DE102016003738A1 - DC link voltage-dependent regenerative energy limit for electric drives - Google Patents

Abstract

The invention relates to a method for operating a load (4) driving electric motor (3) fed by a frequency converter (2) with voltage intermediate circuit (6) and regulated by a frequency converter (2) controlling motor control (10) in its speed is that it returns energy in the voltage intermediate circuit (6) during braking. During energy recovery, a current limit value (Mdlim, iqlim) for a manipulated variable (Mdeset, iqsetpoint) of the motor control (10) is determined as a function of the actual intermediate circuit voltage (Udc) from a differential energy (ΔEcap) which can be fed into the voltage intermediate circuit (6) and for limiting the power fed back into the voltage intermediate circuit (6) from the electric motor (3) at the moment limits a value of the manipulated variable (Mdoll, iqsoll) determined by the motor control (10) to the limit value (Mdlim, iqlim), if the value amounts to the magnitude of the limit value Exceeds (Mdlim, iqlim).

Description

The invention relates to a method for operating a load driving electric motor, which is fed by a frequency converter with voltage intermediate circuit and controlled by a frequency converter controlling motor control in its speed such that it feeds energy back into the voltage intermediate circuit during braking. In particular, the invention relates to a limitation of the energy fed back of the electric motor as a function of the current DC link voltage. Moreover, the invention relates to a drive in which the method according to the invention is implemented.

Today's frequency-driven drives are not only able to convert electrical energy into mechanical energy, but also to convert the energy stored in the rotation back into electrical energy. This is done by active electrical braking, d. H. when the drive is driven at positive speed with a current generating a negative torque. This negative torque then acts as a braking torque, causing the engine to stop faster than unrestrained coasting. The electric motor is then operated in the generator area. The electric power supplied by the electric motor is then fed back into the frequency converter.

Such an operation in an electric motor is called a four-quadrant operation, such a frequency converter is called a four-quadrant controller. This term is based on the four quadrants of a coordinate system in which the speed and the torque are the coordinates. Have speed and torque the same sign, is there a motor operation of the electric motor, the signs are different, there is generator operation.

Frequency converters usually consist of at least four components, a rectifier, an inverter, an intermediate energy storage and control electronics. As a rule, the energy store is one or more capacitors, which are charged by the rectifier to a specific DC voltage. The one or more capacitors thereby form a so-called voltage intermediate circuit.

The rectifier is connected to at least one AC voltage source, for example to an AC voltage network or to a three-phase network. For this reason, the rectifier is also referred to as a network converter. As a rule, it comprises uncontrolled power electronic semiconductor switches, for example diodes. The energy flow is therefore here from the AC voltage network to the DC link.

The inverter generates from the DC voltage of the DC link a single- or multi-phase AC voltage for the corresponding electric motor, which may be, for example, a synchronous motor or an asynchronous motor. The inverter is therefore also referred to as a motor inverter. It comprises controlled power electronic semiconductor switches, for example high-performance transistors such as MOSFETs, IGBTs or IGCTs, which are controlled by the control electronics using a suitable control method.

A common control method is here, for example, the pulse width modulation, which receives a one-or corresponding to the phases to be switched multi-dimensional control variable from a higher-level controller. This controller can, for example, perform a field-oriented control (FOR) or a flow-based control such as Direct Self-Regulation (DSR), Indirect Stator Control (ISR) or Direct Torque Control (DTC). The methods are well known to the person skilled in the art, so that reference is made here to the relevant specialist literature. The regulated quantity is either the current, the torque, the magnetic flux or the power. Since the controller is part of the control electronics, this forms a control and regulation electronics in the true sense. The regulator in turn gets its desired value from a superimposed, drive-related controller, for example a speed controller, by means of which a specific speed is adjusted in the electric motor. The first-mentioned controller is subordinate or underlaid to this second-mentioned controller. Since the second controller controls or regulates a drive-related variable, and not just the frequency converter, the control electronics forms a motor control.

A setpoint input for the drive-related controller can be done, for example, directly to the control and regulation electronics as the desired constant speed. However, it is in the context of another load-related control, for example in pump units for heating or cooling systems, for pressure booster systems in the drinking water supply or wastewater pumps also make a characteristic control in which, for example, a specific setpoint speed from the demand for an operating point on the curve follows , The characteristic can, for example, a relationship between two hydraulic Describe sizes of the pump set, for example the delivery head or the delivery pressure (differential pressure) and the flow rate. The load-controlled variable is then, for example, the delivery pressure or differential pressure between the suction and the pressure side of the pump. The control electronics then forms a motor / load control, in particular a motor / pump control.

Especially with pumps, the regenerative operation of the driving electric motor is important, because it may be that the impeller of the pump is driven by an external flow, so that the impeller rotates faster than desired and consequently must be braked. The pump then acts like an electric power generating turbine. In the simplest case, the electrical energy fed back from the drive into the intermediate circuit can be converted into heat unused, in which a low-impedance braking resistor is temporarily connected in parallel with the capacitor for the braking operation. It goes without saying that this procedure is not economical. Therefore, the aim is to make the recovered energy available for the DC link.

However, there is the problem that the DC link is increasingly charged by the regenerative current and possibly overcharged when a maximum voltage limit is exceeded at the DC link capacitor. For this reason, it is necessary to limit the recovery power. Although, in principle, the energy fed back into the intermediate circuit could be transmitted further by the network converter into the AC voltage network. However, the power converters are usually not capable of being regenerated, since they comprise uncontrolled power electronic components, for example diodes, in order to convert the AC voltage present on the input side into a DC voltage.

The simplest way to prevent overvoltage in the DC link is to prevent the flow of energy into the DC link, ie to prevent the regenerative operation of the electric motor. For this purpose, the frequency converter is controlled such that at positive speed no negative torque generating current, or at negative speed no positive torque generating current is allowed. The electric motor is thus always operated with the torque equivalent torque to the speed or no torque. Such a procedure is from the European patent application EP 1 206 040 A2 known.

