a control unit configured to determine a first parameter from the back EMF signal and to determine at least one second parameter corresponding to the first parameter from the back EMF signal. The control unit further verifies, based on the first parameter, at least one condition containing the at least one second parameter and detects standstill or step loss in the stepper motor based on this verification. The present invention uses pattern recognition by identifying the characteristic features of the back EMF signal peaks, namely sequence, magnitude, and polarity. These characteristic features are independent of rotational speed, backlash, and load, do not require complex calculations, and are very robust even at low rotational speeds.

[0017] US 5,327,053 A discloses a device and a process for detecting a rotor position for improved engine starting. The device detects a target equilibrium position of the rotor using a fixed-gain amplifier, a window comparator, and a slope detector circuit. The window comparator circuit qualifies the peaks of the back EMF signal. The slope detector circuit identifies the peak of the back EMF signal qualified by the window comparator.

[0018] With such brushless motors, overheating can occur if the phases are energized by the controlled frequency converter, but the resulting rotation of the rotor is mechanically blocked or at least hindered. The latter can also be due to stiffness in the rotor or the driven mechanical element. Furthermore, incorrect or faulty control of the controlled frequency converter can also lead to the desired rotation of the rotor not being generated at all or only insufficiently.

[0019] In any case, the continued energizing of the phases can lead to heating of the stator coils, exceeding their permissible temperature and thus overheating the brushless motor. This can damage the coils, the controlled frequency converter, and other electronic components, and may even result in the failure of the brushless motor.

[0020] This can be particularly true for brushless motors, which have a fan driven by their rotor. This fan generates an airflow during rotor rotation, thus cooling the brushless motor automatically. When the rotor stops, this automatic air cooling fails, or its effectiveness is significantly reduced if the rotor speed is too low, preventing the desired cooling effect from being achieved.

[0021] Therefore, it is common practice to provide thermal monitoring for such brushless motors and to automatically switch off the brushless motor or at least reduce its power when a predetermined limit temperature is reached, in order to avoid thermal overload due to overheating.

[0022] Such thermal monitoring is typically implemented indirectly by not directly measuring the temperature of the brushless motor itself, but rather by placing a temperature sensor near the brushless motor or its stator coils, which can measure the ambient air temperature. This allows the sensor to detect the temperature of the air surrounding the stator coils of the brushless motor and react to it as described above.

[0023] Such indirect temperature monitoring can typically provide sufficient protection against overheating in brushless motors if a response is required to react to a slow warming of the brushless motor, which has enough time to raise the ambient temperature and thus be detected by the temperature sensor. This can occur at a reduced rotor speed of the brushless motor.

[0024] If, for example, the stator coils of a brushless motor heat up very rapidly due to a standstill of the rotor, this cannot usually be detected quickly enough by the previously described indirect thermal monitoring system to react and prevent damage to the brushless motor. This is because the thermal coupling between the stator coils of the brushless motor and the temperature sensor, which typically occurs via several centimeters of ambient air, is so poor or slow that the resulting time delay is too great to detect the overheating of the brushless motor coils quickly enough to allow for an effective response by switching off the brushless motor.

[0025] It should also be noted that the temperature sensor in the vicinity of the coils of the brushless motor and its electronics incurs additional costs and requires installation space.

[0026] The invention addresses the problem of providing a method for the thermal monitoring of a brushless motor of at least two phases, as described above, so that its overheating can be detected quickly enough to allow for effective action. This should be implemented in a way that is as simple, cost-effective, and / or flexible as possible. In particular, this should be possible without a temperature sensor. At the very least, an alternative to known methods of this kind should be provided. According to the invention, this problem is solved by a method with the features of claim 1 and by a control device with the features of claim 5. Advantageous embodiments and further developments of the invention are described in the following dependent claims.

