Arrangement and method for controlling an asynchronous motor and a frequency converter

Abstract

The present invention relates to the field of electric drive devices and electric motors for industrial applications, and more particularly to an arrangement and a method for controlling an asynchronous motor and a frequency converter. The arrangement of the present invention for controlling an asynchronous motor (3) comprises an inverter unit (2) arranged to drive said asynchronous motor (3); a flux sensor unit (4) comprising polyphase flux sensor / sensors placed in the airgap of said asynchronous motor (3) arranged to measure the polyphase airgap flux ψ ag,a , ψ ag,b ,... ψ ag,m of the asynchronous motor (3); a flux observer (5); and polyphase current sensors (31); wherein said flux observer (5) is arranged to calculate a flux estimate [I] utilizing a received airgap flux value, at least one received stator current value and either one of a received / calculated stator voltage reference or a received stator voltage value, and wherein said flux observer (5) is used in control of said asynchronous motor (3).

Description

ARRANGEMENT AND METHOD FOR CONTROLLING AN ASYNCHRONOUS MOTOR AND A FREQUENCY CONVERTER FIELD OF THE INVENTION

[0001] The present invention relates to the field of electric drive devices and electric motors for industrial applications, and more particularly to an arrangement and a method for controlling an asynchronous motor and a frequency converter. BACKGROUND OF THE INVENTION

[0002] Electric drives are widely used for industrial applications, e.g. for providing and controlling electrical power and energy to various public and industrial applications as well as for driving and controlling various public and industrial applications. Electric drives are used in industry for different applications, such as for driving motors within the transportation industry, for driving different devices within the process and manufacturing industry as well as within the energy industry.

[0003] Rotating electrical machines are typically controlled using a frequency converter, and for the controlled operation information regarding electrical behaviour of the machine is required. Such information is fed to the controller of the frequency converter in form of electrical parameters of the machine.

[0004] Speed sensorless asynchronous motors may have an unstable operating region at low speeds. Also variations of the stator resistance due to the temperature may cause stability problems at low speeds.

[0005] In prior art solutions for controlling an asynchronous motor, the main problems are the quality of the control as well as the stability of the control.

[0006] Within the technology, there is a specific need for an improved arrangement and a method for controlling an asynchronous motor that would be more stable and reliable compared to the current prior art solutions.BRIEF DESCRIPTION OF THE INVENTION

[0007] The object of the invention is to introduce an improved arrangement and an improved method for controlling an asynchronous motor that would be more stable and reliable compared to the current prior art solutions. Advantageous embodiments are furthermore presented.

[0008] It is brought forward a new arrangement for controlling an asynchronous motor, said arrangement comprising: an inverter unit arranged to drive said asynchronous motor; a flux sensor unit comprising polyphase flux sensor / sensors placed in the airgap of said asynchronous motor arranged tomeasure the polyphase airgap fluxof the asynchronousmotor; a flux observer; and polyphase current sensor / sensors arranged tomeasure polyphase stator current… of said asynchronous motor,wherein said flux observer is arranged to receive an airgap flux value from said flux sensor unit, to receive at least one stator current value from said polyphase current sensor / sensors, and either to receive / calculate a stator voltage reference or to receive a stator voltage value from polyphase voltage sensor / sensors, said polyphase voltage sensor / sensors arranged to measurepolyphase stator voltage , , … of said asynchronous motor, to calculate aflux estimate utilizing said airgap flux value , said at least one stator currentvalue and either one of said received / calculated stator voltage reference or said stator voltage value , and wherein said flux observer is used in control of said asynchronous motor.

[0009] In a preferred embodiment, the airgap flux value is an airgap flux vector signal value in a -reference frame received from said flux sensor unitvia a first abc / -converter; at least one stator current value comprises: a statorcurrent vector signal value in a -reference frame received from saidpolyphase current sensor / sensors via a second abc / -converter, and a statorcurrent vector signal value in a dq-reference frame received from a / dq-converter; and the stator voltage value is a stator voltage vector signal valuein a -reference frame received from polyphase voltage sensor / sensors via a third abc / -converter.

[0010] In a preferred embodiment, said flux observer is arranged to to calculate a motor angular speed estimateutilizing one or more of said airgapflux vector signal value in a -reference frame, said stator current vectorsignal value in a -reference frame, said stator current vector signal value ina dq-reference frame and either one of said received / calculated stator voltage reference or said stator voltage vector signal value in a -reference frame,and wherein said calculated flux estimate and said calculated motor angularbeing used in establishing a stator voltage reference ,vector signal value in a -reference frame, said stator voltage reference ,vector signal value in a -reference frame being used in control of said asynchronous motor.

[0011] In a preferred embodiment, said arrangement comprises polyphase voltage sensor / sensors arranged to measure polyphase stator voltage ,, … of said asynchronous motor, and wherein said flux observer isarranged to receive a stator voltage value from said polyphase voltage sensor / sensors via a third abc / -converter and to utilize said received statorvoltage value in calculating said flux estimate and / or said motor angular speedestimate .

[0012] In a preferred embodiment, said arrangement comprises a current controller arranged to receive / calculate a stator voltage reference value and forward said stator voltage reference value to said flux observer, and wherein said flux observer is arranged to utilize said calculated stator voltage reference value in calculating said flux estimate and / or said motor angular speedestimate .

[0013] In a preferred embodiment, said arrangement comprises a current controller, a / dq-converter and a dq / -converter, wherein said / dq-converter is arranged to receive a stator current vector signal value in a -reference frame from said polyphase current sensor / sensors via a secondabc / -converter, to convert the stator current vector signal value in a -reference frame to a stator current vector signal value in a dq-reference frame,and to forward said stator current vector signal value in a dq-reference frameto the current controller, wherein said current controller is arranged to calculatea stator voltage reference , vector signal value in a dq-reference frameutilizing said stator current vector signal value in a dq-reference frame, an idcurrent reference value , and an iq current reference value , , andwherein said dq / -converter is arranged to receive the calculated stator voltagereference , vector value in a dq-reference frame from said current controller,to convert the stator voltage reference , vector signal value in a dq-reference frame to a stator voltage reference , vector signal value in a -reference frame, and to forward said stator voltage reference , vector signalvalue in a -reference frame for being used in control of said asynchronous motor.

[0014] In a preferred embodiment, said arrangement comprises a flux controller, wherein said flux controller is arranged to receive a flux reference as an input value, to receive the flux estimate from said flux observer, tocalculate an id current reference value , utilizing said flux reference andsaid flux estimate , and to forward said calculated id current reference value,to said current controller.

[0015] In a preferred embodiment, said arrangement further comprises a speed controller and a torque gain unit, wherein said speed controller isarranged to receive a motor angular speed reference , as an input value,to receive the motor angular speed estimate from said flux observer, tocalculate a torque reference value utilizing said motor angular speedreference , and said motor angular speed estimate , and to forward saidcalculated torque reference value to said torque gain unit, and wherein saidtorque gain unit is arranged to receive the calculated torque reference valuefrom said flux observer, to calculate an iq current reference value , utilizingsaid calculated torque reference value , and to forward said calculated iqcurrent reference value , to said current controller.

[0016] In a preferred embodiment of said arrangement, said polyphase flux sensor / sensors of said flux sensor unit comprise two or more hall effect sensors.

[0017] In a preferred embodiment of said arrangement, said arrangement further comprises one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors.

[0018] In a preferred embodiment, said polyphase flux sensor / sensors of said flux sensor unit comprise two or more planar energy harvester coilsarranged to measure the polyphase airgap fluxof theasynchronous motor.

[0019] In a preferred embodiment, said current controller, said / dq-converter, said dq / -converter, said flux controller, said speed controller andsaid torque gain unit are realized in a frequency converter.

[0020] In a preferred embodiment, said arrangement comprises a speed sensor unit placed in said asynchronous motor arranged to measure the angularspeed of the asynchronous motor.

[0021] Furthermore, it is brought forward a new method for controlling an asynchronous motor, in which method: polyphase airgap flux ,, , , … , of the asynchronous motor is measured by a flux sensorunit comprising polyphase flux sensor / sensors placed in the airgap of saidasynchronous motor; polyphase stator current , , … of said asynchronousmotor is measured by polyphase current sensor / sensors; an airgap flux value , at least one stator current value and either one of a received / calculated stator voltage reference or a stator voltage value are received by a flux observer asinput values; a flux estimate is calculated by said flux observer utilizing saidairgap flux value , said at least one stator current value and either one of said received / calculated stator voltage reference or said stator voltage value ; and said flux observer is used in controlling said asynchronous motor.

[0022] In a preferred embodiment, the airgap flux value is an airgap flux vector signal value in a -reference frame; at least one stator current valuecomprises: a stator current vector signal value in a -reference frame anda stator current vector signal value in a dq-reference frame; and the statorvoltage value is a stator voltage vector signal value in a -reference frame.

[0023] In a preferred embodiment of said method, a stator current vectorsignal value in a -reference frame of said measured stator current , , …is received by a / dq-converter from said polyphase current sensor / sensors viaa second abc / -converter, and converted to a stator current vector signalvalue in a dq-reference frame, and forwarded to a current controller; a statorvoltage reference , vector signal value in a dq-reference frame is calculatedby said current controller utilizing said stator current vector signal value in adq-reference frame, an id current reference value , and an iq currentreference value , , and the stator voltage reference , vector signal valuein a dq-reference frame is converted to a stator voltage reference , vectorsignal value in a -reference frame by a dq / -converter and said statorvoltage reference , vector signal value in a -reference frame is forwardedfor being used in control of said asynchronous motor.

