SE9903665A0

SE9903665A0 - A device and a method for estimating the electromagnetic torque of a slip ring asynchronous machine

Info

Publication number: SE9903665A0
Application number: SE9903665A
Authority: SE
Inventors: Lars Tuve Hansson; Alojz Slutej
Original assignee: Abb Ab
Priority date: 1999-10-12
Filing date: 1999-10-12
Publication date: 2001-04-30
Also published as: SE9903665D0; SE9903665L

Abstract

Abstract The invention relates to a device and a method for estimating the electromagnetic torque of a slip ring asynchronous machine (1). The asynchronous machine (1) comprises a stator (2) having a stator winding, arranged to be fed with three-phase voltage and current in order to generate a varying magnetic flux, and a rotor (3) having a rotor winding in which the magnetic flux is arranged to induce a voltage to create an electromagnetic torque. The device comprises a measuring equipment (18) arranged to measure the values of the voltage and current fed to the stator winding, a measuring member (12) arranged to measure a value of a parameter related to the rotation speed of the rotor (2) and a calculating unit (17) arranged, by means of said measured values, to calculate a value of the electromagnetic torque.

Description

B1LAGOR [xj Beskrivning, patentkrav och samman- Ombud: BJERKENS PATENTBYRA KB, Sthlm. drag i tre exemplar [ ] Utlandsk text i 1 exemplar [x] 2 St ritningar i 3 exemplar [ I Overlatelsehandling [ I FuIlmakt [ I Ev sekvenslista i maskinlasbar form [ I Prioritetsbevis Stockholm den 12 oktober 1999 Stefa Ber AVGI FT [x] Anmalningsavgift: SEK 800:- [x] Granskningsavgift: SEK 3000:- [x] Tillaggsavgift, SEK 100:- fOr varje patentkrav utOver tio Ix] Diariebevis: SEK 20:- ix) ITS-granskningsavgift: SEK 5510:- SEK 800:- SEK 3 000:- SEK 1 200:- SEK 20:- SEK 510:- kg/mw Ref.: SE 51162 Sokande: ABB AB A device and a method for estimating the electromagnetic torque of a slip ring asynchronous machine TECHNICAL FIELD OF THE INVENTION AND PRIOR ART The invention relates to a device and a method for 15 estimating the electromagnetic torque of a slip ring asynchronous machine, wherein the asynchronous machine comprises a stator having a stator winding, arranged to be fed with three-phase voltage and current in order to generate a varying magnetic flux, and a rotor having a rotor 20 winding in which the magnetic flux is arranged to induce a voltage to create an electromagnetic torque which drives the rotor with a rotation speed with a slip in relation to a rotation speed of the varying magnetic flux. 25 Such slip ring asynchronous machines are used, for example, in cranes, rolling mills, in heavy industry and in lifting devices like elevators. In a crane may, for example, four asynchronous machines be used to supply a motion to a load lifted by the crane. A control system, for example, ASTAT 30 may be used to control said motion. Usually, the control system controls the machines by the use of a pair of thyristors in each phase for regulating the voltage and current supplied to the stator winding of the machines. The control system is arranged to obtain information about the 35 actual motor speed from measuring members in order to provide a stable control of the load. Such known control 2 systems control the asynchronous machines without knowledge about the actual electromagnetic torque.

SUMMARY OF THE INVENTION The object of the present invention is to provide a device and a method for estimating the electromagnetic torque of a slip ring asynchronous machine, which make it possible to estimate the electromagnetic torque with a high precision 10 and at the same time provide a product which is light, compact and easy to use in a control system.

This object is achieved according to the initially mentioned device, which is characterised in that the device comprises 15 a measuring equipment arranged to measure the values of the voltage and current fed to the stator winding, a measuring member arranged to measure a value of a parameter related to the rotation speed of the rotor and a calculating unit arranged, by means of said measured values, to calculate a 20 value of the electromagnetic torque. By the above measured parameters and by knowledge of the properties of the significant asynchronous machine, it is possible to calculate an actual value of the electromagnetic torque with high precision by means of a calculating unit. Such a 25 calculating unit may include a microprocessor or a computer. The device may include mainly electric components, wherein the device will be manufactured in such a way that it is light and compact. 30 According to a preferred embodiment of the invention, the measuring equipment is arranged to measure the values of at least two of the phases of the voltage and the current. If the asynchronous machine is connected to a conventional power supply of, for example, 380 V, the value of the third 35 phase may be calculated from the two first phases. In order to measure the voltage and current supplied to the stator 3 winding, the measuring equipment may comprises current and voltage transformers. Preferably, the measuring equipment also comprises an isolation amplifier, which provides total isolation between input and output channels.

