TWI754965B - Estimating method for estimating rotor frequency of motor - Google Patents

Abstract

An estimation method for estimating a rotor frequency of a motor during freewheeling, includes steps: sequentially applying a fixed input voltage and one selected from multiple stator frequencies to the motor so as to scan frequency; detecting a stator current value of the motor corresponding to the stator frequency; calculating a stator current slope of the stator current value sequentially; defining a target period from a start point where the stator current slope varies from positive to negative to an end point where the stator current slope varies from negative to positive; and determining a difference between the stator frequencies and the rotor frequency is within a preset value, using any of the corresponding stator frequencies during the target period as an estimated value of the rotor frequency.

Description

Motor rotor frequency estimation method

The present disclosure relates to a method for estimating rotor frequency of an electric machine, and more particularly, to an estimation method for estimating the rotor frequency of an electric machine during freewheeling.

In various applications of motors, in order to save energy, the motors are often accelerated or decelerated according to the load conditions. During this process, when the motor is freewheeling, the rotor frequency of the motor must be estimated, so that the inverter can run at an appropriate speed to control the motor to the target speed. Otherwise, in the process of restarting, it may cause failure to start, or even damage the inverter due to overvoltage and overcurrent.

Therefore, how to accurately estimate the rotor frequency of the motor during free-running is one of the important issues in the art.

One aspect of the present disclosure pertains to an estimation method. The estimation method is applied to a motor control system for estimating the rotor frequency of the motor during free-running. The motor control system includes a controller and a current sensor. The estimation method includes: the controller successively applies a fixed input voltage and selects one of a plurality of stator frequencies to the motor to perform frequency sweep; the current sensor senses the stator current value of the motor corresponding to the stator frequency; The stator current value calculates the stator current slope successively; the controller defines the target period from the starting point of the stator current slope from positive to negative and the end point from negative to positive; and when sweeping the frequency, the controller judges the difference between the stator frequency and the rotor frequency Less than a set value, the stator frequency corresponding to any one of a plurality of time points during the target period is used as an estimated value of the rotor frequency.

To sum up, due to the characteristic that the stator current will decrease as the slip decreases, by applying a fixed input voltage successively and selecting one of multiple stator frequencies for scanning, the time point of the positive and negative conversion of the stator current slope can be determined. Define the target period. Since it is determined that the stator frequency is close to the target period of the rotor frequency, the estimated value of the rotor frequency can be obtained from the designation of the stator frequency at any point in the target period.

The following is a detailed description of the embodiments in conjunction with the accompanying drawings, but the specific embodiments described are only used to explain the present case, and are not used to limit the present case, and the description of the structure and operation is not used to limit the order of its execution. The recombined structures, resulting in devices with equal efficacy, are all within the scope of the present disclosure.

Generally speaking, in order to accurately estimate the rotor frequency of a free-running motor, the rotor frequency is usually calculated according to a parametric model of the motor during design or manufacture. However, in this method, parameters must be set according to motors of different sizes, types, and uses, in order to apply appropriate current and voltage commands for frequency sweeping, and the input current and voltage must be continuously monitored during the frequency sweeping process. The estimation time required is longer, and there are many variables that may cause the estimation to be inaccurate. In addition, more complex calculations may increase costs and delays in obtaining estimates and compensating for them in real time.

In order to solve the above problems, this case will use an induction motor with V/f open-loop control. Under the condition that no motor parameters are required, the operating point at which the stator frequency is close to the rotor frequency is determined by the change of the slope of the stator current, so as to obtain an estimate of the rotor frequency. measured value. The details will be described in the following paragraphs. Here, the operation principle and parameter waveforms of the motor will be explained first.

