Starting an induction machine

Abstract

A method for starting an induction machine without residual flux includes: scanning a state of the induction machine with different stator frequencies by controlling a supply voltage applied to the induction machine; determining whether a slip value, being a difference between a rotor frequency and a stator frequency, is within a slip interval; and, when it has been determined that the slip value is in the slip interval, regulating the slip value towards zero and magnetising the machine to the required level.

Claims

1. A method for starting an induction machine (12), the method comprising: scanning a state of the induction machine with different stator frequencies (.sub.s) by controlling a supply voltage (V.sub.s) applied to the induction machine; during the scanning, determining a torque related quantity (m, i.sub.sq) from measurements in the induction machine, wherein the torque related quantity (m, i.sub.sq) is positively correlated with an electromagnetic torque (m) of the induction machine, so that the torque (m) is raising or falling, when the torque related quantity is increased or decreased, respectively; determining whether a slip value (b), being a difference between a rotor frequency (.sub.r) and a stator frequency (.sub.s), is within a slip interval based on the torque related quantity (m, i.sub.sq); and when it has been determined that the slip value (b) is in the slip interval, regulating the slip value (b) towards zero, wherein the slip value (b) is regulated to zero by regulating a torque related quantity (m, i.sub.sq) to zero; wherein it is determined, whether the slip value (b) is within the slip interval, when at least one of: a derivative of the torque related quantity (m, i.sub.sq) with respect to the stator frequency (.sub.s) is higher than a threshold value; the torque related quantity (m, i.sub.sq) becomes smaller than a maximal absolute value of the torque related quantity (m, i.sub.sq); a magnitude of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine; a magnetising component (i.sub.sd) of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine.

2. The method of claim 1, wherein the slip interval is defined by two critical slip values (b.sub.cg, b.sub.cm) at which an electromagnetic torque (m) of the induction machine is minimal or maximal.

3. The method of claim 2, wherein the torque related quantity is the electromagnetic torque (m), a component of a stator current (i.sub.sq), a component of a rotor current, a component of a stator flux and/or a component of a rotor flux.

4. The method of claim 1, wherein the torque related quantity is the electromagnetic torque (m), a component of a stator current (i.sub.sq), a component of a rotor current, a component of a stator flux and/or a component of a rotor flux.

5. The method of claim 1, wherein the scanning starts with a maximal or minimal frequency, which is decreased or increased during scanning.

6. The method of claim 1, wherein the scanning is performed with a scanning stator flux magnitude (.sub.sm) lower than a nominal stator flux magnitude (.sub.sm) used during normal operation of the induction machine.

7. The method of claim 6, wherein during regulating the slip value (b) to zero, the stator flux magnitude (.sub.sm) is controlled to a desired value corresponding to an operating point of the induction machine.

8. A controller for operating an induction machine comprising: a set of instructions executable by a processor effective to: scan a state of the induction machine with different stator frequencies (.sub.s) by controlling a supply voltage (V.sub.s) applied to the induction machine; during the scanning, determine a torque related quantity (m, i.sub.sq) from measurements in the induction machine, wherein the torque related quantity (m, i.sub.sq) is positively correlated with an electromagnetic torque (m) of the induction machine, so that the torque (m) is raising or falling, when the torque related quantity is increased or decreased, respectively; determine whether a slip value (b), being a difference between a rotor frequency (.sub.r) and a stator frequency (.sub.s), is within a slip interval based on the torque related quantity (m, i.sub.sq); and when it has been determined that the slip value (b) is in the slip interval, regulate the slip value (b) towards zero, wherein the slip value (b) is regulated to zero by regulating a torque related quantity (m, i.sub.sq) to zero; wherein it is determined, whether the slip value (b) is within the slip interval, when at least one of: a derivative of the torque related quantity (m, i.sub.sq) with respect to the stator frequency (.sub.s) is higher than a threshold value; the torque related quantity (m, i.sub.sq) becomes smaller than a maximal absolute value of the torque related quantity (m, i.sub.sq); a magnitude of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine; a magnetising component (i.sub.sd) of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine.