However, since there are electrical losses in the motor and in the frequency converter, this requirement can be softened a bit and at the moment the regeneration of a braking energy can be allowed, which corresponds to the sum of the power losses / loss energy of the engine and the electronics. The DC link voltage does not increase and the electrical losses are covered by the braking energy. This ensures a balanced energy balance in the DC link.

A further development of this approach for DC link voltage limitation is to allow a flow of energy into the DC link and continuously monitor the DC link voltage. Only when a specified limit value for the intermediate circuit voltage is exceeded is the energy flow in the intermediate circuit prevented by the control and regulation electronics of the frequency converter or the motor control, so that an increase in the intermediate circuit voltage is prevented. The torque of the motor is then adjusted so that the energy flow into the DC link becomes zero. This therefore corresponds to a type of 2-point control: below the limit, the entire braking energy is fed fully into the DC link, ie. H. the lower-level controller is not deliberately controlled differently here; If the voltage limit is reached, the energy flow into the DC link is completely prevented.

This procedure can be optimized by using a DC link voltage regulator. For this purpose, an additional control loop is constructed, which receives the difference between the current DC link voltage and a maximum permitted DC link voltage, for example, the said limit, as an input variable. The output variable of the intermediate circuit voltage regulator is then a regenerative power to be provided by the motor control at the respective time.

All solutions have the disadvantage that the physically possible maximum energy recovery in the DC link is not used at the moment, since the motor control consists of the superimposed and the downstream, subordinate controller, the latter adjusts the energy flow in the motor / DC link. Since this subordinate control (as all regulations) has a certain response time until the required size required by it is implemented, all the aforementioned methods can not optimally feed energy back into the DC link.

In the former and in the EP 1 206 040 A2 described method, the feedback is suppressed per se.

With 2-point control, the maximum voltage limit must be selected such that an overshoot of the subordinate controller (and thus an overshoot of the regenerated energy) does not overload the DC link. This means that the limit value for the intermediate circuit voltage with a corresponding safety distance must be selected smaller than the maximum permissible DC link voltage, which essentially corresponds to the rated voltage of the DC link capacitor or the sum of the series-connected DC link capacitors.

When using a DC link voltage regulator, a time constant must be taken into account. Furthermore, the lower-level controller with its specified time constant must also be taken into account here. A DC link voltage regulator can not be readily integrated into the motor control structure described above, because other control objectives of the same control or manipulated variables are given by the existing controller.

In summary, none of the known methods for intermediate circuit voltage limiting takes into account the subordinate controller. This leads to all known methods that at the moment not the maximum possible power can be fed back into the DC link, but less power. This is not optimal.

It is therefore an object of the present invention to provide a method and a drive, in which the frequency converter is controlled so that at any time the maximum possible power is fed back from the electric motor into the DC link.

This object is achieved by a method having the features of claim 1 and by a drive having the features of claim 13. Advantageous developments are specified in the respective subclaims.

According to the invention, a method for operating an electric motor is proposed in which a current limit value for a manipulated variable of the motor control is determined as a function of the current DC link voltage from a differential voltage which can be fed into the voltage intermediate link, and for limiting the energy fed back from the electric motor into the voltage intermediate link from the motor control determined value of the manipulated variable is limited to the limit value if the value exceeds the value of the limit value.

• – Mittel zur Erfassung der aktuellen Zwischenkreisspannung des Spannungszwischenkreises,
• – eine Grenzwertermittlungseinrichtung zur Ermittlung eines aktuellen Grenzwerts für eine Stellgröße der Motorregelung aus einer in den Spannungszwischenkreis einspeisbaren Differenzenergie in Abhängigkeit der aktuellen Zwischenkreisspannung,
• – eine Stellgrößenbegrenzungseinheit zur Begrenzung der Stellgröße auf den ermittelten Grenzwert zur Begrenzung der in den Spannungszwischenkreis vom Elektromotor zurückgespeisten Energie.

The invention further proposes a drive comprising an electric motor for driving a load, a frequency converter with a voltage intermediate circuit for supplying the electric motor and a motor controller for controlling the frequency converter and regulating the rotational speed of the electric motor, wherein the drive further comprises:

• Means for detecting the actual intermediate circuit voltage of the voltage intermediate circuit,
• A limit value determination device for determining a current limit value for a manipulated variable of the motor control from a differential energy which can be fed into the voltage intermediate link as a function of the current DC link voltage,
• A manipulated variable limiting unit for limiting the manipulated variable to the determined limit value for limiting the energy fed back into the voltage intermediate circuit from the electric motor.

The basic idea of the invention is based on determining the maximum permitted amount of energy that can be fed back into the DC link without overloading it, ie. H. to exceed a voltage upper limit. In contrast to the prior art, in the method according to the invention, therefore, no voltage limit value for the intermediate circuit voltage but an energy limit value is used as the basis. Furthermore, no fixed limit value is used whose exceeding causes a change in the engine control. Rather, a voltage limit is used, which is adjusted as a function of the current DC link voltage, in particular is calculated repeatedly. In each case, the energy which can be fed into the voltage intermediate circuit at the current time is determined which may still be fed in until a certain voltage limit is reached. The feedable differential energy is thus the difference between the energy stored in the voltage intermediate circuit at the voltage limit and the energy currently stored in the voltage intermediate circuit.

The solution presented here for limiting the intermediate circuit voltage in a drive system when regenerating energy from the engine into the intermediate circuit takes into account the time constant that the motor control or at least part of the motor control, in particular a subordinate controller, requires to set a specific setpoint. It can thus not come to an overshoot, whereby the voltage intermediate circuit would be compromised. For this reason, no safety distance to a maximum voltage in the DC link is required as in the prior art, so that the inventive method allows and enables the maximum return power from the engine to the DC link at any time. Although the inventive method stationary leads to an identical behavior as in a DC link voltage regulator, but because of the lack of the intermediate circuit voltage controller forming control loop, more energy can be dynamically fed back into the DC link and thus the existing physical drive system can be better utilized.

