Estimating motor slip

Abstract

Embodiments of the present disclosure relate to estimating a slip of a motor. A method, a slip estimation device, a computer program and a computer program for estimating a slip of a motor, and a machine comprising the motor. In the method, a slip estimation function of the slip estimation device obtains (S100) an input current to the machine, obtains (S110) a value of a power input to the machine, and estimates (S130) the slip of the motor based on a first expression defining a relationship between an absolute value of a squared input current space vector of the input current and the input power to the machine.

Description

Technical Field

[0001] The present invention relates to a method, a slip estimation device, a computer program and a computer program for estimating motor slip, and a machine including such a motor. Background Technology

[0002] In many situations, such as when using a soft starter, estimating motor speed is of interest. Of course, sensors can be used to measure motor speed. However, it is often of interest to omit the sensor and estimate motor speed in other ways.

[0003] US 6414463 describes a method for estimating motor speed. In this method, the motor stator is placed in a state where no current flows during motor acceleration. A slowly decaying direct current then flows through the rotor. Thus, the rotor can be considered as a rotating magnet with a nearly constant magnetic flux. The rotation induces a voltage at the motor stator terminals, the frequency of which corresponds to the product of the known number of pole pairs and the rotational speed of the machine under test. This means that, during a time period intentionally generated with the aid of a controllable AC controller, in which no current flows in the stator, the rotor speed is detected using the frequency of the stator voltage space vector.

[0004] However, estimating the rotor speed is also of interest when stator current is present. It may also be of interest to use a method that estimates other characteristics of the motor and derives the rotor speed from them. Summary of the Invention

[0005] Therefore, all aspects of the present invention relate to the problem of how to estimate motor characteristics in the presence of stator current and thereby derive rotor speed.

[0006] According to the first aspect, this objective is achieved by a motor slip estimation method, which is executed in a slip estimation device and includes:

[0007] The slip estimation function of the slip estimation device is used to obtain a measured value of the input current of the machine, which includes the motor.

[0008] The power value input to the machine is obtained through the slip estimation function; and

[0009] The slip estimation function estimates the motor slip based on a first expression, which defines the relationship between the square of the input current, the absolute value of the input current space vector, and the input power to the machine.

[0010] According to the second aspect, this objective is achieved by a slip estimation device for estimating motor slip. The slip estimation device includes at least one processor that performs a slip estimation function, wherein the at least one processor performs the following operations when performing the slip estimation function:

[0011] Obtain a measurement of the input current of the machine, which includes the motor;

[0012] Obtain the value of the power input to the machine; and

[0013] The slip of the motor is estimated based on the first expression, which defines the relationship between the square of the input current, the absolute value of the input current space vector, and the input power to the machine.

[0014] According to the third aspect, this objective is achieved by a machine comprising a motor and a slip estimation device as described in the second aspect.

[0015] According to the fourth aspect, this objective is achieved by a computer program for estimating motor slip, the computer program being configured to perform the operation according to the first aspect when operated by at least one processor of the slip estimation device described in the second aspect.

[0016] According to the fifth aspect, this objective is achieved by a computer program product for estimating motor slip, the computer program product comprising a data carrier having the computer program described in the fourth aspect.

[0017] Input power can be obtained based on measurements of the input voltage and input current to the machine.

[0018] In a first variation of the first aspect, the method further includes: signal conditioning of at least a portion of the first expression via a slip estimation function.

[0019] In the second aspect of the strain type, at least one processor, while implementing the slip estimation function, is also operable to perform signal conditioning on at least a portion of the first expression.

[0020] Signal conditioning can include low-pass filtering.

[0021] The first expression may include a current-based numerator and a power-based denominator. Low-pass filtering may include filtering the numerator and denominator separately, or the quotient formed by them.

[0022] According to a second variation of the first aspect, the method further includes: estimating the rotor speed of the motor based on the estimated slip rate using a slip rate estimation function.

[0023] According to the strain type of the second aspect, at least one processor, while implementing the slip estimation function, can also operate to estimate the rotor speed of the motor based on the estimated slip.

