DE102019200224A1 - Method and device for operating an electrical machine - Google Patents

Abstract

The invention relates to a method for operating an electrical machine (2) by means of power electronics (3), which has at least three half bridges (HB1-HB3), each with a highside semiconductor switch (HS1-HS3) and a low-side semiconductor switch (LS1-LS3) ) for energizing at least three phases (P1-P3) of the electrical machine (2), the semiconductor switches (HS1-HS3, LS1-LS3) being at least essentially pulse-width-modulated as a function of an electrical revolution of an electrical motor rotation angle (φ _Re ) electrical machine (2) can be controlled. It is provided that at least one, in particular each of the semiconductor switches (HS1-HS3, LS1-LS3) is held in the switching position at least once for a predetermined angle of rotation range (a) over several pulse widths.

Description

The invention relates to a method for operating an electrical machine by means of power electronics, which has at least three half bridges, each with a highside semiconductor switch and in each case a low-side semiconductor switch for energizing at least three phases of the electrical machine, the semiconductor switch being at least essentially pulse-width-modulated as a function be controlled several times by one revolution of an electric motor rotation angle.

Furthermore, the invention relates to a device for operating an electrical machine with power electronics, the power electronics having at least three half bridges, each having a highside semiconductor switch and a low side semiconductor switch, each phase of the electrical machine being assigned one of the half bridges, and with a control device that is designed to control the semiconductor switches for operating the electrical machine.

State of the art

Methods and devices of the type mentioned at the outset are known from the prior art. In order to set the rotatably mounted rotor of a three-phase AC machine into a rotational movement, the phases of the machine are typically controlled by sinusoidal AC voltages that are 120 ° out of phase with one another. These AC voltages are generated, for example, from a constant supply voltage using a B6 power output stage. A B6 power output stage has three half-bridges connected in parallel to one another, each of which has two semiconductor switches, a high-side semiconductor switch and a low-side semiconductor switch. MOSFETs are usually used as semiconductor switches. By selectively switching on the semiconductor switches alternately, half-bridge voltages can be generated, the mean values of which, for example, have a sinusoidal profile.

The semiconductor switches are controlled by pulse width modulation with, in particular, a constant period and a variable duty cycle, which specifies the mean voltage curve over the course of the pulse duty factor. If the high-side semiconductor switch is switched on for a relatively long time and the low-side semiconductor switch of a half-bridge is switched off for a relatively long time, the result is a relatively high mean half-bridge voltage. If the low-side semiconductor switch is switched on for a relatively long time and the high-side semiconductor switch is switched off for a relatively long time, the result is a relatively low mean half-bridge voltage. Chained voltages on the electrical machine result from subtraction of the half-bridge voltages. Both positive and negative chained voltages can be generated. In order to generate a somewhat higher chained voltage, a harmonic with a frequency three times higher than the electric motor speed can be modulated onto the half-bridge voltage, for example. However, this harmonic does not appear in the linked voltages, since it is eliminated again by subtracting the half-bridge voltages.

During operation of the power electronics, losses occur in the latter due to the currents flowing through the semiconductor switches. A large part of these losses are switching losses which arise during the non-ideal transition from the conductive state (closed position) to the blocking state (open position) or vice versa. To reduce electromagnetic emissions, however, it may be desirable to provide such a non-ideal transition.

Disclosure of the invention

The method according to the invention with the features of claim 1 has the advantage that the losses at the semiconductor switches generated by switching operations are reduced, as a result of which the efficiency of the power electronics is improved and its service life is increased. The waste heat from the power electronics is also reduced, which is advantageous, for example, for elements adjacent to the power electronics. According to the invention, this is achieved in that at least one, in particular each, of the semiconductor switches within a complete revolution of the electrical angle of rotation, that is to say within a rotation of the angle of rotation by 360 °, at least once for a predetermined range of angle of rotation, and thus for a period of time dependent on the speed of rotation is left in its switching position for several pulse widths. The half bridges of the power electronics are thus controlled in such a way that, within one electrical revolution, at least one semiconductor switch, preferably each of the semiconductor switches of the power electronics, is switched at least once for a predetermined angle of rotation range, and thus for a duration dependent on the speed of rotation, not in a pulse-width-modulated manner, but in a switch position, i.e. opened or closed, left or held. This noticeably reduces switching losses, particularly on average. Maintaining the switching position of the individual semiconductor switch constant is compensated in particular by an adapted pulse width modulation of the other semiconductor switches of the other semiconductor bridges. The Semiconductor switches are preferably open, that is to say without control, open, that is to say non-conductive, semiconductor switches. The semiconductor switches in an optional embodiment as MOSFETs are preferably switched on when activated with approximately 10 V (U_GS) and switched off when activated with approximately 0 V (U_GS).

