DE102014100570A1 - Method for operating an electronically commutated DC motor - Google Patents

Abstract

Method for operating a sensorless, electronically commutated direct current motor, with a start-up phase, in which the commutation takes place according to a predetermined Kommutierungsmuster and an operating phase in which the commutation takes place in dependence on the counterelectromotive force. According to the invention, a short commutation phase is run through between the starting phase and the operating phase, in which the commutation takes place with short commutation steps. In a short commutation step, the commutation takes place before the optimal commutation time, in particular immediately after a zero crossing of the BEMF.

Description

Brushless, electronically commutated direct current motors (BLDC motors) are now widely used.

A BLDC motor has a permanent magnet rotor and a stator with multiple stator windings. Depending on the application of the motor, the stator windings may be arranged in one or more phases. Particularly widespread are three-phase motors. The commutation of the motor is carried out by alternately and successively switching on and off of the correct motor phases. This is usually done via an inverter, which consists of three half-bridges in the case of a three-phase motor. To commutate a BLDC motor, it is also necessary to know the rotor position so that the windings of the motor phases can be switched at the right time, so that a rotating field results and the rotor can be rotated.

For rotor position detection Hall sensors are usually used, which are arranged in the magnetic field of the rotor magnet or a position magnet. In order to obtain a correct position information, for example, in a three-phase motor, three Hall sensors are needed, which are arranged at an angle to each other. The magnetic field creates a Hall voltage in the Hall sensors.

From the Hall voltages of the three sensors, the rotor position can be determined.

Hall sensors are sensitive to external influences, especially they are not suitable for high temperatures. In addition, they are expensive and must be arranged close enough to the rotor magnet, resulting in structural restrictions.

Therefore, the Hall sensors are not the only ones to do without in cost-sensitive applications. A BLDC motor can also be operated without a sensor.

The rotation of the rotor in the stator field induces a voltage in the stator windings which counteracts the applied phase voltage. Therefore, this induced voltage is also called back electromotive force (BEMF = Back ElectroMotive Force). The rotor position can be derived from the voltage curve of the BEMF. The commutation times are determined for sensorless BLDC motors starting from the zero crossings of the BEMF at the respective de-energized motor phase. The optimum commutation time is about 30 ° electrical after a zero crossing of the BEMF voltage. The zero crossing does not have to be at the voltage zero point, but can also be at a finite voltage level.

In the prior art, the sensorless operation of a BLDC motor, especially in three-phase BLDC motors, well known, which is why not go into more detail here.

A fundamental problem with sensorless motors is that the BEMF is not induced until the motor rotates. This means that when the motor is at a standstill there is no positional information from a BEMF from which a correct commutation can be derived. Correct commutation and thus operation of a sensorless motor from standstill is therefore not possible. For this reason, sensorless motors are usually started with a start-up phase in which the engine is operated, for example, like a synchronous motor. The commutation is carried out using a fixed commutation pattern. Therein the commutation times of each engine phase are determined. This start-up phase usually includes some commutation steps. The goal is to accelerate the engine as fast as possible so that a BEMF is quickly available.

In synchronous operation of the start-up phase, the efficiency of the motor due to the forced commutation is poor and the load angle is large. One goal is therefore to keep the start-up phase as short as possible. For this reason, the BEMF is constantly monitored for valid zero crossings during the start-up phase. There are now countless conditions from when the start-up phase can be ended and switched to the operating phase.

For example, a predetermined number of valid zero crossings can be awaited. In the operating phase, the zero crossings of the BEMF are present at each commutation step and thus the commutation time can be determined starting from the zero crossing.

In the start-up phase, the motor is usually operated with a high starting current to ensure that the rotor also follows the applied accelerating rotating field. This high current would lead to a high acceleration during normal operation. However, this theoretically possible acceleration is slowed down again by the given commutation times. This means that the motor would actually turn faster due to the high starting current than the fixed commutation times allow. The result is that the electric and the magnetic field of the motor are no longer in phase, but form an angle. The engine is over- or under-excited, the rotor preferably precedes the rotating field. If the transition from the start-up phase to the operating phase occurs, the angle between the electric and the magnetic field may change too quickly as a result of switching to the correct commutation. This leads to an angle oscillation, which must be prevented at all costs.

If the angle between the electric and the magnetic field is too large, this will cause the electric and magnetic fields to be completely decoupled. The rotor can then no longer follow the stator field and stops. Also, the zero crossings of the BEMF may not be measured correctly at a too large angular vibration of the rotor and make a further commutation impossible.

