DE102011003559A1 - Method and device for controlling a pulse inverter, which serves to control an electrical machine - Google Patents

Abstract

The invention relates to a method and a device for controlling a pulse-controlled inverter (2) which is used to control an electrical machine (1). A sensor unit (4) detects a variable that characterizes the speed (s) of the electrical machine (1) and transmits this to a control unit (3). The control unit (3) then sets a clock frequency (fTakt) of a pulse-width-modulated control signal (SPWM) of the pulse-controlled inverter (2) depending on this variable.

Description

The invention relates to a method and a device for controlling a pulse-controlled inverter, which serves for controlling an electrical machine, as well as a use of the control method according to the invention for avoiding disturbances of a radio transmission system.

State of the art

For the drive in hybrid or electric vehicles electrical machines in the form of induction machines are usually used, which in conjunction with pulse-controlled inverters - often referred to as inverter - are operated. The electrical machines are operated either in motor or generator mode. During engine operation, the electric machine generates a drive torque which, when used in a hybrid vehicle, supports an internal combustion engine, for example in an acceleration phase. In generator mode, the electric machine generates electrical energy that is stored in an energy store, such as a traction battery. To stabilize the battery voltage, an intermediate circuit capacitor is provided. Operating mode and power of the electric machine are set via the pulse inverter. In this case, the pulse-controlled inverter comprises power semiconductor switches-in the following referred to as power switches-such as MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors) or MCTs (MOS Controlled Thyristor), which are controlled by a control unit.

Basically, it is known to control the switching elements of an inverter pulse width modulated (PWM). In order to perform inaudible the timing of the pulse width modulated control signals for humans, usually clock frequencies of 20 to 30 kHz are used. In this case, however, disturbances that act on the electrical system of the motor vehicle, so that sometimes very complex anti-interference measures are required.

It is known to drive pulse-controlled inverters by means of PWM clock frequencies whose integer multiples, which are also referred to as harmonics, coincide with the channel spacing of the carrier frequencies of radio transmission systems, such as amplitude-modulated (AM) broadcasting. For example, in the long wave (LW) - and the medium wave transmission (MW) in Europe, a channel spacing of 9 kHz, after which for LW carrier frequencies of 153 kHz, 162 kHz, 171 kHz, ..., 513 kHz or MW of 531 kHz, 540 kHz, 549 kHz, ..., 1710 kHz. If a fixed value of 9 kHz is now used for the clock frequency of the pulse-width-modulated signal, the 18th harmonic at 162 kHz and the 60th harmonic at 540 kHz fall precisely on the channel spacing of the AM transmitters. In this case, a modulation is excluded, so that it can not come to a disturbance. In America, on the other hand, a channel spacing of 10 kHz is used. Accordingly, a fixed clock frequency of 10 kHz can be provided to exactly hit the carrier frequencies in question and to prevent unwanted modulation. It is also known as the clock frequency not the channel spacing itself, but a multiple thereof, ie z. 27 kHz in Europe and 30 kHz in America.

Another known possibility for avoiding interference consists in the control by means of clock frequencies which correspond to an integer multiple of half the channel spacing (Europe: 4.5 kHz, America: 5 kHz). In this case, half of the harmonics coincides exactly with the carrier frequencies of the AM transmitters, while the other half lies exactly between two adjacent carrier frequencies and is attenuated by an AM filter of the radio receiver.

To ensure that the channel grids of the respective radio transmission systems can be hit exactly, a very high precision of the PWM clock frequency for the power control is required. Even with a tolerance of only 0.01% of the desired clock frequency, the deviation in the upper MW range with a carrier frequency of, for example, 1.6 MHz is about 160 Hz, which leads to clearly audible modulations. Accordingly, highly precise clock regulators with maximum tolerances of 0.001% are required for the aforementioned methods.

From the WO 2008/2008/000655 A2 a method for preventing interference of a radio transmission system with at least one carrier frequency by a pulse width modulated signal is known in which a clock frequency of the pulse width modulated signal is set such that their integer multiple maintains a defined minimum distance to the carrier frequency in question.