Please refer to Figure 1A and Figure 1B. FIG. 1A is a system schematic diagram illustrating an electrical connection between a motor control system 1 and a motor 2 according to some embodiments of the present disclosure. FIG. 1B is an equivalent circuit diagram of the motor 2 according to the partial embodiment of FIG. 1A. As shown in FIG. 1A , the motor control system 1 is connected to the motor 2 . The motor control system 1 is used to control the motor 2 . The motor control system 1 at least includes a controller 11 , current sensors 12 a and 12 b and a power stage circuit 13 . In some embodiments of the present disclosure, the motor control system 1 is, for example, a frequency converter, and the motor 2 is, for example, an asynchronous motor or an induction motor. The motor control system 1 receives an external input power AC, and passes the power and The circuit is converted into output power and supplied to the motor 2. During the operation, the current sensors 12a and 12b respectively sense the feedback currents Ia and Ib of the a-phase and the b-phase, and provide the converted two after analog-to-digital conversion. The phase feedback currents Ia_AD and Ib_AD are calculated by the controller 11 to obtain the stator current Is of the motor 2 , and other details will be described in detail later.

式(1) As shown in Figure 1B, _Vs is the stator voltage, Is is the stator current, jXs is the stator leakage inductance, Rs is the stator resistance, jXm is the excitation inductance, jXr is the rotor leakage inductance, Rr is the rotor resistance, and s is the slip (slip). Among them, the definition of slip s is shown in formula (1).

Formula 1)

An asynchronous motor uses electromagnetic induction in the stator coil to generate an induced current in the rotor, thereby outputting torque to make the motor rotate. In order to generate rotor current, the actual rotational speed of the rotor (eg, n _r in equation (1)) is slower than the rotational speed of the stator magnetic field (eg, _ns in equation (1)). The ratio of the speed difference between the two to the actual speed is called slip s. Therefore, as can be seen from Fig. 1B and equation (1), when the stator frequency Fs is close to the rotor frequency Fr, the smaller the slip s is, the larger the stator equivalent impedance Zs_eq (as shown in Fig. 2A) is.

Please refer to Figure 2A, Figure 2B and Figure 3. FIGS. 2A , 2B and 3 are waveform diagrams illustrating various motor parameters according to some embodiments of the present disclosure. Specifically, in the embodiment of the present case, the controller 11 applies a fixed voltage command to the motor 2 and selects one of a plurality of different stator frequencies Fs to apply to the stator sequentially through the inverter to perform frequency sweeping. For example, the controller 11 first applies a fixed current command to the motor 2 until the motor 2 reaches a steady state, and uses the voltage in the steady state (30 volts as shown in FIG. 2A ) as the fixed voltage command. Next, different frequencies (such as 50 Hz to 0 Hz as shown in Figure 2A) are successively selected and applied to the stator in order from high to low to perform frequency sweeping.

Since the slip is less than zero (ie, the stator frequency Fs is less than the rotor frequency Fr), the electric machine 2 is in generator mode. Therefore, in order to avoid energy storage on the DC link capacitor at the beginning of the frequency sweep, the frequency sweep will be performed in a sequence from high to low to ensure safety. In addition, in this embodiment, the frequency sweeping time is shorter than 0.4 seconds each time. For example, the time required for one frequency sweep can be about 0.3 seconds, but this case is not limited to this, and can be adjusted according to needs.

During the frequency sweep, as shown in Figures 2A and 2B, the stator frequency Fs will gradually approach the rotor frequency Fr until the two are equal, and then each other The values gradually move away. As the stator frequency Fs gradually approaches the rotor frequency Fr, due to the reduction of the slip s, the stator equivalent impedance Zs_eq will rise, and the stator current Is will fall, as shown between the 0.1st and 0.2th seconds. Then, when the stator frequency Fs and the rotor frequency Fr are equal, the stator equivalent impedance Zs_eq reaches the maximum value, and the stator current Is reaches the minimum value, as shown in the 0.2 second. In addition, the input power Ps at this time is zero. Then, when the stator frequency Fs is lower than the rotor frequency Fr and the value is further and further away, the stator equivalent impedance Zs_eq will decrease and the stator current Is will increase, as shown after 0.2 seconds. The aforementioned time, the 0.2 second, can be defined as the time when the input power Ps crosses the zero point. When the frequency sweep is performed, the aforementioned stator current Is reaches the minimum value, or the input power Ps zero-crossing point can be used as a trigger condition to perform the estimated amount compensation for the motor 2 . When the stator frequency Fs and the rotor frequency Fr are close to each other, the difference between the frequencies is less than a set value, such as 2.5 Hz, which can be used as a judgment standard.