Furthermore, it is preferably provided that the power electronics are operated in such a way that at least one semiconductor switch of each half-bridge is held at least once in one switching position during one revolution for or in each case a predetermined range of rotation angles over a plurality of pulse widths. This ensures that the load on the half bridges is reduced. Advantageously, however, all semiconductor switches are left or held once within the revolution for the or one angle of rotation range. The rotation angle ranges or time periods of the different semiconductor switches are preferably the same or different sizes.

According to an advantageous development, the power electronics are operated in such a way that all the semiconductor switches within the electrical revolution are alternately left or held once in a switching position for the or in each case a predetermined range of rotation angles at least over several pulse widths. This results in a uniform load on all semiconductor switches with the aforementioned advantages.

The respective angle of rotation range is particularly preferably selected as a function of the number of semiconductor switches or half bridges present. This ensures that the semiconductor switches are loaded evenly overall or are relieved by leaving them in one switching position.

The angle of rotation range is particularly preferably selected such that it corresponds at most to the quotient of a full electrical revolution and the number of half bridges present. In the case of power electronics which have three half bridges for controlling three electrical phases of the electrical machine, the quotient thus results in the maximum time period of one third of the time for an electrical revolution or a maximum angle of rotation range of one third of the full revolution of the angle of rotation. This ensures a simple calculation and determination of the angle of rotation range or the time duration of the leaving. The method is therefore easily scalable for a different number of semiconductor switches or half bridges. A smaller range of rotation angle or a shorter period of time, for example one sixth of the revolution or the time for an electrical revolution, is preferably used in order to avoid very low or very high duty cycles, because this makes useless due to the short on and off times of the semiconductor switches Switching losses occur.

Furthermore, it is preferably provided that MOSFETs are used as semiconductor switches. With these in particular, there is the advantage of reduced switching losses, which has an advantageous effect on the overall behavior of the power electronics, in particular with regard to its service life.

The respective highside switch is particularly preferably left in the open position and the respective low-side semiconductor switch in the closed position. With reference to a half-bridge, it is therefore provided that when the high-side semiconductor switch is to be left in a switching position for the predetermined period of time, the open position is selected, while the closed position is selected for the low-side semiconductor switch. This has the advantage that the phase current can be measured over the entire time period or the full rotation angle range and thus the full revolution using a shunt resistor, which is typically connected between the low-side semiconductor switch and the reference point of the operating voltage source.

Alternatively, the reverse control takes place, in which the respective high-side semiconductor switch is left in the closed position and the respective low-side semiconductor switch is left in the open position for the predetermined angle of rotation range.

According to an advantageous development of the invention, the semiconductor switches of the half-bridges controlled by pulse modulation, that is to say the half-bridges whose semiconductor switches are not left in a switching position, are controlled by an adapted pulse width modulation in such a way that sinusoidally linked output voltages of the half-bridges result. This results in the already mentioned compensation of the half-bridge that is not continuously pulse-width-modulated / controlled.

Furthermore, it is preferably provided that the method is carried out only when a predeterminable operating current limit of the power electronics is exceeded. The method is therefore only carried out when the operating current flowing through the power electronics exceeds the predetermined operating current limit and thus increased switching losses are feared, which can be compensated for or avoided by the advantageous method. At low operating currents, conventional pulse width modulation is preferred applied because the switching losses that occur may be rather low.

The device according to the invention with the features of claim 11 is characterized in that the control device is specially designed to carry out the method according to the invention. This results in the advantages already mentioned.

Further advantages and preferred features and combinations of features result in particular from what has been described above and from the claims.

• 1 eine Vorrichtung zum Betreiben einer elektrischen Maschine in einer vereinfachten Darstellung,
• 2 ein Schaubild zur Erläuterung von verketteten Ausgangsspannungen,
• 3 eine vereinfachte Darstellung eines vorteilhaften Ansteuerverfahrens der Vorrichtung,
• 4 einen beispielhaften durch das Verfahren erzeugten Spannungsverlauf,
• 5A und 5B Signalverläufe der Vorrichtung, wenn keine Ansteuerung erfolgt,
• 6A bis 6D Signalverläufe der Vorrichtung bei einer halben Ansteuerung und
• 7A bis 7D Signalverläufe der Vorrichtung bei voller Ansteuerung.

