METHOD FOR THERMAL MONITORING OF A BRUSHLESS MOTOR

Description

[0001] The invention relates to a method for thermal monitoring of at least a two-phase brushless motor according to claim 1.

[0002] Electric drives include synchronous motors as single-phase or three-phase synchronous machines in motor operation. The rotor or rotor has a constant magnetization, which is synchronously driven by a moving magnetic rotating field in the stator during operation, resulting in the rotary or rotational movement of the rotor within the stator or stator. In other words, the three-phase winding of the stator, usually with three phases, is controlled by a suitable circuit in such a way that a moving magnetic field is generated by the stator coils, which pulls the rotor along and thereby sets it in rotation. The rotary or rotational movement of the rotor of the running synchronous motor is synchronous with the alternating voltage of the stator windings. The speed of the rotor's rotational movement is linked to the frequency of the alternating voltage via the number of pole pairs in the stator windings.

[0003] Alternatively, 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 can avoid the aforementioned disadvantages and overall keep the weight, installation space, and cost of such permanent-magnet synchronous motors to a minimum. Such permanent-magnet synchronous motors with switching electronics are therefore commonly referred to as brushless motors (brushless direct current motors - BLDC motors for short). The stator coil strands can be controlled via a four-quadrant controller. The electronics for controlling the bridge are a regulated frequency converter. Brushless motors are typically designed as three-phase motors, as they then offer the advantage of a defined rotor position with high dynamics, high torque, and high efficiency.

[0004] A characteristic of brushless motors is their commutation, which, for example, in a three-phase stator, consists of six blocks or sectors per rotating 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 one time, and 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—in the case of a trapezoidal counter voltage—is currently changing polarity is always "floating."

[0005] Because the bridge controller automatically advances, 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 accelerates until its generator voltage equals the supply voltage. The maximum speed, or operating speed, is then reached and maintained constant. The rotor speed can thus be controlled or changed by changing the supply voltage.

[0006] The commutation is controlled depending on the current rotor position, which can be detected by at least one position sensor, for example a Hall sensor, which, however, leads to a corresponding additional effort or costs and requires installation space.

[0007] Alternatively, sensorless commutation can be implemented, eliminating the need for position sensors, by indirectly determining the current rotor position by measuring electrical parameters at the coils. For example, the EMF (electromotive force) induced in the stator coils can be recorded and evaluated by the electronic control circuit, although this can only occur above a certain minimum speed.

[0008] In other words, a brushless motor (BLDC motor) is, from a structural perspective, 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 creates an electrical moment that can lead to rotation.

[0009] A motor inverter is typically required to operate a brushless motor. The motor inverter typically includes a controller and power semiconductors for controlling the motor phases. There is also usually an intermediate circuit from which the brushless motor is powered.

[0010] Unlike motors known as permanent magnet synchronous motors (PMSM) or PMACs, in a brushless motor, for example, only two of three phases are energized simultaneously, while the third phase is de-energized. The selection of the phases to be energized depends on the rotor position. The transition from one sector or block to the next is also called "electronic commutation"; hence the term "brushless" or "electronically commutated." The information about the rotor position required for the commutation of a brushless motor is usually measured using a position sensor such as a Hall sensor, resolver, or incremental encoder.

[0011] Various solutions are known from the state of the art that allow this commutation without a position sensor. The most common 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 transition is used as the reference time for commutation.

[0012] For brushless motors where the star point is not physically connected (e.g., in a delta connection), a star point can be artificially simulated using three resistors connected in star. The voltage potential of the artificial star point is then compared with the voltage potential of the inactive phase, for example, via a comparator. The zero crossing of this difference is used as the reference time for commutation.

[0013] Such brushless motors can overheat or overheat if the phases are energized by the controlled frequency converter as described above, but the resulting rotational movement of the rotor is completely blocked or at least impeded mechanically. This can also be due to stiffness of the rotor or the mechanical element being driven. Furthermore, incorrect or faulty control of the controlled frequency converter can lead to the desired rotational movement of the rotor not being generated at all or only insufficiently.

[0014] In any case, applying current to the phases can cause the stator coils to heat up, exceeding their permissible temperature and thus overheating the brushless motor. This can lead to damage to the coils, the controlled frequency converter, and other electronic components, and possibly even to the failure of the brushless motor.

