JPWO1998011663A1 - Induction motor control device and control method thereof - Google Patents

Abstract

(57) [Abstract] The present invention provides a vector control device with a DC braking function that is trip-less and produces little vibration during stopping, even during DC braking. The device includes a DC control calculator (1) and switches (S1, S3, and S4) controlled thereby. During vector control, the DC control calculator (1) sets all of S1, S3, and S4 to side a, performing vector control. To switch from this state to DC control mode, the phase of the voltage vector is calculated, S1 and S3 are set to side b, and the absolute values of i and q are compared with a reference value (α). If the absolute values of i and q are small, S4 is switched to side b, and only the d-axis ACR is used. This invention provides a vector control device with a trip-less DC braking function and minimizes vibration during stopping, thereby improving the stopping position accuracy of DC braking in emergencies and providing a low-noise system.

Description

[Detailed Description of the Invention] Induction Motor Control Device and Control Method Technical Field The present invention relates to an induction motor control device that performs current control on a rotating coordinate system and has a DC braking function.

BACKGROUND ART Induction motors can be controlled using V/f control, which varies the motor's applied voltage (V) and frequency (f) at a fixed ratio to control the motor's rotation speed, or vector control, which decomposes the motor's current into a magnetic flux component (d-axis) and a torque component (q-axis) and controls them in the same way as DC motors.

The V/f control method has been widely used for a long time due to its simple control method. However, advances in microcomputers have made real-time control calculations possible, and this method has come to be used as a replacement for conventional DC servos.

The former V/f control does not produce torque at low speeds due to factors such as a voltage drop across the armature resistance, and positioning is difficult with V/f control, so DC braking, which applies braking by passing a DC current through the induction motor, has been used.

The latter type of vector control can ensure sufficient torque at low speeds and can also perform positioning. However, because vector control requires speed (position) information, the induction motor needs a position sensor such as an encoder to detect speed, or a speed estimator to estimate speed from current and voltage. If these speed detection methods fail, the aforementioned DC braking is often used for emergency stops and other purposes.

The latter vector control device also uses the same control method as a V/f-controlled induction motor control device during DC braking, so the same control method as before is often used. This DC braking method simply applies a DC voltage to the induction motor corresponding to the desired DC braking force.

FIG. 4 shows a block diagram of a conventional vector control device having a DC braking function.

To implement DC braking functionality, conventional vector control devices include DC braking control calculation unit 1 and switches S1 and S2 in addition to the vector control calculation units 2-11, as shown in Figure 4. During vector control, S1 and S2 are on side a, and during DC braking, S1 and S2 are on side b. By switching S2 to side b, the output of d-q ACR 4 becomes ineffective, a value corresponding to the braking force is input to one voltage (vd*), and the other (vq*) is set to zero, applying DC voltage to induction motor 13. To minimize switching shock, S1 calculates the voltage phase θv in DC braking control calculation unit 1, aligning the d-axis with the voltage phase.

As mentioned above, vector control devices are sometimes used as servos for positioning, and stopping due to overcurrent or other factors is unacceptable. This also applies when the vector control device enters DC braking mode for emergency situations. DC braking is unnecessary because vector control devices can ensure sufficient low-speed torque and perform positioning, but as mentioned above, it is an essential control mode when considering situations such as position (speed) sensor failure.

However, if conventional DC braking techniques are used, after switching from vector control to DC braking, tripping may occur due to overcurrent protection or other reasons. This is likely to occur at high rotation speeds or under heavy loads, where current tends to grow, because applying DC voltage alone, as with conventional DC braking, does not adequately control the current. Because DC braking is used in the event of a fault, as described above, vector control devices cannot be limited to load conditions that are less likely to trip; a DC braking technique that will not trip under any conditions is required.

The primary objective of this invention is to provide a DC braking method that does not trip and is incorporated into a high-performance vector control device. A second objective is to provide a DC braking method that minimizes vibration during stopping.

Disclosure of the Invention To achieve the first objective, a d-axis and q-axis current control device (ACR) in a vector control device is used. DC braking mode is realized by setting either the d-axis or q-axis current command of the ACR to zero and inputting the other current command as a current command corresponding to the DC braking force. Furthermore, to achieve the second objective, a means is provided to monitor the absolute value of one of the currents with the current command set to zero, or to monitor the induction motor rotation speed. If the absolute value is below a predetermined reference value, the ACR for that axis is set to zero (vq* = 0) and the remaining axis controls the current solely through the other axis.