If a pulse-controlled inverter is used to control an electrical machine, it is essential to ensure that the clock frequency of the pulse-width-modulated control signal of the pulse-controlled inverter is high enough to produce a sufficient sinusoidal current even at high speeds of the electric machine and a high stator frequency associated therewith. Accordingly, the clock frequency of the pulse width modulated control signal is designed in conventional systems to a maximum speed. However, a high clock frequency also leads to an increased electromagnetic interference emission.

Disclosure of the invention

The invention provides a method for controlling a pulse inverter, which serves to control an electrical machine. In this case, a clock frequency of a pulse-width-modulated control signal of the pulse inverter is set depending on a variable characterizing a rotational speed of the electrical machine.

The invention also provides a device for controlling a pulse inverter, which serves to control an electrical machine. This comprises a sensor unit for detecting and transmitting a variable characterizing the rotational speed of the electrical machine to a control device. The controller then adjusts a clock frequency of a pulse width modulated control signal of the pulse inverter depending on this size.

Advantages of the invention

At lower speeds basically a smaller clock frequency is sufficient to produce a sufficiently sinusoidal current than at high speeds. An increased clock frequency of the pulse width modulated control signal of the pulse inverter causes an amplified electromagnetic interference emission. So if a fixed clock frequency is set, which is designed for a maximum speed of the electrical, so at lower speeds inevitably leads to unnecessarily high noise emissions. This effect is exacerbated by the fact that, especially in the low speed range, those operating points of the electric machine are at which the largest phase currents occur, which in turn lead to increased interference emission.

This problem is inventively solved in that the clock frequency of the pulse width modulated control signal of the pulse inverter is set in dependence on a maximum allowable torque ripple and thus the speed or the stator frequency of the electric machine. In particular, the clock frequency of the pulse width modulated signal is reduced with decreasing speed. In this way, the amplitudes of the harmonics and thus the electromagnetic interference emission are reduced at decreasing speeds. A reduction in the clock frequency also leads to lower switching losses in the pulse-controlled inverter, which in turn has a positive effect on the thermal behavior of the power components of the pulse-controlled inverter.

A particularly simple realization of the invention provides that the clock frequency of the pulse width modulated signal is reduced in steps. In this case, the clock frequency of the pulse-width-modulated signal can advantageously be reduced in each case when a speed threshold value is reached.

According to a further embodiment of the invention, the clock frequency of the pulse width modulated signal is an integer fraction of a channel spacing of a radio transmission system, such. B. a broadcasting system is. In this way it is achieved that the harmonics of the clock frequency are at transmitter frequencies of a transmitting unit of the radio transmission system, which allows use of the method according to the invention for avoiding disturbances of the radio transmission system.

Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying drawings.

Brief description of the drawings

Show it:

1 a schematic block diagram of a device according to the invention and

2 : Frequency spectra of a pulse width modulated control signal of a pulse inverter as a function of the clock frequency.

An electric machine 1 is through a pulse inverter 2 which in turn is controlled by a control unit via a pulse-width-modulated control signal SPWM 3 is controlled. A sensor unit 4 detects a speed n of the electric machine 1 characterizing size and transmits it to the control unit 3 , The control unit 3 then sets a clock frequency f _{clock of} the pulse width modulated signal SPWM as a function of this size, that is, of the rotational speed n or another variable representing the rotational speed n, a.

The clock frequency f _clock is thereby reduced continuously or stepwise with decreasing speed. It makes use of the fact that at lower speeds n and a lower clock frequency f _{clock of} the pulse width modulated signal SPWM sufficient to achieve a sufficient sine wave of the current. The reduction of the clock frequency f _clock leads to an advantageous reduction of the amplitudes of the harmonics of the clock frequency f _clock and thus to a reduction of the interference emission. A detailed description of this effect is in connection with 2 ,

To realize a stepped reduction can in the control unit 3 Thresholds be deposited. Falls below, the speed n the electric machine 1 characterizing size, one of these thresholds, the clock frequency is reduced by a predetermined amount. The amount of reduction can be constant or variable.