It can be seen from this that, according to the stator speed Fs when the stator equivalent impedance Zs_eq reaches the highest point (that is, the stator current Is reaches the lowest point), whichever is the closest value, the rotor frequency can be roughly estimated. Numerical value of Fr.

As shown in Figure 3, the stator equivalent impedances Z1 to Z5 are the results of sweeping the rotor frequencies of 40 Hz, 30 Hz, 20 Hz, 10 Hz, and 1 Hz, respectively. As shown in Figure 3, the time points when the stator equivalent impedances Z1 to Z5 reach the highest points are X1 to X5. The rotational speeds of the stator corresponding to the time points X1 to X5 are Y1 to Y5, that is, 40 Hz, 30 Hz, 20 Hz, 10 Hz, and 1 Hz, which roughly correspond to the rotor frequencies.

In other words, by applying a fixed voltage command and different frequencies to the stator for frequency sweeping, sensing the minimum value of the stator current and calculating the slope of the stator current, the time interval when the stator frequency is close to the rotor frequency can be determined to obtain the estimated value of the rotor frequency .

Please refer to Figure 4. FIG. 4 is a flowchart illustrating an estimation method 400 according to some embodiments of the present disclosure. For the sake of convenience and clarity, the following estimation method 400 is described in conjunction with the embodiment shown in FIG. 5, but not limited thereto. Anyone skilled in the art will not depart from the spirit and scope of the present disclosure. , when you can make various changes and retouches. As shown in FIG. 4 , the estimation method 400 includes operations S410 , S420 , S430 , S440 and S450 .

First, in operation S410, the controller 11 sequentially applies a selected one of a plurality of stator frequencies to the motor 2 and a fixed input voltage for frequency sweeping. Specifically, as shown in FIG. 5 , during the period from the time point T0 to the time point T2 , the controller 11 selects the stator frequency Fs successively and inputs it to the motor 2 in order from high to low for frequency sweeping. In addition, in some embodiments, in order to prevent the torque generated by the excessive sweep current from affecting the rotational speed of the rotor, the controller 11 assigns the set current to 20% of the rated current value of the motor 2, and waits until the stator current Is enters a steady state. The voltage is used as the fixed input voltage of the sweep to ensure that the stator current Is will keep increasing before the stator frequency Fs is close to the rotor frequency Fr (for example, the stator current Is keeps increasing during the period P0). It is worth noting that the magnitude of the current command is only an example for the convenience of illustration, and is not intended to limit the present case.

Next, in operation S420, the current sensor 12 senses the motor 2 is the stator current value Is. Specifically, the quantum current value Is is sensed during the frequency sweep. In some embodiments, the stator current value Is of the motor 2 may be sensed at any time from the beginning to the end of the estimation method 100 .