According to an embodiment of the invention, the measuring member may be arranged to measure the rotor speed. Such a measuring member may be a tachometer or a kind of optical' revolution indicator. Advantageously, a circuit comprising a 10 variable resistance is connected to the rotor winding. By such a circuit, the start and the control of the asynchronous machine is facilitated. The measuring member may be arranged to measure the voltage in said circuit connected to the rotor winding. It is possible to determine 15 the rotor speed by information of the rotor voltage. Such an electrical measuring member is less sensitive and requires a smaller space than, for example, a tachometer. Advantageously, the measuring member is arranged to measure the value of said voltage during at least one time period. 20 It is possible to observe the fluctuation of the voltage, during the time period, and thus determine the oscillation of the voltage. This may be done by the calculating unit, which calculates the voltage frequency. After that, the calculating unit calculates the slip and finally the rotor 25 speed. Thereafter, the calculating unit may, as mentioned above, calculate the electromagnetic torque. The calculating unit may be arranged to use a mathematical algorithm for the calculating of the electromagnetic torque. By an advanced mathematical algorithm, the calculated value of the 30 electromagnetic torque may be a nearly correct estimation of the real electromagnetic torque.

According to a preferred embodiment of the invention, the device is a part of a control system for at least one slip 35 ring asynchronous machine, which control system comprises a control unit arranged to control the voltage and current fed 4 to the stator winding. A control system may by information about the actual electromagnetic torque additionally improve the ability to control one or several asynchronous machines. Such a control unit may be arranged to control a thyristor 5 device in order to supply said three-phase voltage and current to the stator winding. By the control of a pair of thyristors, in each phase, the supplied voltage and current to the stator winding may not always be sinusoidal. 10 According to a preferred embodiment of the invention, the control unit is arranged to control the resistance in a circuit connected to the rotor winding for varying the resistance in the circuit, due to the value of the calculated electromagnetic torque, in order to minimise the 15 current fed to the stator winding. The control unit may calculate a value of the optimal resistance in the circuit due to the value of the estimated electromagnetic torque. Thereafter, the control unit sends a control signal to a switch device, which connects a resistance in the circuit 20 according to the calculated value. Thereby, the energy wastage may be minimised.

The invention comprises also a method for estimating an electromagnetic torque of a slip ring asynchronous machine. 25 The method relates to the initially defined machine and comprises the step to measure the values of the voltage and current fed to the stator winding, to measure the value of a parameter related to the rotor speed and to calculate a value of the electromagnetic torque by means of said 30 measured values. By the above mentioned measurement and information about properties of the specific asynchronous machine, it is possible to calculate a value of the actual electromagnetic torque for an asynchronous machine. 35 According to a preferred embodiment of the invention, the method may comprise the step to measure the value of at least two phases of the voltages and currents fed to the stator winding. The third phase may be calculated from the two measured phases if the power supply is conventional and is, for example, a three-phase voltage of 380 V.

According to another preferred embodiment of the invention, the method may comprise the step to measure the rotation speed of the rotor. A tachometer or an optical revolution indicator may measure the rotational speed of the rotor. 10 Advantageously, the method comprises the step to measure the voltage in a circuit connected to the rotor winding. It is possible by information of the rotor voltage to calculate the rotor speed. The method may comprise the step to measure the value of the rotor voltage during at least one time 15 period. Thereby, the fluctuation of the voltage may be visible during the time period and it is possible to calculate the oscillation of the rotor and the voltage frequency. Thereafter, the slip may be calculated and finally the rotor speed. By information about the rotor 20 speed and the voltage and current supplied to the stator winding, the value of the electromagnetic torque may be calculated for a specific asynchronous machine by means of a mathematical algorithm. An advanced mathematical algorithm results in a nearly exact estimation of the electromagnetic 25 torque.

According to another preferred embodiment of the invention, the method may be used in a control system, which controls at least one slip ring asynchronous machine. A control 30 system, which obtains information about the actual electromagnetic torque, may use said information to control the electromagnetic torque in the asynchronous machine. If the control system controls several asynchronous machines the electromagnetic torque may be divided equally between 35 the asynchronous machines.

