DE102012006491A1 - Method for detecting position of rotor of electric motor e.g. brushless direct current motor, involves providing electrical isolation unit to temporarily, partially and electrically insulate motor phase from common point - Google Patents

Abstract

The method involves using counter electromagnetic force voltage partially at motor phase of electric motor (5) by scanning the counter-electromagnetic force voltages. A crossing of particular voltage level of counter-electrode magnetic force voltage is detected in which phases of electric motor are connected to each other by common point. An electrical isolation unit is provided to temporarily, partially and electrically insulate the phase from common point. An independent claim is included for electric circuit device.

Description

The invention relates to a method for detecting the position of a rotor of an electric motor. The invention also relates to an electrical circuit capable of detecting the position.

To a brushless DC motor 11 (BLDCM), it is necessary to know the position of the rotor 5 the machine 1 to know. The position of the rotor 5 can be found using sensors. This is not practical in a compressor application because of a very harsh environment and also because of the price. Another way is sensorless access. In the compressor industry, the most commonly used sensorless positioning method is the "direct BEMF method" in which the zero crossings of the induced machine voltage are detected by reading the motor feedback signals 2 to 1 ,

1 shows the motor feedback signals 2 directly on the engine (engine) phases 3 to be read. The feedback signals 2 be through the control unit 4 processed to the appropriate Kommutierungssequenz to the engine 11 to determine.

With the "direct BEMF method", which is used in 1 is shown (BEMF = counterelectromagnetic force), it is possible to determine the position of the rotor 5 to read through every 60 ° electrical interval. A model 7 (a representative equivalent circuit) for a BLDCM is in 2 shown that a 3-phase full bridge inverter 8th and model 7 a BLDCM shows. The induced voltages are denoted by e _a , e _b and e _c .

The drive of the BLDCM 11 is done using a 2-phase line strategy. That means ideally two phases 3a . 3b . 3c to lead at a time. In reality this is not completely the case. During the commutation between the phase sequences, all three phases are conducted 3a . 3b . 3c for a short period of time. The reason for this is that magnetic energy (return energy) in the machine inductances 9 is stored. The "additional" phase 3a . 3b . 3c which conducts for a short period of time (for the so-called demagnetization period) after commutation plays an important role. From this phase 3a . 3b . 3c becomes the position of the rotor 5 derived using the BEMF voltage (zero crossing of the BEMF). This event is essentially used to determine when the phase commutation is taking place.

The following is a description of the drive of the BLDCM. The relevant events associated with the drive are: zero crossing of the BEMF voltage (Z-event), commutation (C-event) and demagnetization (D-event). These events are described below. 3 shows these electrical events for one electrical revolution and is a rough sketch of some of the relevant signals when looking at a BLDCM 11 drives. The solid lines in 3 show the motor's anti-electromagnetic force voltages for three phases 3a . 3b . 3c , The dashed lines show the three motor phase currents I _a , I _b , I _c .

3 shows three events that are relevant to driving a BLDCM 11 are: zero crossing of BEMF (Z-event), commutation (C-event) and demagnetization (D-event). Furthermore, it can be seen that the phase BEMF voltages e _a , e _b , e _c and the phase currents I _a , I _b , I _c repeat each after 360 ° electrically. 3 also shows that the three aforementioned events (ie the Z-event, C-event and D-event) are electrically present every 60 °. The drive of the BLDCM with trapezoidal phase BEMF voltages e _a , e _{b ,} e _c is performed using a 6-step procedure, where conventionally two phases 3a . 3b . 3c each 60 ° electrically conduct. This 6-step mode of operation is shown in Table 1. Degree electrical 60 120 180 240 300 360 Phase A current Positive power supply (+ Ia) Negative power supply (-Ia) Negative power supply (-Ia) Positive power supply (+ Ia) Phase B electricity Positive power supply (+ Ib) Positive power supply (+ Ib) Negative power supply (-Ib) Negative power supply (-Ib) Phase C electricity Negative power supply (-Ic) Negative power supply (-Ic) Positive power supply (+ Ic) Positive power supply (+ Ic) Table 1

Table 1 and 3 show that this strategy is based on 120 ° electrical current pulses in the BLDCM 11 feed.

