System and method for calculating the torque difference between at least two asynchronous induction motors

Abstract

System and method for calculating torque difference between at least two asynchronous induction motors The invention relates to a system for calculating torque difference between at least two asynchronous induction motors connected in parallel on a plurality of phases to the same inverter (16), each connection being made, for each phase, by a phase conductor (18A, 18B, 18C). The system includes a calculation device (38), configured, for a pair comprising a first and a second motor (M1, M2), to receive from at least one current sensor (24A, 32A), a measurement of a current difference, for a single and same chosen phase, between the current supplied respectively to the first motor (M1) by the phase conductor (18A1) of said chosen phase and to the second motor (M2) by the phase conductor (18A2) of said chosen phase, and to calculate a torque difference between said first motor (M1) and second motor (M2) as a function of the current difference.Figure for the abbreviation: Figure 1.

Description

Title of the invention: System and method for calculating the torque difference between at least two asynchronous induction motors

[0001] The present invention relates to a system and a method for calculating the torque difference between at least two asynchronous induction motors connected in parallel over a plurality of phases to the same inverter adapted to supply an alternating voltage to each motor.

[0002] The invention lies in the field of the electrical power supply of asynchronous motors, and finds a particular application in the field of vehicle traction, particularly railway vehicles.

[0003] Indeed, in the field of vehicle traction, particularly railway vehicles, it is usual to use a traction chain comprising traction motors, which are asynchronous induction motors, each motor being linked to wheels of the vehicle through a transmission, to drive the movement of the wheels. In such a traction chain, two or more motors are powered in parallel by the same inverter. If the wheels associated with the motors powered by the same inverter are not precisely of identical diameter, the speeds of the motors are different and the load per motor is different, which leads to a difference in torque between these motors. It is important to control the torque exerted by each of the motors, to avoid overloading one motor relative to another.Indeed, such an overload can cause the asynchronous induction motor to stall under overload, as well as overheating, itself leading to a risk of fire.

[0004] It is known to use instantaneous speed sensors to monitor the load or torque difference between asynchronous induction motors, in addition to current sensors supplied by the inverter. In such a solution, at least two current sensors are required at the inverter, as well as one speed sensor per motor. The dispersion of the speed measurements is used to reconstruct the load dispersion between the motors, in order to protect the most loaded motor.

[0005] To facilitate maintenance and increase the reliability of motor overload monitoring, it is preferable to minimize the number of sensors used.

[0006] Patent application US2019 / 0006967A1 describes a method for detecting the load difference between a first electrical machine and at least one second three-phase electrical machine, powered in parallel by an inverter, using three current measurements, respectively two measurements on two distinct first phases of a first electrical machine and one measurement on the third phase of the second electrical machine, so at least three current sensors are used. According to this method, the amplitude of the sum of the three measured currents provides information on the load dispersion, without however allowing the calculation of the load difference between the electrical machines.

[0007] The invention aims to remedy the drawbacks of the prior art, by reducing the number of sensors used to provide protection for the most heavily loaded motor.

[0008] To this end, the invention proposes, according to one aspect, a system for calculating the torque difference between at least two asynchronous induction motors connected in parallel over a plurality of phases to the same inverter, each connection being made, for each phase, by a phase conductor at the output of the inverter connected, at a connection point, to a conductor of said phase per motor, the inverter being adapted to supply an alternating voltage to each motor, the system comprising a calculation device.For a pair of motors comprising a first motor and a second motor, the calculation device is configured to receive from at least one current sensor, a measurement of a current difference, for a single chosen phase, between the current supplied respectively to the first motor by the phase conductor of said chosen phase and to the second motor by the phase conductor of said chosen phase, and to calculate a torque difference between the first motor and the second motor as a function of said current difference.

[0009] Advantageously, the system of the invention makes it possible to calculate the torque difference between two asynchronous motors connected in parallel to the same inverter, without using a speed sensor. Advantageously, the current measurement of the same phase requires at most two current sensors to calculate the torque difference between two asynchronous motors connected in parallel, thus making it possible to reduce the number of current sensors used for this purpose.

[0010] The torque difference calculation system according to the invention may also have one or more of the characteristics below, taken independently or in any technically conceivable combination.

[0011] The system includes a differential current sensor encircling the phase conductor of said selected phase connected to the first motor and the phase conductor of said selected phase connected to the second motor, said differential current sensor providing said current difference.