According to another preferred embodiment of the invention, the method may be used to calculate the value of the electromagnetic torque for varying the resistance in a circuit connected to the rotor winding in order to minimise 5 the current fed to the stator winding. Thereby, the energy wastage of the asynchronous machines may be minimised.

BRIEF DESCRIPTION OF THE DRAWINGS 10 In the following the invention is described by way of examples with reference to the attached drawings, in which Fig 1shows schematically a control system using a device according to the invention for estimating the rotor speed, Fig 2shows schematically the signal processor in Fig 1, Fig 3shows schematically the use of a monitoring device, which monitors the states of four asynchronous machines.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Fig 1 shows schematically a control system using a device 25 for estimation of the rotor speed of a slip ring asynchronous machine 1. Preferably, said control system may be used for controlling an arbitrary number of asynchronous machines, for examples, four asynchronous machines I in a crane. The asynchronous machine 1 comprises a stator 2 and a 30 rotor 3. The stator 2 has a stator winding and the rotor 3 has a rotor winding. The asynchronous machine 1 is fed with three-phase voltage and current by cables 4a-c. The supplied three-phase voltage and current to the stator winding generates a varying magnetic flux, which induces a voltage 35 in the rotor winding. The voltage in the rotor winding creates an electromagnetic torque, which drives the rotor 3 7 with a rotor speed with a slip in relation to a rotational speed of the varying magnetic flux. A circuit 6 including a plurality of resistors 7 is connected to each phase of the rotor winding. The circuits 6 include a desired number of 5 resistors 7. A switching device 8 is provided to the circuits 6 for connecting a desired numbers of resistors in the circuits 6 in order to vary the resistance.

A thyristor device 9 is provided to control the supply of 10 voltage and current to the stator 2. The thyristor device 9 consists of a pair of thyristors in each phase. By the use of such thyristors, the voltage to the stator 2 may be reduced continuously from a full line voltage to zero. A control unit 10 is provided to control the thyristor device. 15 Such a control unit 10 may be a microprocessor or a computer, which orders the thyristors to switch to an on state by adjusting the phase position of trigger pulses, hence applying more or less stator voltage. 20 An operator of, for example, a crane is by means of an operating device, which may be at least one joystick 11, arranged to control the motion of a load lifted by the crane. The operator may by means of the joystick control the voltage and current supplied to the asynchronous machines 1 25 and thus the load lifted by the crane. The operator commands the joystick 11 to a desired position to control the movement of the load. Thereby, a signal related to the position of the joystick 11 is sent to the control unit 10. Thereafter, the control unit 10 orders the thyristor device 30 9 to supply a required level of voltage and current to the stator winding. In that way, the asynchronous machine 1 obtains a desired speed.

In order to control the speed of the asynchronous machine 1 35 with a high precision, the control system comprises a first device for estimating the actual rotor speed. The first 8 device comprises a measuring member 12. The measuring member 12 is connected to the circuits 6 for measuring the induced voltage in the rotor winding, in each phase. The measuring member 12 measures the rotor voltage during at least one 5 time period. Thereafter, the measuring member 12 outputs an analog signal related to the measured values of the rotor voltage during the time period. This measured analog signal is often fluctuating due to disturbances. Consequently, the signal may not immediately be used for calculating the 10 frequency of the rotor voltage. Therefore, the output signal from the measuring member 12 is led to a signal processor 13. The signal processor 13 is arranged to transform and filter the signal in order to remove the noise. 15 Fig 2 shows schematically a signal processor 13. The signal processor 13 comprises, in this case, an analog signal processor unit 14. The analog signal processor unit 14 is arranged to provide an analog transformation and filtering of the output signal from the measuring member 12. This may 20 be performed by means of operation amplifiers and resistor- capacitor networks. Thereafter, the signal is led to an AID converter 15. The A/D converter converts the analog signal to a digital signal. Finally, the signal processor 13 comprises a digital signal processor unit 16. The digital 25 signal processor unit 16 comprises digital filters and performs a digital filtering of the signal. The output signal from the signal processor 13 is now relatively clear and essentially free from disturbances. The output signal is led to a calculating unit 17. The calculating unit 17 30 calculates the frequency of the rotor voltage by means of this signal. By knowledge of the frequency of the rotor voltage, the calculating 17 unit then calculates the slip of the asynchronous machine 1 and finally the rotor speed. The calculating unit 17 uses a mathematical algorithm for these 35 calculations. Thereafter, the calculating unit 17 outputs a signal related to the actual value of the rotor speed to the 9 control unit 10. The calculating unit 17 and the control unit 10 may be parts of a microprocessor or a computer. This first device, which estimates the actual rotor speed may be very compact and therefore requires only a small space.