The object of the invention is therefore to provide a method for operating a sensorless BLDC motor, in which a safe start of the engine is guaranteed at all times.

This object is achieved by a method with the features mentioned in the main claim.

Also in the method according to the invention, the engine is started by a start-up phase from standstill. The type of start-up phase does not matter. In the start-up phase, the commutation preferably takes place according to a predetermined commutation pattern with fixed commutation times. The start-up phase may, for example, take so long until a predetermined number of zero crossings of the BEMF has been successfully detected, wherein a detection is considered successful, for example, if the commutation took place at the time expected after the predetermined commutation sequence and was clearly recognized.

In the operating phase, the commutation is based on the zero crossings of the BEMF. The exact definition and determination of the commutation times in the start-up phase, or in the operating phase, play no role for the invention, which is why the invention is not limited to one of the numerous methods known in the prior art.

In the method according to the invention, an additional short commutation phase is now passed through between the startup phase and the operating phase.

In this Kurzkommutierungsphase the commutation takes place with short commutation steps, wherein in a short commutation step, the commutation takes place before the theoretically optimal commutation. In particular, it may be advantageous if the commutation takes place in a short commutation step at least 10 ° electrical earlier than the first commutation of the operating phase.

The optimal commutation time is usually 30 ° electrical after the zero crossing of the BEMF. In the short commutation phase, the commutation now takes place before this optimal time, in particular between the zero crossing of the BEMF and the optimal commutation time.

This too early commutation step causes the electric and the magnetic field to equalize again. In the best case, an existing angle is completely reduced by means of the shortened commutation.

As a result, a possible angular vibration of the rotor is damped and / or prevented during the transition to the operating phase. A standstill as described above can not occur. By this simple measure, a significant improvement of the starting behavior of a sensorless motor is possible.

Particularly advantageously, the commutation in the short commutation phase takes place essentially immediately after a zero crossing of the BEMF. The commutation therefore takes place substantially 30 ° electrically earlier than the optimum commutation time would be. If the angle between the electric and the magnetic field in the range of 30 °, then can be done with a single such Kurzkommutierungsschritt an equalization of the electric to the magnetic field.

In an advantageous embodiment of the invention, the short commutation phase therefore contains exactly one short commutation step.

However, if the angle between the electric field and the magnetic field is too large, it may be that the so-called fly back pulse covers the zero crossing of the BEMF. This means that no zero crossing can be detected in this phase. In such a case can not be commuted immediately after the zero crossing.

Appropriately, the commutation then takes place in the Kurzkommutierungsphase then substantially immediately after a freewheeling pulse.

Since this commutation time is later than the zero crossing, it may be that the angle between electric field and magnetic Field is not reduced enough to go into the operational phase.

In a development of the invention, the short commutation phase therefore contains more than one short commutation step. The angle between the electric field and the magnetic field can thus be reduced with each short commutation step so that a safe transition into the operating phase can take place. In particular, the short commutation phase can have between one and six short commutation steps, preferably two to three.

It is possible that the first or the first commutation steps after a freewheeling pulse and the subsequent done after a zero crossing.

Brief description of the drawings

1 shows an exemplary embodiment of an H-bridge for driving a brushless DC motor,

2 shows a commutation pattern according to the inventive method in the form of a graph of the applied voltage as a function of time for the example of a three-phase DC motor.

The invention is explained in more detail with reference to an embodiment.

The example is a flapper for a radiator assembly of an automobile. In modern automobiles, the radiator openings may be closed when the cooling air is not needed to improve the aerodynamics so that the car consumes less gasoline. The cooler has for this purpose flaps, which are driven by an actuator. This actuator has an electronically commutated electric motor, which is connected via a transmission with the adjusting mechanism, the radiator flaps.

To save costs and because of the higher temperatures in the engine compartment, the engine is operated sensorless. As already mentioned above, therefore, no position information is available at standstill, which allow a commutation of the motor when starting. For this reason, the engine is started in a start-up phase. In the start-up phase, a fixed commutation pattern is specified, through which the commutation takes place at fixed times. The aim in the start-up phase is to accelerate the engine as fast as possible so that the BEMF necessary for commutation is available as quickly as possible.

In the example, the zero crossing detection is started on the respectively de-energized motor phase already from the approximately sixth commutation step. Once six consecutive valid zero crossings have been detected, it is assumed that the motor has successfully started. A zero crossing is valid if it occurs on the expected engine phase in the expected sequence. This ensures that the motor is also started in the correct direction of rotation.