[0027] Thus, the invention relates to a method for the thermal monitoring of a brushless motor with at least two phases, preferably without a rotor position sensor, comprising at least the following steps: Energizing the motor, wherein the phase voltage of the energizing process is preferably set by means of PWM control, detecting the phase voltage, detecting the neutral point voltage, determining the differential voltage as the difference between the detected phase voltage and the detected neutral point voltage, detecting a first sign change of the detected differential voltage at a first time point, detecting a second sign change of the same direction of the detected differential voltage at a second time point, determining the time difference between the first time point of the first sign change and the second time point of the second sign change, and determining the speed of the brushless motor based on the determined time difference.

[0028] The present invention is based on the understanding that the thermal state, and in particular the risk of overheating, can be inferred from the rotational speed of a brushless motor, thus enabling thermal monitoring. At the very least, the rotational speed information allows conclusions to be drawn about whether the rotor is stationary despite being energized when the speed is zero. Similarly, even without precise knowledge of the rotational speed, and especially without a temperature sensor, it is possible to infer a potential or imminent overheating of the brushless motor and to take appropriate measures, such as reducing the power output and, in particular, stopping the energization.

[0029] The differential voltage is determined taking into account the measured current, which causes a voltage drop across the resistive part of the winding, and the change in current results in an inductive component of the voltage. The corrected differential voltage corresponds approximately to the induced voltage (EMF).

[0030] The rotational speed of the brushless motor can be determined by measuring the differential voltage and identifying its identical zero crossings, i.e., the periodically repeating changes in the sign of the induced voltage from positive to negative or vice versa. The time difference between two consecutive zero crossings, for example from positive to negative, corresponds to the period of movement or rotation of the brushless motor's rotor. The rotational frequency of the brushless motor's rotor, which corresponds to its rotational speed, is inversely proportional to this period.

[0031] The invention also relates to a method for thermal monitoring of a brushless motor with at least two phases, preferably without a rotor position sensor, comprising at least the following steps: Energizing two phases, wherein the phase voltage of the energizing process is preferably set by means of PWM control, detecting the phase voltage, detecting the neutral point voltage, determining the differential voltage as the difference between the detected phase voltage and the detected neutral point voltage, determining at least one extreme value of the determined differential voltage, and determining the speed of the brushless motor based on the determined extreme value.

[0032] Thermal monitoring based on the determined rotational speed of the brushless motor is achieved by identifying at least one extreme value, in the form of a maximum or minimum amplitude, from the time-dependent profile of the measured differential voltage. Since the differential voltage is proportional to the rotational speed of the brushless motor, the motor's rotational speed can also be determined in this way and used for thermal monitoring as previously described. According to one aspect of the invention, the determination of at least two extreme values ​​of the measured differential voltage is carried out, and the method comprises the following further steps: Determining the difference between the two determined extreme values, comparing the determined difference with a predetermined limit value, and if the predetermined limit value is exceeded or reached,

[0033] Reducing or terminating the current to the two phases. In other words, at least two extreme values, which could be, for example, a maximum and a minimum amplitude, or two maximum or two minimum amplitudes, are compared and evaluated to determine whether their difference lies within a predetermined tolerance. If so, the determination of the extreme values ​​of the measured induced voltage can be considered correct, and the speed of the brushless motor can be determined based on this, as described previously. Otherwise, the determination of the extreme values ​​of the measured induced voltage can be considered erroneous, and appropriate action can be taken, for example, by issuing an error message to the user and, if necessary, reducing or even terminating the current to the two phases to prevent the brushless motor from operating.

[0034] According to another aspect of the invention, the method further comprises at least the following steps: Comparing the determined rotational speed with a predetermined rotational speed, and if the rotational speed falls below or reaches the predetermined speed, reducing or stopping the current flow to the two phases.