[0015] This can be particularly true for brushless motors, which have a fan driven by their rotor, which generates an airflow during the rotor's rotation, thus cooling the brushless motor itself. When the rotor is at a standstill, its own air cooling system fails, or if the rotor speed is too low, its effectiveness is reduced accordingly, so that the desired air cooling effect cannot be achieved.

[0016] It is therefore common practice to provide thermal monitoring for such brushless motors and, when a predetermined limit temperature is reached, to automatically switch off the brushless motor or at least reduce its power in order to avoid thermal overload due to overheating.

[0017] Such thermal monitoring is typically implemented indirectly, by not measuring the temperature of the brushless motor itself, but by installing a temperature sensor near the brushless motor or its stator coils that can measure the ambient air temperature. Thus, the temperature of the air surrounding the coils of the brushless motor's stator is measured by a sensor and reacts to this temperature as described above.

[0018] Such indirect temperature monitoring can typically provide sufficient protection against overheating or overheating in such brushless motors if the system is to react to a slow warm-up of the brushless motor, which has sufficient time to warm up the ambient temperature and thus be detected by the temperature sensor. This can be the case with a reduced speed of the brushless motor's rotor.

[0019] If the stator coils of the brushless motor heat up very quickly, for example, due to a standstill of the rotor of the brushless motor, this cannot usually be detected quickly enough using the indirect thermal monitoring described above to react to it 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 usually occurs over several centimeters of ambient air, is so poor or slow that the resulting time delay is too long to detect the overheating of the coils of the brushless motor quickly enough to effectively respond to it by shutting down the brushless motor.

[0020] It should also be noted that the temperature sensor in the vicinity of the coils of the brushless motor and its electronics causes additional costs and requires installation space.

[0021] US2006197480A1 discloses a control circuit for a brushless electric motor without a rotor position sensor, comprising a circuit for detecting the motor star point voltage and a comparator belonging to a rotor position detector, which compares the star point voltage with a reference voltage.

[0022] The invention addresses the problem of providing a method for thermally monitoring an at least three-phase brushless motor of the type described above, such that overheating can be detected sufficiently quickly to enable effective response. This should be implemented as simply, cost-effectively, and flexibly as possible. In particular, it should be possible to do this without a temperature sensor. At the very least, an alternative to known methods of this kind should be created.

[0023] According to the invention, this problem is solved by a method having the features of patent claim 1. Advantageous embodiments and further developments of the invention emerge from the following subclaims.

[0024] Thus, the invention relates to a method for thermal monitoring of at least two-, preferably three-phase brushless motor, preferably without rotor position sensor, comprising at least the following steps: Energizing the motor, wherein the phase voltage of the energization is preferably adjusted by means of PWM control, detecting the star point voltage and comparing the detected star point voltage with a predetermined limit value.

[0025] According to the invention, the neutral point voltage of the brushless motor is determined during operation and compared with a limit value. Subsequent operation can then be determined depending on the comparison, with the brushless motor continuing or stopping its operation to prevent overheating.

[0026] The invention is based on the finding that during normal operation of a brushless motor with a rotating rotor, the star-point voltage is always significantly lower than that of a powered brushless motor with a stationary rotor. In this case, the star-point voltage fluctuates between zero potential and the full magnitude of the intermediate circuit voltage, whereas the star-point voltage of a powered brushless motor with a rotating rotor remains significantly below the intermediate circuit voltage.

[0027] If, according to the invention, the star-point voltage is detected and compared with a limit value that lies below the intermediate circuit voltage and can enable the previously described differentiation, then, based on the detected star-point voltage, a distinction can be made at least between the states of the energized brushless motor with a rotating rotor and the energized brushless motor with a stationary rotor. This can be used for thermal monitoring of the energized brushless motor by terminating the energization if a stationary rotor is detected despite the brushless motor being energized. This can protect the coils of the stator of the brushless motor from overheating.

[0028] The procedure therefore comprises the following further steps: If the star point voltage is below the predetermined limit during the comparison, continue energizing, or if the star point voltage is at or above the predetermined limit during the comparison, detect a fault condition.

[0029] In this way, the corresponding aspects of the invention described above can be specifically implemented and utilized. Continuing the current supply can alternatively be supplemented or replaced by detecting a normal state.

[0030] If a fault condition is detected that may indicate a motor standstill despite energization, the response can be to immediately stop the energization. Alternatively, a grace period can be allowed, for example, to allow the motor to resume or resume normal operation. Since overheating does not usually occur immediately, this may be acceptable.