[0012] The system includes a first current sensor placed on the phase conductor of said chosen phase at the output of the inverter, and a second current sensor placed either on the phase conductor of said chosen phase connected to the first motor or on the phase conductor of said chosen phase connected to the second motor.

[0013] The system further comprises addition / subtraction units adapted to provide said current difference from the current measurements provided by said

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[0023] first and second current sensors. The calculation device implements a formula using said current difference, a modulation rate of the inverter and a fundamental frequency of the alternating voltage supplied by the inverter. The calculation device implements a formula using said current difference, a modulation rate of the inverter and a fundamental frequency of the alternating voltage supplied by the inverter, said calculation formula further using a stator resistance value. According to another aspect, the invention relates to a method for calculating the torque difference between at least two asynchronous induction motors connected in parallel over a plurality of phases to the same inverter, each connection being made, for each phase, by a phase conductor at the output of the inverter connected, at a connection point, to a conductor of said phase per motor, the inverter being adapted to supply an alternating voltage to each motor. This method comprises, for a pair of motors comprising a first motor and a second motor, steps implemented by a calculation device: -obtaining, from at least one current sensor, a measurement of a current difference, for a single chosen phase, between the current supplied respectively to the first motor by the phase conductor of said chosen phase and to the second motor by the phase conductor of said chosen phase, and -calculating a torque difference between said first and second motors as a function of said current difference. The method for calculating torque deviation according to the invention may also have one or more of the characteristics below, taken independently or in any technically conceivable combination. The torque deviation calculation implements a calculation formula using said current difference, an inverter modulation rate and a fundamental frequency of the alternating voltage supplied by the inverter. The calculation formula is: Æv A i ATQ = 3N p Ijij Where ATQ is the calculated torque deviation, Np the number of magnetic pole pairs of each motor, Aisa the said current difference, Vs is the total voltage applied by the inverter and fs the fundamental frequency of the AC voltage applied by the inverter. The calculation formula is:

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[0034] A i - R. A ï2 ATK = 3N ---:----L=± p ç Where ATQ is the calculated torque gap, Np is the number of magnetic pole pairs of each motor, Ais_a is said current difference, Vs is a total voltage applied by the inverter, fs is the fundamental frequency of the AC voltage applied by the inverter, and Rs is a stator resistance. According to another aspect, the invention relates to a vehicle, in particular a railway vehicle, comprising a traction chain comprising at least two pairs of wheels, and at least two asynchronous induction motors, each asynchronous induction motor being linked by a transmission to at least one pair of wheels and being adapted to drive the rotation of said wheels, and an inverter, said asynchronous induction motors being powered in parallel by said inverter, the traction chain comprising a system for calculating the torque difference between asynchronous induction motors as described above, and a control device, configured to control a voltage supplied to said inverter as a function of the calculated torque difference. Other characteristics and advantages of the invention will emerge from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which: [Fig.l] [Fig.l] is a block diagram of a vehicle powertrain comprising a torque difference calculation system according to a first embodiment; [Fig.2] [Fig.2] is a block diagram of a vehicle powertrain comprising a torque difference calculation system according to a second embodiment; [Fig.3] [Fig.3] is a flowchart of the main steps of a torque deviation calculation process. [Fig.l] schematically illustrates a traction chain 2 of a vehicle, for example a railway vehicle (not shown). The traction chain includes two asynchronous induction motors, respectively a first motor M1 and a second motor M2, each motor M1, M2, being a three-phase, and more generally polyphase, electrical machine. In the example of [Fig.l], the motors Ml, M2 are three-phase, each motor comprising, in a known manner, three stator windings shown schematically, coupled to a conductive metal rotor. The motors Ml, M2 have the same number Np of pairs of magnetic poles each. Each motor M1, M2 is a traction motor, adapted to set in motion pairs of wheels 8, 10, each pair of wheels being linked to an associated motor via a transmission 12, 14.

[0036] The motors are connected in parallel at the output of an inverter 16, via phase conductors 18A, 18B, 18C shown schematically, each phase conductor corresponding respectively to a first phase, denoted phase A, a second phase denoted phase B, and a third phase denoted phase C. Each phase conductor 18A, 18B, 18C is connected between the output of the inverter 16 and a parallel connection terminal 15A, 15B, 15C, from which conductors 18Ai, 18A2; 18Bi, 18B2; 18Ci, 18C2 respectively supply the first phase, second phase, third phase of the first motor M1 and of the second motor M2.