The start-up phase usually takes about 20 commutation steps.

For fast acceleration, the engine is operated with a very high current. The engine is over or under excited during this startup phase. This means it would spin faster due to the high current than forced commutation allows. For this reason, the electric field and the magnetic field form an angle.

In order to reduce this angle, according to the invention, after the six valid zero crossings have been detected, a short commutation step according to the invention is inserted. In this, the next commutation already takes place immediately after the zero crossing of the BEMF. For a normal commutation this commutation would be much too early. In this case, however, it causes the electric and the magnetic field to approach each other. As a rule, a short commutation step is sufficient to reduce the angle so that it no longer plays a role.

After this short commutation step, the operation phase is entered in which the zero crossings of the BEMF are available for commutation. As a rule, a commutation takes place in each case 30 ° electrical after a zero crossing of the BEMF.

In 1 is an example of a drive circuit 1 for commutating a brushless DC motor, which can also be used for the previously described flap plate. By means of semiconductor switches Q of an inverter, for example a bridge circuit, the currents through the phase windings U, V, W of the motor can be controlled and thus the voltage applied to the motor torque and its rotational frequency can be adjusted. In the example, the phase windings U, V, W are connected in a so-called star connection, wherein the phase windings (inductances) in the star point 3 are connected and the respective lower semiconductor switch is grounded GND. The gate inputs of the semiconductor switches Q, for example MOSFET switches, are connected to the drive circuit by the signals BHn (where n = 0.0, 0.1, 0.2, 1.0, 1.1, 1.2).

Furthermore, a virtual star point MTC is provided, which consists of a resistance network, which is connected to the supply lines of the three motor phases. The virtual star point MTC serves as a voltage tap for determining the zero crossing of the BEMF voltage.

Furthermore, the drive circuit 1 an amplifier circuit 4 for measuring the motor current, which is connected on the one hand with measuring resistors R _SHUNT and on the other hand with an analog-to-digital converter input (ADC) of the drive circuit.

2 shows the tapped at the phase windings U, V, W and at the virtual star point MTC voltage of a brushless DC motor as a function of the phase angle in the transition region of the start-up phase to the operating phase. For the sake of simplicity, the freewheeling pulses were faded out and a sinusoidal BEMF was assumed, while the motor was kept at a constant speed.

The commutation pattern shows how shortly after the last predetermined commutation SK of the synchronous phase a short commutation KK occurs. In the example, in the case of the last given commutation SK, the lowside switch of the phase U is opened and the low-side switch of the phase W is closed. In the following short commutation KK, the high-side switch of the phase U is closed while the highside switch is opened for the phase V. For clarity, the distance between the last commutation SK in synchronous mode and the short commutation KK has been exaggerated and the times between the commutations are constant except for the short commutation KK. In the example, the commutation IK after the first zero crossing BEMF0 of the BEMF during the short commutation phase represents the first ideal and sensorless controlled commutation, whereby the lowside switch of the phase V is closed and the low-side switch of the phase W is opened. The energetically ideal commutation of sensorless operation is reflected in the 2 in the symmetrical to the voltage value VCC / 2 behavior of the voltage applied to the virtual neutral point MTC voltage again.

LIST OF REFERENCE NUMBERS

1

 drive circuit

2

 inverter

3

 star point

4

 amplifier circuit

ADC

Analog-to-digital converter input

AND MANY MORE

motor phases

Q

 Semiconductor switches

BHN

 Gate signals of the MOSFET switches

Claims (6)

Method for operating a sensorless, electronically commutated direct-current motor, with a start-up phase in which the commutation takes place according to a predetermined commutation pattern and an operating phase in which the commutation takes place as a function of the counterelectromotive force (BEMF), characterized in that between the start-up phase and the Operating phase is passed through a short commutation phase, in which the commutation is carried out with one or more Kurzkommutierungsschritten, wherein in at least one short commutation step, the commutation takes place at least 10 ° electrically earlier than the first commutation in the operating phase. A method according to claim 1, characterized in that the commutation in the Kurzkommutierungsphase takes place at the time or immediately after a zero crossing of the BEMF. A method according to claim 1, characterized in that the commutation takes place in the short commutation phase substantially immediately after a freewheeling pulse. Method according to one of claims 1 to 3, characterized in that the Kurzkommutierungsphase contains at least one and a maximum of six Kurzkommutierungsschritte. Method according to one of claims 1 to 3, characterized in that the short commutation phase contains exactly one short commutation step.  An electronically commutated DC motor operated in accordance with the method of any one of claims 1 to 5.

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