Next, in operation S430, the controller 11 calculates the stator current slope of the stator current value Is. Specifically, during successive frequency sweeps, the current gradient of the stator current is calculated according to the stator current values sensed this time and the previous time. For example, successive sensing is performed at a sampling frequency of every 100 microseconds, and the stator current value of the current stroke (hereinafter referred to as this time) is compared with the stator current value of the previous stroke (hereinafter referred to as the previous time). If the stator current value sensed this time is smaller than the stator current value sensed last time, the stator current slope this time is less than zero, which is a negative value. If the value of the stator current sensed this time is larger than the value of the stator current sensed last time, the current slope of the stator current is greater than zero, which is a positive value. In other partial embodiments, the average value of the stator current values of the current N pens (ie, the multiple samplings including this time) may be compared with the average value of the previous N pens (ie, the multiple samplings including the previous time). The average value of the stator current value is compared. N is a positive integer greater than one. For example, take N stator current values near time points T3 and T1 in Fig. 5, respectively, calculate the average value and then compare. As shown in Fig. 5, the average value corresponding to time point T3 is smaller than the average value corresponding to time point T1, so it can be determined that the stator current slope is less than zero. In addition to using the number of data records as the sampling standard for calculating the average value, the average calculation can also be performed on the stator current values obtained by successive frequency sweeps within a preset period. For example, in the first period (such as a period including time point T1 in Figure 5), the frequency is swept successively to calculate the first average value, and in the second period (such as a period including time point T3 in Figure 5) Sweep the frequency successively to calculate the second average, The magnitudes of the first average value and the second average value are then compared to determine whether the stator current slope is positive or negative. Wherein, the start time of the first time period is earlier than the start time of the second time period (eg, the time point T1 is earlier than the time point T3). In some embodiments, the first period and the second period may partially overlap. Taking sensing every 100 microseconds as an example, the first period may be 0-1000 microseconds, and the second period may be 500-1500 microseconds. The ten times of the stator current sensed values are averaged and the positive and negative values of the stator current slope are determined by their magnitudes. The first time period and the second time period overlap the 500th to 1000th microseconds. It is worth noting that the above sampling frequency is only an example for convenience of description, and this case is not limited to this. It should be noted that, the definitions of the positive value or the negative value of the current slope in the foregoing various calculation methods are similar, which will not be repeated here.

Next, in operation S440, the controller 11 takes the time when the slope of the stator current changes from positive to negative as a starting point, and takes the time when the slope of the stator current changes from negative to positive as an end point, and the starting point and the ending point define a target period. In other words, the target period is the period from when the stator current slope starts to be less than zero to when the stator current slope turns to be greater than zero, that is, the stator current slope is always negative during the target period.

Specifically, as described in the above paragraph, as the stator frequency Fs approaches the actual rotor frequency Fr, the stator equivalent impedance Zs_eq will increase and the stator current Is will decrease. Therefore, when the stator current slope starts to change from positive to negative, the representative stator equivalent impedance Zs_eq starts to rise, that is, the stator frequency Fs is approaching the actual rotor frequency Fr. Therefore, the starting point of the target period is taken when the slope of the stator current changes from positive to negative (for example, the time point T1 in Fig. 5).

While as stated in the above paragraph, theoretically, at the stator frequency Fs and When the actual rotor frequency Fr is equal, the stator equivalent impedance Zs_eq will reach the maximum value, and the stator current Is will reach the minimum value. Therefore, when the stator current slope starts to change from negative to positive, the highest point representing the stator equivalent impedance Zs_eq has passed, that is, the stator frequency Fs has started to be smaller than the actual rotor frequency Fr. Therefore, the end point of the target period is taken when the stator current slope changes from negative to positive (such as the time point T2 in Fig. 5).

In this way, it can be determined that the stator frequency Fs is close to the rotor frequency Fr in the target period (eg period P1 in FIG. 5 ) according to the time point when the stator current slope is switched between positive and negative values.

Finally, in operation S450, one of the stator frequencies Fs corresponding to the target period is close to the rotor frequency Fr. Therefore, when sweeping the frequency, the controller 11 first determines that the stator frequency Fs and the rotor frequency Fr are close to each other and the difference between the two frequencies is close to each other. The difference is smaller than a set value, such as 2.5 Hz, and any one of the plurality of stator frequencies Fs determined to correspond to the target period is used as the estimated value of the rotor frequency Fr. In a preferred embodiment, when the frequency sweep is performed, the controller 11 uses the stator frequency Fs corresponding to a middle point of the target period as an estimated value of the rotor frequency Fr, usually at the middle point of the target period, The current slope will start to slow down. Specifically, as shown in FIG. 5 , any time point in the period P1 can be selected as the operation point. The stator frequency Fs corresponding to this operating point (ie, the corresponding one between the frequency F1 and the frequency F2 ) is used as the estimated value of the rotor frequency Fr. In other words, after the time point T2, add a compensation value to the stator frequency Fs (ie, the frequency F2) corresponding to the time point T2 as the estimated value of the rotor frequency Fr, wherein the compensation value is the stator frequency corresponding to the operating point Fs (ie, frequency F1 to A value between the frequency F2) and the difference between the frequency F2.