9. A drive system, comprising: an induction machine, an electrical converter, and a controller structured to: scan a state of the induction machine with different stator frequencies (.sub.s) by controlling a supply voltage (V.sub.s) applied to the induction machine; during the scanning, determine a torque related quantity (m, i.sub.sq) from measurements in the induction machine, wherein the torque related quantity (m, i.sub.sq) is positively correlated with an electromagnetic torque (m) of the induction machine, so that the torque (m) is raising or falling, when the torque related quantity is increased or decreased, respectively; determine whether a slip value (b), being a difference between a rotor frequency (.sub.r) and a stator frequency (.sub.s), is within a slip interval based on the torque related quantity (m, i.sub.sq); and when it has been determined that the slip value (b) is in the slip interval, regulate the slip value (b) towards zero, wherein the slip value (b) is regulated to zero by regulating a torque related quantity (m, i.sub.sq) to zero; wherein it is determined, whether the slip value (b) is within the slip interval, when at least one of: a derivative of the torque related quantity (m, i.sub.sq) with respect to the stator frequency (.sub.s) is higher than a threshold value; the torque related quantity (m, i.sub.sq) becomes smaller than a maximal absolute value of the torque related quantity (m, i.sub.sq); a magnitude of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine; a magnetising component (i.sub.sd) of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to a stator flux magnitude (.sub.sm) used for scanning of the induction machine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

(2) FIG. 1 schematically shows a drive system according to an embodiment of the invention.

(3) FIG. 2 shows a diagram indicating the dependence of torque with respect to slip.

(4) FIG. 3A, 3B, 3C show diagrams with electromagnetic quantities of an induction machine, which are generated during performing the method according to an embodiment of the invention.

(5) The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(6) FIG. 1 schematically shows a drive system 10 comprising an induction machine 12, a converter 14 and a controller 16.

(7) The converter 14 may be connected with an AC or DC source 18, such as an electrical grid or DC link, and may generate a supply current i with supply voltage v, which are applied to the stator 20 of the induction machine 12, which may generate electromagnetic fields that drive a rotor 22 of the induction machine 12. In such a way, the induction machine 12 may be operated in a motor mode, in which electrical energy is transferred from stator 20 to mechanical energy of the rotor 22 of the induction machine 12. On the other hand, the induction machine 12 may be operated in a generator mode, in which the rotor 22 is driven by a turbine or other means and mechanical energy from the rotor 22 is transferred to the stator 20 of the induction machine 12 and to the source 18.

(8) For example, the converter 14 may be a three-level or multi-level converter based on half-bridges, which may comprise semiconductor switches controlled by the controller 16. The converter 14 also may be a direct converter.

(9) The operation of the induction machine 12 is controlled by the controller 16, which receives measurement values of the supply voltage current V.sub.s and supply current I.sub.s, which usually are equal to the stator voltage and the stator current. The controller 16 determines from the supply voltage V.sub.s and supply current I.sub.s further quantities of the induction machine 12, and based on these quantities controls the induction machine 12. For example, the controller 16 may comprise a direct torque controller or a current controller.

(10) It has to be noted that the supply voltage V.sub.s and the supply current I.sub.s may be multi-component quantities, i.e. vectors. When the drive system 10 is a three phase system, the measured supply voltage V.sub.s and the measured supply current I.sub.s comprise three components.

(11) During the startup phase of the drive system 10, i.e. when the converter 14 starts to work, there may be the problem, that the controller 16 does not know the rotation speed of the rotor 22. This may be the case, when, for example, the induction machine 12 is a motor, which has lost power supply only a short time ago and the rotor 22 is still rotating. In this case, which may be called flying start, the controller 16 may not be able to correctly calculate specific quantities used for controlling the converter 14 and the induction machine 12.

(12) In the following, a method will be described, how the controller 16 can estimate the rotor speed without the use of a mechanical speed sensor. When performing the method, it is possible that the induction machine 12 is rotating with unknown speed or is standing still. Furthermore, the method is applicable for starting the induction machine 12 without a residual flux.

(13) The flying start method is based on the dependencies of the characteristics of the induction machine 12 from slip . The slip may be defined as the difference between the rotor frequency .sub.r and the stator frequency .sub.s.

(14) The rotor frequency .sub.r may be the mechanical rotational speed of the rotor 22 multiplied by a number of stator pole pairs. The rotor mechanical rotational speed may be the speed with which the rotor 22 is rotating with respect to the stator 20. At the beginning of the method, the controller 16 does not know the rotor frequency .sub.r and the slip .