[0024] Furthermore, it is brought forward a new frequency converter comprising an inverter unit arranged to drive an asynchronous motor; and a flux observer, wherein said flux observer is arranged to receive: an airgap flux value from a flux sensor unit, to receive at least one stator current value from said polyphase current sensor / sensorsvalue , and either to receive / calculate a stator voltage reference or to receive a stator voltage value from polyphase voltage sensor / sensors, said polyphase voltage sensor / sensors arranged to measurepolyphase stator voltage , , … of said asynchronous motor, to calculate aflux estimate utilizing said airgap flux value , said at least one stator currentvalue and either one of said received / calculated stator voltage reference or said stator voltage value , and wherein said flux observer is used in control of said asynchronous motor.

[0025] In a preferred embodiment, the airgap flux value is an airgap flux vector signal value in a -reference frame received from said flux sensor unitvia a first abc / -converter; at least one stator current value comprises: a statorcurrent vector signal value in a -reference frame received from saidpolyphase current sensor / sensors via a second abc / -converter, and a statorcurrent vector signal value in a dq-reference frame received from a / dq-converter; and the stator voltage value is a stator voltage vector signal valuein a -reference frame received from polyphase voltage sensor / sensors via athird abc / -converter.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the following, the present invention will be described in more detail by way of example and with reference to the attached drawings, in which: Figure 1 illustrates an induction motor equivalent circuit T-model in an alpha- beta stator reference frame. Figure 2 illustrates an induction motor equivalent circuit inverse-gamma-model in an alpha-beta stator reference frame. Figure 3 illustrates an embodiment of an example simple phase-lock loop used in a flux observer for the calculation of estimates. Figure 4 illustrates an embodiment of an arrangement for controlling an asynchronous motor according to the present invention. Figure 5 illustrates another embodiment of an arrangement for controlling an asynchronous motor according to the present invention. Figure 6 illustrates a third embodiment of an arrangement for controlling an asynchronous motor according to the present invention.Figure 7 illustrates a fourth embodiment of an arrangement for controlling an asynchronous motor according to the present invention. Figure 8 illustrates a fifth embodiment of an arrangement for controlling an asynchronous motor according to the present invention. Figure 9 illustrates an embodiment of a method for controlling an asynchronous motor according to the present invention. The foregoing aspects, features and advantages of the invention will be apparent from the drawings and the detailed description related thereto. In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings of Figures 1 to 9. DETAILED DESCRIPTION

[0027] The arrangement according to the present invention for controlling an asynchronous motor comprises: an inverter unit arranged to drive said asynchronous motor; a flux sensor unit comprising polyphase flux sensor / sensors placed in the airgap of said asynchronous motor arranged tomeasure the polyphase airgap fluxof the asynchronousmotor; a flux observer; and polyphase current sensor / sensors arranged tomeasure polyphase stator current… of said asynchronous motor,wherein said flux observer is arranged to receive an airgap flux value from said flux sensor unit, to receive at least one stator current value from said polyphase current sensor / sensors, and either to receive / calculate a stator voltage reference or to receive a stator voltage value from polyphase voltage sensor / sensors, said polyphase voltage sensor / sensors arranged to measurepolyphase stator voltage , , … of said asynchronous motor, to calculate aflux estimate utilizing said airgap flux value , said at least one stator currentvalueand either one of said received / calculated stator voltage reference or said stator voltage value, and wherein said flux observer is used in control of said asynchronous motor.

[0028] When referring to the term ”polyphase”, i.e. to the terms“polyphase airgap flux”, “polyphase stator current”, “polyphase current”, “polyphase stator voltage”, “polyphase voltage”, in this application, it is meant to refer to respective terms in any phase count asynchronous motors, this comprising three-phase asynchronous motors and six-phase asynchronous motors among others.

[0029] There are multiple different ways of measuring airgap flux. Onesolution for measuring airgap flux of an asynchronous motor is utilizing a planar energy harvester coil. A planar energy harvester coil can be manufactured as a flexible printed circuit (FPC). This type of planar energy harvester coil can be placed in the airgap of an asynchronous motor between the stator and the rotor. According to Faraday’s law any change in the magnetic field around a coil will induce a voltage into the coil. The induced voltage can be derived from Equation 1 as: == (1)

[0030] where N is the coil turns number, A is area enclosed by the coil,is air gap flux, and B is airgap flux density. The flux linkage can be derived from Equation 2 by integrating the induced voltage as:

[0031] Laboratory measurements have showed that a planar energyharvester coil can measure the induced voltage in the airgap with a wide spectrum. A planar energy harvester coil is cost-efficient, safe, and reliable power source that can be utilized by add-on motor electronics as well as by embedded motor electronics.

[0032] However, one practical problem with using a planar energyharvester coil is the zero-speed operation. If there would be any offset error in the measurement, the error will be accumulated in time by integration in accordance with the Equation 2. This will lead to gradual drifting of the flux.Additionally, according to the Equation 1, at standstill harvester coil cannot measure any voltage, thus the integrator output will be zero. To overcome these problems, a Hall effect sensor can be used.

[0033] The planar energy harvester coil can be equipped or replaced witha miniature hall effect sensor. Hall effect sensors usually output the flux density. Therefore, the airgap flux quantity can be obtained from Equation 3 as: =(3)

[0034] where A is the effective area of the sensor. As a result, theintegration drift problem will be disappeared. Additionally, the DC component of the flux can be accurately measured in standstill.

[0035] The modern miniature hall effect sensors are very energy efficientand may only require from µA to a few of mA current. An AC / DC converter circuit can convert the AC power harvested by an electromagnetic harvester coil to be used by a hall effect sensor. Even in the zero-speed operation, due to the presence of the modulation ripple in flux, voltage will be induced to the coils of the hall effect sensor. This voltage can be utilized as a power source for the hall effect sensor. Alternatively, or in the case where there is no harvester coil, a small battery can be used as an external power source of the hall effect sensor.

[0036] Figure 1 illustrates an induction motor equivalent circuit T-model10 in an alpha-beta stator reference frame. In the equivalent circuit T-model 10 of Figure 1 the airgap flux is represented along a dashed line 11.

[0037] The stator fluxcan be derived from the airgap flux of Figure1. The airgap flux of a three-phase asynchronous motor can be acquired inaccordance with the procedure described in the following. First the three-phaseairgap voltage vector , is measured. The measured the three-phase airgapvoltage vector , is then utilized in the space-vector transformation (abc / )shown in Equation 4 as:

[0038] Consequently, the airgap flux is acquired from Equation 5 as:

[0039] Alternatively, when utilizing a hall effect sensor the airgap fluxdensity of each phase is first measured, and the measured airgap flux iscalculated from Equation 6 as:

[0040] Consequently, after establishing the airgap flux the stator fluxcan be derived from the airgap flux .

[0041] In an embodiment with a polyphase asynchronous motor, theelectrical angle between the hall sensors can be calculated as = 360° ,where is the number of phases. The number of flux sensors could be as= , or = 1. Respectively, the mechanical angle between theadjacent hall sensors can be calculated as = , where is the number ofpole pairs.

[0042] In an exemplary embodiment with a three-phase asynchronousmotor having six poles distributed around the stator, the electrical anglebetween the hall sensors can be calculated as = 360°3 = 120°, where = 3being the number of phases. Respectively, the mechanical angle betweenthe adjacent hall sensors positioned around the circumference of the stator canbe calculated as = 120°3 = 40°, where = 3 being the number of polepairs.

[0043] In another exemplary embodiment with a six-phase asynchronousmotor having four poles distributed around the stator, the electrical anglebetween the hall sensors can be calculated as = 360°6 = 60°, where = 6being the number of phases. Respectively, the mechanical angle betweenthe adjacent hall sensors positioned around the circumference of the stator canbe calculated aswhere = 2 being the number of pole pairs.

[0044] In yet another further exemplary embodiment the number of fluxsensors could also be selected as = 2, or = 3, regardless of the numberof phases. Therefore, for calculation of the airgap flux vector, Equation 6 will suffice regardless of the number of phases.

[0045] Figure 2 illustrates an induction motor equivalent circuit inverse-gamma-model 12 in an alpha-beta stator reference frame. The vector-oriented control is usually based on the inverse-gamma-model 12. It can be seen fromFigure 1 and Figure 2 that the stator flux is the same quantity in both models.Consequently, the stator flux can be derived from the airgap flux ofFigure 1 and be used in the inverse-gamma-model 12 of Figure 2 if the statorleakage inductance is known.