If you study phase B, you can see that the first event that takes place is the demagnetization (at the interval between 0 ° electrical - which is "identical" to 360 ° electrical) and 60 ° electrical directly following the previous interval, in particular, the preceding interval between 240 ° 360 ° electrical, where a negative current -I _{b is} fed). This completed Demagnetisierungsintervall takes place because of the energy storage in the inductance 9b , The current sequence starts from (+ I _a , -I _b ) (between 300 ° and 360 ° electrical) to (+ I _a , -I _c ) (between 0 ° = 360 ° electrical and 60 ° electrical). This means that at this point no further current I _b in phase B is fed. The phases B inductance 9b contains magnetic field energy. This magnetic field energy is released (flyback effect) back into the three-phase full bridge inverter 8th , The demagnetization end can be considered as a jump on the motor potential voltages e _b on the BLDCM 11 because the energy path is normally through the freewheeling diodes 10 in the three-phase full bridge inverter 8th and this means that the motor potential voltages e _a, e _b, e _c are in the positive or negative DC connecting rail at first (in this case, a voltage drop across the freewheeling diodes 10 omitted from consideration) until the respective phase inductance 9a . 9b . 9c is made without energy. When the phase 3a . 3b . 3c is made energy-free, returns the respective motor potential e _a , e _{b ,} e _c back to roughly half the DC bus voltage V _dc .

The next event that is present is the zero crossing of the BEMF voltage e _b of phase B (between 0 ° electrical and 60 ° electrical). This event can easily be seen at the motor potential e _{b of} the phase B, because the phase B is made energy-free (no current I _b flows). Here you can see that the slope of the B BEMF voltage is increasing. The zero crossing is searched. This zero crossing is caused at a certain electrical angle (theoretically at 30 ° electrical) after the commutation event. This means that the current sequence goes from (+ I _a , -I _c ) to (+ I _b , -I _c ) and so on. The consequence here is to feed I _a , I _b , I _c into the phase at the right time to produce an optimal engine torque. Here it should be noted that the instantaneous power to the shaft 5 can be calculated as:

Equation 1:

• P _wave = I _a * e _a + I _b * e _b + I _c * e _c

• • I_a, I_b und I_c der augenblickliche Motorphasenstrom (gemessen in Ampere [A]) ist.
• • e_a, e_b und e_c die augenblickliche Spannung pro Phase (gemessen in Volt [V]) ist.

Wherein

• • I _a , I _b and I _{c is} the instantaneous motor phase current (measured in amperes [A]).
• • e _a , e _b and e _{c is} the instantaneous voltage per phase (measured in volts [V]).

It is possible to set up the following table (ie Table 2), which is a general picture for the control of a BLDC motor 11 shows. Signals \ degrees electrical [° electrical] 60 120 180 240 300 360 Phase A BEMF voltage Falling BEMF \ Rising BEMF / Phase B BEMF voltage Rising BEMF / Falling BEMF \ Phase C BEMF voltage Rising BEMF / Falling BEMF \ Phase A current Positive power supply Negative power supply Negative power supply Positive power supply Phase B electricity Positive power supply Positive power supply Negative power supply Negative power supply Phase C electricity Negative power supply Negative power supply Positive power supply Positive power supply Table 2

In order to produce optimum engine torque, it is essential that the events mentioned in Table 2 be properly handled. The two events that need to be captured and validated are demagnetization (D-event) and zero-crossing (Z-event) of BEMF. The commutation event (C-event) is based on the zero crossing of the BEMF. To capture and validate these events, some external circuitry is necessary along with a processor with a built-in comparator (eg, a control unit 4 ).