[0037] The inverter 16 is, in a known manner, a device for generating alternating voltages and currents from an electrical energy source of different voltage or frequency. The electrical energy source is not shown.

[0038] The inverter 16 is configured to deliver the same alternating voltage to the terminals of each phase of each of the motors M1, M2.

[0039] The voltage delivered by the inverter 16 is controlled by a control device 20, via a pulse width modulation device 22 or PWM device.

[0040] The control device 20 is adapted to control the modulation rate mod and the fundamental frequency fs of the voltage to be applied, transmitted to the amplitude width modulation device 22 which is adapted to convert a voltage vector into three control signals of the power transistors (eg IGBT) of the inverter 16.

[0041] The control device 20 and the amplitude width modulation device 22 control the traction motors M1, M2 without a speed sensor.

[0042] The control device 20 receives as input current values ​​Isa and Is[3, obtained from current measurements on two output phases of the inverter 16, respectively phase A and phase B, denoted respectively Isa_inv and Isb_inv. These current measurements are carried out respectively by current sensors 24A, 24B of the conductors 18A, 18B.

[0043] The current values ​​Isa and Is[3 are obtained from the measurements of currents I sa inv and Isb inv by a unit 26 which performs a transformation of three-phase quantities into a two-dimensional vector, for example by applying the Concordia transform.

[0044] The current values ​​Isa and Is[3 are used to define the voltage to be applied and are sufficient in the absence of overload on one of the controlled motors.

[0045] The drive train 2 further comprises a system 30 for calculating the torque difference between the motors M1 and M2, the torque difference being noted ATQ.

[0046] The torque difference calculation system 30 advantageously implements two current sensors applied to a single phase of the motors M1, M2 powered by the inverter 16.

[0047] In the example illustrated, the phase chosen is the first phase A, but it is understood that this works in a similar manner with the second phase B or with the third phase C, and more generally with a single phase in the case of polyphase motors supplied in parallel by the same inverter.

[0048] In the example of [Fig. 1], the system 30 includes the current sensor 24A, adapted to provide a measurement of the output current of the inverter on the first phase A, and another current sensor 32A, which provides a measurement of the current Isa mot2 supplied to the second motor M2 on the first phase A. The current sensor 32A measures the current on the conductor 18A2.

[0049] The system 30 further comprises addition / subtraction units 34, 36.

[0050] Unit 34 receives as input the current measurement Isa inv on the first phase A and the current measurement Lancet provides as output: Lamoti=La_inv-Lamot2, where La moti is the current value on conductor 18Ai.

[0051] Unit 36 ​​receives as input the current value ISa_moti and the measurement La mot2 and provides as output: Ais a, the current difference between the current supplied to the first motor Ml (current on conductor 18Ai) and the current supplied to the second motor M2 (current on conductor 18A2):

[0052] [Math.l] A ^s_a sa - word I " sa - word2

[0053] Of course, in a similar manner, it is possible to measure the current supplied to the first motor Ml on the conductor 18A1 and to calculate: La mot2=Lajnv-La moti by the unit 34.

[0054] The system 30 further includes a torque deviation calculation device 38, which is an electronic calculation device, for example a microcontroller, comprising a calculation processor 40 and a memory 42.

[0055] The calculation device 38 also receives as input the values ​​of modulation rate mod and fundamental frequency fs of the voltage supplied by the control device 20.

[0056] The calculation device 38 implements the following formula for calculating the torque difference between the first motor and the second motor, adapted to the case of three-phase motors:

[0057] [Math.2] JÎV A i A TQ = 3N p Ittj

[0058] Where Np is the number of pole pairs of the motors Ml, M2, Vs a is the total voltage applied by the inverter on phase A, A i„ a is the calculated current difference, fs the fundamental frequency of the applied voltage.

[0059] The ATQ value thus calculated corresponds to an estimate of the torque difference.

[0060] The total voltage Vs a is calculated by:

[0061] [Math.3] a ~ ■ Uf ' Sin (ua)

[0062] Where mod is the modulation rate, Uf the inverter bus voltage and Ua the voltage applied to phase A.

[0063] Thus, the torque difference is calculated using the proposed formula [MATH 2], using the current difference on a single phase and values ​​characterizing the voltage supplied by the inverter.