[0035] In this way, the conclusion drawn from the determined rotational speed regarding a possible thermal overload of the brushless motor, as previously mentioned, can be implemented for each of the two previously described methods according to the invention. According to a further aspect of the invention, the method further comprises at least the following steps: Determining the difference between the two measured rotational speeds, comparing the determined difference with a predetermined limit value, and if the predetermined limit value is exceeded

[0036] Reducing or stopping the current flow to the two phases. In this way, the two previously described methods can be combined to verify each other. The two methods can be executed in parallel and independently. The two results, in the form of the respective measured speeds of the brushless motor, can then be compared, as they must, as previously described, lead to the same result independently. This can be checked within a predetermined tolerance. If an unacceptably large deviation is detected, it can be concluded that one of the two measured speeds is faulty. Appropriate action can then be taken, for example, by issuing an error message to the user and, if necessary, reducing or even stopping the current flow to the two phases as a precautionary measure to shut down the operation of the brushless motor.Otherwise, both determined rotational speeds can be considered correct and one of the two rotational speeds, or a value based on both rotational speeds, for example as an average, can be used to further carry out the procedure as described above.

[0037] According to a further aspect of the invention, the phase voltage is acquired using a filter. For this purpose, a filter with a predetermined, known characteristic can be used. Preferably, the amplitude of the phase voltage can be attenuated more strongly at higher frequencies than at lower frequencies. By taking the frequency into account, the measured phase voltage can be corrected for the attenuation of the filter. This increases the accuracy of the acquired phase voltage, which in turn can improve the accuracy of the resulting subsequent process steps.

[0038] According to a further aspect of the invention, the neutral point voltage is measured using a filter. This allows the previously described aspects of filtered phase voltage measurement to be applied to the neutral point voltage.

[0039] The present invention also relates to a control device for controlling a brushless motor with at least two phases and without a rotor position sensor, preferably according to a method as described above, comprising a control unit configured to energize the motor, wherein the phase voltage of the energizing process is preferably set by means of PWM control, and comprising a scanning device configured to detect the phase voltage and the neutral point voltage, wherein the control unit is further configured to determine the differential voltage as the difference between the detected phase voltage and the detected neutral point voltage, to determine a first sign change of the determined differential voltage at a first time point, and to determine a second sign change of the same direction of the determined differential voltage at a second time point.to determine the time difference between the first time of the first sign change and the second time of the second sign change, and to determine the rotational speed of the brushless motor based on the determined time difference.

[0040] This allows a control device to be provided to implement the corresponding aspects of the invention described above or to implement the previously described method according to the invention in a brushless motor with at least two phases.

[0041] The present invention also relates to a control device for the thermal monitoring of a brushless motor with at least two phases and without a rotor position sensor, according to a method as described above, comprising a control unit configured to energize two phases, wherein the phase voltage of the energizing is preferably set by means of PWM control, and comprising a scanning device configured to detect the phase voltage and the neutral point voltage, wherein the control unit is further configured to determine at least one extreme value of the determined induced voltage and to determine the speed of the brushless motor based on the determined extreme value.

[0042] This allows a control device to be provided to implement the corresponding aspects of the invention described above or to implement the previously described method according to the invention in a brushless motor with at least two phases.

[0043] The present invention also relates to a brushless motor with at least two phases, preferably without a rotor position sensor, with a control device as described above. This allows a brushless motor with at least three phases to be provided in order to implement the aspects of the invention described above or the method described above according to the invention.

[0044] The present invention also relates to a household appliance with at least one at least two-phase brushless motor as described above. This allows a household appliance to be provided for implementing the aspects of the invention described above or for implementing the method described above according to the invention.

[0045] According to one aspect of the invention, the household appliance has an electrical energy storage device, preferably rechargeable, and is designed to be operated by means of the stored electrical energy of the electrical energy storage device.

[0046] This aspect of the invention is based on the understanding that battery-powered household appliances often use relatively low-power brushless motors, which typically have a comparatively low ohmic resistance and a comparatively low inductance, thus favoring the proportionality between induced voltage and speed. Therefore, the corresponding aspects of the invention described above, which are based on or utilize the proportionality between induced voltage and speed, can be successfully applied and utilized, particularly in such brushless motors of battery-powered household appliances.