(15) The stator frequency .sub.s may be the frequency of the stator flux .sub.s of the stator 22 and/or of the stator current I.sub.s flowing in the rotor 22. The stator current I.sub.s may be controlled by the controller 16 and measurement values for the stator current I.sub.s may be received continuously by the controller 16. Therefore, the stator current I.sub.s and the stator flux .sub.s which is derivable from the stator voltage V.sub.s and the stator current I.sub.s, are known to the controller 16.

(16) FIG. 2 shows the dependency of the stator current

(17) $I_s=\left[\begin{array}{c}{i}_{\mathrm{sd}}\\ i_{\mathrm{sq}}\end{array}\right]$
(with components in the DQ coordinate frame) and the electromagnetic torque m with respect to the slip , in case of a nominal stator flux magnitude .sub.sm and a nominal stator frequency .sub.s. It has to be noted that the curve shapes stay substantially the same for a wide range of stator frequency and stator flux magnitude combination. FIG. 2 shows the quantities in normalized units (p.u.). In the example of FIG. 2: the stator frequency is .sub.s=0.5; the stator flux magnitude is .sub.sm=1, and the slip is variable.

(18) In FIG. 2, the current components i.sub.sd and i.sub.si, of the stator current I.sub.s were calculated in the DQ coordinate frame rotating with the stator frequency .sub.s. In particular, the controller 16 may measure the stator current I.sub.s in three components and may determine the current components in the DQ coordinate frame. Furthermore, the controller 16 may then estimate the stator flux also in the DQ coordinate frame

(19) ${}_s=\left[\begin{array}{c}{}_{\mathrm{sd}}\\ {}_{\mathrm{sq}}\end{array}\right].$

(20) The DQ coordinate frame may be aligned with the stator flux .sub.s, vector so that
.sub.sd=.sub.sm,.sub.sq=0,(1)
where .sub.sm is the stator flux magnitude, i.e. the length of the stator flux vector. The torque m and the stator current I.sub.s are coupled by:
m=.sub.sdi.sub.sq.sub.sqi.sub.sd(2)
The DQ coordinate frame orientation condition (1) results in a proportional dependency
m=.sub.sdi.sub.sq.(3)

(21) Thus, the components of the stator current I.sub.s become a torque producing/torque related current component i.sub.sq and a magnetizing current component i.sub.sd.

(22) The curves of the electromagnetic torque m and torque related current component i.sub.sq can be split into three parts. The first one lies between two critical slip values .sub.c, and .sub.cm. .sub.cm corresponds to the motor mode of the induction machine 12, .sub.cg corresponds to the generator mode of the induction machine 12. Here, i.sub.sq and m are raising from maximum generating value to maximum motoring. The corresponding two torque values are also called pull-out torques. The second part is between and .sub.cg, the third one between .sub.cm and +.

(23) Between the two critical slip values .sub.cg and .sub.cm, the torque m and torque related current component i.sub.sq is increasing, while outside of this interval between two critical slip values .sub.cg and .sub.cm, the torque m and torque related current component i.sub.sq is decreasing. Thus, inside the slip interval 24, defined between the two critical slip values .sub.cg and .sub.cm, the slip and the torque m are coupled by a positive possibly non-constant coefficient, so that the torque is raising (or falling), when the torque related quantity is increased (or decreased). Thus, when the torque m or a torque related quantity, such as the torque related current component i.sub.sq, are regulated towards zero, the slip also is regulated to zero.

(24) However, this regulation should be performed, when the actual value of the slip is already inside the slip interval 24. This may be determined by the controller 16 based on the form of curves of the torque m and/or a torque related quantity, such as the torque related current component i.sub.sq.

(25) In particular, the controller 16 may scan a range of rotor frequencies .sub.r by changing the stator frequency .sub.s. During scanning, it is determined, when the stator frequency .sub.s appears within the slip interval
.sub.r+.sub.cg<.sub.s<.sub.r+.sub.cm(4)

(26) Then, the controller 16 may regulate the slip to zero directly or indirectly. For example, this may be performed with a subcontroller that is a torque or an i.sub.sq-current controller, which may be supplied with a zero reference. No additional control loop for the controller 16 is required. A torque or i.sub.sq-current subcontroller already used by the controller 16 during normal operation (i.e. after startup and before shutdown) of the drive system 10 may be used.