[0024] The rotor speed can be estimated based on a second expression, which defines a function of 1 minus the estimated slip. This second expression used to estimate the rotor speed can be applied between a lower limit of 0 and an upper limit of 1.

[0025] The slip can be estimated using a first expression adjusted for gain.

[0026] According to the third variation of the first aspect, the method further includes: determining the gain through a slip estimation function.

[0027] According to the strain type of the second aspect, at least one processor can also be operated to determine the gain when implementing the slip estimation function.

[0028] Determining the gain can include: initializing the gain to a value higher than a threshold; reducing the gain using the integral of a third expression, which defines a function of 1 minus the estimated slip; and stopping the reduction of the gain and freezing the integral when the gain reaches the threshold.

[0029] This threshold can be based on the sum of the stator resistance and the rotor resistance, modified by a first constant, which can depend on the space vector scaling constant. More specifically, the first constant can be inversely proportional to the square of the space vector scaling constant.

[0030] The third expression used to determine the gain can be applied to a range from negative infinity at the lower limit to zero at the upper limit.

[0031] The motor can be an electric motor. The electric motor can also be an induction motor.

[0032] Alternatively, the machine may consist of only the motor. Therefore, there may be only one entity in the machine—the motor—in which case the input current to the machine is the input current to the motor, and the power input to the machine is the power input to the motor.

[0033] The machine may also include a soft starter connected to the motor. In this case, the input current to the machine may be the same as the input current to the soft starter, and the power input to the machine may be the same as the power input to the soft starter.

[0034] When the machine includes a soft starter, the soft starter may include triggerable rectifier elements such as thyristors or triacs, which are triggered by an applied trigger angle.

[0035] In this case, during the initialization phase, the initialization and reduction of gain can be accomplished using the set trigger angle of the soft starter.

[0036] In this case, the method may further include: during the initialization phase, controlling the trigger angle of the soft starter to a set trigger angle through the controller function of the slip estimation device.

[0037] In this case, at least one processor of the slip estimation device is also operable to implement the control function of the machine, wherein the at least one processor is operable to control the trigger angle of the soft starter to a set trigger angle when performing the control function.

[0038] The set trigger angle can be at least half of the maximum trigger angle. For example, the set trigger angle can be 50% to 100% of the maximum trigger angle. Therefore, the set trigger angle can be in the range of 0.5 to 1.0 times the maximum trigger angle. For example, the set trigger angle can also be the same as the maximum trigger angle.

[0039] The slip ratio can be determined continuously.

[0040] After the initialization phase, the trigger angle can be freely controlled.

[0041] For example, the trigger angle of the soft starter can be controlled to achieve the desired control objective.

[0042] One possible control objective of interest is the desired torque. Other possible control objectives are controlling the motor to achieve a desired speed profile, desired acceleration, desired voltage profile, or desired current profile.

[0043] In this case, at least one processor, while performing the control function, can also operate to control the trigger angle of the soft starter after the initialization phase in order to achieve the desired control objective.

[0044] The machine may also include a motor driver to replace or supplement a soft starter.

[0045] Generally, unless otherwise expressly defined herein, all terms used in the claims shall be interpreted in accordance with their ordinary meaning in the technical field. Unless otherwise expressly stated, all references to “element, device, component, part, etc.” shall be interpreted as referring to at least one instance of “element, device, component, part, etc.” Attached Figure Description

[0046] Specific embodiments of the inventive concept will now be described by way of example with reference to the accompanying drawings, wherein:

[0047] Figure 1A single-line diagram of the static inverse equivalent circuit of a machine in the form of a soft starter-motor combination is schematically shown.

[0048] Figure 2 A slip estimation device is schematically illustrated.

[0049] Figure 3 The controller used to control the machine is shown schematically;

[0050] Figure 4 A first implementation of a slip estimation device is schematically shown, including functions for implementing the slip estimation device and functions for implementing the controller;

[0051] Figure 5 A data carrier containing computer program code in the form of a CD-ROM is shown for implementing the functions of a slip estimation device and controller.

[0052] Figure 6 Several method steps in a method for estimating the slip of a motor in the machine are illustrated schematically.