In 2 are schematic frequency spectra of the pulse width modulated control signal SPWM of the pulse inverter 2 shown. It is denoted by the reference numeral 20 denotes a first frequency spectrum f _{Takt_0} arises at a first clock frequency. A second frequency spectrum, which results at a second clock frequency f _{clock_1} , with f _{clock_1} = ½ × f _{clock_0} , is _denoted by the reference numeral 21 designated. It can be clearly seen that a reduction of the clock frequency f _clock , in the example case a halving, leads to a shift of a first bending frequency from a value f _{Knick_0} to a smaller value f _{Knick_1} . Since the amplitudes A of the harmonics but decrease from the first kink frequency f _kink , this shift of the knee frequency f _kink ultimately leads to a lower interference emission.

A preferred application of the invention results, if in the environment of the pulse inverter 2 controlled electric machine 1 also a receiving and / or transmitting device of a radio transmission system 5 is operated, the function of which can be affected by the frequency of the clock frequency f _{clock of} the pulse width modulated signal SPWM stemming emissions. The radio transmission system can be z. B. radio transmission system or a radio clock system. However, the invention can also be advantageously used in connection with other types of radio transmission systems. For example, there is no restriction on unidirectional transmission as used in broadcasting or radio clocks. Likewise, the invention also for bidirectional radio transmission systems, such as the radio or the like, apply.

A particularly advantageous embodiment of the invention will be explained below in connection with a radio receiver, which in the environment by the pulse inverter 2 controlled electric machine 1 is operated. Such a constellation arises, for example, in a hybrid or electric vehicle, in which the electric machine 1 is used as a drive unit and which are usually equipped with a radio receiver. Of course, the invention can also be used for other applications outside of the automotive sector in which pulse width modulated drive signals are used.

Is a basic clock frequency f _{reason of} the pulse width modulated signal SPWM an integer multiple of the channel spacing, ie z. B. used in Europe 9 kHz, the harmonics come to lie on the transmitter frequencies of the ampiltudenmodulierten radio station, which only leads to a slight disturbance of the radio reception. If the clock frequency f _{clock of} the pulse width modulated signal SPWM now starting from this basic clock frequency f _reason with decreasing speed n of the electric machine 1 always reduced such that each new clock frequency is an integer fraction of the channel spacing of the radio transmission system, the harmonics continue to be on the transmitter frequencies of the amplitude modulated radio transmitter. The disturbance of the radio reception can thus be reduced on the one hand, that the harmonics are respectively on the transmitter frequencies and on the other hand in that the amplitudes of the harmonics with decreasing speed of the electric machine 1 be reduced.

Concretely, this can, for. B. in the form that the controller initially sets a basic clock frequency f _reason of 9 kHz. When the speed n drops to a first predetermined speed threshold value, the clock frequency f _{clock is} lowered to 1/2 × 9 kHz = 4.5 kHz. If the speed continues to fall below a second predetermined speed threshold value, the clock frequency f _{clock is} reduced to 1/3 × 9 kHz = 3 kHz. This process can of course be continued depending on the application.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2008/2008/000655 A2 [0007]

Claims (7)

Method for controlling a pulse-controlled inverter ( 2 ), which for controlling an electrical machine ( 1 ), characterized in that a clock frequency (f _clock ) of a pulse width modulated control signal (SPWM) of the pulse inverter ( 2 ) depending on a speed (n) of the electric machine ( 1 ) characterizing size is set. A method according to claim 1, characterized in that the clock frequency (f _clock ) of the pulse width modulated signal (SPWM) with decreasing speed (n) is reduced. A method according to claim 2, characterized in that the clock frequency (f _clock ) of the pulse width modulated signal (SPWM) is reduced in steps. A method according to claim 3, characterized in that the clock frequency (f _clock ) of the pulse width modulated signal (SPWM) is reduced in each case upon reaching a speed threshold. Method according to Claim 3 or 4, characterized in that the clock frequency (f _clock ) of the pulse-width-modulated signal (SPWM) is in each case an integral fraction of a channel spacing of a radio transmission system ( 5 ). Device for controlling a pulse-controlled inverter ( 2 ), which for controlling an electrical machine ( 1 ), with a sensor unit ( 4 ) for detecting and transmitting a speed (n) of the electric machine ( 1 ) characterizing size to a control unit ( 3 ), whereby the control unit ( 3 ) a clock frequency (f _clock ) of a pulse width modulated control signal (SPWM) of the pulse inverter ( 2 ) depending on this size. Use of a control method according to claim 5 for avoiding disturbances of the radio transmission system ( 5 ).

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