The difference energy may be, for example, the energy difference between the energy stored in the voltage intermediate circuit at the time of the current intermediate circuit voltage and the energy stored at a maximum intermediate circuit voltage in the voltage intermediate circuit. In particular, the difference energy can be calculated from this energy difference. As already mentioned, this achieves the result that the regenerative energy limit according to the invention has no fixed limit value for limiting the regenerative energy. Rather, the energy recovery is allowed at any time, provided that the voltage intermediate circuit is still able to absorb more energy. The maximum DC link voltage is determined by the rated voltage of the capacitor or capacitors installed in the DC link, or by other limiting components.

Alternatively, the differential energy can be calculated from the energy difference between the energy stored at the instantaneous DC link voltage in the voltage intermediate circuit and an energy which is smaller than that stored at a maximum DC link voltage in the voltage intermediate circuit, for example 1% to 5% smaller. This ensures that the DC link is not charged to its full voltage. The capacitor or capacitors are thereby spared and last longer.

Preferably, the motor control may have a subordinate first controller and a higher-level second controller, wherein the first controller is arranged downstream of the second controller control technology. In this case, the manipulated variable may be the output variable of the second regulator, which is then an input variable of the first regulator, d. H. forms a setpoint for the first controller. It follows that in this embodiment, the manipulated variable limiting unit is located between the first and the second controller. An output of the first regulator can then act directly or indirectly on the frequency converter.

In the manipulated variable limiting unit, the current value of the manipulated variable can be compared with the current limit value and, when it is exceeded, preferably equated with the current limit value. The current limit value can be made available to the manipulated variable limiting unit by a limit value determination device, which in turn is supplied with the current intermediate circuit voltage. The DC link voltage can be measured directly or calculated from another measured variable or alternatively determined from an observer.

According to one embodiment, the higher-level second controller may be a speed controller. This can receive its desired value, for example, by means of a manual specification or by a load controller which is the primary controller of the second controller. This load controller can perform, for example, a characteristic control, in the case of a pump unit as a load, for example, one of the usual pump characteristics control. Known pump characteristic curves are, in particular, keeping the differential pressure constant over the volume flow (Δp-c) or regulating the differential pressure according to a dependency, for example a linear dependence on the volume flow (Δp-v). The actual value, d. H. the current speed, the higher-level second controller can be obtained from a speed measurement in the drive or from an observer, who mathematically describes a technical model of the drive. At its output, the higher-level speed controller provides the manipulated variable for the subordinate first controller.

According to another embodiment variant, the higher-order second controller may be a power controller which adjusts a specific mechanical or hydraulic power on the electric motor or pump unit. Analogous to the speed controller, the power controller can also receive its setpoint value, for example, by means of a manual specification or by a load controller which is the primary controller of the second controller. Again, the load controller, for example, a characteristic control, in the case of a pump unit as a load beispielseiweise perform the above-mentioned pump characteristics control. The actual value, ie the current hydraulic or mechanical power, the higher-level second controller can be obtained from a power measurement in the drive or from an observer, who mathematically describes a technical model of the drive. At its output, the higher-level power controller provides the manipulated variable for the subordinate first controller.

The lower-level first controller depends on the control and control mode of the frequency inverter, which in turn takes into account the type of electric motor. According to one embodiment, the subordinate first controller is a current regulator. This suitably sets the current that is necessary to build up a torque in the electric motor. This allows a field-oriented control (FOR) of the frequency converter, including the electric motor, to be carried out. The subordinate first regulator may alternatively be a flow regulator which, for example, adjusts the stator flux. Hereby, a flow-based control, for example an ISR or DTC of the frequency converter, including the electric motor, can be carried out.

In the embodiment of the motor control with a subordinate first controller and a superimposed second controller, the second controller outputs the manipulated variable as the desired value for the subordinate first controller. In this embodiment, then the limitation of the value of the manipulated variable to the limit value preferably takes place between the second controller and the first controller, so that the first controller always receives only a maximum permissible value of the manipulated variable at the moment so as not to overload the intermediate circuit.

The task of the current controller is to impress a torque-forming current in the electric motor, d. H. To energize the electric motor so that it builds up a certain torque to accelerate or decelerate the rotor to achieve or maintain a certain speed. For this purpose, the current controller either directly set the target torque to be set or a desired current, with which the torque to be set is achieved. In terms of its physical mode of operation, the current controller is thus a torque controller.

The task of the flow regulator is to form a torque-forming magnetic flux in the electric motor. The flux controller acts to regulate the difference in the magnetic flux between the rotor and stator in such a way that a certain torque is achieved. However, the controlled variable here is not the torque or the torque-generating current but the torque-carrying magnetic flux. For this purpose, the flow controller can either be given a specific setpoint flow to be set with which the torque to be set is achieved, or alternatively a setpoint torque can be preset.

According to the aforementioned embodiments of the first and second controllers, the manipulated variable may be, for example, a torque, a torque-forming current component or a torque-equivalent variable, such as a magnetic flux. This torque, the current component or the torque equivalent quantity then form a desired value for the subordinate first controller.

According to a preferred embodiment, the engine control performs a field-oriented control, which is well known to the skilled person. In this case, the stator current is considered vectorially, with the two current components forming a current vector being transformed into a coordinate system rotating with the rotor. This results in a first current component i _d , also called field-forming current or d-current, which causes exclusively the formation of the electromagnetic field, and an orthogonal thereto second current component i _q , also called torque-generating current or q-stream, which exclusively the formation of the Torque causes. d and q form the axes in the rotating rotor-related coordinate system. However, d and q are not exclusive to field and torque. For example, the d-component can also be torque-forming. This is the case, for example, with the MMPA method (maximum torque per ampere).

According to the embodiment in which the motor control performs a field-oriented control, the manipulated variable may be the second current component i _q .