The detection is handled as a comparison between the motor potential voltages e _a , e _{b ,} e _c and the potential of the engine neutral point 12 . 15 , These aspects are described below.

• • Die 6-Schritt-Betriebsweise ist auf eine Zwei-Phasen-Leitungsstrategie gestützt, wenn man die Demagnetisierung außer Betracht lässt.
• • Die energielos gemachten Phasen 3a, 3b, 3c werden verwendet, um den Nullpunktdurchgang der BEMF zu erfassen, um eine optimale Momenterzeugung sicherzustellen.

The following should be remembered:

• • The 6-step operation is based on a two-phase conduction strategy, disregarding demagnetization.
• • The energy-free phases 3a . 3b . 3c are used to detect the zero crossing of the BEMF to ensure optimal torque production.

The magnitude of the BEMF voltages e _a , e _{b ,} e _c is machine speed dependent. That means that at low engine speeds 11 the BEMF voltages e _a , e _{b ,} e _{c are} small. That's why it's relatively difficult to get a BLDCM 11 due to the lack of BEMF voltages e _a , e _{b ,} e _c (or to be more specific: the BEMF voltages e _a , e _{b ,} e _c are not large enough to pass through the control unit 4 to be processed with sufficient accuracy). The motor feedback signals 2 in 1 should be considered very carefully because this is the key to success when capturing the zero point crossing of the BEMF. Some examples of engine feedback signals 2 are presented below.

Examples of motor feedback signals:

New Compressor Technology Brushless DC Motors (Motors) 11 (BLDC) with considerably higher moments of inertia have since been introduced. This gives rise to some critical challenges. In particular, when starting methods known in the art are used, it is necessary to build up engine speed (to be able to detect the zero cross point of the BEMF voltage with sufficient accuracy) by running in a step mode for a longer period of time (a procedure for this initial startup phase of a BLDC 11 is known per se in the art). This is very critical because there is a potential risk to the position of the rotor 5 due to unknown compressor load (or other mechanical load).

A typical example of a previously used BEMF circuit 13 is in 4 shown. With this circuit 13 , in the 4 is shown, the motor feedback signals e _a , e _b , e _c of the phases 3a . 3b . 3c damped by the voltage divider 14 (R10 and R13; R11 and R14; R12 and R15). This is not ideal for these BLDCMs 11 with high inertia because of the longer step-by-step operation and the possibility of the position of the rotor 5 to lose".

One possible solution to the challenge of high inertia might be to lower the ground resistances (R13, R14, R15 in 4 R4, R5, R6 are in 5 ) and the control unit 4 to allow, to detect the un-suppressed BEMF signals. This procedure is in 5 presented. In this method, zero cross detection is synchronized with the pulse width modulation (PWM) of the three-phase full bridge inverter 8th made in the off-time, where the zero crossing of the BEMF voltage can refer to ground.

The downside is that it is not possible to use 100% PWM, which results in a degradation in the maximum engine speed that could be achieved.

To get (essentially) 100% PWM at higher speeds, it is necessary to connect the lower resistors to ground (as with R13, R14, R15 in 4 is made) and to suppress the motor feedback signals. The detection of the zero crossing of the BEMF voltage is then made synchronous with the PWM in on-time. The reference at high speeds then becomes the actual machine star point 12 be that one 4 is produced.

The interruption / connection of the lower resistors (R4, R5, R6 in 5 ) with ground is made via some digital pins on the control unit, compare 5 showing a modified BEMF circuit.

The construction in 5 is, however, will not work because signal on EMK_A is affected by the signal level on EMK_B or EMK_C through the resistors (R7, R8, R9 in FIG 5 ), which are used to the star point 12 to create. 325 V DC will be on one of the other motor phases 3a . 3b . 3c be present and this will significantly disturb the signal intended to be measured.