[0031] According to the invention, a reaction to the comparison only occurs if the result of the comparison is present over a predetermined period of time or a predetermined number of times. If the phases are energized in a pulsed manner, for example, using pulse width modulation (PWM), this also includes considering the immediately consecutive PWM pulses over the predetermined period of time, i.e., without taking into account the intervening periods in which no pulses are present.

[0032] In this way, it is possible to prevent the single occurrence of reaching or exceeding the predetermined limit from leading to the termination of the brushless motor's power supply, since such a single occurrence could be caused, for example, by measurement errors. Rather, the comparison over the predetermined period of time must consistently produce the same result, or a certain number of exceedances of the limit must occur to trigger the termination of the brushless motor's power supply. This prevents the brushless motor from being terminated unjustifiably. This can increase the reliability and robustness of the method according to the invention.

[0033] According to a further aspect of the invention, after the detection and before the comparison, the further step is carried out: Taking into account the detected exceedances of the limit value by the star point voltage over a predetermined period, where the comparison is based on consideration.

[0034] In this way, the properties and advantages described above can alternatively be implemented by taking them into account over the predetermined period of time in order to also exclude the possibility that a one-time event of a measurement error could trigger a termination of the power supply to the brushless motor.

[0035] For example, it can be determined that a certain maximum number of limit value violations occurs, or that a certain number of limit value violations occur per unit of time before an error condition is detected.

[0036] According to a further aspect of the invention, the comparison is performed only after a predetermined period of time, which is longer than the start-up period of the brushless motor. This further prevents the inventive method, as described above, from incorrectly classifying an intentional start-up or startup of the brushless motor as an unintentional standstill and counteracting this.

[0037] According to a further aspect of the invention, the predetermined limit value is a percentage of the intermediate circuit voltage. This aspect of the invention is based on the finding that, in a powered brushless motor with a stationary rotor, the starpoint voltage fluctuates between zero potential and the full level of the intermediate circuit voltage. Thus, if the predetermined limit value is selected as a percentage of the intermediate circuit voltage, a limit value can be easily predetermined, which can be used for differentiation as described above. The limit value is preferably between 70 and 90%, most preferably approximately 80% of the full level of the intermediate circuit voltage.

[0038] The following embodiments of a control device, a brushless motor and a household appliance are not in accordance with the invention and are for illustrative purposes only.

[0039] A control device for thermally monitoring an at least two-, preferably three-phase brushless motor, preferably without a rotor position sensor, according to a method as described above, is described, comprising a control unit which is designed to energize the motor, wherein the phase voltage of the energization is preferably set by means of PWM control, and comprising a sampling device which is designed to detect the star point voltage of the energized phases, wherein the control unit is further designed to compare the detected star point voltage with a predetermined limit value.

[0040] In this way, a control device can be provided in order to implement the corresponding aspects of the invention described above or in order to implement the method according to the invention described above in an at least two-phase brushless motor.

[0041] The invention relates to an at least two-phase brushless motor, preferably without a rotor position sensor, with a control device as described above. This allows an at least two-phase brushless motor to be provided in order to implement the above-described aspects of the invention or the above-described method according to the invention.

[0042] A household appliance with at least one at least two-phase brushless motor as described above is described. This allows a household appliance to be provided for implementing the above-described aspects of the invention or for implementing the above-described method according to the invention.

[0043] The household appliance may have a preferably rechargeable electrical energy storage device and be designed to be operated using the stored electrical energy of the electrical energy storage device. This allows the present invention to be implemented and used particularly in such household appliances.

[0044] An embodiment of the invention is shown purely schematically in the drawings and is described in more detail below. It shows Figure 1 shows a circuit of a known brushless motor; Figure 2 shows the phase voltages of the three phases of the brushless motor of the Figure 1 ; Figure 3 a diagram of the star point voltage of a powered brushless motor with a rotating rotor over time; Figure 4 a diagram of the star point voltage of a powered brushless motor with a stationary rotor over time; Figure 5 the diagram of the Figure 3with a predetermined limit value; Figure 6 shows a circuit of a brushless motor; and Figure 7 shows a flowchart of a method according to the invention.