(27) For example, within the slip interval 24 and only there, the following conditions are true:

(28) $\begin{array}{cc}\frac{{\mathrm{di}}_{\mathrm{sq}}}{d}>0\mathrm{and}& \left(5\right)\\ \frac{dm}{d}>0.& \left(6\right)\end{array}$

(29) For example, if torque control is based on a current control with flux orientation, then condition (5) may be used. In the case of a direct torque control, the condition (6) may be used.

(30) During scanning, the change of .sub.r may be assumed to be comparatively slow, thus the assumption .sub.r=const is acceptable. Then the following relation can be used:
d=d.sub.s.(7)

(31) When the scanning starts outside of the slip interval 24, which is the case, when the controller 16 starts at a maximal or minimal possible stator frequency, the fractions (5) and/or (6), which may be determined by the controller as discrete derivatives, turn from negative to positive, thus entering the slip interval 24 may be detected, by comparing the corresponding fraction (5) and/or (6) with a positive threshold >0, which may provide noise immunity.

(32) For example, in case of a flux oriented control using current control loops, during scanning, the i.sub.sq-current subcontroller may be disabled. The subcontroller may be enabled again with zero reference when the condition

(33) $\begin{array}{cc}\frac{{\mathrm{di}}_{\mathrm{sq}}}{d{}_s}>\mathrm{.Math.}& \left(8\right)\end{array}$

(34) becomes true, i.e. when the entering of the slip interval 24 is detected.

(35) In case of direct torque control, during scanning, the subcontroller may work in scalar mode. The stator frequency .sub.s (for example via the stator flux angle reference) may be forced by a generator. The torque control loop of the subcontroller may be fed by a virtual estimated torque and a zero torque reference.

(36) When the condition

(37) $\begin{array}{cc}\frac{dm}{d{}_s}>\mathrm{.Math.}& \left(9\right)\end{array}$
becomes true, the torque control loop feedback may be switched to an estimate of the actual machine torque.

(38) The conditions (8) and (9) may not give ideal rotor frequency detection with =0 but may be used to enable a torque or i.sub.sq-current controller, when it can operate normally and regulate the slip value to zero.

(39) FIG. 3A to 3B show diagrams, in which electromagnetic quantities of the induction machine 12 are depicted over time t, during which the startup method is performed. The method will be explained with respect to these diagrams.

(40) In a first step, between time t=D to time t=t.sub.1, the controller 16 scans a state of the induction machine 12 with different stator frequencies .sub.s by controlling the supply voltage V.sub.s applied to the induction machine 12, which is generated by the converter 14.

(41) For example, the scanning may start with a maximal or minimal frequency 26 (in FIG. 3A normalized to 1), which is decreased or increased during scanning. Furthermore, the scanning frequency 28 may be linearly increased or decreased during the scanning. This may be done by the controller 16 by regularly increasing or decreasing the scanning frequency 28.

(42) During the first step, i.e. during scanning, the controller furthermore determines whether the slip value , being the difference between the rotor speed multiplied by a number of stator pole pairs .sub.r and the actual stator frequency .sub.s set for scanning, is within the slip interval 24. As can be seen from FIG. 3A, the slip value , when entering the slip interval 24, is rather small compared to the slip value at the maximal or minimal frequency 26.

(43) Furthermore, the scanning may be performed with a scanning stator flux magnitude .sub.sm, 30 lower than a desired value 32 for the stator flux magnitude .sub.sm used during normal operation of the induction machine 12. For example, the scanning stator flux magnitude 30 may be about 10% or less of the desired value 32 for the stator flux magnitude corresponding to the operating point of the induction machine.

(44) There are several possibilities, how it may be determined, whether the slip value has entered the slip interval 24.

(45) From FIG. 3B, one can see that the torque producing current i.sub.sq has passed a maximum, which is an indicator for the slip value entering the slip interval 24. For example, this may be determined by a derivative of the current i.sub.sq and comparing it with a positive threshold value. As shown in FIG. 3C, the same is true for the electromagnetic torque m, and in the example, the controller 16 determines the entering of the slip interval 24, by comparing the derivative of the electromagnetic torque m with respect to the actual scanning stator frequency .sub.s, 28 with a threshold value 34 of, for example, 0.1.