[0046] Assuming the stator and rotor leakage inductances to be equal, i.e.= , and for obtaining the stator leakage inductance a transformationcoefficient can be derived from Equation 7 as:

[0047] In Equation 7, is the magnetizing inductance and is theleakage inductance of the inverse-gamma-model 12. These inductances are estimated during the identification run of the asynchronous motor. Respectively,the stator rotor leakage inductance and the rotor leakage inductance areobtained as presented in Equation 8:

[0048] It can be shown that the Equation 8 is robust against errors in themagnetizing inductance and can therefore be presented as in Equation 9:

[0049] In Equation 9, the magnetizing inductance appears in thedenominator of the second term. Typically, in induction motors, >> , sothe second term in Equation 9 will be very small. Consequently, the variationsin the magnetizing inductance will have a negligible effect on the accuracy ofthe stator leakage inductance . Even though the magnetizing inductancehas a negligible effect on calculating the stator leakage inductance as shownin Equation 9, the assumption that the stator and rotor leakage inductances areequal, i.e. = , is not always correct in practice and can introduce furthererror in obtaining the stator leakage inductance . To overcome theinaccuracies and to remove the dependency on the magnetizing inductance ,an estimate for the stator leakage inductance can be identified during theidentification run of the asynchronous motor using the measured airgap flux

[0050] Consequently, the stator flux of the inverse-gamma-model 12is then derived from the measured airgap flux and stator current aspresented in Equation 10:

[0051] In accordance with the present invention a flux observer can beutilized for stator flux estimation. The stator flux estimate is obtained fromEquation 11, where is the stator voltage vector, is the stator resistanceestimate, and is the voltage correction vector, as:

[0052] The error between the stator flux estimate and the measuredstator flux is an error vector obtained from Equation 12 as:= (12)

[0053] The correction vector is then defined in the stator referenceframe as a function of the error vector as presented in Equation 13:= , + , = ( ) (13)

[0054] The rotor flux estimate can be obtained from Equation 14 as:

[0055] The rotor flux angle estimate can be obtained from Equation 15as:

[0056] The rotor angular speed estimate can be obtained fromEquation 16 as:

[0057] Figure 3 illustrates an embodiment of an example simple phase-lock loop used in a flux observer for the calculation of estimates. In an alternative embodiment, a PLL algorithm (PLL, phase-locked loop) may be used in a phase-lock loop 20 to obtain the rotor flux angle estimate and the rotor angular speedestimate . The PLL algorithm gives a smoother result as compared to thecalculations using 2 function (Equations 15 and 16). The phase-lock loop20 according to the presented embodiment comprises an / dq-converter 21, a PI controller 22 (PI, Proportional Integral) and an integrator 23.

[0058] In the presented embodiment, the rotor flux estimate is firstbrought to the / dq-converter 21. Said / dq-converter 21 receives the rotor flux estimate , and converts / transforms said rotor flux estimate in a -reference frame to a rotor flux signal value in a dq-reference frame, i.e. inrotor coordinates. The converted / transformed rotor flux signal valuecomprises both the rotor flux d-axis component and the rotor flux q-axiscomponent , of which the rotor flux q-axis component is forwarded tothe PI controller 22. Said PI controller 22 estimates the rotor angular speed estimate and forwards said estimated rotor angular speed estimate to anintegrator 23. The integrator 23 then estimates the rotor flux angle estimate as an output of the PLL algorithm. The estimated rotor flux angle estimate isthen looped to the / dq-converter 21 at the input of the phase-locked loop.

[0059] In the presented embodiment of Figure 3, the PLL algorithmestimates the rotor flux angle estimate by driving the error to the rotor flux q-axis component zero. This will lead to= (17)and to

[0060] The above illustrated embodiment of Figure 3 is illustrated as asimple example. In practise there can be more filtering used in the phase-lock loop.

[0061] In accordance with the presented embodiment of Figure 3, therotor flux amplitude estimate and the stator flux amplitude estimate areestimated in a flux observer as:and as:

[0062] Furthermore, the slip frequency estimate can be obtained from

[0063] In accordance with the presented embodiment of Figure 3, themotor angular speed estimate is obtained in a flux observer from equation22 as:

[0064] Figure 4 illustrates an embodiment of an arrangement forcontrolling an asynchronous motor according to the present invention. In the presented embodiment an arrangement for controlling a three-phase asynchronous motor is illustrated. The arrangement according to the present embodiment comprises a PWM unit 1 (PWM, Pulse Width Modulator) and an inverter unit 2. Said arrangement is used for controlling an asynchronous motor 3. The PWM unit 1 of the presented embodiment is only example of a control unit for providing control instruction, i.e. switching instruction to the inverter unit 2. There are several other control / modulation techniques equally suited for providing switching instruction to the inverter unit 2. As an example, such control / modulation techniques would include direct torque control technique or vector modulation technique among others.

[0065] In the presented embodiment, the asynchronous motor 3 is drivenby said inverter unit 2, which inverter unit 2 is controlled by said PWM unit 1. Said PWM unit 1 receives the measured input voltage of said inverter unit 2from a voltage sensor in the input of said inverter unit 2. Said PWM unit 1 and said inverter unit 2 may be realized in a frequency converter. The arrangement according to the present embodiment also comprises a flux sensor unit 4 and a flux observer 5. Said flux observer 5 may be realized in a frequency converter.

[0066] Said flux sensor unit 4 comprises three-phase flux sensor / sensorsplaced in the airgap of said asynchronous motor 3 arranged to measure thethree-phase airgap flux , , , , , of the asynchronous motor 3. Saidthree-phase flux sensor / sensors of said flux sensor unit 4 may comprise two or more hall effect sensors. The arrangement according to the present embodiment may also comprise one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors. Said three-phase flux sensor / sensors of said flux sensor unit 4 may also comprise two or more optionalenergy harvester coils arranged to measure the airgap fluxthe asynchronous motor 3. In yet another different embodiment, said arrangement comprises polyphase flux sensor / sensors placed in the airgap of said asynchronous motor 3 arranged to measure the polyphase airgap fluxof the asynchronous motor.

[0067] Said three-phase flux sensor / sensors of said flux sensor unit 4measure the three-phase airgap flux of said asynchronous motor 3 and providea measured airgap flux , , , , , to said flux observer 5. Said fluxobserver 5 receives an airgap flux value of the asynchronous motor from said flux sensor unit 4. In yet another different embodiment, said arrangement further comprises two or more planar energy harvester coils placed in the airgap of said asynchronous motor arranged to measure the airgap flux value of the asynchronous motor.

[0068] The arrangement according to the present embodiment alsocomprises two or more current sensors 31 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. Said two or more current sensors 31 measure the three-phase stator current of saidasynchronous motor 3 and provide a measured stator current , , as at leastone stator current value to said flux observer 5. Said flux observer 5 receives at least one stator current value from said two or more current sensors 31. In yet another different embodiment, said arrangement comprises polyphase currentsensor / sensors 31 arranged to measure polyphase stator current , , … ofsaid asynchronous motor

[0069] The arrangement according to the present embodiment alsocomprises two or more optional voltage sensors 32 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. Said two or more optional voltage sensors 32 are arranged to measure the three- phase stator voltage of said asynchronous motor 3 and provide a measuredstator voltage , , to said flux observer 5. Said flux observer 5 receives astator voltage value from said two or more optional voltage sensors 32. In an alternative embodiment, the stator voltage can be determined using the swich state information together with the measured DC voltage. In yet another different embodiment, said arrangement comprises polyphase voltage sensor / sensors 32, said polyphase voltage sensor / sensors 32 arranged to measure polyphasestator voltage , , … of said asynchronous motor.

[0070] In the arrangement according to the present embodiment said fluxobserver 5 calculates a flux amplitude estimate and a motor angular speedestimate utilizing airgap flux value, said at least one stator current value andsaid stator voltage value. In the calculation of said flux amplitude estimate andsaid motor angular speed estimate said flux observer 5 utilizes Equations 6to 16 and / or Equations 17 to 22.

[0071] In the present embodiment said calculated flux estimate is arotor flux estimate . In yet another embodiment said calculated flux estimatemay e.g. also be a stator flux estimate . Said flux observer 5 is arranged toforward and forwards said calculated rotor flux estimate and said calculatedmotor angular speed estimate for the calculation of a stator voltage referencevalue, said calculated stator voltage reference value being utilized in PWM unit1 in controlling said asynchronous motor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0072] The arrangement according to the present embodiment alsocomprises a flux controller 6, a speed controller 7, a current controller 8 and a torque gain unit 9. In the present embodiment said flux observer 5 is arrangedto forward said rotor flux estimateto said flux controller 6 and to said torquegain unit 9, and arranged to forward said motor angular speed estimate tosaid speed controller 7.

[0073] Said flux controller 6 receives a flux reference as an inputvalue. In the present embodiment said received flux reference is a rotor fluxreferenceanother embodiment said received flux reference maye.g. also be a stator flux reference , . Said flux controller 6 also receives therotor flux estimate from said flux observer 5. Said flux controller 6 calculatesan id current reference value , utilizing said rotor flux reference , andsaid rotor flux estimate . Said flux controller 6 is arranged to forward saidcalculated id current reference value , to said current controller 8.

[0074] Said speed controller 7 receives a motor angular speed reference,as an input value. Said speed controller 7 also receives the motor angularspeed estimate from said flux observer 5. Said speed controller 7 calculatesa torque reference value utilizing said motor angular speed reference ,and said motor angular speed estimate . Said speed controller 7 is arrangedto forward said calculated torque reference value to said torque gain unit 9.

[0075] Said torque gain unit 9 receives the calculated torque referencevalue from said speed controller 7 and said rotor flux estimate from saidflux observer 5. Said torque gain unit 9 calculates an iq current reference value ,utilizing said calculated torque reference value in accordance withEquation 23. Said torque gain unit 9 is arranged to forward said calculated iqcurrent reference value , to said current controller 8.

[0076] Said current controller 8 receives at least one stator current valuefrom said two or more current sensors 31, the calculated id current referencevalue , from said flux controller 6 and the calculated iq current referencevalue , from said torque gain unit 9.

[0077] Said current controller 8 calculates a stator voltage referenceutilizing said at least one stator current value, said calculated id current referencevalue , and said calculated iq current reference value , . Said currentcontroller 8 is arranged to forward said calculated stator voltage reference to the PWM unit 1.