[0045] Figure 1 shows a circuit of a known brushless motor M. The brushless motor M, or BLDC motor M for short, is controlled or operated via a control unit S in the form of a regulated frequency converter S. A three-phase rotating field is applied to the BLDC motor M from a direct voltage V DC as the intermediate circuit voltage V DC via a B6 bridge with six transistors T1-T6 and six diodes D1-D6. This causes a rotation of a rotor (not shown) of the BLDC motor M during operation.

[0046] In order for the rotating field applied to the stator (not shown) of the BLDC motor M to match the rotor position, the rotor position must be determined. In the past, this was done using a sensor on the BLDC motor M. With the advent of sensorless control, the sensor was replaced by the measurement of electrical quantities in combination with a software motor model by the control unit S. For this purpose, the phase voltages U Phase of the three phases U, V, and W, as well as the star point voltage, were measured. The star point voltage can either be measured at the star point of the BLDC motor M or simulated and measured using an "artificial star point." The circuit of the ohmic resistors R8-R10 and the capacitor C5 represents the artificial star point. In a BLDC motor M with a delta connection, only the artificial star point is usually used.

[0047] Figure 2shows the waveforms of the phase voltages U Phase of the three phases U, V, W of the brushless motor M of the Figure 1 As can be seen from the time course of the phase voltage U phase of phase U, phase U is actively subjected to a positive voltage for 120° (two sectors). For one sector, phase U is de-energized and "floating." During this time, the induced voltage (EMF) of phase U can be measured, which is also referred to as the electromotive force (EMF).

[0048] The zero line for the phase voltage U Phase is the star point voltage. Where the phase voltage U Phase crosses the star point voltage, there is a so-called zero crossing of the respective phase voltage U Phase. This point in time of the zero crossing of the phase voltage U Phase is the moment when the stator and rotor are aligned with each other. Measurements are made at these zero crossings. This means that 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.

[0049] Such BLDC motors M are usually monitored for excessive heating or overheating, i.e., thermal protection of the BLDC motor M is usually provided. If, for example, the BLDC motor M is blocked and is still energized, this can lead to very high currents in the stator coils and thus to overheating of the BLDC motor M. When the BLDC motor M rotates, a voltage is induced in the stator windings, which can be referred to as induced voltage (EMF). This induced voltage (EMF) ensures that the current in the stator winding is limited. If the BLDC motor M M because the BLDC motor M, for example, is very stiff and is still energized, this can lead to very high currents and thus to overheating of the stator winding.

[0050] Thus, the information about whether the rotor of the BLDC motor M is rotating, stationary, or rotating very slowly is very useful for thermal protection. If the BLDC motor M does not start up despite the control or energization of phases U, V, and W, the BLDC motor M should be shut down after a short time to prevent it from overheating. Therefore, it is advantageous to detect a standstill of the BLDC motor M.

[0051] According to the invention, to prevent thermal overload of a BLDC motor M, a method as in Figure 7shown in such a way that energization 100 of two phases U, V, W takes place, wherein the phase voltage U Phase of the energization 100 is set by means of PWM control. Furthermore, the star point voltage of the energized phases U, V, W is detected 200 and the detected star point voltage is taken into account 250 over a predetermined period of time in order to compensate for measurement fluctuations or measurement errors. The detected star point voltage is compared 300 with a predetermined limit value, which is a percentage value of the intermediate circuit voltage V DC. For example, the focus can be on a certain maximum number of limit value violations occurring, or on a certain number of limit value violations occurring per unit of time, before an error condition is detected.

[0052] If it is determined during comparison 300 that the star point voltage is below the predetermined limit value according to the criteria mentioned above, then the energization 100 is continued 400, since this can be used to conclude that the rotor of the BLDC motor M is actually rotating. However, if it is determined during comparison 300 that the star point voltage is at or above the predetermined limit value, i.e., reaches or exceeds the predetermined limit value, then an error state is detected, and one reaction to this can be to terminate 500 the energization 100 of the two phases U, V, W, since this can be used to conclude that the rotor of the BLDC motor M is stationary despite the energization. Since in this case the continued energization 100 can lead to overheating of the coils of the stator of the BLDC motor M, the energization 100 is terminated, thereby preventing thermal overload of the BLDC motor M.According to the invention, thermal overload protection or thermal monitoring of the BLDC motor M can be implemented in this way.