In the method according to the invention, it is particularly advantageous if the determination of the limit value takes place on the assumption of a lossless electric motor and / or frequency converter. As a result, despite the use of the maximum voltage limit of the voltage intermediate circuit for calculating the differential energy, there is a safety distance to the overload limit of the intermediate circuit. For a real consideration of the drive energy losses or efficiency losses must be considered, which act in the case of energy recovery from the electric motor to the intermediate circuit advantageous because they reduce the incoming energy in the DC link.

oder

ermittelt werden, wobei

C_DC

eine Kapazität des Spannungszwischenkreises ist,

U_DC

die aktuelle Spannung im Spannungszwischenkreis (6),

U_DC,max

eine maximale Spannung im Spannungszwischenkreis (6),

p

die Polpaarzahl des Elektromotors,

i_d

eine Feld bildende Komponente des Motorstroms,

i_q,lim

der Grenzwert einer Drehmoment bildenden Komponente des Motorstroms,

Md_lim

der Grenzwert für das Drehmoment,

L_d

die Statorinduktivität des Elektromotors in Richtung der d-Achse eines mit dem Rotor umlaufenden Koordinatensystems,

L_q

die Statorinduktivität des Elektromotors in Richtung der q-Achse eines mit dem Rotor umlaufenden Koordinatensystems,

ψ_PM

der magnetische Fluss eines permanenterregten Rotors des Elektromotors,

Ω

die aktuelle mechanische Winkelgeschwindigkeit des Rotors des Elektromotors, und

Δt

ein Zeitraum ist, während dem Energie in den Spannungszwischenkreis gespeist wird und den die Motorregelung benötigt, um einen vorgegebenen Sollwert von einem beliebigen Istwert zu erreichen.

For example, the limit may be one of the equations

or

be determined, where

C _DC

is a capacitance of the voltage intermediate circuit,

U _DC

the current voltage in the voltage intermediate circuit ( 6 )

U _{DC, max}

a maximum voltage in the voltage intermediate circuit ( 6 )

p

the pole pair number of the electric motor,

i _d

a field forming component of the motor current,

i _{q, lim}

the limit of a torque-forming component of the motor current,

Md _lim

the limit value for the torque,

L _d

the stator inductance of the electric motor in the direction of the d-axis of a coordinate system rotating with the rotor,

L _q

the stator inductance of the electric motor in the direction of the q-axis of a coordinate system rotating with the rotor,

ψ _PM

the magnetic flux of a permanent-magnet rotor of the electric motor,

Ω

the current mechanical angular velocity of the rotor of the electric motor, and

.delta.t

is a period of time during which energy is fed into the voltage intermediate circuit and which the motor control requires in order to achieve a predetermined desired value from any actual value.

It should be noted that in the present equations, in addition to neglecting the efficiency losses, the change of the current within the period Δt is neglected, resulting in a simplification of the equations and a further safety margin to the maximum intermediate circuit voltage.

The limit value Md _lim can be used both as a limit value for a current controller and for a flux controller as a subordinate first controller, which has the torque as an input variable.

In this case, the period .DELTA.t may preferably be three times the time constant (r) of the subordinate first regulator. For example, if the subordinate first controller is a PI controller, then the period Δt = 3τ is the time required to reach 95% of the setpoint after a sudden change in the setpoint. For example, if 0 A is required, it takes this period Δt = 3τ until this value is adjusted to 5%. If, for example, 5 A could still be supplied at the current time, but the control is only called again after the time period Δt = 3τ, then the intermediate circuit voltage value would be too large. For this reason, the calculation must be carried out in every controller cycle because the DC link voltage is constantly changing. Consequently, the adaptive limit values must be recalculated at each controller clock.

When calculating the current limit, it may still make sense to limit it to a maximum value. This is advantageous if, for physical reasons, the drive can dynamically feed back only a limited amount of energy per unit of time into the DC link and this limited amount could well be exceeded. The current limit itself does not take this case into account. It can be higher than a maximum value, for example when the DC link is charged to less than 30% of its rated voltage. In this case, an excessively high regenerative current at the moment can, for example, severely burden the electronic power semiconductor switches of the motor converter. In order to prevent this, the ascertained current limit value may be limited to this maximum limit value when exceeding a maximum limit value. This limitation may preferably also be carried out by the limit value determination device.

Further features and advantages of the method according to the invention are explained below with reference to an embodiment and the attached figures. Show it:

1 : Block diagram of an exemplary construction of a drive according to the invention

2 : Exemplary profile of a limit value for the torque as manipulated variable as a function of the DC link voltage during electric braking and positive speed

3 : Exemplary profile of a limit value for the torque as manipulated variable as a function of the DC link voltage during electric braking and negative speed

4 : History of the torque limit analog 1 with additional, exemplary torque actual value.

5 : Flowchart of an exemplary sequence of the method according to the invention

1 shows a block diagram of an exemplary construction of a drive according to the invention 1 consisting of a frequency converter 2 an electric motor 3 , a load machine 4 and a control and elec- tronics, here because of their function as a motor control 10 referred to as. The electric motor 3 , For example, a permanent magnet synchronous motor is mechanically with the load machine 4 coupled and drives this. The load machine 4 is for example a pump unit. The electric motor 3 is from the frequency converter 2 fed, at its output three phase voltages for the three-phase motor 3 outputs. According to another embodiment, the electric motor 3 be single phase.

The frequency converter 2 consists of a rectifier 5 , an inverter 7 and a voltage intermediate circuit between them 6 , The voltage intermediate circuit comprises at least one capacitance C _DC , which in turn is provided by one or more capacitors 8th is formed. In addition, a braking resistor 9 parallel to the capacitor 8th lie.

The rectifier 5 is intended to be connected to an alternating voltage network which supplies, for example, an alternating voltage of 230 V or 110 V. Alternatively, a rectifier can also be used 5 used, which is operable on a three-phase network. The rectifier 5 includes uncontrolled electronic power semiconductor switches, such as diodes, which convert the AC voltage of the AC voltage network into a DC voltage and thus the DC link 6 charge. Instead of the diodes, the rectifier can also have controlled semiconductor switches, for example MOSFETs. From the perspective of the DC link 6 is the rectifier 5 So a DC voltage source. The energy flow is therefore here from the AC voltage network to the DC link 6 , Due to the lack of controllability of the semiconductor switch is the rectifier 5 unable to feed energy back from the DC link to the AC mains.

The inverter 7 generated from the DC voltage of the DC link 6 here a three-phase AC voltage for the electric motor 3 , It includes controlled power semiconductor electronic switches, such as high-performance transistors such as MOSFETs, IGBTs or IGCTs used by the engine electronics 10 be controlled.