In order to control the rotor speed with an additional precision, the control system comprises a second device for estimation of the actual electromagnetic torque of the asynchronous machine 1. The second device comprises 10 measuring equipment 18. The measuring equipment 18 is arranged to measure at least two phases of the voltage and current supplied to the stator 2. If the asynchronous machine 1 is connected to a three-phase conventional power supply of, for example, 380 V, the values of the third phase 15 may be calculated from the first and second phases. Such measuring equipment 12 may consist of current and voltage transformers and an isolation amplifier. The measuring equipment 18 is arranged to send an output signal to the calculating unit 17, which is related to the measured values 20 of the voltage and current fed to the asynchronous machine 1. Thereby, the calculating unit 17 is arranged to obtain information about the actual voltage and current supplied to the stator 2. At the same time, the calculating unit 17 is arranged to obtain information about the actual rotor speed 25 from the first device. The calculating unit 17 may, by this information and the knowledge of the properties of the specific asynchronous machine 1, calculate the actual electromagnetic torque for the asynchronous machine. The calculating unit 17 uses also in this case a mathematical 30 algorithm for the calculation. This second device for estimation of the actual electromagnetic torque may also be very compact and thus requires only a small space.

The control unit 10 may, by the information about the actual 35 electromagnetic torque and the position of the joystick 11, control the voltage and current supplied to the stator 2 by the thyristor device 9 in order to control the speed of at least one asynchronous machine 1. In, for example, a crane, an operator may regulate two joysticks controlling four asynchronous machines in order to move a load. By means of 5 the calculated values of the electromagnetic torque for each asynchronous machine 1, the control system will distribute the torque essentially equally to the asynchronous machines 1. 10 A display 19 is connected to the control unit 10. The display 19 is arranged to display comprehensive information about the state of the asynchronous machine 1. Red warning lights will, for example, be actuated at a machine or cable overtemperatur, brake fault, rotor/speed fault, overload, 15 overspeed and thyristor overload. In other cases, a green light will indicate that the control system is OK. Other comprehensive information, which may be displayed, is motor speed, shaft torque and stator voltage and current. 20 As mentioned above, a number of asynchronous machines 1 may be used in a crane for providing a desired motion to a load lifted by the crane. Fig 3 shows schematically a monitoring device, which monitor the state of each of the asynchronous machine la-d. A measuring member 12a-d measures the rotor 25 voltage of the asynchronous machines la-d. Thereafter, the measuring members 12a-d output a signal corresponding to the rotor voltage of each asynchronous machines la-d. This signal may be treated in a signal processor 13 before the signal is sent to the calculating unit 17 and to the control 30 unit 10. In this way, it is possible to monitor the function of each of the asynchronous machines la-d. Thereby, it is possible to discover a fault of any asynchronous machine lad and prevent the other asynchronous machines la-d from working at an unnecessary high torque. 11 In order to minimise the energy wastage, the control unit is arranged to vary the resistance in the circuits 6 in response to the calculated electromagnetic torque. In such a way, the control unit 10 minimises the current in the stator 5 winding by selecting as high resistance as possible in the circuits 6 due to the actual electromagnetic torque. Thereby, the control unit 10 calculates the optimal resistance in the circuits 6 and sends a control signal to the switching device 8. The switching device 8 connects so 10 many resistors 7 in the circuits 6 as possible due to the actual calculated electromagnetic torque.

The invention is not in any way restricted to the embodiment described in the figures, but may be varied freely within 15 the scope of the claims. 12

Claims

1. A device for estimating an electromagnetic torque of a slip ring asynchronous machine (1), wherein the asynchronous machine (1) comprises a stator (2) having a stator winding, arranged to be fed with three-phase voltage and current in order to generate a varying magnetic flux, and a rotor (3) having a rotor winding in which the magnetic flux is arranged to induce a voltage to create an electromagnetic torque which drives the rotor (3) with a rotor speed with a slip in relation to a rotation speed of the varying magnetic flux, characterised in that the device comprises a measuring equipment (18) arranged to measure the values of the voltage and current fed to the stator winding, a measuring member (12) arranged to measure a value of a parameter related to the rotation speed of the rotor (2) and a calculating unit (17) arranged, by means of said measured values, to calculate a value of the electromagnetic torque.