One solution could be the resistance to the star point 12 to a level where this disturbance is no longer a problem, but because the upper voltage divider resistors (R1, R2, R3 in FIG 5 ) are in the megohm range, to limit losses, the resistors (R7, R8, R9 in 5 ) to the star point 12 Be tens of megohms to limit the influence. When you do this, the star point becomes 12 Very sensitive to coupled interference and resistance values in this range are not very stable when it comes to dust and moisture, which is the star point 12 makes it unreliable.

Another solution is to move a star point 15 to generate in 6 is shown.

In the 6 The solution shown will work, but will twice as many losses in the BEMF circuit, thereby making them unattractive when high-efficiency circuits are needed.

A third solution will be to generate the engine star point (not shown) using the DC bus voltage around a shifted neutral point 15 to create. However, this could possibly give false detections of the zero crossings of the BEMF voltage.

Therefore, there is still a need for an improved method for detecting the position of a rotor of an electric motor. Likewise, there is a need for an improved circuit for detecting the position of a rotor of an electric motor.

It is therefore an object of the invention to provide a method for detecting the position of a rotor of an electric motor, which is improved over known from the prior art method. Furthermore, it is an object of the invention to provide an electrical circuit capable of detecting the position of a rotor of an electric motor and improved over electrical circuits known from the prior art.

The invention solves this problem.

It is proposed to carry out a method of detecting the position of a rotor of an electric motor which at least partially uses a counterelectromagnetic force voltage of at least one motor phase of the electric motor by sensing at least one counterelectromotive force voltage of at least one motor phase and detecting a passage a certain voltage level of the counter-magnetic-force voltage, wherein the phases of the electric motor are connected to each other via a common star in such a way that at least one motor phase is at least temporarily partially electrically decoupled from the neutral point by at least one electrical decoupling device. In principle, the electric motor may have a substantially random number of motor phases to have. As an example, it may have two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, or even more motor phases (the typical setup being an even number of motor phases). The sampling of the at least one counterelectromagnetic force voltage of at least one motor phase may be made by substantially all means known in the art. For example, a voltage measurement via a resistor or the measurement of a magnetic field (strength) and / or a magnetic field (strength) and / or a magnetic field (strength) variation can be envisaged. The scan can be read out and processed by analog and / or digital circuitry. In particular, digital processing units (such as a single-board computer or the like) may be used. Such devices may also be used to control the current supplied to the electric motor. As is usual with electric motors, the different phases of the electric motor are connected together via a common star. This does not necessarily mean that they are directly connected, for example by (substantially) shorting the respective lines. Instead, it is possible for resistors or other devices to be placed in series with and / or in parallel with the respective lines. The star point itself may be grounded (probably by a resistor or the like) and / or it may float (by being disconnected from ground) and / or by using a resistor with a high resistance, for example in the megohm or even higher range addition). According to the present proposal, at least one phase is at least temporarily partially electrically decoupled from the neutral point by at least one electrical decoupling device. The electrical decoupling can be done in a wide variety of ways. In particular, it is possible for one direction of the electrical current to be preferred over the other (an extreme shape would be a diode where the preference is essentially "100", but "smaller" benefits are also possible). It is also possible that a connection (including a connection that prefers one direction) is set up only during certain time intervals. Furthermore, it is possible that the partial electrical decoupling is performed by a non-linear behavior of the electrical coupling (as a means of comparison, a standard electrical resistor should exhibit a linear behavior between the applied voltage and the current passing through the resistor ). Non-linear behavior may also include gain means (including gain means which favor one direction over another).