In further detail, it can be seen from the waveform diagram of the stator frequency and the actual rotor frequency (such as Fig. 2A), although ideally when the stator frequency Fs and the actual rotor frequency Fr are equal, the stator current Is should be the minimum value. However, in practice, Due to transients or filtering, the signal will lag behind, causing a delay in the time point when the stator current reaches the minimum value, as shown by the stator current Ir in Figure 2B. Therefore, if the controller 11 takes the time point when the current sensor 12 senses that the stator current is the minimum value as the operating point, the obtained estimated value will be lower than the actual rotor frequency Fr.

In the embodiment of FIG. 5, when the stator current Is has just started to decrease (eg time point T1), it will not be the time when the stator frequency Fs is closest to the rotor frequency Fr. In addition, due to the signal delay, when the stator current Is reaches the lowest point (eg, time point T2 ), it will not be the time when the stator frequency Fs is closest to the rotor frequency Fr. Therefore, the operating point may be selected in the period P1 not close to the time point T1, nor to another time point close to the time point T2, such as one of the periods P1a.

For example, in some embodiments, the operating point may be selected at the middle point of the period P1, such as the time point T3. According to the estimated value of the rotor frequency Fr, the stator frequency Fs corresponding to the time point T3 (ie, the frequency F3 ) is used as the estimated value of the rotor frequency. Alternatively, in some other embodiments, the operating point may be selected at a time point in the second half of the period P1 but close to the middle point, such as one of the periods P1b, that is, starting from the time point T3 but not exceeding the time point T4 One of them to avoid the estimation being affected by the aforementioned signal delay. According to one of the periods P1b The stator frequency Fs corresponding to the time point (ie the corresponding one between the frequency F3 and the frequency F4) is used as the estimated value of the rotor frequency Fr.

In this way, when the stator frequency is close to the rotor frequency, the stator equivalent impedance Zs_eq will increase and the stator current will decrease, and the fixed input voltage is applied to the stator at different frequencies for scanning, and the positive and negative values of the stator current slope can be obtained. At the time of conversion, it is estimated that the stator frequency is close to the target period of the rotor frequency, and then an appropriate operating point is selected to obtain the estimated value of the rotor frequency. At this time, the stator frequency Fs and the rotor frequency Fr are close to each other, and the difference between the frequencies is smaller than a set value, such as 2.5 Hz.

Please refer to Figure 6. FIG. 6 is a schematic diagram illustrating a waveform of a motor speed versus time according to some embodiments of the present disclosure. As shown in FIG. 6, in phase S1, the motor 2 is accelerated from a standstill to a rated rotational speed. In phase S2, the motor is operated with mains power. In stage S3, when the mains is cut off, the motor will free-run. At this time, the above estimation method 100 can be used to estimate the actual rotational speed of the rotor to obtain an estimated value of the rotor frequency Fr. In the stage S4, the frequency converter or the controller 11 of the present application may perform a speed chase operation according to the estimated value of the rotor frequency. In this way, in the stage S4, the motor 2 can be decelerated to the target low speed.

Although the disclosed methods are shown and described herein as a series of steps or events, it should be understood that the order in which the steps or events are shown should not be construed in a limiting sense. For example, some of the steps may occur in a different order and/or concurrently with other steps or events other than those shown and/or described herein. In addition, implementing the descriptions herein Not all steps shown herein are required for one or more aspects or embodiments. Furthermore, one or more of the steps herein may also be performed in one or more separate steps and/or stages.

To sum up, in this case, by applying the above-mentioned various embodiments, by applying a fixed input voltage and a plurality of stator frequencies for scanning, the stator current can be converted into positive and negative according to the slope of the stator current due to the characteristic that the stator current will decrease when the slip decreases. At the time point, it is determined that the stator frequency is close to the target period of the rotor frequency, so as to obtain the estimated value of the rotor frequency. Without using motor parameters, the estimation time is effectively shortened and the estimation accuracy is improved.