[0064] According to a variant, the accuracy of calculating the torque difference is improved by taking into account the losses in the stator resistance, by the following formula:

[0065] [Math.4] JÎV A i - R, A z 2 A TQ = 3N --?--LA

[0066] Where Rs is the stator resistance, the term ^j2 representing the loss of stator power. The value of the stator resistance Rs is a characteristic value of each asynchronous motor, for example provided by the manufacturer. For example, several asynchronous motors of the same type, with the same electrical characteristics, are connected in parallel for propulsion, and they all have the same stator resistance.

[0067] The ATQ value calculated by the formula [MATH 4] is an estimate of the value of the torque deviation, refined with respect to the value calculated by the formula [MATH 2] by the term corresponding to the power loss at the stator, divided by 2irfs.

[0068] In one application, the calculated torque deviation value is provided to the control device 20 to ensure that neither of the motors M1, M2 produces excessive torque, exceeding the motor's thermal limit or a torque close to motor stall. This makes it possible to implement a control system for a voltage supplied to an inverter that powers at least two asynchronous induction motors in parallel, which has the advantage of preventing overloading of one of the motors.

[0069] Alternatively or additionally, the torque difference thus calculated is transmitted to another monitoring and control device not shown, to carry out other processing, for example to raise an alert to warn an operator.

[0070] [Fig.2] illustrates a second embodiment of a traction chain comprising a system 50 for calculating the torque difference between the first motor M1 and the second motor M2. The elements common to the two embodiments retain the same reference.

[0071] In the embodiment of Figure 2, the system 50 comprises a differential current sensor 52, embracing the conductors 18Ai, 18A2, which provides in a single measurement the current difference: A fa = Isa _ motA - Isa _ mof2

[0072] This current difference value is provided directly as an input to the calculation device 38.

[0073] Thus, in this second embodiment, the 50 system of torque difference between the two motors is simplified.

[0074] The formulas for calculating torque deviation are the same, and are implemented in a similar manner.

[0075] Fig. 3 is a synoptic diagram of the main steps of a process according to the invention, implemented by the computing device 38.

[0076] The method comprises, for a selected motor pair comprising a first motor and a second motor, and for a given phase, obtaining 60 a current difference between the current supplied to the first motor and the current supplied to the second motor.

[0077] The method also includes obtaining 62 a modulation rate mod and a fundamental frequency of the voltage fs.

[0078] The process then includes, according to a first variant, the calculation 64 of a torque difference according to the formula [MATH 2].

[0079] According to a second variant, the calculation 64 of a torque difference implements the formula [MATH 4].

[0080] Obtaining 60 a current difference between the current supplied to the first motor and the current supplied to the second motor is implemented with a chosen frequency measurement, so as to obtain almost instantaneous monitoring.

[0081] Thus, a quasi-instantaneous torque deviation calculation is obtained.

[0082] In one embodiment, the method of the invention is implemented in the form of software, which is stored in the memory 42 of the computing device 38, and executed by the processor 40 when the computing device 38 is powered on.

[0083] Alternatively, the calculation device 38 is produced in the form of programmable logic components, such as FPGAs (Field Programmable Gate Arrays), or even dedicated integrated circuits, such as ASICs (Application Specific Integrated Circuits).

[0084] The method for calculating the torque difference described applies, more generally, to any pair of motors Mi, Mk in a case where N asynchronous motors are supplied in parallel by the same inverter, with N an integer greater than or equal to 2.

[0085] For example, in the case where N is strictly greater than 2, one of the motors is chosen as the reference motor, for example the motor with index i.

[0086] The torque difference ATQ between the motor Mi and each other motor of index k (motor Mk), with ki is calculated by the formula [MATH 2] or by the formula [MATH 4], depending on the variant chosen.

[0087] The method optionally includes a calculation (66) of the difference d_max between the average torque produced by N motors in parallel and the torque of the most loaded motor, which makes it possible to adjust the control voltage of the inverter.

[0088] This difference is calculated based on the torque differences between motors, using the following formula:

[0089] [Math.5] 1 V d_max = ^max k = àT Q ik i & k

[0090] The method described applies to a number N of polyphase motors. It has been described above in the three-phase case, but it applies in the case of motors with Q phases, where Q is greater than or equal to 2, the calculations being applied in a similar manner to a chosen phase.

[0091] The torque difference calculated in step 64, or, where appropriate, the difference d_max between the average torque achieved by N motors in parallel and the torque of the most loaded motor calculated in step 66, is supplied to the control device for controlling a voltage supplied to the inverter. Thus, the invention makes it possible to control a voltage supplied to an inverter supplying at least two asynchronous induction motors in parallel, avoiding an overload of any of the controlled asynchronous induction motors.