(46) In general, only a quantity that is positively correlated to the electromagnetic torque m, such as the torque producing current i.sub.sq, has to be investigated for reaching an extremum.

(47) Thus, in general, during the scanning, the controller 16 determines a torque related quantity m, i.sub.sq from measurements in the induction machine 12, wherein the torque related quantity m, i.sub.sq is positively correlated to an electromagnetic torque m of the induction machine 12, and determines, whether the slip value is within the slip interval 24 based on the torque related quantity m,i.sub.sq.

(48) As shown in FIG. 3C, the controller 16 may determine a derivative of the torque related quantity m, i.sub.sq with respect to the stator frequency .sub.s and may determine whether the slip value is within the slip interval 24, when the derivative is higher than a threshold value 34.

(49) In general, the criteria (8) and (9) above are not the only options for detection of the slip value entering the slip interval 24, i.e. condition (5). For example, using a peak detector for torque m or current i.sub.sq, or in general a torque related quantity, one can substitute (8) and (9) correspondingly by:
|i.sub.sq|<ki.sub.sq peak.(10)
and
|m|<km.sub.peak(11)

(50) Here, m.sub.peak, i.sub.sq peak are the respective absolute maximum values 36 of torque m and current i.sub.sq, which are detected during scanning. k is a margin parameter or factor, 1>k>0, which is multiplied with the absolute maximum value 36 and/or under which the torque related quantity has to fall relative to the absolute maximum value 36 for detecting the slip value entering the slip interval 24.

(51) In general, the controller 16 may determine a maximal absolute value 36 of the torque related quantity m, i.sub.sq and may determine whether the slip value is within the slip interval 24, when the absolute value of the torque related quantity m, i.sub.sq becomes again smaller than the maximal absolute value 36.

(52) A further criterion for detecting that the slip value enters the slip interval 24 may be based on a stator current magnitude:
I.sub.sm<kI.sub.sc m,(12)

(53) where I.sub.sm is the stator current magnitude and I.sub.sc m is a magnitude of a stator short circuit current corresponding to the scanning stator flux magnitude .sub.sm, 30. The stator short circuit current magnitude I.sub.sc m may be calculated or measured by the controller 16.

(54) Summarized, the controller 16 may determine whether the slip value is within the slip interval 24, when a magnitude of an actual stator current becomes smaller than a magnitude of a short circuit stator current.

(55) It is also possible to use the magnetizing current component i.sub.sd instead of the stator current magnitude I.sub.sm in (12) by
i.sub.sd<kI.sub.sc m.(13)
The controller 16 may determine whether the slip value is within the slip interval 24, when a component i.sub.sd of an actual stator current becomes smaller than a magnitude of a short circuit stator current corresponding to the scanning stator flux magnitude .sub.sm, 30 by the factor 1>k>0.

(56) In a second step, after the time t.sub.1, when it has been determined that the slip value is in the slip interval 24, the controller 16 regulates the slip value towards zero. This may be performed by regulating the torque m or the torque related quantity to zero, for example the same torque related quantity i.sub.sq, which has been used for detecting the slip value entering the slip interval 24.

(57) Furthermore, during the second step, the stator flux magnitude .sub.sm, may be controlled to a desired value corresponding to the operating point of the induction machine 12.

(58) For example, with reference to FIGS. 3A and 3C, at the time t.sub.1, when the torque derivative reaches the threshold value 34 of 0.1, an i.sub.sq-current controller with zero reference may be enabled and the stator flux magnitude .sub.sm is ramped up to the nominal value of 1.0. At t.sub.2, the induction machine 12 is magnetized and the drive system 10 is ready for normal operation.

(59) After the induction machine 12 is magnetized, the flying start method may be finished.

(60) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

(61) 10 drive system 12 induction machine 14 converter 16 controller 18 source 20 stator 22 rotor V.sub.s supply/stator voltage I.sub.s supply/stator current i.sub.sd magnetizing current component i.sub.sq torque producing/related current component m electromagnetic torque slip/slip value .sub.cg critical slip value in generator mode .sub.cm critical slip value in motor mode 24 slip interval .sub.s stator frequency .sub.r rotor frequency .sub.sm stator flux magnitude 26 minimal/maximal frequency 28 scanning frequency 30 scanning stator flux magnitude 32 desired value for stator flux magnitude 34 threshold value 36 maximal absolute value