[0047] The induced voltage is always proportional to the rotational speed. The induced voltage is normally determined by measuring the phase voltage of an unenergized phase and calculating the difference with the neutral point voltage. According to the invention, however, the phase voltage is measured continuously, i.e., both when the phase is energized and when it is unenergized. Therefore, the measured voltage does not exactly correspond to the induced voltage or electromotive force (EMF). The induced voltage can be derived with a comparatively low ohmic resistance and a comparatively low inductance. Alternatively, the measured current can be taken into account when determining the induced voltage. However, this does not yield the exact value of the induced voltage (EMF).

[0048] Without knowledge of the current, it can also happen that a very high current flows through the winding, leading to a voltage difference across the winding, which is also interpreted as a rotating motor. Therefore, the current can be taken into account. If the ratio of current to voltage difference is not within a certain range, the motor will either stop or rotate slowly, and a high current will flow. In this case, the motor can also be switched off according to the invention.

[0049] Several embodiments of the invention are shown schematically in the drawings and are described in more detail below. It shows Figure 1: A circuit diagram of a known brushless motor; Figure 2: Phase voltage waveforms of the three phases of the brushless motor. Figure 1Figure 3 shows a circuit diagram of a brushless motor according to the invention; Figure 4 shows a diagram of the phase voltage versus the speed of a brushless motor; Figure 5 shows the curves of induced voltage, phase voltage and neutral point voltage of a brushless motor over time; and Figure 6 shows a flowchart of several methods according to the invention.

[0050] Figure 1 Figure 1 shows a circuit diagram of a known brushless motor M. The brushless motor M, or BLDC motor M for short, is controlled and operated by a control unit S in the form of a regulated frequency converter S. A three-phase rotating magnetic field is applied to the BLDC motor M from a DC voltage VDC as the intermediate circuit voltage VDC via a B6 bridge with six transistors T1-T6 and six diodes D1-D6. This causes a rotor (not shown) of the BLDC motor M to rotate during operation.

[0051] To ensure that the rotating magnetic field applied to the stator (not shown) of the BLDC motor M is aligned with the rotor position, the rotor position must be determined. Previously, this was done using a sensor on the BLDC motor M. With the advent of sensorless control, the sensor was replaced by measuring electrical quantities in combination with a software motor model provided by the control unit S. For this purpose, the phase voltages Uphase of the three phases U, V, and W, as well as the neutral point voltage, were measured. The neutral point voltage can either be measured at the neutral point of the BLDC motor M or simulated and measured using an "artificial neutral point." The circuit consisting of resistors R8-R10 and capacitor C5 represents the artificial neutral point. For a BLDC motor M with a delta connection, only the artificial neutral point is typically used.

[0052] Figure 2shows the phase voltage curves Uphase of the three phases U, V, W of the brushless motor M. Figure 1 As can be seen from the time course of the phase voltage U, phase U is actively subjected to a positive voltage for 120° (two sectors). For one sector, phase U is without current and voltage. During this time, the induced voltage ΔU of phase U can be measured, which is also referred to as electromotive force (EMF).

[0053] The zero line for the phase voltage Uphase is the neutral point voltage. Where the phase voltage Uphase crosses the neutral point voltage, a so-called zero crossing of the respective phase voltage Uphase occurs. This zero crossing point of the phase voltage Uphase is the moment when the stator and rotor are aligned. Measurements are taken at these zero crossings. Thus, in phase U of a single-pole BLDC motor M, a zero crossing can be measured twice during one mechanical revolution (0° to 360°). If the current is measured in all three phases U, V, W, the rotor position can be determined six times during one mechanical revolution. All control units S of conventional BLDC motors M are based on this principle.

