The invention relates to estimation of a rotation speed of an electric motor, and particularly to estimation of a rotation speed of an electric motor supplied by an inverter.
An inverter is an electric device, with which it is possible to supply a load with a voltage that has a variable frequency. Inverters are typically used in connection with frequency converters.
Inverters and, more generally, electric drives comprising inverters typically involve various safety functions which aim to control and ensure the safety of the drive in various operation conditions. As control methods have developed, electric drives without movement sensors have become more common in applications in which traditional solutions have required a feedback coupling, such as a tachometer, from the motor's shaft or from mechanics connected thereto. In this kind of sensorless applications, a possible fault situation of the inverter control system may lead to an overspeed of the motor and, as a result, to severe personal injury and/or considerable damage to property, e.g. in the case of lifts and cranes. Therefore, such sensorless solutions typically employ a separate safety circuit or safety supervision system, which is independent of the electric drive and which supervises or verifies the rotation speed or movement of the motor or mechanics connected thereto independently of the inverter by utilizing alternative speed determination. For example, if the determined alternative rotation speed value exceeds a predetermined threshold value, the safety circuit may activate a safety function, which typically comprises activation of an emergency braking of the drive. In addition or alternatively, the safety circuit may use both the alternative rotation speed value and the rotation speed value determined by the inverter for implementing various safety functions. For example, these two values could be compared to one another, and if they differ, certain safety functions might be activated.
A separate speed measurement adds to the cost of the safety function, however, and therefore some solutions are based on monitoring the voltage and current supplied to the motor by the inverter. The safety circuit thus uses the same signals as the inverter controlling the motor and calculates a signal proportional to the motor's speed. When this signal value exceeds a predetermined threshold value, the safety function is activated. The calculation of the signal proportional to the speed is preferably not dependent on motor parameters and it should substantially differ from the speed estimation algorithm used in the inverter in order that the safety circuit would be able to control the operation of the inverter. One solution is to calculate the signal proportional to the motor's speed simply on the basis of the fundamental wave frequency of the voltage or current vector, i.e. the supply frequency. In the case of a synchronous machine, the supply frequency corresponds directly to the speed of the machine. In the case of an asynchronous machine, there is a deviation corresponding to the slip frequency. This deviation, however, is considered negligible to the correct operation of the safety function in such solutions. An example of a solution utilizing the supply frequency of the machine for the estimation of the rotation speed is disclosed in U.S. Pat. No. 6,745,083.
Speed estimation solutions based on the monitoring of the frequency of the fundamental wave of the supply voltage or current have two problems. In the case of an asynchronous machine, such solutions are incapable of detecting a fault situation of the control of the frequency converter in which the actual speed of the machine has increased to a high value while the control erroneously assumes that the speed is close to zero and the frequency converter is supplying low-frequency voltage to the machine. In this situation the speed estimation based on the monitoring of the frequency of the fundamental wave of the supply voltage cannot detect that the speed of the machine is high, because it can only see the frequency supplied by the frequency converter to the machine. In reality, the operating point is on the wrong side of the breakover point and the speed increases in an uncontrollable manner because the accelerating torque caused by the load (e.g. in crane applications) is greater than the counter-torque produced by the machine. In the case of a synchronous machine, the stator current does not necessarily have a magnetization component and, therefore the current vector length is almost zero during zero moment and it is impossible to detect the fundamental wave on the basis thereof. At zero speed also the voltage vector length is zero and thus the voltage vector does not indicate anything about the speed either.
The object of the invention is thus to provide a method and an apparatus for implementing the method so as to solve or at least alleviate the above problems. The object of the invention is achieved with a method, computer program product, and an apparatus that are characterized by what is stated in the independent claims. Preferred embodiments of the invention are described in the dependent claims.
The invention is based on monitoring the angular speed of the back-emf (electromotive force) of an electric motor for estimating a rotation speed of the electric motor. The idea of the invention is to determine a time derivative of a stator current vector of the electric motor during a zero vector state of an inverter supplying the electric motor, when the motion voltage is directly proportional to the determined time derivative, whereupon the estimate for the rotation speed of the electric motor can be determined on the basis of the thus determined time derivative of the stator current vector.