[0078] Said PWM unit 1 receives said calculated stator voltage referencefrom said current controller 8. In accordance with the present invention said PWM unit 1 utilizes said stator voltage reference in controlling said asynchronous motor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0079] In the presented embodiment of Figure 4 an arrangement forcontrolling a three-phase asynchronous motor is illustrated. In a similar fashion the arrangement for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0080] Figure 5 illustrates another embodiment of an arrangement forcontrolling an asynchronous motor according to the present invention. In the presented embodiment an arrangement for controlling a three-phase asynchronous motor is illustrated. The arrangement according to the present another embodiment comprises a PWM unit 1 (PWM, Pulse Width Modulator) and an inverter unit 2. Said arrangement is used for controlling an asynchronous motor 3. The PWM unit 1 of the presented embodiment is only example of a control unit for providing control instruction, i.e. switching instruction to the inverter unit 2. There are several other control / modulation techniques equally suited for providing switching instruction to the inverter unit 2. As an example,such control / modulation techniques would include direct torque control technique or vector modulation technique among others.

[0081] In the presented embodiment, the asynchronous motor 3 is drivenby said inverter unit 2, which inverter unit 2 is controlled by said PWM unit 1.Said PWM unit 1 receives the measured input voltage of said inverter unit 2from a voltage sensor in the input of said inverter unit 2. Said PWM unit 1 and said inverter unit 2 may be realized in a frequency converter. The arrangement according to the present another embodiment also comprises a flux sensor unit 4 and a flux observer 5. Said flux observer 5 may be realized in a frequency converter.

[0082] Said flux sensor unit 4 comprises three-phase flux sensor / sensorsplaced in the airgap of said asynchronous motor 3 arranged to measure thethree-phase airgap flux , , , , , of the asynchronous motor 3. Thethree-phase airgap flux measurement of an asynchronous motor 3 can be arranged with two or three sensors. With three sensors each sensor is used tomeasure airgap flux aligned with each phase , , , , , of theasynchronous motor 3 separately. With two sensors the sensors are used to measure airgap flux aligned with two phases and the third phase is constructed using these two measurements. For example, first the airgap flux aligned withphase a and phase b are measured with two sensors , , , . As ideally thesum of the fluxes is zero+ , = 0, the airgap flux aligned withc-phase can be constructed

[0083] Said three-phase flux sensor / sensors of said flux sensor unit 4 maycomprise two or more hall effect sensors. The arrangement according to the present another embodiment may also comprise one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors. Said three-phase flux sensor / sensors of said flux sensor unit 4 may also comprise two or more energy harvester coils arranged to measure theairgap flux , , , , , of the asynchronous motor 3.

[0084] Said three-phase flux sensor / sensors of said flux sensor unit 4measure the three-phase airgap flux of said asynchronous motor 3 and provide ameasured airgap flux , , , , , to said flux observer 5 via a first abc / -converter 45. Said first abc / -converter 45 receives the measured airgap flux, , , , , from said flux sensor unit 4, converts / transforms the measuredairgap flux , , , , , in abc-reference frame to a measured airgap fluxvector signal value in a -reference frame, i.e. in stationary space vector coordinates, and forwards said converted measured airgap flux vectorsignal value in a -reference frame to said flux observer 5.

[0085] Said flux observer 5 receives the measured airgap flux of theasynchronous motor 3 from said flux sensor unit 4. In another embodiment, said arrangement further comprises two or more planar energy harvester coils placed in the airgap of said asynchronous motor arranged to measure the airgap flux asynchronous motor.The arrangement according to the present another embodimentalso comprises two or more current sensors 31 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. The three-phase current measurement of an asynchronous motor 3 is usually arranged with two or three sensors. With three sensors each sensor is used to measure each phase of the current , , of the asynchronous motor 3separately. With two sensors the sensors are used to measure two phases of the current and the third phase is constructed using these two measurements. For example, first the currents of phase a and phase b are measured with two sensors , . As ideally the sum of the currents is zero + + = 0, the c-phase current can be constructed as = ( + ).

[0087] Said two or more current sensors 31 measure the three-phasestator current of said asynchronous motor 3 and provide a measured stator current , , to said flux observer 5 via a second abc / -converter 43. Saidsecond abc / -converter 43 receives the measured stator current , , fromsaid two or more current sensors 31, converts / transforms the measured statorcurrent , , in abc-reference frame to a stator current vector signal valuein a -reference frame, i.e. in stationary space vector coordinates, and forwardssaid converted stator current vector signal value in a -reference frame tosaid flux observer 5.

[0088] The arrangement according to the present another embodimentalso comprises two or more optional voltage sensors 32 arranged in the three- phase motor input between said inverter unit 2 and said asynchronous motor 3. Said two or more optional voltage sensors 32 are arranged to measure the three- phase stator voltage of said asynchronous motor 3 and provide a measuredstator voltage , , to said flux observer 5 via a third abc / -converter 44.Said third abc / -converter 44 receives the measured stator voltage , ,from said two or more optional voltage sensors 32, converts / transforms themeasured stator voltage , , in abc-reference frame to a stator voltagevector signal value in a -reference frame, i.e. in stationary space vectorcoordinates, and forwards said converted stator voltage vector signal valuein a -reference frame to said flux observer 5.

[0089] In the presented embodiment said said flux observer 5 receivesthe converted stator voltage vector signal value in a -reference framebeing converted from the measured stator voltage , , . In an alternativeembodiment, the stator voltage can be determined using the swich state information together with the measured DC voltage.

[0090] The arrangement according to the present another embodimentalso comprises an / dq-converter 41. Said / dq-converter 41 receives themeasured stator current vector signal value from said second abc / -converter 43, converts / transforms the stator current vector signal value in a-reference frame, i.e. in stationary space vector coordinates, to a statorcurrent vector signal value in a dq-reference frame, i.e. in rotor coordinates,and forwards said converted stator current vector signal value in a dq-reference frame to said flux observer 5.

[0091] Said flux observer 5 receives the measured stator currentvector signal value from said two or more current sensors 31 via said secondabc / -converter 43 and the measured stator voltage , , from said two ormore optional voltage sensors 32 via said third abc / -converter 44. Said fluxobserver 5 also receives the converted stator current vector signal value in adq-reference frame from said / dq-converter 41.

[0092] In an alternative embodiment the stator voltage in a xy-referenceframe can be calculated based on measured line-to-line voltages. Furthermore, in yet another alternative embodiment the stator voltage in a xy-reference frame can be calculated based on measured DC voltage and the output signals of said PWM unit 1.

[0093] In the arrangement according to the present another embodimentsaid flux observer 5 calculates a flux amplitude estimate utilizing saidmeasured airgap flux, said measured stator current vector signal valuein a -reference frame, said stator current vector signal value in a dq-reference frame and said measured stator voltage , , . In the calculationof said flux amplitude estimate said flux observer 5 utilizes Equations 6 to 16and / or Equations 17 to 22.

[0094] Said flux observer 5 may calculate said flux amplitude estimate ,e.g. the rotor flux amplitude estimate in accordance with Equation 19 as:

[0095] In the present another embodiment said calculated flux amplitudeestimate is a rotor flux amplitude estimate . In yet another embodiment saidcalculated flux amplitude estimate may e.g. also be a stator flux amplitudeestimate . In yet another embodiment both amplitude estimates, i.e. a rotorflux amplitude estimate and a stator flux amplitude estimate , may becalculated as said calculated flux amplitude estimate . Said flux observer 5may calculate said flux amplitude estimate , e.g. the stator flux amplitudeestimate in accordance with Equation 20 as:= + (20)

[0096] Said flux observer 5 forwards said calculated rotor flux amplitudeestimate for the calculation of a stator voltage reference , vector signalvalue in a -reference frame, said calculated stator voltage reference ,vector signal value being utilized in PWM unit 1 in controlling said asynchronous motor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0097] The arrangement according to the present another embodimentalso comprises a flux controller 6, a current controller 8 and a torque gain unit 9. In the present another embodiment said flux observer 5 is arranged to forward said rotor flux amplitude estimate to said flux controller 6 and to said torquegain unit 9.

[0098] Said flux controller 6 receives a flux reference as an inputvalue. In the present another embodiment said received flux reference is ayet another embodiment said received fluxreference may e.g. also be a stator flux reference , . Said flux controller6 also receives the rotor flux amplitude estimate from said flux observer 5.Said flux controller 6 calculates an id current reference value , utilizing saidrotor flux reference , and said rotor flux amplitude estimate . Said fluxcontroller 6 is arranged to forward said calculated id current reference value ,to said current controller 8.

[0099] Said torque gain unit 9 receives a torque reference as an inputvalue. Said torque gain unit 9 also receives the rotor flux amplitude estimatefrom said flux observer 5. Said torque gain unit 9 calculates an iq current reference value , utilizing said torque reference in accordance with

[0100] Said torque gain unit 9 is arranged to forward said calculated iq current reference value , to said current controller 8.

[0101] The arrangement according to the present another embodiment also comprises an / dq-converter 41 and a dq / -converter 42.

[0102] In the arrangement according to the present another embodiment said flux observer 5 calculates a rotor flux angle estimate utilizing saidmeasured stator current , , . Said flux observer 5 is arranged to forward andforwards said calculated rotor flux angle estimate to said / dq-converter 41and to said dq / -converter 42.

[0103] Said / dq-converter 41 receives the measured stator current ,, from said two or more current sensors 31, converts / transforms the statorcurrent , , in a -reference frame, i.e. in stationary space vectorcoordinates, to a stator current vector signal value in a dq-reference frame,i.e. in rotor coordinates, and forwards said converted stator current vectorsignal value in a dq-reference frame to the current controller 8.

