DE102011052274A1 - Method and device for protection time compensation - Google Patents

Abstract

The invention relates to a method and a device for protection time compensation in an inverter for supplying an electric motor, in particular for a three-phase inverter with a pulse width modulation control. Moreover, the invention relates to a control for an electric motor. In the method for protection time compensation in an inverter for supplying a rotor having an electric motor (6) with phase voltages, in particular for a three-phase inverter with a pulse width modulation control (8), a by a guard time (T _A ) caused disturbance (Vs ) of a motor torque of the electric motor (6) is calculated taking into account a sampling time of the drive (8), whereby a voltage distortion (V _q ) considered in the dq system is minimized depending on the position of the rotor to provide a compensation of the phase voltages between two sampling times.

Description

The invention relates to a method and a device for protection time compensation in an inverter for supplying an electric motor, in particular for a three-phase inverter with a pulse width modulation control. Moreover, the invention relates to a control for an electric motor.

In the general state of the art, inverters are known that are suitable for converting a DC voltage into an AC voltage. Three-phase inverters are used, in particular, for controlling motors, which, on the one hand as a frequency converter, enable a frequency-dependent control of a three-phase motor and are generally used in applications with a high power requirement.

In its simplest form, a three-phase inverter has three inverters connected in parallel, each providing one phase of the output voltage. The inverters used are usually provided as a series connection of two power transistors. A center tap of the series circuit represents an output of the inverter circuit. The input voltage is applied across both transistors of the series circuit. The two transistors are usually provided as MOS-FETs or as IGBTs.

For driving the power transistors of a three-phase inverter, pulse width modulation methods (PWM) are known in which each branch of the three-phase inverter is individually controlled, so that the desired temporal development of the output voltage results. The so-called protection time must be observed. The guard time is defined as the amount of time to wait between turning off a MOSFET in one inverter branch and turning on the complementary MOSFET to prevent a hot branch. Thus, during the guard period, both MOSFETs of one inverter branch are turned off, resulting in a phase voltage disturbance. The fault occurs only at the time of the protection time, but can be converted to a complete PWM pulse.

It is furthermore known from the general state of the art that a transformation into the dq system can be carried out to describe the disturbances. In this case, for a control engineering view of three-phase systems, a geometric mapping is made to a two-dimensional d-q coordinate system which essentially corresponds to a transformation of three phase variables into polar-fixed coordinates. From this it can be seen that the phase voltage disturbances lead to the moment orders 6, 12, 18 ... (electrical).

The previously made compensation of the phase voltage disturbances takes place by means of negative connection of the calculated phase voltage disturbances to the reference voltages. Known priorities in the development of compensation methods so far have been the determination of the phase voltage disturbances and the determination of the sign changes of phase currents.

Thus, there is a need in the art to achieve further improvement in phase voltage noise.

The present invention is therefore based on the object to provide a method for protection time compensation in an inverter for supplying an electric motor, which is easily integrated into existing systems and also has an improved compensation of phase voltage disturbances.

This object is achieved by a method for protection time compensation in an inverter for supplying a rotor having an electric motor with phase voltages, in particular for a three-phase inverter with a pulse width modulation control, in which caused by a protection time disturbance of an engine torque of the electric motor, taking into account a Sampling time of the drive is calculated, wherein a dq system considered voltage distortion as a function of the position of the rotor is minimized to provide a compensation of the phase voltages between two sampling times.

In one embodiment of the invention, minimization is accomplished by calculating the effect of field weakening operation of the motor during the guard time.

In a further embodiment of the invention, the minimization takes place in that the disturbance is calculated for the current sampling instant, the preceding sampling instant and without compensation, the value subsequently being selected for further compensation which causes the least disturbance.

In a further embodiment of the invention, the minimization takes place in that the disturbance of the engine torque over a sampling pulse is minimal, preferably has the value zero.

In a further embodiment of the invention, the minimization takes place in that the disturbance of the engine torque takes place over one motor revolution by calculating a Fourier transform of the voltage distortion considered in the dq system.

In a further embodiment of the invention, the amplitude and a second correction factor adjusts the phase position by means of a first correction factor, so that the sampled compensation signal corresponds to the temporally continuous disturbance.

In a further embodiment of the invention, the compensation of the phase voltages due to a sign change of phase currents during the sampling pulse is required.