[0053] Figure 7 Several methodological steps for determining the gain used in slip estimation are illustrated schematically; and

[0054] Figure 8 The simulation results of the machine are shown. Detailed Implementation

[0055] Combining a motor (such as an induction motor) with a soft starter can offer advantages. Figure 1 A schematic single-line diagram of machine 10 (which is also a soft starter-motor combination 10) is shown, represented by a static inverse Γ equivalent circuit diagram. Therefore, machine 10 includes a motor M, which in this case is an induction motor. In this case, machine 10 also includes a soft starter SS connected to motor M.

[0056] The first terminal of the soft starter SS forms the first input terminal of the machine 10, and the second terminal of the soft starter SS is connected to the stator resistor. R s The first terminal, stator resistor R s The second end is then connected to the nominal leakage reactance. X σ The first terminal of the parallel circuit is then connected to the nominal leakage reactance. X σ The second terminal, wherein the second terminal of the parallel circuit constitutes the second input terminal of machine 10. This parallel circuit includes a rotor resistor. R RDivide by the motor slip s, and the resulting resistance R R / s With nominal magnetizing reactance X M in parallel.

[0057] like Figure 1 As can be seen, a soft starter SS typically includes thyristors or triacs connected in anti-parallel, these components being connected in series with the AC motor in phases. Therefore, a soft starter SS includes triggerable rectifier elements. As the name suggests, the purpose of a soft starter SS is to limit inrush current and / or electrical torque during startup. This is achieved by delaying the thyristor's turn-on time by a so-called "firing angle," which is defined relative to the zero-crossing point of the line voltage. This reduces the voltage applied to the motor M. v s As the rotational speed increases, the firing angle gradually decreases, eventually reaching its maximum when a full-line voltage is applied. v l The time reaches zero. Once this happens, the soft starter SS will be bypassed to reduce losses.

[0058] The firing angle can be adjusted without rotor speed measurement. However, in some applications, especially where a starting torque greater than the nominal value is sometimes required, knowing the speed is beneficial. An example is a stone crusher, where larger stones can cause blockages and jam the rotor. By knowing the speed accurately, it can be detected that applying the nominal starting torque is not causing any rotation. The torque limit can then be temporarily increased to alleviate the blockage.

[0059] Installing a speed encoder is not a practical option because it increases costs. In many cases, it is simply not feasible. Therefore, speed estimation must be resorted to. Since the 1990s, speed estimation for inverter-powered induction motors, known as "sensorless control," has been extensively studied.

[0060] However, unlike inverter-powered motors, soft-starter-powered motors have a stator voltage... v s Normally unavailable. Only line voltage can be measured. v l That is, the voltage on the SS line side of the soft starter. On the other hand, the stator current. i s With line current i l The same. In addition, because the losses of the soft starter are very low, the active power on both sides of the soft starter SS can be approximately equal with good accuracy.

[0061] Therefore, one problem is how to estimate the rotor speed based solely on line-side voltage and current measurements.

[0062] Various aspects of this disclosure are intended to estimate such speeds using estimates of motor slip. However, slip estimation may also be of interest in other cases where rotor speed is not a primary consideration.

[0063] As mentioned above, Figure 1 The diagram illustrates a single-line representation of a soft-start induction motor. This equivalent circuit is considered under steady-state conditions, where the stator frequency is constant and equal to the line frequency. Because the stator frequency is constant and equal to the line frequency, therefore... X σ and X M These are the nominal leakage reactance and the magnetizing reactance, respectively. R s and R R These are the stator resistance and the rotor resistance, respectively.

[0064] (1)

[0065] This refers to the previously mentioned slip, where ω1 is the stator angular frequency, and... ω r This refers to the electric rotor speed. Speed ​​estimation is particularly important in the low-speed range (as mentioned, even with applied torque, it's necessary to detect whether the motor has stopped running), therefore a slip rate close to 1 can be considered. s .because X M > 1 per unit (pu) and R R ≪ 1p.u., therefore R R / s≪X M This allows the excitation branch to be ignored; almost the entire stator current flows through the rotor branch. Therefore, the input impedance seen from the stator terminals can be approximated as...