This means that even during DC braking, the ACR of the vector control device is used, so the current can be controlled regardless of the load condition, and there is no risk of tripping due to overcurrent.

Furthermore, because switching to DC braking simply involves manipulating the ACR command current, there is no strange discontinuity in the power converter voltage before and after switching, which minimizes switching shock and makes switching trips less likely.

Furthermore, when DC braking is performed on both the d-axis and q-axis ACRs, voltage vectors can be output in the full 360° phase direction of the motor, causing the rotor rotational position to vibrate slightly just before coming to a stop. For this reason, when the current on the axis that is set to zero or the induction motor's RPM falls below a reference value, control is performed solely by the ACR on the other axis. This ensures that the voltage output direction is limited to the other axis controlled by this ACR, preventing the rotor from vibrating when stopped.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a control block diagram of one specific embodiment of the present invention.

FIG. 2 is a flow chart showing the operation of a DC braking calculator according to a specific embodiment of the present invention.

FIG. 3 is a vector diagram of voltage and current on the d and q axes.

Figure 4 is a control block diagram of a conventional control device. Best Mode for Carrying Out the Invention One embodiment of the present invention will be described below with reference to Figure 1. Figure 1 shows a vector control device for an induction motor that controls an induction motor 13 using a power conversion circuit 7. For ease of understanding, this embodiment will be described based on a speed control system.

The vector controller of this embodiment, shown in Figure 1, is identical to a well-known vector controller equipped with a speed sensor, with the d-axis and q-axis coordinate systems rotating in synchronization with the rotating magnetic field of the induction motor. Speed control calculation unit 2 calculates a torque current (torque equivalent) command iq* based on a speed command ω* input from a system external to the vector controller and a detected speed value ωr input from speed detection calculation unit 10. Speed detection calculation unit 10 calculates speed using a signal from a position detector 12, such as an encoder, or estimates speed from current or other sources in the case of a position-detection-less system. This iq* is input to magnetic flux calculation unit 3, which calculates a magnetic flux current command id* and slip frequency ωs that satisfy the vector control conditions, as is well known, and outputs id*, iq*, and ωs. The d-q axis ACR 4 calculates voltage commands vd* and vq* that cause the detected current values id and iq from the rotating coordinate d-q transformation unit 11 to follow these id* and id*.

These vq* and vq* are converted into three-phase voltage commands by an inverse d-q converter 5, which converts the rotating coordinate d-q axes into three phases. The PWM calculation unit 6 then calculates the firing pattern for turning on/off the switching elements of the power converter 7. Additionally, the ωs output from the magnetic flux calculation unit 3 is input to a slip compensation calculation unit 8, which calculates the induction motor's primary frequency ωl. This is then integrated by an integrator 9 to calculate the d-axis phase θl, which is then output to the d-q transformation unit 11 and the inverse d-q transformation unit 5.

The present invention comprises a DC braking control calculator 1 and switches S1, S3, and S4 controlled thereby. The DC braking control calculator 1 receives a DC braking force target value and a DC braking command (not shown) for transitioning to DC braking operation. During vector control, the DC braking control calculator 1 switches S1, S3, and S4 to side a, and vector control is performed as described above. The operation of the DC braking control calculator 1 when switching from this state to the DC braking control state is explained using the flow chart shown in Figure 2.

The DC braking control calculator 1 calculates the phase of the voltage vector θv (step 101). This step 101 only needs to be calculated at the moment of switching from vector control to DC braking. The relationship between the induction motor's three-phase windings (u, v, w phases), the d-q axis voltage vector V1, and the current vector I1 is as shown in Figure 3. From the diagram, θv can be calculated using the following formula: θv = θl + tan ^-1 (Vq/Vd) ... (1). Vq and Vd use the d- and q-axis voltage values calculated immediately before switching. Next, switch S1 is switched to the b side, and θv is input to θdq (step 102). This aligns the d-axis with the phase of the voltage vector V1, as shown in Figure 3, and makes it the d'-axis, preventing discontinuity in the voltage vector V1 when switching to DC braking. This reduces the shock during switching, making it less likely to trip. In this example, the voltage vector V1 is made continuous, but the current vector I1 may also be switched continuously, in which case V in equation (1) should be replaced with i. Furthermore, these switches may be unnecessary if the ACR response is fast because they will automatically function to make it continuous, but this switching is particularly effective if the ACR response is slow.