Furthermore, a storage medium is specified on which instructions are stored which are suitable for carrying out the method on a computer, preferably a microcontroller.

In addition, a drive for an electric motor is provided, which is designed so that it can perform the method.

According to the invention, the above object is also achieved by a device having an inverter for supplying a rotor having an electric motor with phase voltages, in particular for a three-phase inverter with a pulse width modulation control, in which a caused by a protection time disturbance of an engine torque of the electric motor is calculable taking into account a sampling time of the drive, so that a voltage distortion considered in the dq system is minimized as a function of the position of the rotor in order to output a compensation of the phase voltages between two sampling times.

The invention will be explained in more detail by means of embodiments with reference to the accompanying drawings.

In the drawings show:

1 a schematic representation of a device according to the invention or for carrying out a method according to the invention;

2 a diagram for comparison between compensation without consideration of the sampling time and without compensation; and

3 a diagram illustrating the function of the device according to the invention.

With reference to 1 An embodiment of the invention is shown below. 1 is a schematic representation of an apparatus for carrying out the method according to the invention.

1 shows a device 2 that is a voltage source 4 includes. The voltage source 4 is to supply an engine 6 , For example, a servo motor, provided. The servo motor is supplied with a three-phase supply line (in 1 labeled a, b and c). The motor 6 Provides a balanced, star-connected load of the power supply 4 and is from a control unit 8th controlled. The control unit 8th For example, it can perform pulse width modulation and have a microcontroller. In addition, the control unit 8th with the gate terminal of a variety of transistors 10 connected.

The transistors 10 are designed as three parallel inverters, each with gate lines 12 . 14 and 16 be controlled and provide a phase of the output voltage. Other input or Output signals can be sent to the control unit 8th be fed as in 1 schematically by an arrow 18 is indicated.

The inverters are each a series connection of two transistors 10 and the respective center taps of the series connection provide a phase of the output voltage across the output lines 20 . 22 and 24 to disposal. Because the input voltage across both transistors 10 the series circuit is applied, must be between turning off a transistor 10 in an inverter branch and turning on the complementary transistor 10 the guard time to wait for a hot branch, ie a short circuit of the power supply 4 , to prevent.

As already mentioned, the switching off of both transistors 10 an inverter branch during the protection period to a phase voltage fault. The fault occurs only at the time of the protection time and can be converted to a complete PWM pulse as a disturbance of the phase voltages ΔV _{a0, b0, c0} : ΔV _{a0, b0, c0} = -T _SZ · f _PWM · U _ZK · S _{a, b, c} = -ΔV · S _{a, b, c}

In the following description, the inverter is considered ideal except for the guard times. It contains no parasitic inductances or voltage drops. It always comes from a three-phase inverter with three inverter branches 20 . 22 . 24 went out.

In this case, T _SZ denotes the duration of the protection time for the transistors 10 , f _PWM the PWM frequency of the control circuit 8th , U _ZK DC link voltage and s _{a, b, c} the signum function of the phase currents a, b and c.

The sampling with the sampling time T _A by the microcontroller of the control unit 8th is limited by a given clock and complicates the corrections of the calculated phase voltage noise. If there is a sign change of s _{a, b, c} between two sampling steps, an incorrect compensation value is applied for the duration until the next sampling step. The consequences are in turn the moment orders 6, 12, 18 ..., which, despite known ΔV and s _{a, b, c} , become so great that compensation could remain ineffective.

The phase voltage disturbances can be derived from the phase voltage disturbances and result as follows:

After a transformation from the three-phase system (a, b, c) into the pole-fixed dq coordinate system, the voltage disturbances in the dq system can be represented:

In this equation, by considering φ, ie the angle of the current vector in the dq system, the dependence of the voltage disturbances in the dq system on the field weakening can be taken. The size k is calculated from the electrical rotor position angle and the angle of the current vector in the dq system. Assuming that ΔV _q mainly contributes to torque disturbances, there is a significant increase in field weakening disturbances.

The sign changes of s _{a, b, c} always occur at the rotor position positions θ _x when k takes the value 0. The sampling by the microcontroller before the sign change takes place at the rotor position positions: θ _- = θ _x - lωT _A and again after the sign change at θ ₊ = θ _x + (1 - l) ωT _A , where ω is the electrical angular velocity of the motor 6 and l takes a value between 0 and 1 and indicates where the current zero crossing lies between sampling times. The value 0 corresponds to a current zero crossing directly after the sampling and the value 1 only shortly before the next sampling.