[0066] (2)

[0067] Now consider the space vector v associated with phase voltage and phase current. s and i s For the stator current space vector i s = i α + ji β ,in

[0068] (3)

[0069] in K It is the space vector scaling constant, while { i a , i b , i c} represents the phase current. The stator voltage space vector is defined similarly. It can be shown that the active input power at the stator terminals is given by the following equation:

[0070] (4)

[0071] And among them κ It depends on the space vector scaling constant. K The first constant. As can be seen in equation (4), the first constant κ With the square space vector scaling constant K It is inversely proportional to, that is, it is inversely proportional to, the space vector scaling constant. K It is inversely proportional to the square of the product.

[0072] Because of v s = Z i s ,

[0073] (5)

[0074] If this relation is divided by P and multiply by s Then we can obtain

[0075] (6)

[0076] Because it is particularly close to 1 s I am interested, therefore sR s ≈ R s Normally, R s < R R This observation further reinforces the validity of this approximation. Therefore, from equation (6), the slip estimate can be obtained as:

[0077] (7)

[0078] Therefore, the rotational speed can be estimated as follows: Note that, based on the assumption that the power input to the soft starter can be considered equal to the power input to the stator (i.e., the power on both sides of the soft starter SS can be considered equal), i can be obtained from the phase voltage and phase current on the line side. s as well as P .

[0079] Equation (7) suggests that you need to know R s and R R The value, but this can be avoided in the following way. Since startup always begins from a quiescent state (at which point...) s = 1), so the following algorithm can be considered.

[0080] Apply a larger trigger angle α (A firing angle such as at least half the maximum permissible firing angle, or a firing angle in the range of 0.5 to 1.0 times the maximum permissible firing angle) will result in low voltage and low torque. Therefore, the speed will remain at or near zero. Slip estimation is achieved by forming a first expression that depends on the absolute value of the square space vector of the input current and the input power. This first expression defines the relationship between the absolute value of the square space vector of the input current and the input power of the machine. This first expression can be based on the quotient of the absolute value of the square space vector of the input current and the input power. More specifically, this first expression can include a numerator and a denominator, where the numerator is based on the absolute value of the square space vector of the input current and the denominator is based on the power. More specifically, it can be a quotient| i s | 2 / P The first expression can also be multiplied by a variable gain. ,get:

[0081] (8)

[0082] As indicated by "FILT", it can be applied to | i s | 2 and P And / or its quotient is low-pass filtered to suppress noise caused by stator current harmonics, which are caused by the non-sinusoidal stator voltage resulting from the soft starter SS. Low-pass filtering is a signal conditioning method that can be performed on the first expression. Furthermore, the rotational speed can be estimated as:

[0083] (9)

[0084] in It is the second expression.

[0085] This means that the upper and lower limits of the parameters in the second expression are two values, 0 and 1. If the value of the second expression is less than 0, then the estimated rotational speed... The stator angular frequency ω1 is set to 0, i.e., set to 0; if the value of the second expression is greater than 1, the estimated rotational speed is... It is set to the stator angular frequency ω1 multiplied by 1, that is, set to ω1. The estimated rotational speed lies between these two values. It is set to the stator angular frequency ω1 multiplied by the second expression. It can also be considered a saturation function.

[0086] Since the initial slip estimate is greater than 1 (see below), it is important to apply a lower limit of 0 to prevent the estimation of negative speed.

[0087] Gain It should be initialized to be guaranteed to be greater than κ(R s +R R ) The value. Then you can tune it as follows. :

[0088] (10)

[0089] in k It is a positive gain. As long as > κ ( R s + R R If this is done, an incorrect estimate of the slippage rate will be obtained. Then, the integrand in equation (10) is negative, which will lead to Decrease. Once achieve κ(R s +R R ) ,get And the integral in equation (10) is frozen. If k If the choice is appropriate, then The adjustment will occur very quickly, during which time the motor will be kept within an extremely low speed range. s Very close to 1. Once Once frozen, it can begin to shrink. α This results in higher torque and (typically) higher speed.