An advantage of the invention is that an estimate for the rotation speed of an electric motor can be determined in a reliable way also in case the frequency of the fundamental wave of the supply voltage or current cannot be reliably detected or otherwise used for the speed estimation. The invention can be used in connection of any application in which sensorless speed estimation is preferable either for principal or redundant speed determination purposes.
In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawings, in which
The example of a frequency converter connection shown in
The invention is based on monitoring the angular speed of the back-emf (electromotive force) of an electric motor for estimating a rotation speed of the electric motor. When the behaviour of the back-emf is examined in the case of an asynchronous machine, the following equation applies in a stator coordinate system:
wherein:
ūs=stator voltage
īs=stator current
R=rotor flux
Rs=stator resistance
RR=rotor resistance
Lσ=leakage inductance
τr=rotor time constant
ωm=mechanical speed
By assuming that the losses of the machine are small and the rotor time constant is great, the following approximate formula is obtained:
Thus, the time derivative of the stator current changes in proportion to the difference of momentary voltage pulses ūs supplied into the machine and the motion voltage jωm
The motion voltage comprises information about the speed of the machine in two different ways. First of all, its amplitude in the short run is directly proportional to the speed of the machine (when the changes of the rotor flux are assumed to be slow). On the other hand, in a short-period observation the rotor flux tends to rotate along with the rotor if the speed of the machine overspeeds because of a malfunction of the control. The same turning of the phase angle may also be noticed in the motion voltage having the direction of a tangent of the rotor flux and thus the speed of the machine can be estimated directly from the angular speed of the motion voltage.
The motion voltage could be calculated on the basis of the derivative of the current and voltage pulses supplied into the machine according to equation 2. The inaccuracy of measurement of the derivative of the current might be a problem because the voltage pulses are narrow at a low speed. Another possible problem is the leakage inductance Lσ, which should be known with a sufficient accuracy such that the portion of the motion voltage from the derivative of the current could be calculated. These problems can be avoided by performing the measurement of the current when the voltage vector is zero. The frequency of use of zero vectors naturally depends on the modulation index of the inverter 40. However, at a low speed, when the probability of a malfunction of a sensorless control is highest, the zero vectors are well available because their duration is longer at low speeds. Thus, the stator current can be determined frequently enough during a zero vector state when the motion voltage is directly proportional to the determined derivative (dīs/dt)z:
In addition, the coefficient −Lσ may be ignored because the angular speed of the motion voltage follows the angular speed of the derivative of the current. The above is applicable not only in the case of an asynchronous machine but also in the case of a synchronous machine since the approximate formula of equation 3 is also valid for synchronous machines (RR=0, τr=∞) In addition, in the case of synchronous machines, it is possible to estimate the speed on the basis of the derivative of the current even if the component of the fundamental wave is zero (cos φ=1 & torque=0).
The operation of the speed estimation arrangement according to various exemplary embodiments is given in the following with reference to
Due to the approximation of equation 2, in some cases the resistive losses (=(Rs+RR)īs) of the machine may significantly deteriorate the accuracy of the speed estimate determined on the basis of equation 3. The accuracy of the speed estimate can be improved, however, if the parameters that are needed when taking into account the resistive losses are known. The most significant loss term in equation 1 is caused by the stator resistance since the terms RRīs and
By assuming that during a zero vector state the stator current vector and the time derivative of the stator current vector are īs,Z and (dīs/dt)z, respectively, it follows from equation 4 that during the zero vector state the motion voltage is:
Thus, the motion voltage can be determined by determining the stator current vector during the zero vector state in addition to the time derivative of the stator current vector, and then determining the motion voltage on the basis of the stator current vector, the time derivative of the stator current vector, the stator resistance Rs, and the leakage inductance Lσ according to equation 5. Naturally this embodiment requires that the stator resistance and the leakage inductance are known. The signs of the factors Rs and Lσ in equation 5 may be ignored because the angular speeds of the motion voltage vector jωm
The operation of the speed estimation arrangement according to various exemplary embodiments utilizing equation 5 is given in the following with reference to
It should be noted that in the embodiments of
The speed estimation arrangement or the functionality thereof could further be used for the control of the inverter 40 supplying the electric motor 50. In this case the speed estimate is conveyed from the speed estimation arrangement 10 to the inverter 40 as illustrated in
An apparatus according to any one of the above embodiments, or a combination thereof, may be implemented as one unit or as two or more separate units that are configured to implement the functionality of the various embodiments. Here the term ‘unit’ generally refers to a physical or logical entity, such as a physical device or a part thereof or a software routine. One or more of these units may reside in an inverter or a frequency converter, for example.