After this, S3 is switched to side b, the d-axis ACR command i d* is input to the d-q-axis ACR 4 at a value corresponding to the braking force input to DC braking control calculator 1, and the q-axis ACR command i q* is set to 0 (step 103). Unlike conventional systems, the d-axis and q-axis ACRs are active even during DC braking, so the current grows under load in the AC motor, and tripping due to overcurrent is not possible.

The absolute values of i and q are then compared with a predetermined reference value α (step 104). If they are greater than this, S4 is switched to side a, and the output of d-q MAC R4 is sent to PWM calculation unit 6 as described above (step 105a). Conversely, if they are less than reference value α, S4 is switched to side b, the d-axis ACR output is activated, and the q-axis voltage command Vq* = 0 is set, i.e., the q-axis ACR is disabled, and a voltage command is output to PWM calculation unit 6 (step 105b). These steps 104 and 105 prevent vibration during rotor positioning and stopping of the AC motor. The reason for this vibration during positioning and stopping is that if both d- and q2-axis ACRs are left open, the d- and q-axis components are output in all 360° directions, as can be seen from the voltage vectors in Figure 3. This causes the ACR output to oscillate due to the q-axis voltage, resulting in rotor wobble.

Furthermore, while the reference value α is used as the threshold for S4 switching, in some cases the current may become oscillatory or noise may be introduced, causing the switching to be jerky. This can be prevented by adding a hysteresis of several to several tens of percent to the reference value α. Furthermore, vibration during stopping can be reduced by varying the reference value α depending on the braking force. For example, when the braking force is 40% and 80% of the induction motor's rated current, the d-axis current command will be 40% and 80%, respectively. Since the interference voltage of the d-axis current at the same rotation speed is different, increasing the reference value α so that switching to only the d-axis occurs earlier when the braking force is 80% compared to 40%, can reduce vibration during stopping.

Furthermore, the reference value α should naturally be smaller than the overcurrent detection level. If it is set to 80% or less of the overcurrent detection level, it is less likely to trip and vibration when stopped can be reduced.

Here, for convenience of explanation, the q-axis ACR command iq* is set to 0, but the same can be achieved with the d-axis ACR command id*. In this case, instead of aligning the d'-axis with V1 in Figure 3, calculate θv as follows to align the q'-axis with V1, and it is clear that the d and q suffixes in the flow chart in Figure 2 should be changed accordingly.

θ v = θ l + tan ⁻¹ ( Vq / Vd ) − π/2 ...................................(2) Also, when making the current vector I1 continuous as described above, V in equation (2) can be replaced with i, as described above.

Furthermore, when applying an ACR on the d- and q-axes, it is advisable to calculate the d- and q-axis interference voltages and add this to the ACR output as a feedforward, thereby canceling out the d- and q-axis interference voltages through decoupling control. In this invention, since the ACR is applied on the d- and q-axes even during DC braking, this decoupling control is effective. This decoupling reduces the oscillations in the d- and q-axis currents, thereby suppressing vibrations during braking. In this case, the interference voltage can be calculated with the primary frequency ωl = 0. Various equations for calculating the interference voltage are possible, but as an example, let's assume that the interference voltage Vzdq is decoupling as shown in the following equation:

where lσ is leakage inductance, M is mutual inductance, L2 is secondary self-inductance, T2 is secondary time constant, ωr is rotor electrical angular frequency, and φ* is secondary magnetic flux command.

In the above equation, ω ₁ = 0 ………………(4) This can be achieved by decoupling.

Although the above explanation focuses on vector control with a speed sensor, the present invention can be applied to all vector control systems, including those without a speed sensor.

Furthermore, in the above embodiment, when DC braking the induction motor, the phase of the current command is first fixed, and then the phase of the voltage command is fixed when the actual current component orthogonal to this fixed current command phase falls below a predetermined value. However, because current growth is more likely at high rotational speeds, overcurrent and vibration can be similarly prevented by first fixing the phase of the current command when DC braking the induction motor, and then switching S4 to side b to fix the phase of the voltage command when the induction motor's rotational speed falls below a predetermined value. In this case, too, when the induction motor is rotated by a load from a state below the predetermined rotational speed and again exceeds the predetermined rotational speed, S4 is returned to side a. In this embodiment, the speed detection value ωr from speed detection calculation unit 10 is input to DC braking control calculation unit 1, which determines when the rotational speed has fallen below the predetermined speed and outputs a signal to switch S4.