[0104] Said current controller 8 receives the converted stator current vector signal value in a dq-reference frame from said / dq-converter 41, the calculated id current reference value , from said flux controller 6 and thecalculated iq current reference value , from said torque gain unit 9.

[0105] Said current controller 8 calculates a stator voltage reference ,vector signal value in a dq-reference frame utilizing said converted stator current vector signal value in a dq-reference frame, said calculated id currentreference value , and said calculated iq current reference value , . Saidcurrent controller 8 is arranged to forward said calculated stator voltagereference , vector signal value in a dq-reference frame to the PWM unit 1via said dq / -converter 42.

[0106] Said current controller 8 forwards said calculated stator voltagereference , vector signal value in a dq-reference frame to the dq / -converter 42. Said dq / -converter 42 receives the calculated stator voltagereference , vector signal value in a dq-reference frame from said currentcontroller 8, converts / transforms the stator voltage reference , vector signalvalue in a dq-reference frame, i.e. in rotor coordinates, to a stator voltagereference , vector signal value in a -reference frame, i.e. in stationaryspace vector coordinates, and forwards said converted stator voltage reference , vector signal value in a -reference frame to the the PWM unit 1.

[0107] Said PWM unit 1 receives said converted stator voltage reference ,vector signal value in a -reference frame from said dq / -converter 42.In accordance with the present invention said PWM unit 1 utilizes said statorvoltage reference , vector signal value in controlling said asynchronousmotor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0108] In the presented embodiment of Figure 5 an arrangement for controlling a three-phase asynchronous motor is illustrated. In a similar fashion the arrangement for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0109] Figure 6 illustrates a third embodiment of an arrangement for controlling an asynchronous motor according to the present invention. In the presented embodiment an arrangement for controlling a three-phase asynchronous motor is illustrated. The arrangement according to the present third embodiment comprises a PWM unit 1 (PWM, Pulse Width Modulator) and an inverter unit 2. Said arrangement is used for controlling an asynchronous motor 3. The PWM unit 1 of the presented embodiment is only example of a control unit for providing control instruction, i.e. switching instruction to the inverter unit 2. There are several other control / modulation techniques equally suited for providing switching instruction to the inverter unit 2. As an example, such control / modulation techniques would include direct torque control technique or vector modulation technique among others.

[0110] In the presented embodiment, the asynchronous motor 3 is driven by said inverter unit 2, which inverter unit 2 is controlled by said PWM unit 1.Said PWM unit 1 receives the measured input voltage of said inverter unit 2from a voltage sensor in the input of said inverter unit 2. Said PWM unit 1 and said inverter unit 2 may be realized in a frequency converter. The arrangement according to the present third embodiment also comprises a flux sensor unit 4 and a flux observer 5. Said flux observer 5 may be realized in a frequency converter.

[0111] Said flux sensor unit 4 comprises three-phase flux sensor / sensors placed in the airgap of said asynchronous motor 3 arranged to measure thethree-phase airgap flux , , , , , of the asynchronous motor 3. Saidthree-phase flux sensor / sensors of said flux sensor unit 4 may comprise two or more hall effect sensors. The arrangement according to the present third embodiment may also comprise one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors. Said three- phase flux sensor / sensors of said flux sensor unit 4 may also comprise two or more optional energy harvester coils arranged to measure the airgap flux ,, , , , of the asynchronous motor 3.

[0112] Said three-phase flux sensor / sensors of said flux sensor unit 4 measure the three-phase airgap flux of said asynchronous motor 3 and providea measured airgap flux , , , , , to said flux observer 5 via a first abc / -converter 45. Said first abc / -converter 45 receives the measured airgap flux, , , , , from said flux sensor unit 4, converts / transforms the measuredairgap flux , , , , , in abc-reference frame to a measured airgap fluxvector signal value in a -reference frame, i.e. in stationary space vectorcoordinates, and forwards said converted measured airgap flux vectorsignal value in a -reference frame to said flux observer 5.

[0113] Said flux observer 5 receives the measured airgap flux of theasynchronous motor 3 from said flux sensor unit 4. In yet another different embodiment, said arrangement further comprises two or more planar energy harvester coils placed in the airgap of said asynchronous motor arranged tomeasure the airgap flux of the asynchronous motor.

[0114] The arrangement according to the present third embodiment also comprises two or more current sensors 31 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. Said two or more current sensors 31 measure the three-phase stator current of saidasynchronous motor 3 and provide a measured stator current , , to said fluxobserver 5 via a second abc / -converter 43. Said second abc / -converter 43receives the measured stator current , , from said two or more currentsensors 31, converts / transforms the measured stator current , , in abc-reference frame to a stator current vector signal value in a -referenceframe, i.e. in stationary space vector coordinates, and forwards said convertedstator current vector signal value in a -reference frame to said flux observer5.

[0115] The arrangement according to the present third embodiment also comprises two or more optional voltage sensors 32 arranged in the three-phasemotor input between said inverter unit 2 and said asynchronous motor 3. Said two or more optional voltage sensors 32 are arranged to measure the three- phase stator voltage of said asynchronous motor 3 and provide a measuredstator voltage , , to said flux observer 5 via a third abc / -converter 44.Said third abc / -converter 44 receives the measured stator voltage , ,from said two or more optional voltage sensors 32, converts / transforms themeasured stator voltage , , in abc-reference frame to a stator voltagevector signal value in a -reference frame, i.e. in stationary space vectorcoordinates, and forwards said converted stator voltage vector signal valuein a -reference frame to said flux observer 5.

[0116] In the presented embodiment said said flux observer 5 receivesthe converted stator voltage vector signal value in a -reference framebeing converted from the measured stator voltage , , . In an alternativeembodiment, the stator voltage can be determined using the swich state information together with the measured DC voltage.

[0117] The arrangement according to the present third embodiment also comprises an / dq-converter 41. Said / dq-converter 41 receives themeasured stator current vector signal value from said second abc / -converter 43, converts / transforms the stator current vector signal value in a-reference frame, i.e. in stationary space vector coordinates, to a statorcurrent vector signal value in a dq-reference frame, i.e. in rotor coordinates,and forwards said converted stator current vector signal value in a dq-reference frame to said flux observer 5.

[0118] Said flux observer 5 receives the measured stator current vector signal value from said two or more current sensors 31 via said secondabc / -converter 43 and the measured stator voltage , , from said two ormore optional voltage sensors 32 via said third abc / -converter 44. Said fluxobserver 5 also receives the converted stator current vector signal value in adq-reference frame from said / dq-converter 41.

[0119] In the arrangement according to the present third embodiment saidflux observer 5 calculates a flux amplitude estimate and a motor angular speedestimateutilizing said measured airgap flux , said measured statorcurrent vector signal value in a -reference frame, said stator currentvector signal value in a dq-reference frame and said measured stator voltage ,, . In the calculation of said flux amplitude estimate and said motorangular speed estimatesaid flux observer 5 utilizes Equations 6 to 16 and / orEquations 17 to 22.

[0120] Said flux observer 5 may calculate said flux amplitude estimate ,e.g. the rotor flux amplitude estimate in accordance with Equation 19 as:

[0121] Said flux observer 5 may calculate said motor angular speedestimate in accordance with Equation 22 as:

[0122] In the calculation, the slip frequency estimate can be obtainedfrom Equation 21 as:

[0123] In the present third embodiment said calculated flux estimate isa rotor flux estimate . In yet another embodiment said calculated flux estimatemay e.g. also be a stator flux estimate . Said flux observer 5 is arranged toforward and forwards said calculated rotor flux estimate and said calculatedmotor angular speed estimatefor the calculation of a stator voltage reference,vector signal value in a -reference frame, said calculated stator voltagereference , vector signal value being utilized in PWM unit 1 in controllingsaid asynchronous motor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0124] The arrangement according to the present third embodiment also comprises a flux controller 6, a speed controller 7, a current controller 8 and a torque gain unit 9. In the present third embodiment said flux observer 5 isarranged to forward said rotor flux estimate to said flux controller 6 and tosaid torque gain unit 9, and arranged to forward said motor angular speedestimate to said speed controller 7.

[0125] Said flux controller 6 receives a flux reference as an inputvalue. In the present third embodiment said received flux reference is arotor flux reference , . In yet another embodiment said received fluxreference may e.g. also be a stator flux reference , . Said flux controller6 also receives the rotor flux estimate from said flux observer 5. Said fluxcontroller 6 calculates an id current reference value , utilizing said rotor fluxreferenceand said rotor flux estimate . Said flux controller 6 is arrangedto forward said calculated id current reference value , to said currentcontroller 8.

[0126] Said speed controller 7 receives a motor angular speed reference , as an input value. Said speed controller 7 also receives the motor angularspeed estimate from said flux observer 5. Said speed controller 7 calculatesa torque reference value utilizing said motor angular speed reference ,and said motor angular speed estimate . Said speed controller 7 is arrangedto forward said calculated torque reference value to said torque gain unit 9.

[0127] Said torque gain unit 9 receives the calculated torque referencevalue from said speed controller 7 and said rotor flux estimate from saidflux observer 5. Said torque gain unit 9 calculates an iq current reference value ,utilizing said calculated torque reference value in accordance withEquation 23 as:

[0128] Said torque gain unit 9 is arranged to forward said calculated iqcurrent reference value , to said current controller 8.

[0129] The arrangement according to the present third embodiment alsocomprises an / dq-converter 41 and a dq / -converter 42.