While it is sufficient in principle that at least one counter-electromagnetic force split is scanned and / or at least one motor phase is at least temporarily partially electrically decoupled from the star point, it is preferred if a plurality and / or (substantially) all counterelectromagnetic Force-voltage sampled or when a plurality and / or (substantially) all motor phases are partially electrically decoupled. As a rule of thumb, the more counterelectromotive force voltages sampled and / or the more motor phases partially electrically decoupled, the better the position detection. As voltage level used for detection, substantially all voltage levels can be used. In particular, certain levels are preferred, such as a level (dense) at the occurring maximum and / or minimum voltage. Particularly preferably, a "zero point crossing" is detected. The "zero point crossing" may be a zero mass level and / or a voltage level that is a DC power supply between two potentials and / or a voltage level that is between a maximum voltage level and a minimum voltage level, respectively. Other potentials are also possible.

In particular, the method can be used in such a way that at least one counter electromotive force voltage is compared with a reference voltage. In this way, a particularly simple embodiment can be envisaged. Normally, reference voltages can be generated comparatively easily. A comparison can be made on a "zero difference" comparison. Such a comparison can be particularly precise and / or particularly simple.

Although it is possible to detect a wide variety of voltage levels, it is preferable to use the method in such a manner that a zero crossing of at least one counterelectromagnetic force voltage is detected. Such a zero crossing is typically very important for determining the position of the rotor. The term "zero crossing" does not necessarily mean a passage of a mass level potential. Instead, especially if an electric motor is used which is galvanically isolated from ground level, other potentials may be used. A particularly well-suited embodiment can be obtained if a medium voltage potential level between the two potentials of a power supply (in particular a DC power supply and / or the "extremes" of an AC power supply) is used as a zero level for detecting the zero crossover.

It is preferred if at least one electrical decoupling device is at least partially designed as a non-linear electrical decoupling device. Initial experiments have shown that such nonlinear behavior may be sufficient to improve detection. In particular, nonlinearity may occur by having a non-linear behavior between applied voltage and the resulting current and / or when one direction is preferred over the other. To simply cite some typical electronic components that exhibit non-linear behavior, diodes, Zener diodes, DIACs, TRIACs, IGBTs, transistors, FETs, MOSFETs, operational amplifiers, or the like can be used.

In particular, it is preferable to carry out the method in such a way that at least one electrical decoupling device is at least partially designed as an amplifying device, in particular as an operational amplifier device. In particular, such amplification means may exhibit non-linear behavior (although linear gain behavior, at least over a wide interval) which may also be used to perform the method. To simply name a few electronic components that exhibit gain performance, transistors, FETs, MOSFETs, IGBTs, operational amplifiers, or the like may be used. Experiments have shown that, in particular, operational amplifiers (or electronic components having a similar amplification behavior) are particularly well suited to carrying out the presently proposed method.

It is further proposed that at least one counterelectromagnetic force voltage is measured at least at times during a substantially full-pulse-ready modulation sampling cycle. Nowadays (especially when inverters are used to drive electric motors), electric motors are quite often controlled by pulse width modulation (PWM). Previously proposed methods of detecting the position of a rotor of an electric motor typically have the disadvantage that position detection can not be performed (with sufficient precision or at all) when the motor is driven at (substantially) 100% pulse width modulation sampling cycle. However, this is possible with the method proposed here. Therefore, the power and / or torque of the electric motor can be fully utilized, which is particularly advantageous.

Furthermore, it is proposed to use the method at least temporarily during operation of the electric motor at low speed. This is another advantage of the presently proposed method since initial experiments have shown that the measured accuracy of the proposed method is comparatively high even at low rotational speeds of the electric motor. In particular, in combination with the use of a 100% PWM sampling cycle, this is very advantageous. As a rule of thumb, if the method is used very early, an initial startup phase (using, for example, a stepper motor behavior) may be abbreviated, making it possible to shorten the startup time.