The invention will be explained in more detail below with reference to the drawing. Show this

• 1 a device for operating an electrical machine in a simplified representation,
• 2nd a diagram for explaining chained output voltages,
• 3rd a simplified representation of an advantageous control method of the device,
• 4th an exemplary voltage curve generated by the method,
• 5A and 5B Signal curves of the device when there is no activation,
• 6A to 6D Signal curves of the device with half a control and
• 7A to 7D Signal curves of the device with full control.

1 shows a simplified representation of an advantageous device 1 for operating an electrical machine 2nd , which is designed in particular as an electric motor. The device 1 has power electronics 3rd on by a control unit 4th is operated and with a voltage source 5 connected is. The power electronics 3rd has a B6 bridge circuit 6 as the power output stage, which has three half bridges HB1, HB2, HB3. Each of the half bridges HB1, HB2, HB3 has a high-side semiconductor switch HS1, HS2, HS3 and a low-side semiconductor switch LS1, LS2, LS3. The high-side semiconductor switch HS1, HS2, HS3 is in each case in series with the associated low-side semiconductor switch LS1, LS2, LS3 in parallel with the voltage source 5 switched with the supply voltage U _B. There is a phase between the respective highside semiconductor switch HS1, HS2, HS3 and the associated low-side semiconductor switch LS1, LS2, LS3 of the respective half-bridge HB1, HB2, HB3 P1 , P2 , P3 of the electrical machine 2nd connected. So far is the electrical machine 2nd designed as a three-phase electric motor, each phase being controlled by a half bridge HB1, HB2, HB3, so that the respective phase P1 , P2 , P3 with a half-bridge _voltage U _HB1 , U _HB2 , U _{HB3 of} the respective half-bridge HB1, HB2, HB3.

Typically the electrical machine 2nd controlled by sinusoidal AC voltages U _HB1 , U _HB2 , U _{HB3 which} are phase-shifted by 120 °. By selectively switching on the high-side and low-side semiconductor switches, half-bridge voltages are generated, the mean value of which has a sinusoidal profile. To generate higher linked voltages on the electrical machine 2nd (For example U ₁₂ = U _HB1 - U _HB2 or U ₂₃ = U _HB2 - U _HB3 or U ₃₁ = U _HB3 - U _HB1 ) an additional harmonic with a frequency three times higher than the electric motor speed can be modulated onto the half-bridge _voltages . 2nd shows in simplified form the meaning of the linked voltages U ₁₂ , U ₂₃ , U ₃₁ . However, this harmonic does not appear in the linked voltages, since it is eliminated again by subtracting the half-bridge voltages. The linked voltages of the electrical machine 2nd should advantageously have a sinusoidal shape without harmonics.

The control of the semiconductor switch HS1 to LS3 that are presently formed as MOSFETs is made by a pulse width modulation (PWM) with constant period duration T _PWM and variable-time t _a (with, for example T _PWM = 1 / (16 kHz) = 62.5 .mu.s ), which follows the mean voltage curve over the course of the duty cycle t _HSxein / T _PWM = 1 - t _LSxein / T _PWM . If the high-side semiconductor switch HS1, HS2, HS3 is switched on for a relatively long time and the associated low-side semiconductor switch LS1, LS2, LS3 is switched off for a relatively long time (t _HSxein / T _PWM > 50%), the result is a relatively high half-bridge voltage (50% U _B ≤ U _HB ≤ U _B ). If the respective low-side semiconductor switch LSx (x = 1 to 3) is switched on for a relatively long time and the associated high-side semiconductor switch HSx (x = 1 to 3) is switched off for a relatively long time (t _HSxein / T _PWM <50%), the result is one relatively low mean half-bridge voltage (50% U _B ≥ U _HB ≥ 0V). The linked voltages of the electrical machine result from subtraction of the half-bridge voltages, as in 2nd shown. Both positive and negative chained voltages can be generated.

Typically, the half-bridges are driven with a duty cycle of 50% in order to generate linked voltages of 0V. If linked voltages greater or less than 0V are to be generated, the half-bridges are controlled with a pulse duty factor greater than or less than 50%. This means that the control is symmetrical from the middle (50%) of the duty cycle.