Although the present disclosure has been disclosed in the above embodiments, it is not intended to limit the present disclosure. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present disclosure. The scope of protection of the disclosed contents shall be determined by the scope of the appended patent application.

1: Motor control system

11: Controller

12a, 12b: Current sensor

13: Power stage circuit

2: Motor

Ia,Ib,Ia_AD,Ib_AD: Feedback current

Vs: stator voltage

Is: stator current

jXs: stator leakage inductance

Rs: stator resistance

jXm: Excitation inductance

jXr: rotor leakage inductance

Rr: rotor resistance

s: slip

Fs: stator frequency

Fr: rotor frequency

Zs_eq: stator equivalent impedance

Is, Ir: stator current

Ps: input power

X1~X5: time point

Y1~Y5: stator frequency

Z1~Z5: Equivalent impedance of stator

400: Estimation Methods

S410, S420, S430, S440, S450: Operation

P0,P1,P1a,P1b: Period

T0,T1,T2,T3,T4: time points

F1,F2,F3,F4: Frequency

S1, S2, S3, S4: Stages

FIG. 1A is a system schematic diagram illustrating an electrical connection between a motor control system and a motor according to some embodiments of the present disclosure. FIG. 1B is an equivalent circuit diagram of a motor according to some embodiments of FIG. 1A. FIGS. 2A , 2B and 3 are waveform diagrams illustrating various motor parameters according to some embodiments of the present disclosure. FIG. 4 is a flowchart illustrating an estimation method according to some embodiments of the present disclosure. FIG. 5 is a schematic diagram illustrating a waveform of an estimated parameter according to some embodiments of the present disclosure. FIG. 6 is a schematic diagram illustrating a waveform of a motor speed versus time according to some embodiments of the present disclosure.

400: Estimation Methods S410, S420, S430, S440, S450: Operation

Claims (10)

An estimation method applied to a motor control system for estimating a rotor frequency of a motor during a free-running period, the motor control system comprising a controller and a current sensor, the estimation method comprising: The controller sequentially applies a fixed input voltage and one selected from a plurality of stator frequencies to the motor for frequency sweeping; The current sensor senses the stator current value of the motor corresponding to the stator frequency; The controller calculates the stator current slope successively according to the stator current value; the controller defines a target period from a start point of the stator current slope from positive to negative and an end point from negative to positive; and When sweeping the frequency, the controller determines that the difference between the stator frequency and the rotor frequency is less than a set value, and uses the stator frequency corresponding to any one of a plurality of time points during the target period as the rotor frequency. an estimate. The estimation method of claim 1, wherein the controller uses the stator frequency applied to the motor at a midpoint of the target period as the estimation value. The estimation method as claimed in claim 1, wherein the controller uses the stator frequency applied to the motor at a first time point as the estimation value when the controller performs frequency sweep, and the first time point is at the second half of the target period. The estimation method of claim 3, wherein when the controller performs frequency sweeping, the estimated value is the stator frequency applied to the motor at a second time point, the second time point being at the middle of the target period. The estimation method as claimed in claim 1, wherein when the controller performs frequency sweep, the plurality of stator frequencies are successively selected in descending order and applied to the motor. The estimation method of claim 1, wherein the fixed input voltage is a voltage value when the motor reaches a steady state after the controller applies a current command to the motor, and the current command is a rated current value of the motor twenty percent. The estimation method of claim 1, wherein the set value is 2.5 Hz. The estimation method of claim 2, wherein when the controller performs the frequency sweep, each time is shorter than 0.4 seconds. The estimation method of claim 1, wherein calculating the stator current slope comprises: when the controller performs frequency sweep, comparing a first average value including the plurality of stator current values of this time with a second average value including the plurality of stator current values of the previous time; If the first average value is less than the second average value, the controller determines that the current slope of the stator current is a negative value; and If the first average value is greater than the second average value, the controller determines that the current slope of the stator current is a positive value. The estimation method of claim 9, wherein the controller performs frequency sweeping during a first period to calculate the first average value, and performs frequency sweeping during a second period to calculate the second average value, wherein the The first period partially overlaps the second period.

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