[0054] BLDC motors of this type are typically monitored for excessive heating or overheating; that is, thermal protection is usually in place. If the BLDC motor is blocked, for example, but still energized, this can lead to very high currents in the stator coils and thus to overheating of the BLDC motor. When the BLDC motor rotates, a voltage is induced in the stator windings. This induced voltage limits the current in the stator winding. If the BLDC motor is stationary or rotating very slowly, for example, because it is very stiff, and is still energized, this can lead to very high currents and thus to overheating of the stator winding.

[0055] Therefore, knowing whether the rotor of the BLDC motor M is rotating, stationary, or rotating very slowly is very helpful for thermal protection. If the BLDC motor M does not start up despite being controlled or energized to phases U, V, and W, it should be switched off after a short time to prevent overheating. Thus, it is advantageous to detect when the BLDC motor M is stationary.

[0056] To detect an unintentional standstill of the BLDC motor M, one or more methods as described below can be carried out using safety-related software, in which the filtered neutral point voltage via the capacitor C6 and via the ohmic resistor R12 and a filtered phase voltage U Phase via the capacitor C7 and via the ohmic resistor R13 of the BLDC motor M are detected by the control unit S.

[0057] The neutral point voltage and the phase voltage Uphase are filtered to filter the PWM control. The filters used for neutral point and phase voltage measurement have a specific, known characteristic. The voltage amplitude is attenuated more strongly at higher frequencies than at lower frequencies. By measuring the frequency, the measured voltage can therefore be corrected for the filter attenuation. This further increases the accuracy of the methods according to the invention, which are described below.

[0058] How to determine the equivalent circuit diagram or circuit of a known star-connected BLDC motor M Figure 1The difference in voltages is the phase voltage of the BLDC motor M. Neglecting the voltage drop across the resistor and inductor, the measured voltage difference approximately corresponds to the induced EMF voltage of the BLDC motor M. The induced EMF voltage is proportional to the rotational speed of the BLDC motor M, which is why, from this proportional relationship, see Figure 4 , the rotational speed of the BLDC motor M can be determined. Both the ohmic resistance and the inductance of the BLDC motor M should be relatively low so that the method can be applied as reliably and accurately as possible. This is usually the case with compact BLDC motors M, which are used in cordless vacuum cleaners.

[0059] Based on this, a method can be carried out according to the invention in which, firstly, an energizing 100 of two phases U, V, W, a detection 200 of the phase voltage Uphase, a detection 250 of the neutral point voltage, a determination 300 of the differential voltage ΔU as the difference between the detected phase voltage Uphase and the detected neutral point voltage, a detection 400a of a first sign change of the determined differential voltage ΔU at a first time point, a detection 450a of a second sign change of the same direction of the determined differential voltage ΔU at a second time point, a determination 500a of the time difference between the first time point of the first sign change and the second time point of the second sign change, and a determination 550a of the rotational speed of the brushless motor M based on the determined time difference are carried out.

[0060] In this way, the frequency of the differential voltage ΔU can be measured by triggering a comparator upon a sign change when comparing the filtered phase voltage Uphase and the filtered neutral voltage. Triggering occurs only on either the positive or negative edge, thus initiating an interrupt. A coupled counter records the number of clock cycles until the next interrupt occurs and thereby determines the frequency of the differential voltage ΔU. The rotational speed measurement is also possible. Figure 5 Presented in an exemplary (idealized) manner.

[0061] Furthermore, according to the invention, a method can be carried out by energizing 100 the motor M, detecting 200 the phase voltage U Phase, detecting 250 the star point voltage, determining 300 the differential voltage ΔU as the difference between the detected phase voltage U Phase and the detected star point voltage, determining 400b at least one extreme value of the determined differential voltage ΔU and determining 450b the speed of the brushless motor M based on the determined extreme value.

[0062] In this way, a voltage measurement can be performed by appropriately sampling the differential voltage, thereby also recording the course of the differential voltage ΔU from the control unit S and determining both the maximum and minimum values ​​in one period. The differential voltage ΔU is proportional to the rotational speed, and thus the rotational speed can be determined using one or both values.