[0090] Therefore, the slip can be estimated, which is useful in several situations where it may be necessary to estimate the rotor speed.

[0091] The slip ratio can be estimated in a slip ratio estimation device that includes a slip ratio estimation device.

[0092] Figure 2 The slip estimation device SED 12 is schematically shown. It receives the input current. i s and input voltage v l And based on these quantities, the slip ratio ŝ is estimated.

[0093] Sometimes, this estimate may depend on the firing angle used to control the soft starter SS. Therefore, the slip estimation device may also include a controller for controlling the soft starter motor assembly 10.

[0094] Figure 3 The diagram schematically illustrates a controller CTRL 14 that receives filtered air-gap active power. P af And determine the firing angle that enables machine 10 to achieve the desired control objective (e.g., provide the desired torque). α .

[0095] It should be recognized here that this is just one example of a possible control objective. Other possible control objectives are to control the motor to have a desired speed curve, a desired acceleration, a desired motor voltage curve, or a desired motor current curve.

[0096] The slip estimation device 12 and the controller 14 can each be implemented as a processor, with software operating on the processor implementing the slip estimation function and the controller function, respectively. These functions can be provided using separate processors. Alternatively, they can be provided using the same processor. Therefore, the slip estimation device may include at least one processor.

[0097] Figure 4 An implementation of a slip estimation device SEA 16 is schematically shown. SEA 16 includes a processor PR 18 and a memory M 20. The memory M 20 includes a first computer program CP1 22 that implements the slip estimation function and a second computer program CP2 24 that implements the controller function. The first computer program 22 can thus have computer program code, software code, or computer instructions for implementing the slip estimation function, while the second computer program 24 can have computer program code, software code, or computer instructions for implementing the controller function. Therefore, the processor 18 implements the slip determination function when running the first computer program 22, and implements the controller function when running the second computer program 24.

[0098] It should be recognized that processor 18 can be a central processing unit (CPU), multiprocessor, digital signal processor (DSP), application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or other processors, as long as they can implement the controller function. Memory 20 can be random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), or flash memory.

[0099] Each computer program can also be implemented as a computer program product, which includes a data carrier carrying the computer program. Figure 5 The diagram schematically illustrates a data carrier 26, or a non-volatile or non-transitory storage medium, in the form of a CD-ROM optical disc including first and second computer programs 22, 24. It should be understood that this data carrier is not limited to a CD-ROM optical disc, but may also be implemented, for example, as a USB (Universal Serial Bus) memory or flash memory.

[0100] The slip estimation device 16 (which has a slip estimation device 12 and optionally a controller 14) may be part of the machine 10.

[0101] The operation of slip estimation device 12 can be as follows:

[0102] Operation can begin by obtaining a measurement of the current input to machine 10 from slip estimation device 12, S100. Slip estimation device 12 can also obtain the power input to machine 10. P The value of S110. The input current can be the same as the stator current i. s The same line-side current, but the input power can be based on the line-side voltage. v l and line-side current i s Obtained in a known manner.

[0103] The slip estimation device 12 can also determine the gain. S120. Gain The method for determining this gain will be described in detail later. This gain is typically determined during the initialization phase, in which the soft-starter's trigger angle is set to the trigger angle. α Therefore, the controller 14 can also set a trigger angle during this initialization phase. α To control the soft starter SS.

[0104] The set trigger angle can be at least half of the maximum trigger angle. Therefore, it can be in the range of 50% to 100% of the maximum trigger angle. Alternatively, the set trigger angle can also be in the range of 0.5 to 1.0 times the maximum trigger angle. For example, the set trigger angle can be the maximum trigger angle.

[0105] In gain Once determined, the slip estimation device 12 estimates the slip of the motor M based on the previously mentioned first expression. S130, where the first expression can define the absolute value of the square space vector of the input current and the input power. P The quotient or relationship between them can be the square space vector current i s The absolute value divided by power P .

[0106] To perform slip estimation, the slip estimation device can first determine the absolute value of the square space vector of the input current based on the current measurement.