In the embodiment described here, the motor control controls 10 the inverter due to a field-oriented control of the electric motor 3 , In this type of control, the flux and the torque of the electric motor 3 regulated separately from each other. It is a so-called vector control in which the engine sizes are considered as vectors with magnitude and angle. In the field-oriented control, therefore, on the one hand, the amplitude and the orientation of the flow and, on the other hand, the amplitude and the orientation of the torque are adjusted separately from one another as a function of the corresponding actual values.

It should be noted, however, that the method according to the invention can also be used in other types of control. At the in 1 as a block illustrated engine control 10 only those control components are shown that are relevant to the practice of the invention and to understanding. However, other components may also be present or other components are present in another type of control. So is in 1 For example, no flow controller, but only the torque controller shown.

Concretely, the motor control according to the example described here at relevant components comprises a first controller 11 and a second 12 , a modulator 15 and a limit value determination unit according to the invention 13 together with control variable limiting device 14 , The second controller 12 is the first controller superimposed / superior, so that the first controller 11 forms a subordinate controller. According to the embodiment in 1 the first controller is a torque controller for setting a torque Md at the engine 2 , This is done by the first controller for each of the three phases of the electric motor to be controlled outputs a control voltage u _a , u _b , u _c , which then in the first controller 11 downstream modulator 15 is converted into switching signals, by means of which in turn the semiconductor switches of the inverter 7 be controlled. The modulator 15 controls the semiconductor switch with one of the usual modulation method for variable speed drive, for example, with a pulse width modulation.

The second controller 12 is a speed controller, by means of which a certain speed at the electric motor 3 is set. The input quantity is the speed controller 12 the actual speed n _is that it receives from a speed measurement or from an observer. The speed controller 12 gives at its output a setpoint Md _soll for the subordinate torque controller 11 out.

The core of the method according to the invention is the inter-circuit voltage-dependent limitation of a control value of the motor control 10 to limit the recovery of energy from the electric motor 3 in the DC link 6 , wherein the control value in this embodiment corresponds to the aforementioned torque setpoint Md _soll , ie that of the second controller 12 to the subordinate first controller 11 predetermined setpoint.

This is done by the DC link capacitor 8th calculates absorbable differential energy .DELTA.E _cap , which can be impressed in the _DC link to charge him from the current voltage U _DC to the predetermined maximum voltage U _{DC, max} . The in the DC link 6 stored energy is that in the capacitor 8th stored energy, which can be calculated as follows: E _cap = 1 / 2C _DC U 2 / DC

A calculation option for the differential energy ΔE _cap is therefore given by the following equation G1, which is the energy difference of the maximum voltage U _{DC, max} in the _{DC link} 6 stored energy and at the current intermediate circuit _voltage U _DC in the _DC link 6 stored energy describes. dE _cap = 1 / 2C _DC U 2 / DC, max - 1 / 2C _DC U 2 / DC (G1)

Determination of mechanical power P _{mech, the engine} of any engine 3 , such as As a permanent magnet synchronous machine, asynchronous machine, reluctance machine or the like, can be represented as a function f _P of the rotational speed n and the torque Md. P _{mech, motor} = f _P (M _d , n) (G 2) where Md is the torque and n is the speed.

It takes into account that the engine 3 assigned torque control 11a a period .DELTA.t is required to reach a predetermined setpoint Md _, starting from any actual value, the .DELTA.t in this intermediate period 6 powered energy E _{mech, engine described} by a function f _P ':

The period Δt is suitably three times the time constant τ of the subordinate first regulator 11 selected. After this period, the actual value has reached 95% of the setpoint to be set. In order to meet the specification of a _DC bus voltage U _DC that is limited at all times, it must be the one from the motor 3 over the period Δt = 3τ in the DC link 2 traceable power can be limited at any time to a value less than or equal to the maximum allowable amount of absorbed energy ΔE _{cap of} the DC _link . This means that the electric motor 3 must be regulated so that in the intermediate circuit 6 powered energy E _{mech, motor} smaller than the maximum allowable amount of energy ΔE _cap , ie less than the differential energy is ΔE _cap or greater than -ΔE _cap . This applies to both quadrants, in which the electric motor can run in generator mode, so you have to work with the amounts here. | E _{mech, motor} | ≤ | ΔE _cap | (G4)

This inequality can be solved analytically or numerically and z. B. are resolved according to the torque Md, if, as in the present embodiment, the manipulated variable to be limited is the torque. It then follows from inequality G5: | M _d | ≤ | f _E (Δ _cap, n, At) | (G5)

Alternatively, equation (G4) may also be resolved after the recovery power P _{mech, motor,} or other corresponding quantities, and then limited. In this case, this regenerative power or the other size form the manipulated variable to be limited according to the invention.

With equation (G5) one obtains a limitation for the torque. This defines a limit value Md _lim for the manipulated variable torque: | Md _lim | = | f '/ E (ΔE _cap , n, Δt) | (G5a) Md _lim = f '/ E (ΔE _cap , n, Δt) for n <0 (G5b) -Md _lim = -f '/ E (ΔE _cap , n, Δt) for n> 0 (G5c)

Since the differential energy ΔE _{cap is} dependent on the current _DC link voltage U _DC , the limit value Md _lim is also a currently valid variable, ie variable as a function of the intermediate circuit _voltage U _DC .

The limitation is then z. B. on the input of the subordinate torque controller 11 be applied so that only the one regenerative (regenerative) operation causing torque component is limited. Thus, at positive speed, a limitation of the negative torque (Md not less than -Md _lim ), at negative speed, a limitation of the positive torque (Md not greater than + Md _lim ) reached.