[0063] This second method can be extended to include determining 400b for at least two extreme values ​​of the determined differential voltage ΔU, determining 500b the difference between the two determined extreme values, comparing 550b the determined difference with a predetermined limit value, and, if the predetermined limit value is exceeded or reached, triggering 580b a reaction, preferably reducing 580b or terminating 580b the current 100 of the two phases U, V, W.

[0064] In other words, the determined maximum and minimum values ​​of the measured differential voltage ΔU during one period must be nearly identical in magnitude. If the difference between these two values ​​exceeds a certain limit, the voltage measurement is incorrect, or an error has occurred. This allows the second method to be validated in itself, thus preventing the determination of an incorrect rotational speed using the second method. This can increase the safety and reliability of this second method.

[0065] Both previously described methods according to the invention can also be carried out in parallel and their determined rotational speeds can be validated against each other by determining the difference between the two determined rotational speeds, comparing the determined difference with a predetermined limit value, and, if the predetermined limit value is exceeded, triggering a reaction, preferably a reduction or termination of the current.

[0066] This involves a combination of the frequency measurement from the first method and the voltage measurement of the measured voltage difference from the second method. As described previously in the first method, the rotational speed is determined from the frequency measurement of the calculated differential voltage ΔU. Simultaneously, the rotational speed is also determined from the amplitude of the calculated differential voltage ΔU, as described previously in the second method. Both calculated rotational speeds must be nearly identical. If the deviation of the values ​​exceeds a certain limit, an error exists in the measurement data acquisition and / or evaluation. In this case, reliable temperature monitoring of the BLDC motor M based on its calculated rotational speed is not possible.

[0067] The advantages of the methods according to the invention, as well as the corresponding control devices, lie in the fact that the rotational speed of the BLDC motor M can be determined and independently verified by measuring two filtered electrical quantities. From an approval perspective, this verification is essential when safety-related software is used for the thermal protection of the BLDC motor M. The rotational speed measurement allows the BLDC motor M to be detected and shut down if it comes to a standstill. Furthermore, speed-dependent current limits can be set in the software, which can also be advantageous for safeguarding the BLDC motor M. Reference numeral list (part of the description)

[0068] C1-C7 Capacitors D1-D6 B6 bridge diodes T1-T6 B6 bridge transistors R1-R13 Ohmic resistors V DC intermediate circuit voltage U Phase V Phase W Phase U Phase Phase voltage ΔU Induced voltage; electromotive force M (three-phase) brushless motor; BLDC motor S Control unit; computer; regulated frequency converter 100 Energize 200 Determine phase voltage U 250 Determine neutral point voltage 300 Determine differential voltage ΔU 400a Determine a first sign change at a first time 400b Determine at least one extreme value 450a Determine a second sign change at a second time 450b Determine rotational speed based on the determined extreme value 500a Determine the time difference between the first time of the first sign change and the second time of the second sign change 500b Determine the difference between the two determined extreme values ​​550a Determine the rotational speed based on the determined time difference 550b Compare the determined difference with a predetermined limit value 580b Trigger a reaction 600c Compare the determined rotational speed with a predetermined rotational speed 600d Determine the difference between the two determined rotational speeds 650d Compare the determined difference with a predetermined limit value700c Reducing or stopping the current 100 700d Triggering a reaction