2. A device according to claim 1, characterised in that the measuring equipment (18) is arranged to measure the values of at least two of the phases of the voltage and the current.

3. A device according to claim 2, characterised in that the measuring equipment (18) comprises current and voltage transformers.

4. A device according to claim 2 or 3, characterised in that 30 the measuring equipment (18) comprises an isolation amplifier.

5. A device according to any one of the preceding claims, characterised in that the measuring member (12) is arranged 35 to measure the rotor speed. 13

6. A device according to any one of the claims 1 to 4, characterised in that a circuit (6) comprising a variable resistance is connected to the rotor winding.

7. A device according to claim 6, characterised in that the measuring member (12) is arranged to measure the voltage in said circuit (6).

8. A device according to claim 7, characterised in that the 10 measuring member (12) is arranged to measure the value of said voltage during at least one time period.

9. A device according to any one of the preceding claims, characterised in that the calculating unit (17) is arranged 15 to use a mathematical algorithm for the calculating of the electromagnetic torque.

10. A device according to any one of the preceding claims, characterised in that the device is a part of a control system for at least one slip ring asynchronous machine (1), which control system comprises a control unit (10) arranged to control the voltage and current fed to the stator winding.

11. A device according to claim 9, characterised in that the control unit (10) is arranged to control a thyristor device (9) in order to supply said three-phase voltage and current to the stator winding.

12. A device according to claims 6 and 9 or 10, characterised in that the control unit (10) is arranged to control the resistance in the circuit (6) connected to the rotor winding for varying the resistance in the circuit (6), due to the value of the calculated electromagnetic torque, in order to minimise the current fed to the stator winding. 14

13. A method for estimating an electromagnetic torque of a slip ring asynchronous machine, wherein the machine comprises a stator having a stator winding, arranged to be fed with three-phase voltage and current in order to generate a varying magnetic flux, and a rotor having a rotor winding in which the magnetic flux is arranged to induce a voltage to create an electromagnetic torque which drives the rotor with a rotor speed with a slip in relation to a rotation speed of the varying magnetic flux, characterised in that the method comprises the step to measure the values of the voltage and current fed to the stator winding, to measure the value of a parameter related to the rotation speed of the rotor and to calculate a value of the electromagnetic torque by means of the above measured values.

14. A method according to claim 13, characterised by measuring the value of at least two of the phases of the voltage and current fed to the stator winding.

15. A method according to claim 13 or 14, characterised by measuring the rotation speed of the rotor.

16. A method according to claim 13 or 14, characterised by measuring the voltage in a circuit connected to the rotor winding.

17. A method according to claim 16, characterised by measuring the value of the voltage during a time period.

18. A method according to any one of the claims 13-17, characterised by calculating the value of the electromagnetic torque by means of a mathematical algorithm.

19. A method according to any one of the claims 13-18, characterised by using the method in a control system, which controls at least one slip ring asynchronous machine.

20. A method according to claims 19, characterised by using the calculated value of electromagnetic torque for varying the resistance in a circuit connected to the rotor winding in order to minimise the current fed to the stator winding.

21. Use of a device according to any one of the claims 1-12, in order to control at least one asynchronous machine.

22. Use according to claim 21, characterised in that the at least one asynchronous machine is controlled in order to 15 move a load. 16 Abstract The invention relates to a device and a method for estimating the electromagnetic torque of a slip ring asynchronous machine (1). The asynchronous machine (1) comprises a stator (2) having a stator winding, arranged to be fed with three-phase voltage and current in order to generate a varying magnetic flux, and a rotor (3) having a rotor winding in which the magnetic flux is arranged to induce a voltage to create an electromagnetic torque. The device comprises a measuring equipment (18) arranged to measure the values of the voltage and current fed to the stator winding, a measuring member (12) arranged to measure a value of a parameter related to the rotation speed of the rotor (2) and a calculating unit (17) arranged, by means of said measured values, to calculate a value of the electromagnetic torque.