Advantageously, in the above derivation, the losses of engine 3 and electronics 2 neglected. The energy which can be fed into the intermediate circuit is thus equated with the mechanical power of the electric motor. Because in the case of consideration, only more energy could be fed back, because these "extra energy" fed back by the engine is in any case only converted into the losses mentioned, that is, not at all to increase the intermediate circuit voltage. In that regard, an execution of the method due to the inequality G5 always automatically a safety margin to the maximum voltage limit of the DC link 6 held. For an exact calculation, however, these losses must also be taken into account. In this case, inequality G4 is to be extended as follows: | E _{mech, engine} | <| ΔE _cap | + | E _{loss, elec} | + | E _{loss, engine} | (G6) where E _{loss, elec} the energy loss of motor inverter 7 and DC link 6 in the period Δt and E _{loss, engine} the loss energy of the engine 3 in the period .DELTA.t are. In the case of an exact calculation of the energy, it would also be necessary to take into account a Δt at which 100% of the new setpoint value is reached in order to avoid an overshoot of the intermediate circuit voltage.

Accordingly, analogously to inequality G5, the limit value for the torque Md is: | M _d | <| f '/ E (ΔE _cap , n, Δt, E _{loss, elec} ' E _{loss, engine} ) | (G7) | Md _lim | = | f '/ E (ΔE _cap , n, Δt, E _{loss, elec} , E _{loss, motor} ) | (G7a)

As is clear from inequalities G5a and G7a in conjunction with equation (1), the limit value _Mdlim for the torque Md selected here as the manipulated variable changes, in particular depending on the actual intermediate circuit voltage U _DC, until the limit value _Mdlim becomes zero, if the _DC link voltage U _DC has reached exactly the maximum intermediate circuit _voltage U _{DC, max} .

A graphic representation of the limit value course 16 , ie the calculated maximum braking torque Md _lim as a function of the intermediate circuit _voltage U _DC is for positive speeds as an example in 2 shown. A corresponding representation of the limit value course 17 for negative speeds shows by way of example 3 , However, the gradients may also look different and are intended to be illustrative only.

The limit value curves show a limit value Md _{lim which} becomes smaller as the _DC link voltage U _{DC increases} . If the intermediate circuit voltage U _{DC reaches} the maximum intermediate _{circuit voltage} limit U _{DC, max} , the limit value becomes zero in each case. In addition, the limit Md _lim in the direction of increasing limits, ie falling _DC link voltage U _DC limited by a maximum value Md _max . The maximum torque Md _{max is} a size predefined by the drive system. As the intermediate circuit voltage U _{DC increases} , a stricter maximum torque limitation determined from the equations G5a, G7a derived above is applied. The limitation of the limit value Md _lim to the physically determined torque _maximum Md _max occurs in the figures in each case starting at a first intermediate circuit voltage U _{DC, Limit, 1} .

Below this voltage _limit U _{DC, limit, 1} , more energy per unit of time could in principle be fed into the _{DC link} than the system can endure mechanically or electrically.

The calculation of the current limit value Md _lim for the manipulated variable, in this case for the setpoint torque Md _soll , takes place in a limit value determination device, which in 1 with the reference number 13 is provided.

4 shows in extension the 1 a graphical representation of the intermediate circuit voltage dependent limit value curve 16 Md _lim (U _DC ) for positive speeds with additional, purely exemplary course 18 of any computed from the speed control _to the target torque Md. In this representation, it becomes clear that, although there is a current limit value Md _lim at any time, it is not absolutely necessary to permanently limit the manipulated variable, ie here the setpoint torque Md _soll . Because below a second intermediate circuit _voltage U _{DC, 2} is the speed controller 12 Determined setpoint torque Md _should not be greater than the corresponding limit value Md _lim .

A limitation is only effected between the second intermediate circuit _voltage U _{DC, 2} and a third intermediate circuit _voltage U _{DC, 3} . This bounding area is indicated by reference numeral 19 provided and hatched in 3 highlighted. Even above the third intermediate circuit voltage U _{DC, 3 there} is no limitation of the setpoint torque Md _soll , because it is smaller than the current manipulated variable limit value. The limitation according to the invention therefore does not have to have an immediate effect on the engine control. In the example of 3 is from the lower-level torque controller 11 the setpoint Md _should correspond to the curve 18 implemented. Only when the setpoint Md _{is to reach} the limit value Mdlim recalculated for each intermediate circuit _voltage U _DC does a limitation of the setpoint value of the subordinate control loop occur, hatched area 19 ,

With regard to the block diagram in 1 is the limitation of the desired torque Md _soll of a manipulated variable limiting device 14 performed, the control technology between the first and the second controller 11 . 12 or with regard to the signal flow before the first controller 11 and behind the second regulator 12 lies.

The inventive method will now be described with reference to the flowchart in 5 explained, according to the embodiment according to 1 the manipulated variable the setpoint torque Md _{is intended} for the drive 1 is which of a first regulator 11 set and by a second regulator 12 provided.

_Is Starting n of the current speed and to reach a rotational speed n _soll determined, the speed controller 12 in step 12a the necessary torque Md _{is supposed} to be the electric motor 3 to accelerate from the actual speed to the set speed (acceleration torque) or decelerate (braking torque). In the command value limiting unit 14 then takes place in step 14a checking whether the determined setpoint torque Md _{should be} greater than that of the limit value determination device 13 certain current limit Md _lim is.

If this is not the case (NO branch), the determined torque setpoint Md _shall remain unchanged at the torque controller 11 given, the appropriate torque control 11a performs. Consequently, that remains of the speed controller 12 Determined setpoint torque Md _is the input variable for the torque controller 11 , Here, it determines the reaching of the torque Md _to necessary phase voltages u _a, u _b, u _c, he the downstream modulator 15 in turn, as nominal values passes. The modulator 15 controls now the inverter 7 according to a modulation type, for example a pulse width modulation in which the semiconductor switches of the inverter are switched accordingly.

If, however, the determined nominal torque Md _is to be greater than that of the limit value determination device 13 certain current limit value Md _lim is (YES branch) so the setpoint torque Md _{soll is} in the manipulated variable limiting unit 14 limited to the current limit Md _lim and the torque controller 11 this limit Md _{lim is set} as the setpoint, which restricts the return of energy.

The determination of the current limit value Md _lim takes place in the context of an intermediate circuit _{voltage-dependent} limit value determination 13a which repeats this limit value Md _lim with the control execution _{speed of} the subordinate torque controller 11 calculated.