[0130] In the arrangement according to the present third embodiment saidflux observer 5 calculates a rotor flux angle estimate utilizing said measuredstator current , , . Said flux observer 5 is arranged to forward and forwardssaid calculated rotor flux angle estimate to said / dq-converter 41 and tosaid dq / -converter 42.

[0131] Said / dq-converter 41 receives the measured stator current vector signal value from said two or more current sensors 31 via said secondabc / -converter 43, converts / transforms the received stator current vectorsignal value in a -reference frame, i.e. in stationary space vector coordinates,to a stator current vector signal value in a dq-reference frame, i.e. in rotorcoordinates, and forwards said converted stator current vector signal value ina dq-reference frame to the current controller 8.

[0132] Said current controller 8 receives the converted stator current vector signal value in a dq-reference frame from said / dq-converter 41, thecalculated id current reference value , from said flux controller 6 and thecalculated iq current reference value , from said torque gain unit 9.

[0133] Said current controller 8 calculates a stator voltage reference ,vector signal value in a dq-reference frame utilizing said converted stator current vector signal value in a dq-reference frame, said calculated id currentreference value , and said calculated iq current reference value , . Saidcurrent controller 8 is arranged to forward said calculated stator voltagereference , vector signal value in a dq-reference frame to the PWM unit 1via said dq / -converter 42.

[0134] Said current controller 8 forwards said calculated stator voltagereference , vector signal value in a dq-reference frame to the dq / -converter 42. Said dq / -converter 42 receives the calculated stator voltagereference , vector signal value in a dq-reference frame from said currentcontroller 8, converts / transforms the stator voltage reference , vector signalvalue in a dq-reference frame, i.e. in rotor coordinates, to a stator voltagereference , vector signal value in a -reference frame, i.e. in stationaryspace vector coordinates, and forwards said converted stator voltage reference , vector signal value in a -reference frame to the the PWM unit 1.

[0135] Said PWM unit 1 receives said converted stator voltage reference ,vector signal value in a -reference frame from said dq / -converter 42.In accordance with the present invention said PWM unit 1 utilizes said statorvoltage reference , vector signal value in controlling said asynchronousmotor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0136] In the presented embodiment of Figure 6 an arrangement for controlling a three-phase asynchronous motor is illustrated. In a similar fashion the arrangement for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0137] Figure 7 illustrates a fourth embodiment of an arrangement for controlling an asynchronous motor according to the present invention. In the presented embodiment an arrangement for controlling a three-phase asynchronous motor is illustrated. The arrangement according to the present fourth embodiment comprises a PWM unit 1 (PWM, Pulse Width Modulator) and an inverter unit 2. Said arrangement is used for controlling an asynchronous motor 3. The PWM unit 1 of the presented embodiment is only example of acontrol unit for providing control instruction, i.e. switching instruction to the inverter unit 2. There are several other control / modulation techniques equally suited for providing switching instruction to the inverter unit 2. As an example, such control / modulation techniques would include direct torque control technique or vector modulation technique among others.

[0138] In the presented embodiment, the asynchronous motor 3 is driven by said inverter unit 2, which inverter unit 2 is controlled by said PWM unit 1.Said PWM unit 1 receives the measured input voltage of said inverter unit 2from a voltage sensor in the input of said inverter unit 2. Said PWM unit 1 and said inverter unit 2 may be realized in a frequency converter. The arrangement according to the present fourth embodiment also comprises a flux sensor unit 4 and a flux observer 5. Said flux observer 5 may be realized in a frequency converter.

[0139] Said flux sensor unit 4 comprises three-phase flux sensor / sensors placed in the airgap of said asynchronous motor 3 arranged to measure thethree-phase airgap flux , , , , , of the asynchronous motor 3. Saidthree-phase flux sensor / sensors of said flux sensor unit 4 may comprise two or more hall effect sensors. The arrangement according to the present fourth embodiment may also comprise one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors. Said three- phase flux sensor / sensors of said flux sensor unit 4 may also comprise two or more optional energy harvester coils arranged to measure the airgap flux ,, , , , of the asynchronous motor 3.

[0140] Said three-phase flux sensor / sensors of said flux sensor unit 4 measure the three-phase airgap flux of said asynchronous motor 3 and providea measured airgap flux , , , , , to said flux observer 5 via a first abc / -converter 45. Said first abc / -converter 45 receives the measured airgap flux, , , , , from said flux sensor unit 4, converts / transforms the measuredairgap flux , , , , , in abc-reference frame to a measured airgap fluxvector signal value in a -reference frame, i.e. in stationary space vectorcoordinates, and forwards said converted measured airgap flux vectorsignal value in a -reference frame to said flux observer 5.

[0141] Said flux observer 5 receives the measured airgap flux of theasynchronous motor 3 from said flux sensor unit 4. In yet another different embodiment, said arrangement further comprises two or more planar energy harvester coils placed in the airgap of said asynchronous motor arranged tomeasure the airgap flux of the asynchronous motor.

[0142] The arrangement according to the present fourth embodiment also comprises two or more current sensors 31 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. Said two or more current sensors 31 measure the three-phase stator current of saidasynchronous motor 3 and provide a measured stator current , , to said fluxobserver 5 via a second abc / -converter 43. Said second abc / -converter 43receives the measured stator current , , from said two or more currentsensors 31, converts / transforms the measured stator current , , in abc-reference frame to a stator current vector signal value in a -referenceframe, i.e. in stationary space vector coordinates, and forwards said convertedstator current vector signal value in a -reference frame to said flux observer5.

[0143] The arrangement according to the present fourth embodiment also comprises a flux controller 6, a speed controller 7, a current controller 8 and a torque gain unit 9. In the arrangement according to the present fourth embodiment said current controller 8 is arranged to forward a calculated statorvoltage reference , vector signal value in a dq-reference frame to said fluxobserver 5. Said flux observer 5 receives the calculated stator voltage reference , vector signal value in a dq-reference frame from said current controller 8.

[0144] The arrangement according to the present fourth embodiment also comprises an / dq-converter 41. Said / dq-converter 41 receives the measured stator current vector rvalue from said second abc / -converter 43,converts / transforms the stator current vector value in a -reference frame,i.e. in stationary space vector coordinates, to a stator current vector signalvalue in a dq-reference frame, i.e. in rotor coordinates, and forwards said converted stator current vector signal value in a dq-reference frame to saidflux observer 5.

[0145] Said flux observer 5 receives the measured stator current vector signal value from said two or more current sensors 31 via said second abc / -converter 43 and the calculated stator voltage reference , vectorsignal value in a dq-reference frame from said current controller 8. Said flux observer 5 also receives the converted stator current vector signal value in adq-reference frame from said / dq-converter 41.

[0146] In the arrangement according to the present fourth embodiment said flux observer 5 calculates a flux amplitude estimate and a motor angularspeed estimate utilizing said measured airgap fluxsaid measured statorcurrent vector signal value in a -reference frame, said stator currentvector signal value in a dq-reference frame and said calculated stator voltage reference , vector signal value in a dq-reference frame. In the calculation ofsaid flux amplitude estimate and said motor angular speed estimatesaidflux observer 5 utilizes Equations 6 to 16 and / or Equations 17 to 22.

[00147] Said flux observer 5 may calculate said flux amplitude estimate,e.g. the rotor flux amplitude estimate in accordance with Equation 19 as:

[0148] Said flux observer 5 may calculate said motor angular speed estimate in accordance with Equation 22 as:

[00149] In the calculation, the slip frequency estimate can be obtained(21)

[0150] In the present fourth embodiment said calculated flux estimate is a rotor flux estimate . In yet another embodiment said calculated fluxestimate may e.g. also be a stator flux estimate . Said flux observer 5 isarranged to forward and forwards said calculated rotor flux estimate and saidcalculated motor angular speed estimate for the calculation of a statorvoltage referencevector signal value in a -reference frame, saidcalculated stator voltage reference , vector signal value being utilized inPWM unit 1 in controlling said asynchronous motor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0151] In the present fourth embodiment said flux observer 5 is arranged to forward said rotor flux estimate to said flux controller 6 and to said torquegain unit 9, and arranged to forward said motor angular speed estimate tosaid speed controller 7.

[00152] Said flux controller 6 receives a flux reference as an inputvalue. In the present fourth embodiment said received flux reference is arotor flux referenceyet another embodiment said received fluxreference may e.g. also be a stator flux reference , . Said flux controller6 also receives the rotor flux estimate from said flux observer 5. Said fluxcontroller 6 calculates an id current reference value , utilizing said rotor fluxreference , and said rotor flux estimate . Said flux controller 6 is arrangedto forward said calculated id current reference value , to said currentcontroller 8.

[0153] Said speed controller 7 receives a motor angular speed reference , as an input value. Said speed controller 7 also receives the motor angularspeed estimate from said flux observer 5. Said speed controller 7 calculatesa torque reference value utilizing said motor angular speed referenceand said motor angular speed estimate . Said speed controller 7 is arrangedto forward said calculated torque reference value to said torque gain unit 9.

[0154] Said torque gain unit 9 receives the calculated torque referencevalue from said speed controller 7 and said rotor flux estimate from saidflux observer 5. Said torque gain unit 9 calculates an iq current reference value ,utilizing said calculated torque reference value in accordance withEquation 23 as:

[0155] Said torque gain unit 9 is arranged to forward said calculated iqcurrent reference value , to said current controller 8.

[0156] The arrangement according to the present fourth embodiment alsocomprises an / dq-converter 41 and a dq / -converter 42.