An apparatus according to any one of the embodiments may be implemented at least partly by means of one or more computers or corresponding digital signal processing (DSP) equipment provided with suitable software, for example. Such a computer or digital signal processing equipment preferably comprises at least a working memory (RAM) providing storage space for arithmetical operations, and a central processing unit (CPU), such as a general-purpose digital signal processor. The CPU may comprise a set of registers, an arithmetic logic unit, and a control unit. The control unit is controlled by a sequence of program instructions transferred to the CPU from the RAM. The control unit may contain a number of microinstructions for basic operations. The implementation of microinstructions may vary depending on the CPU design. The program instructions may be coded by a programming language, which may be a high-level programming language, such as C, Java, etc., or a low-level programming language, such as a machine language, or an assembler. The computer may also have an operating system which may provide system services to a computer program written with the program instructions. The computer or other apparatus implementing the invention, or a part thereof, further preferably comprises suitable input means for receiving, for example, measurement and/or control data, which input means thus enable e.g. the monitoring of current and voltage quantities, and output means for outputting control data, for instance. It is also possible to use a specific integrated circuit or circuits, or discrete electric components and devices for implementing the functionality according to any one of the embodiments.
The invention according to any one of the embodiments, or any combination thereof, can be implemented in existing system elements, such as inverters or frequency converters, or control units thereof, or similar devices, or by using separate dedicated elements or devices in a centralized or distributed manner. Present devices for electrical drives, such as inverters and frequency converters or control units or systems thereof, typically comprise processors and memory that can be utilized in the functions according to the embodiments of the invention. Thus, all modifications and configurations required for implementing an embodiment of the invention, for instance, in existing devices 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 a 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 a 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.
It is obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in a variety of ways. Consequently, the invention and its embodiments are not restricted to the above examples, but can vary within the scope of the claims.
Number | Date | Country | Kind |
---|---|---|---|
08170357 | Dec 2008 | EP | regional |
Number | Name | Date | Kind |
---|---|---|---|
5473240 | Moreira | Dec 1995 | A |
5654624 | Schroderus | Aug 1997 | A |
6014006 | Stuntz et al. | Jan 2000 | A |
6137258 | Jansen | Oct 2000 | A |
6745083 | Eckardt et al. | Jun 2004 | B2 |
7061134 | Hiti et al. | Jun 2006 | B2 |
8008878 | Gotz et al. | Aug 2011 | B2 |
20020161492 | Eckardt et al. | Oct 2002 | A1 |
20070145939 | Tajima et al. | Jun 2007 | A1 |
20080265817 | Palma | Oct 2008 | A1 |
Number | Date | Country |
---|---|---|
1 793 486 | Jun 2007 | EP |
Entry |
---|
Mohapatra et al. “A sensorless speed control using stator ripple currents for an induction motor drive with space phasor PWM” on pp. 81,ISSN : 0537-9989,Print ISBN: 0-86341-383-8,INSPEC Accession No. 8141135,Digital Object Identifier : 10.1049/cp:20040263 Date of Current Version : Nov. 1, 2004,Issue Date : Mar. 31-Apr. 2, 2004. |
Hiti et al. “Zero vector modulation method for voltage source inverter operating near zero output frequency”, ISSN: 0197-2618, INSPEC Accession No. 8181096, 10.1109/IAS.2004.1348404, Nov. 1, 2004, Issue Date: Oct. 3-7, 2004. pp. 171-176. |
S. Ogasawara “A novel PWM scheme of voltage source inverters based on space vector theory” vol. 74, No. 1 (1990), 33-41, DOI: 10.1007/BF01573229. |
European Search Report for EP 08170357.1 completed May 11, 2009. |
Number | Date | Country | |
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20100134064 A1 | Jun 2010 | US |