It should be noted that this consideration is by no means limited to synchronous motors. Although the terms rotor position, motor rotation and the like are used below, this is not meant to be limiting. For example, the rotor position can generally be described as a position position of an excitation coil in a magnetic field, so that the following consideration, for example, for linear motor is feasible. The same applies to asynchronous motors.

According to previous compensation methods, the interference between the rotor position positions θ _- θ _x and properly compensated. However, an incorrect compensation value is impressed between the positions θ _x and θ ₊ since the sign change of S _{a, b, b can} only be taken into account with the following sampling. The resulting voltage distortion of V _q can be represented as follows for k = 0 ... (1-l) ωT _A : ΔV _q = -ΔV · 4 / 3⌊sin (-φ + k + 5 / 3π) - sin (-φ + k + 1 / 3π + 5 / 3π) ⌋

The voltage distortion results in a disturbance of the engine torque. One indicator of the size of the fault is the volt-seconds (Vs) applied to the motor 6 due to the voltage distortion. In 2 the values of Vs in a diagram are shown both without (right side) and with (left side) compensation of the guard times.

It can be seen that the values for Vs despite compensation for certain φ and (1 - l) ωT _{A have} larger values than without compensation. This effect stems from the sampling time T _{A of} the microcontroller of the control unit 8th and the associated false compensation input after the sign change from s _{a, b, c} to the next sample.

According to a first embodiment of the invention, when scanning the rotor position, the voltage distortion may be compensated either for the current rotor position, for the position at the next sample, or not at all. The selection of one of the three cases is made after calculating the resulting Vs for all three cases. The case with the smallest Vs is selected for the compensation.

The Vs without any compensation can be determined as follows: V without / s = sin (φ) - sin (φ + 1 / 3π + lωT _A ) + sin (-φ + 1 / 3π + (1 - l) ωT _A )

For the Vs in compensation with the current rotor position results: V actual / s = sin (φ) + sin (-φ + (1-l) ωT _A ).

And for the Vs in compensation with the rotor position at the next sampling results: V next / s = sin (φ) + sin (-φ - IωT _A ).

This method has the advantage that the 6th electrical order becomes minimal. However, higher orders 12, 18, ... are reflected by the constant controller frequency of the microcontroller and can also cause low-frequency interference, but the 6th electrical order is largely minimized.

A good approximation of this method gives the compensation with the rotor position: θ _{approximation} = θ _- + 1 / 2ωT _A.

In the following, a second embodiment of the invention will be explained. This method can not achieve such a large minimization of the 6th electrical order. However, in this method, the higher orders 12, 18, .... are not reflected by the controller frequency of the microcontroller of the control unit 8th ,

Accordingly, the ΔV required for compensation is calculated so that the Vs averaged over a PWM pulse give Vs = 0. The voltage distortion with the compensation according to this method is positive before the sign change of s _{a, b, c} and negative after the sign change. The correction factor results in the following relationship:

In the following, a third embodiment of the invention will be explained in more detail. As a result, a very good compensation of the 6th order is achieved and no further low-frequency disturbances occur.

durch die Fourierreihenentwicklung bis zur ersten Ordnung entwickelt. Weitere Ordnungen waren prinzipiell möglich sind aber im Allgemeinen nicht von Interesse, da die Amplituden im Vergleich zu der ersten Ordnung klein sind. Die Fourierreihenentwicklung ergibt für die Störung:

In contrast to the first two embodiments, in the third embodiment, not a single sampling step is considered, but an electric motor revolution. For this purpose, the voltage disturbances in the dq system are off

developed by the Fourierreihenentwicklung to the first order. Other orders were possible in principle but are generally not of interest because the amplitudes are small compared to the first order. The Fourier series evolution gives for the error:

A negative connection of the calculated disturbance to the nominal voltages in the dq system would compensate the 6th electrical order in the engine torque at very high sampling rates. At low (or realistic) sample rates, the calculated noise must be adjusted to compensate. For this purpose, the adjusted compensation signal is calculated, wherein a first correction factor adjusts the amplitude and a second correction factor, the phase position, so that the sampled compensation signal corresponds to the temporally continuous disturbance. With this compensation signal, it is now possible, despite low sampling rates, to very well compensate the 6th electrical order in the engine torque without causing further low-frequency interference.