[0157] In the arrangement according to the present fourth embodimentsaid flux observer 5 calculates a rotor flux angle estimate utilizing saidmeasured stator current , , . Said flux observer 5 is arranged to forward andforwards said calculated rotor flux angle estimate to said / dq-converter 41and to said dq / -converter 42.

[0158] Said / dq-converter 41 receives the measured stator current vector signal value from said two or more current sensors 31 via said secondabc / -converter 43, converts / transforms the received stator current vectorsignal value in a -reference frame, i.e. in stationary space vector coordinates,to a stator current vector signal value in a dq-reference frame, i.e. in rotorcoordinates, and forwards said converted stator current vector signal value ina dq-reference frame to the current controller 8.

[0159] Said current controller 8 receives the converted stator current vector signal value in a dq-reference frame from said / dq-converter 41, thecalculated id current reference value , from said flux controller 6 and thecalculated iq current reference value , from said torque gain unit 9.

[0160] Said current controller 8 calculates a stator voltage reference ,vector signal value in a dq-reference frame utilizing said converted stator current vector signal value in a dq-reference frame, said calculated id currentreference value , and said calculated iq current reference value , . Saidcurrent controller 8 is arranged to forward said calculated stator voltagereference , vector signal value in a dq-reference frame to the PWM unit 1via said dq / -converter 42.

[0161] Said current controller 8 forwards said calculated stator voltagereference , vector signal value in a dq-reference frame to the dq / -converter 42. Said dq / -converter 42 receives the calculated stator voltagereference , vector signal value in a dq-reference frame from said currentcontroller 8, converts / transforms the stator voltage reference , vector signalvalue in a dq-reference frame, i.e. in rotor coordinates, to a stator voltagereference , vector signal value in a -reference frame, i.e. in stationaryspace vector coordinates, and forwards said converted stator voltage reference , vector signal value in a -reference frame to the the PWM unit 1.

[0162] Said PWM unit 1 receives said converted stator voltage reference ,vector signal value in a -reference frame from said dq / -converter 42.In accordance with the present invention said PWM unit 1 utilizes said statorvoltage reference , vector signal value in controlling said asynchronousmotor 3, i.e. in controlling said inverter unit 2 in driving said asynchronous motor 3.

[0163] In the presented embodiment of Figure 7 an arrangement for controlling a three-phase asynchronous motor is illustrated. In a similar fashion the arrangement for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0164] Figure 8 illustrates a fifth embodiment of an arrangement for controlling an asynchronous motor according to the present invention. In the presented embodiment an arrangement for controlling a three-phase asynchronous motor is illustrated. The arrangement according to the present fifth embodiment comprises a PWM unit 1 (PWM, Pulse Width Modulator) and an inverter unit 2. Said arrangement is used for controlling an asynchronous motor 3. Said asynchronous motor 3 is driven by said inverter unit 2, which inverter unit 2 is controlled by said PWM unit 1. Said PWM unit 1 receives themeasured input voltage of said inverter unit 2 from a voltage sensor in theinput of said inverter unit 2. Said PWM unit 1 and said inverter unit 2 may be realized in a frequency converter.

[0165] The arrangement according to the present fifth embodiment is a same embodiment which has been presented in Figure 6 with the exception that in addition to the embodiment of Figure 6 the present fifth embodiment of Figure 8 also comprises a speed sensor unit 33 placed in said asynchronous motor 3arranged to measure the angular speed of the asynchronous motor 3. Saidspeed sensor unit 33 is arranged to measure the angular speed of theasynchronous motor 3 and provide the measured angular speed to a speedcontroller 7.

[0166] In the arrangement according to the present fifth embodiment saidspeed controller 7 receives a motor angular speed reference , as an inputvalue. Said speed controller 7 also receives the measured angular speedfrom said speed sensor unit 33. Said speed controller 7 calculates a torquereference value utilizing said motor angular speed reference , and saidmeasured angular speed . Said speed controller 7 is arranged to forward saidcalculated torque reference value to a torque gain unit 9 as in theembodiment of Figure 6.

[0167] In the presented embodiment of Figure 8 an arrangement for controlling a three-phase asynchronous motor is illustrated. In a similar fashion the arrangement for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0168] Figure 9 illustrates an embodiment of a method for controlling an asynchronous motor according to the present invention. In the presented embodiment a method for controlling a three-phase asynchronous motor is illustrated. In the method according to the present embodiment airgap flux of an asynchronous motor 3 is first measured 51 by a flux sensor unit 4. In the presented embodiment said flux sensor unit 4 comprises two or more hall effect sensors placed in the airgap of said asynchronous motor 3. Said flux sensor unit 4 may also comprise two or more optional energy harvester coils. Said fluxsensor unit 4 measures 51 the airgap flux of the asynchronous motor 3 andforwards the measured airgap flux to a flux observer 5.

[0169] Thereafter, in the method according to the present embodimentthe three-phase stator current , , of said asynchronous motor 3 is measured52 by two or more current sensors 31. In the presented embodiment said two ormore current sensors 31 are arranged in the three-phase motor input between an inverter unit 2 and said asynchronous motor 3. Said two or more currentsensors 31 measure 52 the three-phase stator current , , of saidasynchronous motor 3 and forward the measured the three-phase stator current ,, to said flux observer 5.

[0170] Also, in the method according to the present embodiment thethree-phase stator voltage , , of said asynchronous motor 3 is measuredby three-phase voltage sensor / sensors 32 arranged in the three-phase motor input between said inverter unit 2 and said asynchronous motor 3. Alternatively,a stator voltage reference , vector signal value in a dq-reference frame iscalculated in a current controller 8. Said three-phase voltage sensor / sensors 32measure and forward the measured the three-phase stator voltage , , tosaid reduced order flux observer 5. Alternatively, said current controller 8calculates and forwards the stator voltage reference , vector signal value ina dq-reference frame to said flux observer 5.

[0171] Thereafter, in the method according to the present embodiment aflux estimate and a motor angular speed estimateare calculated 53 bysaid flux observer 5 utilizing said measured airgap flux, said measuredstator current , , and said measured stator voltage . In the presentembodiment said calculated flux estimate may be e.g. a rotor flux estimate, or a stator flux estimate . In the calculation 53 of the flux estimate anda motor angular speed estimate a calculated stator voltage reference ,vector value in a dq-reference frame may be utilized, or optionally a measuredstator voltage , , may be utilized. Said flux observer 5 forwards saidcalculated flux estimate and said calculated motor angular speed estimateto one or more controllers.

[0172] After this, in the method according to the present embodiment astator voltage reference , vector signal value in a -reference frame iscalculated 54 by one or more controllers utilizing said flux estimate and saidmotor angular speed estimate .

[0173] Thereafter, in the method according to the present embodiment, said asynchronous motor 3 is controlled 55, wherein in said controlling 55 saidcalculated stator voltage reference , vector signal value is utilized in PWMunit 1 to control said asynchronous motor 3, i.e. to control said inverter unit 2 in driving said asynchronous motor 3.

[0174] In the presented embodiment of Figure 9 a method for controlling a three-phase asynchronous motor is illustrated. In a similar fashion the method for controlling a three-phase asynchronous motor may be used with any other multiphase asynchronous motors, this comprising six-phase asynchronous motors among others.

[0175] The solution according to the present invention provides a high- quality and stable arrangement and method for controlling an asynchronous motor.

[0176] With the help of the solution for controlling an asynchronous motor according to the present invention the previously presented problems of quality of the control as well as the stability of the control are solved and / or reduced.

[0177] The invention can be implemented in existing frequency converters. Present frequency converters comprise processors and memory that can be utilized in the functions according to embodiments of the invention. Thus, all modifications and configurations required for implementing an embodiment of the invention may be performed as software routines, which may be implemented as added or updated software routines. If the functionality of the invention is implemented by software, such software can be provided as a computer program product comprising computer program code which, when run on a computer, causes the computer or corresponding arrangement to perform the functionality according to the invention as described above. Such a computer program code may be stored or generally embodied on a computer readable medium, such as suitable memory, e.g. a flash memory or a disc memory from which it is loadable to the unit or units executing the program code. In addition, such a computer program code implementing the invention may be loaded to the unit or units executing the computer program code via a suitable data network, for example, and it may replace or update a possibly existing program code.

[0178] It is to be understood that the above description and the accompanying Figures are only intended to teach the best way known to the inventors to make and use the invention. It will be apparent to a person skilled in the art that the inventive concept can be implemented in various ways. The above-described embodiments of the invention may thus be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that the invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims and their equivalents. In the following a list of used symbols: Symbols in three-phase system: current in phase a, current in phase b, current in phase c, voltage of phase-a (phase to ground), voltage of phase-b (phase to ground), voltage of phase-c (phase to ground), , measured airgap flux in the direction of phase a, , measured airgap flux in the direction of phase b, , measured airgap flux in the direction of phase c,Symbols in coordinates:stator current vector in coordinates,measured stator voltage vector in coordinates,, stator voltage reference vector in coordinates,measured airgap flux in coordinates,stator flux vector in coordinates,estimated stator flux vector in coordinates,stator flux estimate vector in coordinates,rotor flux estimate component,rotor flux estimate component,Symbols in coordinatesstator current vector in coordinates,, stator voltage reference vector in coordinates,measured current d component, , current reference d component, measured current q component, , current reference q component, General Symbols rotor flux amplitude estimate, stator flux amplitude estimate, , rotor flux amplitude reference, , stator flux amplitude reference, rotor flux angle, stator frequency estimate, slip frequency estimate, rotor angular speed estimate, rotor angular speed, T model stator leakage inductance, Inverse- model leakage inductance.