[0107] More specifically, slip can be estimated using gain. The first expression to be adjusted can be the first expression multiplied by the gain. Alternatively, signal conditioning can be applied to the first expression multiplied by the gain. For example, this signal conditioning could involve low-pass filtering.

[0108] Therefore, the slip estimation function can low-pass filter at least a portion of the first expression. This low-pass filtering may include filtering the numerator and denominator of the first expression, or the quotient formed therefrom.

[0109] Optionally, the estimated slip can also be used to estimate the rotor speed based on the estimated slip, S140. This can be done by multiplying the stator angular frequency by a second expression, which can be defined as a function of 1 minus the estimated slip. The second expression can also be applied between a lower limit of 0 and an upper limit of 1. That is, when the second expression is greater than 1, it is assigned the value 1; and when it is less than 0, it is assigned the value 0.

[0110] As can be seen above, the estimation mentioned above is made under the condition that the trigger angle is the set trigger angle, such as setting the trigger angle to be in the range of 0.5 to 1.0 times the maximum trigger angle, or setting the trigger angle to be the maximum trigger angle.

[0111] After the gain has been determined, the slip (and optionally the rotor speed) can be further determined by the slip estimation device 12, while the output torque of the motor M is determined by the controller 14 using the firing angle. αControl is performed by gradually decreasing the firing angle from a set angle until it reaches zero. In this example, this control, performed after the initialization phase, controls the firing angle of the soft starter to achieve the desired control objective. This control can also involve closed-loop control. Therefore, controller 14 can use the firing angle... α To control machine 10 and continuously estimate slip (and optionally rotor speed).

[0112] As in the example given here, the control objective is to obtain the desired torque.

[0113] Next, we will refer to Figure 7 To describe how to determine the gain Determining the gain may include initializing the gain to a value higher than a threshold, which may be... κ(R s + R R ) That is, the sum of the stator resistance and the rotor resistance multiplied by a first constant, S200. Therefore, this threshold can be based on the sum of the stator resistance and the rotor resistance multiplied by a first constant.

[0114] Following this, the gain is reduced using the integration of the third expression, S210, where the third expression defines a function of 1 minus the estimated slip. Unlike the case for estimating rotor speed, the third expression is applied between a lower limit of negative infinity and an upper limit of zero. The integration serves to reduce the gain. .

[0115] When the gain reaches a threshold, the slip estimation device 12 stops reducing the gain and freezes the integration, S220, where the threshold is in this case. κ(R s + R R ) Then, the frozen gain is used to estimate the slip. Therefore, the initialization and reduction of the gain are completed during the initialization phase, at which time the set trigger angle of the soft starter is used.

[0116] As can be seen, it's unnecessary to know the stator and rotor resistances. Only a general range for their sum is needed. The initial gain can be chosen to be much higher than this general range. Then, the gain can be gradually reduced until the slip becomes equal to 1.

[0117] To verify these estimates, simulations were performed.

[0118] A soft starter plus induction motor system was simulated, and its PU parameters are as follows: R s = 0.02、 R R = 0.05、X σ = 0.2, and X M = 3. Inertia is set to 4000 pu. Maximum (and initial) trigger angle. α It was set to 2 radians. An application was then performed on it. α = 2 After an initial phase of 1 second in radians, closed-loop torque control begins (using estimated and filtered air gap active power). P af As feedback), where the torque reference value T e The value gradually increases from 0 to 1 pu within 3 seconds.

[0119] In the first simulation (see...) Figure 8 There is no load torque. This allows the speed to stabilize at 1 pu in less than 15 seconds. It can be seen that... It converges from the initial value 1 to [value] in less than 1 second. R s + R R (In simulation) κ =1). As expected, with s As the speed decreases and the magnetization branch current (neglected in the simplified model) increases, the error between the estimated and actual speeds gradually increases, while the rotor branch current decreases. (Approaching...) s = 0 ( ω r When = 1 pu), the speed estimate collapses.

[0120] Simulation results demonstrate that the proposed slip and speed estimation algorithms are effective. While the estimation error increases with increasing speed, the algorithm remains particularly useful for detecting whether a motor is rotating.