Furthermore, an electric circuit device for detecting the position of an electric motor using at least partially a counterelectromagnetic force voltage of at least one motor phase of the electric motor is proposed, wherein the phases of the electric motor are connected to each other by a common neutral point, the at least one scanning device for scanning at least one counter-electromagnetic force voltage of at least one motor phase and at least one evaluation device for detecting a passage of a certain voltage level of the counter-electromagnetic force voltage, wherein at least one electrical decoupling device is used, which is used for at least one motor phase and partially electrically at least temporarily this motor phase decoupled from the neutral point. Using such an electrical circuit device, the features and advantages described above can usually also be achieved, at least in an analogous manner. Furthermore, it is possible to modify the electrical circuit device in the sense described above, at least in an analogous manner.

In particular, it is possible to modify the electrical circuit device in such a way that the at least one electrical decoupling device is at least partially designed as an amplifying device, in particular as an operational amplifier device. Using such an embodiment, the features and advantages described above may also be commonly used, at least in an analogous manner.

In particular, it is advantageous if the electrical circuit arrangement is "completed" by an electric motor. In particular, the electric motor may be connected to a compressor, which leads to a typically particularly advantageous embodiment. In particular, it is possible to have a preassembled group comprising an electrical circuit device and / or an electric motor and / or a compressor device as a prefabricated group and / or as a construction kit.

The present invention and its advantages will become clearer by considering the following description of possible embodiments of the invention, which will be described with reference to the accompanying drawings, in which:

1 a possible embodiment of an electric motor with a control unit in a schematic circuit diagram;

2 a possible embodiment of an electrical circuit of an electrical inverter for driving a three-phase electric motor;

3 a possible control scheme for a brushless DC motor and the return by the electric motor;

4 a first embodiment of an electrical circuit for obtaining a counterelectromagnetic force voltage signal;

5 a second embodiment of an electrical circuit for obtaining a counterelectromagnetic force voltage signal;

6 a third embodiment of an electrical circuit for obtaining a counterelectromagnetic force voltage signal;

7 a fourth embodiment of an electrical circuit for obtaining a counterelectromagnetic force voltage signal;

7 shows an embodiment of an improved circuit 17 for detecting the zero crossing (Z-event) of the BEMF. The proposed circuit 17 , in the 7 is shown has operational amplifier 16 on for partial electrical decoupling of the star point 12 ,

• • Minimierung des Schritt-Betriebsweisen-Betriebs wegen der Fähigkeit, den Nullpunktdurchgang (Z-Ereignis) der BEMF-Spannung bei niedrigen Geschwindigkeiten zu erfassen. Erfassung des Nullpunktdurchgangs der BEMF-Spannung bei niedrigen Geschwindigkeiten ist kritisch, um in der Lage zu sein, mit Maschinen hoher Trägheit, beispielsweise Kompressoren, zu arbeiten.
• • Betrieb mit niedriger Geschwindigkeit und hoher Geschwindigkeit unter Verwendung von 100% PWM, die keine unzulässige Verschlechterung in der Geschwindigkeitsleistung ergibt.
• • Schaltung mit niedrigen Verlusten wegen eines nichtgetrennten Sternpunkts.
• • Aufrechterhalten eines korrekten Sternpunkts 12, um inkorrekte Erfassung des Nullpunktdurchgangs des Sternpunkts 12 zu vermeiden.

In the 7 shown circuit 17 makes it possible to combine the following.

• • Minimization of step-mode operation due to the ability to detect the zero-crossing (Z-event) of the BEMF voltage at low speeds. Detecting the zero crossing of the BEMF voltage at low speeds is critical to being able to work with high inertia machines such as compressors.
• • Low speed, high speed operation using 100% PWM, which does not result in undue degradation in speed performance.
• • circuit with low losses due to a non-separated star point.
• • Maintain a correct star point 12 to incorrect detection of the zero point crossing of the neutral point 12 to avoid.

• – Abtasten einer Gegen-EMF-Spannung einer Motorphase während 100% PWM Abtastzyklus,
• – Vergleichen der abgetasteten Gegen-EMF-Spannung mit einer Referenzspannung,
• – Erfassen eines Nullpunktdurchgangs, gestützt auf den Vergleich der Gegen-EMF-Spannung und der Referenz-Spannung und
• – Entkoppeln eines Sternpunkts mit Hilfe von Verstärker, Motorphasen und dem Sternpunkt.