Claims

CLAIMS1. An arrangement for controlling an asynchronous motor (3), saidarrangement comprising:- an inverter unit (2) arranged to drive said asynchronous motor (3);a flux sensor unit (4) comprising polyphase flux sensor / sensors placed in the airgap of said asynchronous motor (3) arranged to measure thepolyphase airgap flux , , , , … , of the asynchronous motor (3);- a flux observer (5); and- polyphase current sensor / sensors (31) arranged to measurepolyphase stator current , , … of said asynchronous motor (3),- wherein said flux observer (5) is arranged:- to receive an airgap flux value from said flux sensor unit (4),- to receive at least one stator current value vector from saidpolyphase current sensor / sensors (31), and -either to receive / calculate a stator voltage reference or toreceive a stator voltage value from polyphase voltage sensor / sensors (32), said polyphase voltage sensor / sensors (32) arranged to measure polyphase stator voltage , , … of saidasynchronous motor (3), -to calculate a flux estimate utilizing said airgap flux value ,said at least one stator current value and either one of said received / calculated stator voltage reference or said stator voltage value, and- wherein said flux observer (5) is used in control of saidasynchronous motor (3).

2. The arrangement according to claim 1, wherein:- the airgap flux value is an airgap flux vector signal value in a-reference frame received from said flux sensor unit (4) via a firstabc / -converter (45);- at least one stator current value comprises:- a stator current vector signal value in a -referenceframe received from said polyphase current sensor / sensors (31) via a second abc / -converter (43), and- a stator current vector signal value in a dq-reference framereceived from a / dq-converter (41); and- the stator voltage value is a stator voltage vector signal valuein a -reference frame received from polyphase voltage sensor / sensors(32) via a third abc / -converter (44).

3. The arrangement according to claim 2, wherein said flux observer(5) is arranged to to calculate a motor angular speed estimateutilizingone or more of said airgap flux vector signal value in a -referenceframe, said stator current vector signal value in a -reference frame,said stator current vector signal value in a dq-reference frame andeither one of said received / calculated stator voltage reference or saidstator voltage vector signal value in a -reference frame, andwherein said calculated flux estimate and said calculated motor angularspeed estimate being used in establishing a stator voltage reference,vector signal value in a -reference frame, said stator voltagereference , vector signal value in a -reference frame being used incontrol of said asynchronous motor (3).

4. The arrangement according to any of the claims 1 to 3, whereinsaid arrangement comprises polyphase voltage sensor / sensors (32)arranged to measure polyphase stator voltage , , … of saidasynchronous motor (3), and wherein said flux observer (5) is arranged to receive a stator voltage value from said polyphase voltagesensor / sensors (32) via a third abc / -converter (44) and to utilize saidreceived stator voltage value in calculating said flux estimate and / orsaid motor angular speed estimate .

5. The arrangement according to claim 1, which arrangementcomprises a current controller (8) arranged to receive / calculate a stator voltage reference value and forward said stator voltage reference value to said flux observer (5), and wherein said flux observer (5) is arranged to utilize said calculated stator voltage reference value in calculating saidflux estimate and / or said motor angular speed estimate .

6. The arrangement according to any of the claims 1 to 5, whicharrangement comprises a current controller (8), a / dq-converter (41)and a dq / -converter (42),- wherein said / dq-converter (41) is arranged to receive a statorcurrent vector signal value in a -reference frame from saidpolyphase current sensor / sensors (31) via a second abc / -converter(43), to convert the stator current vector signal value in a -referenceframe to a stator current vector signal value in a dq-reference frame,and to forward said stator current vector signal value in a dq-referenceframe to the current controller (8),- wherein said current controller (8) is arranged to calculate a statorvoltage reference , vector signal value in a dq-reference frameutilizing said stator current vector signal value in a dq-reference frame,an id current reference value , and an iq current reference value , ,and- wherein said dq / -converter (42) is arranged to receive thecalculated stator voltage reference , vector signal value in a dq-reference frame from said current controller (8), to convert the statorvoltage reference , vector signal value in a dq-reference frame to astator voltage reference , vector signal value in a -reference frame,and to forward said stator voltage reference , vector signal value ina -reference frame for being used in control of said asynchronous motor (3).

7. The arrangement according to claim 6, which arrangementcomprises a flux controller (6), wherein said flux controller (6) is arrangedto receive a flux reference as an input value, to receive the fluxestimate from said flux observer (5), to calculate an id current referencevalue , utilizing said flux reference and said flux estimate , andto forward said calculated id current reference value , to said currentcontroller (8).

8. The arrangement according to claim 6 or to claim 7, whicharrangement further comprises a speed controller (7) and a torque gain unit (9),- wherein said speed controller (7) is arranged to receive a motorangular speed reference , as an input value, to receive the motorangular speed estimate from said flux observer (5), to calculate atorque reference value utilizing said motor angular speed referenceand said motor angular speed estimate , and to forward saidcalculated torque reference value to said torque gain unit (9), and- wherein said torque gain unit (9) is arranged to receive thecalculated torque reference value from said flux observer (5), tocalculate an iq current reference value , utilizing said calculatedtorque reference value , and to forward said calculated iq currentreference value , to said current controller (8).

9. The arrangement according to any of the claims 1 to 8, whereinsaid polyphase flux sensor / sensors of said flux sensor unit (4) comprise two or more hall effect sensors.

10. The arrangement according to claim 9, which arrangement further comprises one or more planar energy harvester coil / coils arranged to provide power to said two or more hall effect sensors.

11. The arrangement according to any of the claims 1 to 8, wherein said polyphase flux sensor / sensors of said flux sensor unit (4) comprise two or more planar energy harvester coils arranged to measure thepolyphase airgap flux , , , , … , of the asynchronous motor (3).

12. The arrangement according any of the claims 9 to 11, wherein saidcurrent controller (8), said / dq-converter (41), said dq / -converter(42), said flux controller (6), said speed controller (7) and said torque gain unit (9) are realized in a frequency converter.

13. The arrangement according any of the claims 1, 2, 4 to 7 or 9 to 12, wherein said arrangement comprises a speed sensor unit (33) placed in said asynchronous motor (3) arranged to measure the angular speed of the asynchronous motor (3).

14. A method for controlling an asynchronous motor (3), in which method:- polyphase airgap flux , , , , … , of the asynchronousmotor (3) is measured (51) by a flux sensor unit (4) comprising polyphase flux sensor / sensors placed in the airgap of said asynchronous motor (3);- polyphase stator current , , … of said asynchronous motor (3)is measured (52) by polyphase current sensor / sensors (31);- an airgap flux value , at least one stator current value and eitherone of a received / calculated stator voltage reference or a stator voltage value are received (53) by a flux observer (5) as input values;- a flux estimate is calculated (54) by said flux observer (5) utilizingsaid airgap flux value , said at least one stator current value and eitherone of said received / calculated stator voltage reference or said stator voltage value; and- said flux observer (5) is used in controlling (55) said asynchronousmotor (3).

15. The method according to claim 14, wherein:- the airgap flux value is an airgap flux vector signal value in a-reference frame;- at least one stator current value comprises: a stator currentvector signal value in a -reference frame and a stator current vectorsignal value in a dq-reference frame; and- the stator voltage value is a stator voltage vector signal valuein a -reference frame.

16. The method according to claim 15, in which method:- a stator current vector signal value in a -reference frame ofsaid measured stator current , , … is received by a / dq-converter(41) from said polyphase current sensor / sensors (31) via a secondabc / -converter (43), and converted to a stator current vector signalvalue in a dq-reference frame, and forwarded to a current controller (8);- a stator voltage reference , vector signal value in a dq-reference frame is calculated by said current controller (8) utilizing saidstator current vector signal value in a dq-reference frame, an id currentreference value , and an iq current reference value , , and- the stator voltage reference , vector signal value in a dq-reference frame is converted to a stator voltage reference , vectorsignal value in a -reference frame by a dq / -converter (42) and saidstator voltage reference , vector signal value in a -reference frameis forwarded for being used in control of said asynchronous motor (3).

17. A frequency converter comprising:- an inverter unit (2) arranged to drive an asynchronous motor (3);and- a flux observer (5),- wherein said flux observer (5) is arranged:- to receive an airgap flux value from a flux sensor unit (4),- to receive at least one stator current value from saidpolyphase current sensor / sensors (31), and -either to receive / calculate a stator voltage reference or toreceive a stator voltage value from polyphase voltage sensor / sensors (32), said polyphase voltage sensor / sensors (32) arranged to measure polyphase stator voltage , , … of saidasynchronous motor (3), -to calculate a flux estimate utilizing said airgap flux value ,said at least one stator current value and either one of said received / calculated stator voltage reference or said stator voltage value, and- wherein said flux observer (5) is used in control of saidasynchronous motor (3).

18. The frequency converter according to claim 17, wherein:- the airgap flux value is an airgap flux vector signal value in a-reference frame received from said flux sensor unit (4) via a firstabc / -converter (45);- at least one stator current value comprises:- a stator current vector signal value in a -referenceframe received from said polyphase current sensor / sensors (31) via a second abc / -converter (43), and- a stator current vector signal value in a dq-reference framereceived from a / dq-converter (41); and- the stator voltage vector is a stator voltage vector signal valuein a -reference frame received from polyphase voltage sensor / sensors(32) via a third abc / -converter (44).

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