[0121] The inventive concept has been primarily described above with reference to several examples. However, those skilled in the art will readily recognize that, within the scope of the inventive concept as defined in the appended claims, other embodiments besides those disclosed above are equally feasible.

[0122] For example, the slip of a motor not powered by a soft starter can be estimated. Therefore, the machine may consist only of the motor. The machine may also include a motor driver to replace or supplement the soft starter. Similarly, only the slip can be estimated without estimating the rotor speed.

Claims

1. A method for estimating the slip of a motor (M), said method being performed in a slip estimation device (16) and comprising: The slip ratio estimation function of the slip ratio estimation device (16) is used to obtain (S100) the input current to the machine (10). i s The machine (10) includes the motor (M). Through the slip estimation function, the power input to the machine (10) is obtained (S110). P The value of ); as well as The slip ratio of the motor (M) is estimated (S130) based on a first expression using the slip ratio estimation function. This first expression defines the absolute value of the squared space vector of the input current (| i s |) and the relationship between the input power to the machine (10).

2. The method according to claim 1, further comprising: The rotor speed of the motor (M) is estimated (S140) based on the estimated slip rate using the slip rate estimation function.

3. The method of claim 2, wherein the estimation of the rotor speed is based on a second expression, the second expression defining a function of 1 minus the estimated slip.

4. The method according to any one of the preceding claims, wherein the first expression comprises a numerator based on the current and a denominator based on the power.

5. The method according to any one of the preceding claims, wherein the slip is estimated using gain ( The first expression is adjusted.

6. The method according to claim 5, further comprising: The gain is determined (S120) by the slip estimation function.

7. The method of claim 6, wherein determining the gain comprises: The gain is initialized (S200) to a value higher than the threshold. The gain is reduced (S210) by the integral of a third expression, which defines a function of 1 minus the estimated slip; and the reduction of the gain is stopped (S220) and the integral is frozen when the gain reaches the threshold.

8. The method of claim 7, wherein the threshold is based on a first constant ( κ Corrected stator resistance ( R s ) and the resistance of the rotor ( R R The sum of ) and the first constant depends on the space vector scaling constant (K).

9. The method according to claim 7 or 8, wherein the machine (10) includes a soft starter (SS) connected to the motor (M), and the initialization and reduction of the gain are performed with a set trigger angle of the soft starter during the initialization phase.

10. The method of claim 9, further comprising: During the initialization phase, the trigger angle of the soft starter is controlled to the set trigger angle by the controller function of the slip estimation device (16).

11. The method of claim 10, wherein the slip ratio is continuously determined, and the method further comprises: After the initialization phase, the trigger angle of the soft starter is controlled by the controller function to achieve the desired control objective.

12. A slip estimation device (16) for estimating the slip rate of a motor (M), the slip estimation device (16) comprising at least one processor (18) implementing a slip estimation function, the at least one processor (18) being operable to: The input current of the machine (10) is obtained. i s The machine (10) includes the motor (M). The power input to the machine (10) is obtained. P The value of ) and The slip of the motor is estimated based on a first expression, which defines the absolute value of the squared space vector of the input current (| i s |) and the relationship between the input power to the machine (10).

13. The slip estimation device (16) according to claim 12, wherein the machine (10) includes a soft starter (SS) connected to the motor (M), the gain being initialized and reduced with a set trigger angle of the soft starter during an initialization phase, and the at least one processor of the slip estimation device (16) is operable to implement a control function of the machine, wherein the at least one processor is operable to control the trigger angle of the soft starter to the set trigger angle when performing the control function.

14. A machine (10) comprising a motor (10) and a slip estimation device (16) according to claim 12 or 13.

15. A computer program (22, 24) for estimating the slip of a motor (M), the computer program (22, 24) being configured to perform the operation according to any one of claims 1-10 when operated by the at least one processor (18) of the slip estimation device (16) according to claim 13 or 14.

16. A computer program product for estimating the slip of a motor (M), the computer program product comprising a data carrier (26) having a computer program (22, 24) according to claim 15.

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