Claims

Claims

1. System for calculating the torque difference between at least two asynchronous induction motors connected in parallel over a plurality of phases to the same inverter (16), each connection being made, for each phase, by a phase conductor (18A, 18B, 18C) at the output of the inverter connected, at a connection point, to a conductor (18Ai, 18A2; 18Bi, 18B2;18Ci, 18C2) of said phase per motor (Ml, M2), the inverter (16) being adapted to supply an alternating voltage to each motor, the system comprising a calculation device (38) and being characterized in that, for a pair of motors (Ml, M2) comprising a first motor (Ml) and a second motor (M2), the calculation device (38) is configured to receive from at least one current sensor (24A, 32A, 52), a measurement of a current difference, for a single and same chosen phase, between the current supplied respectively to the first motor (Ml) by the phase conductor (18Ai) of said chosen phase and to the second motor (M2) by the phase conductor (18A2) of said chosen phase, and to calculate a torque difference between said first motor (Ml) and second motor (M2) as a function of said current difference.;

2. A system according to claim 1, comprising a differential current sensor (52) interlacing the phase conductor (18Ai) of said selected phase connected to the first motor and the phase conductor (18A2) of said selected phase connected to the second motor, said differential current sensor (52) providing said current difference.

3. System according to claim 1, comprising a first current sensor (24A) placed on the phase conductor (18A) of said chosen phase at the output of the inverter (16), and a second current sensor (32A) placed either on the phase conductor (18Ai) of said chosen phase connected to the first motor or on the phase conductor (18A2) of said chosen phase connected to the second motor.

4. A system according to claim 3, further comprising addition / subtraction units (34, 36) adapted to provide said current difference from the current measurements provided by said first and second current sensors (24A, 32A).

5. A system according to any one of claims 1 to 4, wherein the calculating device (38) implements a formula using said current difference, a modulation rate of the inverter and a fundamental frequency of the alternating voltage supplied by the inverter.

6. A system according to any one of claims 1 to 5, wherein the calculating device implements a formula using said current difference, a modulation rate of the inverter and a fundamental frequency of the alternating voltage supplied by the inverter, said calculating formula further using a stator resistance value.

7. Method for calculating the torque difference between at least two asynchronous induction motors connected in parallel over a plurality of phases to the same inverter, each connection being made, for each phase, by a phase conductor at the output of the inverter connected, at a connection point, to a conductor of said phase per motor, the inverter being adapted to supply an alternating voltage to each motor, characterized in that it comprises, for a pair of motors comprising a first motor and a second motor, steps implemented by a calculation device: -obtaining (60), from at least one current sensor, a measurement of a current difference, for a single and same chosen phase, between current supplied respectively to the first motor by the phase conductor of said chosen phase and to the second motor by the phase conductor of said chosen phase,and -calculating (64) a torque difference between said first and second motors as a function of said current difference.,

8. The method of claim 7, wherein the torque deviation calculation implements a calculation formula using said current difference, a modulation rate of the inverter and a fundamental frequency of the alternating voltage supplied by the inverter.

9. The method of claim 8, wherein the calculation formula is: aSv A i ATQ = 3N - - “ p Zttj Where ATQ is the calculated torque deviation, Np the number of magnetic pole pairs of each motor, Aisa said current difference, Vs a total voltage applied by the inverter and fs the fundamental frequency of the alternating voltage applied by the inverter.

10. A method according to claim 8, wherein the calculation formula is: Æv A i - R- A i2 ATQ-3N ----£=4 p Iîtj Where ATQ is the calculated torque difference, Np is the magnetic pole pairs of each motor, Ais_a is the said current difference, Vs is the total voltage applied by the inverter, fs is the fundamental frequency of the AC voltage applied by the inverter and Rs is a stator resistance.

11. Vehicle, in particular a railway vehicle, comprising a traction chain comprising at least two pairs of wheels, and at least two asynchronous induction motors, each asynchronous induction motor being linked by a transmission to at least one pair of wheels and being adapted to drive the rotation of said wheels, and an inverter, said asynchronous induction motors being powered in parallel by said inverter, the traction chain comprising a system for calculating the torque difference between asynchronous induction motors according to claims 1 to 6, and a control device, configured to control a voltage supplied to said inverter as a function of the calculated torque difference.

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