In particular, the invention can be considered as follows:
A method of detecting a zero crossing of a back EMF, the method comprising:

• Sampling of a counter-EMF voltage of a motor phase during 100% PWM sampling cycle,
• Comparing the sampled back EMF voltage with a reference voltage,
• - detecting a zero crossover based on the comparison of the back EMF voltage and the reference voltage and
• - decoupling a neutral point with the aid of amplifiers, motor phases and the star point.

Additional information can be found in the applications filed under references 10 2012 006 492.6 and 10 2012 006 495.0 on the same day as the present application. The disclosure of these applications is incorporated by reference in the present application. In particular, it is possible to combine the proposals that have been made in all of these three applications.

Claims (9)

Method for detecting the position of the rotor ( 5 ) of an electric motor ( 11 ) that at least partially a counter-electromagnetic-force-voltage (e _a , e _b , e _c ) at least one motor phase ( 3a . 3b . 3c ) of the electric motor ( 5 ) is used by sensing at least one counterelectromagnetic force voltage (e _a , e _b , e _c ) of at least one motor phase ( 3a . 3b . 3c ) and detecting a crossing of a certain voltage level of the counter-electromagnetic force voltage (e _a , e _b , e _c ), wherein the phases ( 3a . 3b . 3c ) of the electric motor ( 5 ) by a common star point ( 12 . 15 ), characterized in that at least one motor phase ( 3a . 3b . 3c ) at least at times partially electrically from the star point ( 12 . 15 ) is decoupled by at least one electrical decoupling device ( 16 ). A method according to claim 1, characterized in that at least one counterelectromagnetic force voltage (e _a , e _b , e _c ) is compared with a reference voltage A method according to claim 1 or 2, characterized in that a zero point crossing of the at least one counterelectromagnetic-force voltage (e _a , e _b , e _c ) is detected. Method according to one of the preceding claims, characterized in that at least one electrical decoupling device ( 16 ) at least partially as a non-linear electrical decoupling device ( 16 ) is trained. Method according to one of the preceding claims, in particular according to claim 4, characterized in that at least one electrical decoupling device ( 16 ) at least partially as an amplifying device ( 16 ), in particular as an operational amplifier device ( 16 ). Method according to one of the preceding claims, characterized in that at least one counterelectromagnetic force voltage (e _a , e _b , e _c ) is measured at least at times during a substantially full-pulse width modulation sampling cycle. Method according to one of the preceding claims, characterized in that the method is used, at least partially, during operation of the electric motor ( 5 ) at low speed. Electrical circuit device ( 1 ) for detecting the position of an electric motor ( 5 ) using, at least in part, a counterelectromagnetic force voltage (e _a , e _b , e _c ) of at least one motor phase ( 3a . 3b . 3c ) of the electric motor ( 5 ), wherein the phases ( 3a . 3b . 3c ) of the electric motor ( 5 ) by a common star point ( 12 . 15 ) are connected to each other, the at least one scanning device ( 2 ) for sensing at least one counterelectromagnetic force voltage (e _a , e _b , e _c ) of at least one motor phase ( 3a . 3b . 3c ) and at least one evaluation device ( 4 ) for detecting a passage of a certain voltage level of the counter-electromagnetic force voltage, characterized by at least one electrical decoupling device ( 16 ) for at least one engine phase ( 3a . 3b . 3c ) is used and the partially electrically, at least temporarily, the motor phase ( 3a . 3b . 3c ) from the star point ( 12 . 15 ) decoupled. Electrical circuit device ( 1 ) according to claim 8, characterized in that at least one electrical decoupling device, at least partially, as an amplifying device ( 16 ) in particular as an operational amplifier device ( 16 ), is trained.

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