Systems that utilize this invention include elevators such as cranes. Cranes and other systems often use induction motors for the drive units that raise and lower loads due to their high capacity. In these systems, DC braking is used to prevent loads from falling in the event of a position sensor or other failure. In this case, the motor is always loaded in the direction of gravity, preventing tripping at switching times or tripping constantly, even during braking. The present invention is effective in these cases, as the two-axis ACR allows the ACR to suppress growing currents, enabling trip-free suppression even in emergencies, preventing loads from falling. Furthermore, even when the system is in a steady-state braking state and load fluctuations occur due to lateral load sway, DC braking is effective because the two-axis ACR suppresses current growth. Furthermore, while gears are often used in these systems, this reduces vibration during stopping, thereby reducing noise.

Industrial Applicability This invention provides a vector control device with tripless DC braking. It also minimizes vibration during stopping, improving the stopping position accuracy of DC braking in emergencies and providing a low-noise system.

───────────────────────────────────────────────────── (Note) This publication is based on the publication published internationally by the International Bureau of Patents (WIPO). The effect of the international publication of the Japanese patent application (Japanese utility model registration application) related to this publication arises pursuant to Article 184-10, Paragraph 1 of the Patent Act (Article 48-13, Paragraph 2 of the Utility Model Act) and is unrelated to this publication.

Claims (12)

[Claims] 1. An induction motor control device comprising an induction motor and a power converter, which controls the current supplied to the induction motor from the power converter by dividing it into vector components, and which also comprises means for controlling the current phase deviation after a DC braking command to be equal to or less than a predetermined value. 2. An induction motor control device comprising an induction motor and a power converter, which controls the current supplied to the induction motor from the power converter by dividing it into vector components, and which includes means for fixing the voltage phase after a DC braking command while the phase of the current command is fixed. 3. An induction motor control device in which the current supplied to an induction motor is controlled by separating it into vector components for magnetic flux and torque using a power conversion device that converts AC power of any frequency, the control device comprising: means for fixing the phase of the current command in response to a DC braking command; and means for fixing the phase of the voltage command when the actual current of the phase component orthogonal to the fixed phase of the current command falls below a predetermined value. 4. An induction motor control device in which the current supplied to an induction motor is controlled by a power converter that converts AC power of any frequency and separates it into vector components for magnetic flux and torque, the control device comprising: means for fixing the phase of the current command in response to a DC braking command; and means for fixing the phase of the voltage command when the rotation speed of the induction motor falls below a predetermined value during DC braking operation. 5. An induction motor control device for controlling the rotation of the induction motor, comprising: a power converter; an induction motor connected to the output of the power converter; a d-q converter that vector-resolves the induction motor's current into a d-axis current for generating magnetic flux and a q-axis current for generating torque; a current command calculator that calculates and outputs d- and q-axis current command values from a torque command value; and a dq-axis current control calculator that calculates d- and q-axis voltages so that the dq-axis currents follow the dq-axis current command values. The current command calculator receives a DC braking command and outputs one of the dq-axis current commands as a 0-axis current command and the other as a command value corresponding to the braking force. The dq-axis current control calculator controls the induction motor using only the current of the other axis when the absolute value of the current of one axis for which the current command value is 0 is equal to or less than a predetermined value. 6. In the induction motor control device of claim 5, the dq-axis current control calculator compares the absolute value of the current with the predetermined value using comparison means having a hysteresis characteristic. 7. In the induction motor control device according to claims 5 and 6, the predetermined value is set to a value corresponding to the braking force commanded during DC braking. 8. An induction motor control device according to any one of claims 5 to 7, wherein the current compared with the predetermined value during DC braking is the q-axis current. 9. An induction motor control device according to any one of claims 5 to 8, wherein the predetermined value is set to 80% or less of the overcurrent detection level. 10. A control method for an induction motor comprising an induction motor and a power converter, in which the current supplied to the induction motor from the power converter is divided into vector components and controlled, wherein when DC braking the induction motor, the phase of the current command is first fixed, and then the phase of the voltage command is fixed when the actual current of the phase component orthogonal to the fixed phase of the current command falls below a predetermined value. 11. A control method for an induction motor comprising an induction motor and a power converter, in which the current supplied to the induction motor from the power converter is divided into vector components and controlled, wherein when DC braking the induction motor, the phase of the current command is first fixed, and then the phase of the voltage command is fixed when the rotation speed of the induction motor falls below a predetermined value. 12. A control method for an induction motor comprising an induction motor and a power converter, in which the current supplied to the induction motor from the power converter is divided into vector components and controlled, the control method fixing the current phase when DC braking the induction motor.