This will be done in a first step 13.1 the current _{DC link} voltage U _DC determined. In a second step 13.2 the differential energy ΔE _{cap is} calculated, which is still in the _DC link until a maximum voltage limit U _{DC, max} is reached 6 can be fed. This then becomes the current manipulated variable limit in step 13.3 calculated. Finally, then in step 13.4 a check as to whether the calculated current limit value Mdlim _exceeds a physical-mechanical / electrical maximum torque limit Md _max . If this is the case, the limit value Md _{lim is} limited to this maximum torque limit Md _max , step 13.5 , The limit determination device 13 then gives this limited limit Md _lim to the manipulated variable limiting unit 14 , If this is not the case, the current limit value Md _{lim is} unchanged at the manipulated variable limiting unit 14 output. This process is repeated continuously.

According to another embodiment, the lower-level first controller regulates 11 the torque Md due to a given current command value i _{q, is intended} for the current component i _q forming the torque Md, so that it forms a current regulator. This embodiment mainly relates to a permanent magnet synchronous motor, which is controlled by means of a field-oriented control. Here, the mechanical power P _mech can be calculated from equation G2a. P _mech = Md · Ω = 3 / 2pi _q [i _d (L _d -L _q ) + ψ _PM ] · Ω (G2a)

In this case, p is the number of pole pairs of the electric motor, i _{q is} the current components forming the torque, i _{q is} the component of current forming the field, L _{d is} the inductance of the stator in the direction of the d axis, and L _{q is} the inductance of the stator in the direction of the q axis with the rotor rotating coordinate system with the axes d and q, ψ _PM the flux linkage of the rotor and Ω the angular velocity / speed of the rotor.

In the calculation, it is assumed that the mechanical power P _mech is constant over a control cycle, although it decreases due to the braking because the current control will adjust the torque-forming current component i _q with respect to the target current limit. This is a worst-case assumption, which leads to a conservative calculation of the limit value i _{q, lim} for the nominal current I _{q, soll} .

After a period Δt of three times the time constant of the current controller, that is in the DC link 6 fed energy according to equation G3a approximately E _mech = 3 / 2pi _q [i _d (L _d - L _q) + ψ _pm] · Ω · .DELTA.t (G3a)

This is an approximation. For one thing, the period .DELTA.t is a free but meaningfully selected period of time, on the other hand, the torque-forming current iq is not constant within this period of time .DELTA.t, so that the energy fed back is not constant during the time interval .DELTA.t. Furthermore, during the period .DELTA.t also the rotational speed and thus the mechanical power change, so that therefore also the energy fed back during the time interval .DELTA.t is not constant. Finally, the losses of the electric motor and the power electronics are neglected here, which lead to a reduced intermediate circuit voltage. For a precise calculation, these factors can all be taken into account. However, the solution presented here leads to a large safety margin to exact voltage limits, to exceed the maximum voltage limit of the DC link 6 even if prevent the value of the DC link capacitance 8th or the time constant of the current controller 11 are inaccurate. In addition, the presented calculation method is particularly simple, that is, less complex than a realistic simulation of all physical properties of the drive simulation method. The method according to the invention can thus be carried out without problems on a conventional microcontroller of the pump electronics.

Starting from equation G3a, analogously to equation G4, it is now necessary to request that the values in the DC link 6 regenerated energy -E _mech (the minus sign follows from the reverse energy flow) is always smaller than the differential energy ΔE _cap , as long as the upper voltage limit U _{DC, max} in the _{DC link} 6 is not reached. Thus applies -E _mech <ΔE _cap (G4a)

By substituting the equations G1 and G3a in equation G4a, one obtains as a requirement for the setpoint value iq _{, should} the torque-forming current component i _q at the current _DC link voltage U _DC , and at the current speed Ω and with a time constant of the current controller corresponding time period .DELTA.t 11 according to inequality G8:

It follows from equation G8 that the current limit i _{q, lim is} equal to the right term of the inequality G8. This inequality applies to positive speeds. In the case of negative speeds, the sign of the term on the right-hand side and the reference symbol must be reversed.

In the embodiment described, the manipulated variable to be limited is the setpoint value i _{q, ought to be} the torque-forming current component i _{q of} the stator current. This is from the second controller 12 is output and is then analogous to the previous example input variable for the subordinate first controller 11 , The inventive method is then identical to the sequence in 5 , wherein only the torque is to be replaced by the torque-forming current component i _q .

Although in 6 the steps 13.2 and 13.3 are shown as individual calculation steps, it becomes clear from inequality G8 that, due to the intermingling of the equations G1 and G3a, only a single equation results for the limit value i _{q, lim} , so that step 13.2 for calculating the differential energy also as part of the step 13.3 , to manipulated variable limit calculation can be understood.

In the case of the torque Md as a manipulated variable to be limited according to the invention, the limit value _Mdlim for a PM synchronous motor with field-oriented control can be determined from equation G9:

With the method according to the invention a maximum feedback is always allowed in the DC link. Furthermore, an overshoot of the intermediate circuit voltage is excluded by taking into account the time constant of the subordinate torque controller. In addition, an intermediate circuit voltage regulator and thus an increase in system complexity are dispensed with. The method according to the invention can thus be implemented particularly simply and brings about a reliable and reliable limitation of the energy feedback into the voltage intermediate circuit of the frequency converter.