When operating the power electronics, losses occur in the current due to the currents flowing through the semiconductor switches HS1-LS3. Some of these losses are switching losses which arise when the transition from the conductive to the blocking state or vice versa is not ideal. Using this method, each of the semiconductor switches is switched on and off once within a PWM period.

The switching losses at the semiconductor switches can be reduced if the semiconductor switches are switched on and off less frequently. This can be achieved by changing or optimizing the courses of the duty cycles, that is to say by changing the PWM.

In the present case, optimized duty cycle or PWM curves are used, in which at least one semiconductor switch HS1 to LS3 of each half-bridge HB1 to HB3 is not switched in an angle of rotation range of, for example, one sixth of the electric motor revolution, or is held in a switch position, opened or closed . The LS semiconductor switch advantageously remains switched on and the HS semiconductor switch remains switched off. The reverse case is also possible. The other two half bridges are controlled with an adapted pulse width modulation, so that sinusoidal chained voltages continue to result. The control is not symmetrical from the middle (50%) of the duty cycle, but one-sided from 0%, but avoiding very small duty cycles, since these are ineffective due to the non-disappearing switching times. Because a half bridge is not switched over, for example, 1/6 of the electrical revolution, the switching losses are reduced overall by a factor of 1/6 (17%), for example.

3rd shows in a simplified exemplary representation the switching pattern of the pulse width modulation for operating the half bridges HB1, HB2 and HB3 via the electrical motor angle φ _Re over a full revolution (0 ° to 360 °). A double arrow indicates the rotation angle range a with respect to a full rotation of 360 °, within which the respective semiconductor switch HS1-LS3 is left in a switch position, open or closed, and not according to the pulse width modulation curve otherwise indicated by horizontal bars is switched. The resulting maximum relative time t ₀ (based on the time for a full revolution of the motor angle φ _Re ) or the maximum angle of rotation a result from the quotient of the full revolution of 360 ° and the number of half bridges present (calculated without dimension) . 3rd however shows an example of a switching pattern with a = 60 °. It follows from this that the method can also be easily applied to power electronics which have more than three half bridges. This makes scaling to different power output stages easy.

$${\text{U}}_{\text{HB2}}={\text{t}}_{\text{HS2}\_\text{ein}}/{\text{T}}_{\text{PWM}}$$

$${\text{U}}_{\text{HB3}}={\text{t}}_{\text{HS3}\_\text{ein}}/{\text{T}}_{\text{PWM}}$$

Wobei in 4 der elektrische Drehwinkel φ_Re für eine Umdrehung um 360° in Prozent angezeigt ist. 4th shows an example from the in 3rd _Desired advantageous control result of the course of the half-bridge _voltages U _HB1 to UHB3, where: $${\text{U}}_{\text{HB1}}={\text{t}}_{\text{HS1}\_\text{a}}/{\text{T}}_{\text{PWM}}$$

$${\text{U}}_{\text{HB2}}={\text{t}}_{\text{HS2}\_\text{a}}/{\text{T}}_{\text{PWM}}$$

$${\text{U}}_{\text{HB3}}={\text{t}}_{\text{HS3}\_\text{a}}/{\text{T}}_{\text{PWM}}$$

Where in 4th the electrical angle of rotation φ _Re is indicated for a revolution through 360 ° in percent.

5A and 5B show the courses of the duty cycles on the half bridges or the course of the average half bridge voltage ( 5A) and the courses of the averaged linked voltages ( 5B) in the event that no control of the electrical machine 2nd he follows. The addition “av” in the legend of the figures means that the mean values in question are concerned (av = average = average or mean).

6A and 6B show the course of the duty cycle ( 6A) and the linked voltages ( 6B) in the event that no harmonics are modulated, with half control of the electrical machine 2nd , and 6C the duty cycle and 6D the course of the linked voltages with half control with harmonics.

7A and 7B accordingly show the course of the duty cycle and the linked voltages with full control without harmonics, and 7C and 7D the courses of the duty cycles and the linked voltages with full control with harmonics.

Optionally, the described method is only used with a relatively high modulation or with large currents, because only then the switching losses play a decisive role. For this purpose, for example, the operating current of the power electronics is monitored and the method is only carried out when a predefinable operating current limit is exceeded. For operating currents that are below the operating current limit, a conventional pulse width modulation is used in particular.