The above calculations of the first and second correction factors are based on the simplification that the transformation from the dq system into the abc system is time-discrete, ie. H. only for every sampling step. For this reason, the 6th electric is not 100% compensated in practice. An adapted calculation of the first and the second correction factor makes it possible to take into account the effects of a continuous coordinate transformation.

It was described how the 6th electrical order can be compensated in the engine torque. The guard times but also higher orders, such as 12th, 18th, etc., the result. To compensate for this, the Fourier series must be further developed to the next order and the first and second correction factors adjusted accordingly.

The presented methods make it possible to minimize interference caused by a sign change of S _{a, b, c} between two sampling steps. The methods can be used as a program in the microcontroller of the control unit 8th be executed. Essentially, the following steps are performed.

In the first embodiment of the invention, a disturbance of the engine torque is calculated in each case without any compensation, with compensation for the current rotor position and with compensation for the next rotor position for each scan. Thereafter, that correction is chosen that causes the least voltage distortion.

In the second embodiment, the voltage required for compensation is determined so that the disturbance of the engine torque over a sampling pulse is minimal.

In the third embodiment, not a single sampling step is considered, but an electric motor revolution and the voltage disturbances in the dq system are developed from the Fourier series development to the first order.

The results of the presented compensation methods are in 3 summarized. It shows 3A a frequency spectrum that occurs without any compensation. 3B corresponds to the compensation method according to the first embodiment, 3C according to the second and 3D according to the third embodiment of the invention.

LIST OF REFERENCE NUMBERS

2

contraption

4

power supply

6

engine

8th

control unit

10

transistor

12

first gate lines

14

second gate lines

16

third gate lines

18

arrow

20

first output line

22

second output line

24

third output line

Claims (12)

Method for protection time compensation in an inverter for supplying an electric motor ( 6 ), in particular for a three-phase inverter with a pulse width modulation control ( 8th ), in which a by a guard time (T _A ) caused disturbance (Vs) of an engine torque of the electric motor ( 6 ) taking into account a sampling time of the drive ( 8th ), wherein a voltage distortion (V _q ) considered in the dq system is minimized to provide compensation for the phase voltage noise between two sampling instants. Method according to Claim 1, in which the minimization takes place in that the effect of a field weakening operation of the engine ( 6 ) during the guard time (T _A ). Method according to Claim 1 or 2, in which the minimization is carried out by calculating the disturbance (Vs) for the current sampling instant, the preceding sampling instant and without compensation, and subsequently selecting the value which causes the least disturbance for further compensation. Method according to Claim 1 or 2, in which the minimization takes place in that the disturbance (Vs) of the engine torque over a sampling pulse is minimal, preferably has the value zero. Method according to Claim 1 or 2, in which the minimization takes place in that the disturbance (Vs) of the engine torque takes place over one motor revolution by calculating a Fourier transform of the voltage distortion (V _q ) considered in the dq system. Method according to Claim 5, in which the Fourier transform uses a first correction factor to adjust the amplitude and a second correction factor adjusts the phase position, so that the sampled compensation signal corresponds to the time-continuous disturbance. Method according to one of claims 1 to 4, in which the compensation of the phase voltages due to a sign change of phase currents during the sampling pulse is required. Storage medium on which instructions are stored which are suitable for carrying out the method according to claims 1 to 7 on a computer, preferably a microcontroller. Control for an electric motor, which is designed so that it can perform the method according to claims 1 to 7. Device with an inverter for supplying a rotor having an electric motor ( 6 ) with phase voltages, in particular for a three-phase inverter with a pulse width modulation control ( 8th ), in which a by a guard time (T _A ) caused disturbance (Vs) of an engine torque of the electric motor ( 6 ) taking into account a sampling time of the drive ( 8th ) is calculable so that a voltage distortion (V _q ) considered in the dq system is minimized as a function of the position of the rotor in order to output a compensation of the phase voltages between two sampling instants. Apparatus according to claim 10, wherein the minimization is performed by calculating the disturbance (Vs) for the current sampling instant, the previous sampling instant and without compensation to output the value for compensation which causes the least disturbance. Apparatus according to claim 10, wherein the minimization is effected in that the disturbance (Vs) of the engine torque over a sampling pulse is minimal, preferably zero.

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