[0053] The invention is based on the finding that in a BLDC motor M the star point voltage as in the Figure 3 shown when the BLDC motor M is energized and its rotor is rotating as intended. In this case, the star point can only be potential-wise between GND and the intermediate circuit voltage V DC or the bus voltage (voltage at capacitor C1, see for example Figure 1 ). Since the phases are controlled by a clocked signal, the potential of the star point also changes with the clock frequency. For example, if the BLDC motor M is controlled at 16 kHz, the 16 kHz can also be seen at the star point.

[0054] When the BLDC motor M rotates, the height of the pulses at the star point varies with the rotor position of the BLDC motor M. The width of the pulses varies with the current level in the phase. However, a characteristic of a rotating BLDC motor M is that the pulses never reach the intermediate circuit voltage V DC, but always remain some distance below it.

[0055] In contrast, with a stationary BLDC motor M the star point voltage oscillates between the bus voltage and the GND, see Figure 4 The reason for the different curves is the EMF, or rather the induced voltage EMF, of the BLDC motor M. When the BLDC motor M is stationary, the EMF is zero; when the BLDC motor M is rotating, the EMF is always present. The EMF changes depending on the speed.

[0056] According to the invention, these two signal forms can be differentiated from each other at the star point and the BLDC motor M can be switched off or continued to operate accordingly. This can be easily achieved by integrating a boundary line into the bus voltage curve, see the dashed line of the Figure 5 If the voltage pulses at the star point are permanently below the limit line, the BLDC motor M rotates. If the pulses at the star point are permanently / a certain number of times above the limit line, the BLDC motor M stops despite current being supplied.

[0057] The decision whether the BLDC motor M or its rotor is rotating or stationary is not made depending on one or ten voltage pulses, but preferably on a large number of voltage pulses in order to avoid a reaction to measurement errors.

[0058] The evaluation of the voltage pulses can be realized via a comparator, see Figure 6The mentioned starpoint voltage pulses are applied to the comparator. The limit value is the percentage value of the bus voltage and is always relative to the bus voltage. The limit value can be set as shown in the Figure 6 shown, are formed using a voltage divider. The comparator compares the two quantities.

[0059] The timer behind it ensures that the decision is not made dependent on a voltage pulse, but the timer averages over a fixed time.

[0060] The timer also functions to suppress the start-up of a BLDC motor M. Every motor start-up begins with a stationary rotor. The short start-up period must be suppressed and must not cause the BLDC motor M to shut down.

[0061] Thus, the timer has a dual function: to suppress start-up and to delay the detection of whether the BLDC motor M is stationary or rotating. List of reference symbols (part of the description)

[0062] C1-C7 Capacitors D1-D6 Diodes of the B6 bridge T1-T6 Transistors of the B6 bridge R1-R13 Ohmic resistors GND Ground potential; zero potential V DC Intermediate circuit voltage U Phase V Phase W Phase U Phase Phase voltage EMI Induced voltage; electromotive force M (three-phase) brushless motor; BLDC motor S Control unit; computer; controlled frequency converter 100Energize two phases U, V, W 200Detect the starpoint voltage of the energized phases U, V, W 250Average the detected starpoint voltage over a predetermined period 300Compare the detected starpoint voltage with a predetermined limit value 400Continue energizing 100 500End energizing 100

Claims

1. Method for thermally monitoring an at least two-phase brushless motor (M), preferably without a rotor position sensor, the method comprising at least the steps of: • 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 star point voltage, • comparing (300) the detected star point voltage with a predetermined threshold value, and • continuing (400) the energisation (100) if, during comparison (300), the star point voltage is below the predetermined threshold value, characterised in that, • an error condition is detected if, during the comparison (300), the star point voltage is at or above the predetermined threshold value, and a response to the comparison (300) occurs only if the result of the comparison (300) is present over a predetermined period of time or a predetermined number of times.

2. Method according to claim 1, characterised by the further step, after detecting (200) and before comparing (300), of: considering (250) the detected instances of the star point voltage exceeding the threshold value over a predetermined period of time, the comparison (300) being based on the consideration.

3. Method according to any of the preceding claims, characterised in that the comparison (300) takes place only after a predetermined period of time, which is longer than the start-up period of the brushless motor (M).

4. Method according to any of the preceding claims, characterised in that the predetermined threshold value is a percentage value less than 100%, preferably between 70 and 90%, most preferably approximately 80%, of the intermediate circuit voltage (VDC).

5. Method according to any of the preceding claims, characterised in that, when an error condition is detected, the energisation is terminated (500).