LIST OF REFERENCE NUMBERS

1

drive

2

frequency converter

3

electric motor

4

Load, pump unit

5

Rectifier, network converter

6

Voltage link

7

Inverter, motor inverter

8th

Capacitor, DC link capacitance

9

braking resistor

10

motor control

11

First controller, torque controller

11a

torque control

12

Second controller, speed controller

12a

Target torque determination

13

Threshold detection means

133

DC link-dependent limit value determination

13.1

Intermediate circuit voltage determination

13.2

Differential energy calculation

13.3

Determination of the current command value limit value

13.4

limit Compare

13.5

Manipulated variable threshold limit

14

Output limiting unit

14a

Command value limit

15

modulator

15a

modulation

16

exemplary DC link-dependent command value maximum curve for positive speeds

17

exemplary DC link-dependent manipulated variable maximum curve for negative speeds

18

Possible torque curve

19

Range of action of the command value limit

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

• EP 1206040 A2 [0011, 0016]

Claims (17)

Method of operating a load ( 4 ) driving electric motor ( 3 ), by a frequency converter ( 2 ) with voltage intermediate circuit ( 6 ) and by a frequency converter ( 2 ) controlling engine control ( 10 ) is regulated in its rotational speed in such a way that, during braking, it transfers energy into the voltage intermediate circuit ( 6 ) fed back, characterized in that in dependence on the current _{DC link} voltage (U _dc ) from one in the voltage intermediate _circuit ( 6 ) input energy difference (ΔE _cap ) a current limit (Md _lim , iq _lim ) for a manipulated variable (Md _soll , iq _soll ) of the motor control ( 10 ) and that for limiting the voltage in the voltage intermediate circuit ( 6 ) from the electric motor ( 3 ) fed back energy from the engine control ( 10 ) value of the manipulated variable (Md _soll , iq _soll ) is limited to the limit value (Md _lim , iq _lim ), if the value exceeds the amount of the limit value (Md _lim , iq _lim ). A method according to claim 1, characterized in that the differential energy (ΔE _cap ), the energy difference between the at the current DC link voltage (U _dc ) in the voltage intermediate _circuit ( 6 ) and at a maximum intermediate _circuit voltage (U _{DC, max} ) in the voltage intermediate circuit ( 6 ) stored energy. Method according to Claim 1 or 2, characterized in that the manipulated variable (Md _soll , iq _soll ) _is transmitted from a higher-order second controller ( 12 ) of the engine control ( 10 ) as setpoint for a subordinate first controller ( 11 ) of the engine control ( 10 ), wherein an output variable (U _{a, b, c} ) of the first regulator ( 11 ) directly or indirectly to the frequency converter ( 2 ) acts. Method according to Claim 3, characterized in that the second controller ( 11 ) is a speed controller or a power controller. Method according to Claim 3 or 4, characterized in that the manipulated variable (Md _soll , iq _soll ) is a torque (Md _soll ), a torque-forming current component (iq _soll ) or a torque-equivalent variable, in particular a magnetic flux. Method according to one of the preceding claims, characterized in that the engine control ( 10 ) performs a field-oriented regulation. Method according to one of the preceding claims, characterized in that the determination of the limit value (Md _lim , iq _lim ) assuming a lossless electric motor ( 3 ) and / or frequency converter ( 2 ) he follows. Method according to one of the preceding claims, characterized in that the limit value (Md _lim , iq _lim ) from one of the equations

where C _{DC is} a capacitance of the voltage intermediate circuit, U _{DC is} the actual voltage in the voltage intermediate circuit ( 6 ), U _{DC, max} a maximum voltage in the voltage intermediate circuit ( 6 ), p is the number of pole pairs of the electric motor, i _{d is} a field-forming component of the motor current, i _{q, lim is} the limit value of a torque-forming component of the motor current, Md _{lim is} the limit value for the torque, Pm _{lim is} the limit value for the mechanical motor power, L _{d is} Statorinduktivität the electric motor in the direction of the d-axis of a rotating with the rotor coordinate system, L _{q is} the stator inductance of the electric motor in the direction of the q-axis of a coordinate system rotating with the rotor, ψ _{PM is} the magnetic flux of a permanent-magnet rotor of the electric motor, Ω is the current mechanical angular velocity of the rotor of the electric motor, and Δt is a period during which energy is in the voltage supply circuit is fed and needs the motor control to achieve a predetermined setpoint from any actual value to a certain percentage. Method according to Claims 3 and 8, characterized in that the time period Δt is three times a time constant (τ) of the lower-level first controller ( 11 ). Method according to one of the preceding claims, characterized in that the determined current limit value (Md _lim , iq _lim ) is limited to this maximum limit value (Md _max , iq _max ) when exceeding a maximum limit value (Md _max , iq _max ). Drive ( 1 ) comprising an electric motor ( 3 ) for driving a load ( 4 ), a frequency converter ( 2 ) with voltage intermediate circuit ( 6 ) for feeding the electric motor ( 3 ) and a motor control ( 10 ) for controlling the frequency converter ( 2 ) and control of the speed of the electric motor ( 3 ), characterized by - means for detecting the actual intermediate _circuit voltage (U _DC ) of the voltage intermediate circuit ( 6 ), - a limit value determination device ( 13 ) for determining a current limit value (Md _lim , iq _lim ) for a manipulated variable (Md _soll , iq _soll ) of the motor control ( 10 ) from one into the voltage intermediate circuit ( 6 ) feed-in differential energy (ΔE _cap ) as a function of the current _DC link voltage (U _DC ), - a command value limiting unit ( 14 ) for limiting the manipulated variable (Md _soll , iq _soll ) to the determined limit value (Md _lim , iq _lim ) for limiting in the voltage intermediate _circuit ( 6 ) from the electric motor ( 3 ) fed back energy. Drive ( 1 ) according to claim 11, characterized in that the engine control ( 10 ) a subordinate first controller ( 11 ) for controlling the frequency converter ( 2 ) and a first controller ( 11 ) superordinate second controller ( 12 ) for outputting the manipulated variable as setpoint for the first controller ( 11 ) having. Drive ( 1 ) according to claim 12, characterized in that the manipulated variable limiting unit ( 14 ) between the first and the second controller ( 11 . 12 ) lies. Drive ( 1 ) according to claim 12 or 13, characterized in that the first controller ( 11 ) is a current regulator or a torque controller. Drive ( 1 ) according to claim 12, 13 or 14, characterized in that the second controller ( 11 ) is a speed controller. Drive ( 1 ) according to one of claims 13 to 15, characterized in that the engine control ( 10 ) is arranged to carry out the method according to one of claims 1 to 10. Centrifugal pump unit with a pump unit ( 4 ) and a drive ( 1 ) according to one of claims 11 to 16, wherein the pump unit ( 4 ) of the electric motor ( 2 ) powered load ( 4 ).

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