Claims

1. Method for thermally monitoring an at least two-phase brushless motor (M) without a rotor position sensor, the method comprising at least the following steps: • energising (100) the motor (M), wherein the phase voltage (UPhase) of the energisation (100) is preferably adjusted by means of PWM control, • detecting (200) the phase voltage (UPhase), • detecting (250) the star point voltage, • determining (300) the differential voltage (ΔU) between the detected phase voltage (UPhase) and the detected star point voltage, • determining (400a) a first sign change of the determined differential voltage (ΔU) at a first point in time, • determining (450a) a second sign change in the same direction of the determined differential voltage (ΔU) at a second point in time, • determining (500a) the time difference between the first point in time of the first sign change and the second point in time of the second sign change, • determining (550a) the rotational speed of the brushless motor (M) based on the determined time difference, characterised by • determining (400b) at least one extreme value of the determined differential voltage (ΔU), • determining (450b) the rotational speed of the brushless motor (M) based on the determined extreme value, • comparing (600c) the determined rotational speed in each case with a predetermined rotational speed, reducing (700c) or terminating (700c) the energisation (100) of the motor (M) when the predetermined rotational speed is not met or is reached, respectively, determining (600d) the difference between the two determined rotational speeds, comparing (650d) the determined difference with a predetermined threshold value, and triggering (700d) a reduction (700d) or termination (700d) of the energisation (100) of the motor (M) when the predetermined threshold value is exceeded.

2. Method according to claim 1, characterised in that the determination (400b) is carried out for at least two extreme values of the determined differential voltage (ΔU) and comprising the further steps: • determining (500b) the difference between the two determined extreme values, • comparing (550b) the determined difference with a predetermined threshold value, and • triggering (580b) a reaction, preferably reducing (580b) or terminating (580b) the energisation (100) of the motor (M), when the predetermined threshold value is exceeded or reached.

3. Method according to either of the preceding claims, characterised in that the detection (200) of the phase voltage (UPhase) is filtered.

4. Method according to any of the preceding claims, characterised in that the detection (250) of the star point voltage is filtered.

5. Control apparatus for thermally monitoring an at least two-phase brushless motor (M) without a rotor position sensor, comprising a control unit (S) which is designed to energise the motor (M), wherein the phase voltage (UPhase) of the energisation (100) is preferably adjusted by means of PWM control, and comprising a scanning device which is designed to detect the phase voltage (UPhase) and the star point voltage, wherein the control unit (S) is further designed to determine the differential voltage (ΔU) as the difference between the detected phase voltage (UPhase) and the detected star point voltage, to determine a first sign change of the determined voltage (ΔU) at a first point in time, to determine a second sign change in the same direction of the determined voltage (ΔU) at a second point in time, to determine the time difference between the first point in time of the first sign change and the second point in time of the second sign change, and to determine the rotational speed of the brushless motor (M) based on the determined time difference, to determine at least one extreme value of the determined differential voltage (ΔU), to determine the rotational speed of the brushless motor (M) based on the determined time difference, characterised in that the control unit (S) is further designed to determine at least one extreme value of the determined differential voltage (ΔU), to determine the rotational speed of the brushless motor (M) based on the determined extreme value, to compare (600c) the determined rotational speed with a predetermined rotational speed in each case, to reduce (700c) or terminate (700c) the energisation (100) of the motor (M) when the predetermined rotational speed is not met or is reached, respectively, to determine (600d) the difference between the two determined rotational speeds, to compare (650d) the determined difference with a predetermined threshold value, and to trigger a reduction (700d) or termination (700d) of the energisation (100) of the motor (M) when the predetermined threshold value is exceeded.

6. Control apparatus according to claim 5, wherein the control unit (S) is further designed to carry out the determination (400b) for at least two extreme values of the determined differential voltage (ΔU), to determine (500b) the difference between the two determined extreme values, to compare (550b) the determined difference with a predetermined threshold value, and to trigger a reduction (700d) or termination (700d) of the energisation (100) of the motor (M) when the predetermined threshold value is exceeded or reached.

7. Brushless motor (M), which is at least three-phase, without a rotor position sensor, comprising a control apparatus according to claim 5 or 6.

8. Domestic appliance comprising at least one at least two-phase brushless motor (M) according to claim 7.

9. Domestic appliance according to claim 8, characterised in that the domestic appliance comprises a preferably rechargeable electrical energy store and is designed to be operated by means of the stored electrical energy of the electrical energy store.