The advantageous method results in a reduction in switching losses of 17% or 1/6 with a time duration t ₀ or expressed as the angle of rotation range a with respect to a full electrical revolution of 360 ° of, for example, a = 60 °, so that a lower total power consumption whereby the efficiency of the device 1 is improved. In addition, there is less heating of the semiconductor switches, in particular the MOSFETs, as a result of which they can be manufactured in a smaller design, if necessary, at a lower price. In addition, space may be saved. Furthermore, there are lower cross currents in a non-ideal switching process from a high-side semiconductor switch to a low-side semiconductor switch of one of the half bridges HB1, HB2, HB3 (and vice versa), since one of the half bridges does not switch over, for example, 60 ° electrical rotation or in the switch position is left. The motor phase current can, for example, via a shunt resistor, which is connected between the respective low-side semiconductor switch LS1, LS2, LS3 and the reference point of the supply voltage U _B , in the time in which the low-side semiconductor switch is permanently switched on, can be measured statically. This means that sampling that is synchronized with pulse width modulation is not required.

Claims (12)

Method for operating an electrical machine (2) by means of power electronics (3), which have at least three half bridges (HB1-HB3), each with a highside semiconductor switch (HS1-HS3) and a low-side semiconductor switch (LS1-LS3) for energizing has at least three phases (P1-P3) of the electrical machine (2), the semiconductor switches (HS1-HS3) being controlled at least essentially in a pulse-width-modulated manner as a function of an electrical revolution of an electrical motor rotation angle (φ _Re ) of the electrical machine (2), characterized in that at least one, in particular each of the semiconductor switches (HS1-HS3, LS1-LS3) is held in the switching position at least once for a predetermined angle of rotation range (a) over a plurality of pulse widths within the electrical revolution. Procedure according to Claim 1 , characterized in that the power electronics (3) is operated in such a way that at least one semiconductor switch (HS1-HS3, LS1-LS3) of each half-bridge (HB1-HB3) at least once for the predetermined or a predetermined angle of rotation range (a) over several pulse widths is held away in a switch position. Method according to one of the preceding claims, characterized in that the power electronics (3) are operated in such a way that all semiconductor switches (HS1-HS3, LS1-LS3) alternate at least once in one switching position for the predetermined or a predetermined angle of rotation range within one revolution ( a) are held across multiple pulse widths. Method according to one of the preceding claims, characterized in that the predetermined angle of rotation range (a) is selected as a function of the number of half bridges (HB1-HB3) present. Method according to one of the preceding claims, characterized in that the angle of rotation range (a) is specified such that it corresponds at most to the quotient of an electrical revolution and the number of half bridges (HB1-HB3) present. Method according to one of the preceding claims, characterized in that MOSFETs are used as semiconductor switches (HS1-HS3, LS1-LS3). Method according to one of the preceding claims, characterized in that the respective highside switch (HS1-HS3) is left in the open position and the respective low-side semiconductor switch (LS1-LS3) in a closed position for the rotation angle range (a). Procedure according to one of the Claims 1 to 6 , characterized in that the respective highside switch (HS1-HS3) is left in the closed position and the respective low-side semiconductor switch (LS1-LS3) in the open position for the rotation angle range (a). Method according to one of the preceding claims, characterized in that the semiconductor switches (HS1-HS3, LS1-LS3) of the half-bridges (HB1-HB3) controlled by pulse modulation for the angle of rotation range (a) are controlled by an adapted pulse width modulation in such a way that sinusoidal linked output voltages (U ₁₂ , U ₂₃ , U ₃₁ ) of the half bridges (HB1-HB3). Method according to one of the preceding claims, characterized in that the method is carried out only when a predetermined operating current limit of the power electronics (3) is exceeded. Device for operating an electrical machine, with power electronics (3), which has at least three half bridges (HB1-HB3), each with a highside semiconductor switch (HS1-HS3) and a low-side semiconductor switch (LS1-LS3), Each phase (P1-P3) of the electrical machine (2) is assigned one of the half bridges (HB1-HB3), and with a control device (4) which is designed to operate the semiconductor switches (HS1-HS3) for operating the electrical To control the machine (2), characterized in that the control device (4) is specially designed to carry out the method according to one of the Claims 1 to 10th perform. Steering system for a vehicle, in particular a motor vehicle, with at least one steering actuator having an electrical machine, the electrical machine having a device Claim 11 having.

Images