Electric braking method of permanent magnet synchronous motor based on current control

Abstract

According to the electric braking method of the permanent magnet synchronous motor based on current control, when electric braking is required, an upper pipe or a lower pipe is blocked, a current controller with a current amplitude limiting value is adopted to control the braking torque, and the current amplitude limiting value is based on a current value corresponding to the maximum torque of electronic star sealing braking; psi r is the flux linkage of the rotor, L is the inductance of the motor, RS is the phase resistance of the motor, and omega sigma is the electrical angular velocity corresponding to the maximum torque of star-sealing braking. Electric braking is achieved without an independent switch, electric braking is achieved purely through the frequency converter, cost and space are saved, and reliability is improved; the thermal risk caused by overspeed due to large current during high-speed sudden stop star sealing and too small star sealing braking torque when a high-speed brake fails and too long star sealing braking time due to too low speed after the star sealing braking is stable is solved.

Description

Technical Field:

[0001] The present invention relates to a method for controlling the braking torque of a permanent magnet synchronous motor drive system, and particularly to an electric braking method for a permanent magnet synchronous motor based on current control. Background Art:

[0002] With the development of elevator technology, the safety requirements for elevators are getting higher and higher. In order to ensure that the elevator can effectively decelerate or limit the elevator creep speed, usually the three-phase input of the motor is short-circuited, and the braking force generated by the permanent magnet of the permanent magnet synchronous motor (PM motor) is used to decelerate the elevator or limit the elevator creep speed (i.e., "star sealing technology").

[0003] One of the commonly used star sealing methods in current elevator control systems is to use a "star sealing contactor". A star sealing contactor is added between the inverter and the PM motor to achieve short-circuiting of the three-phase input of the motor. However, this star sealing contactor must be selected to withstand the motor current and be able to connect and / or cut off this current. These contactors are often large and costly. In addition, contactors including moving parts have a certain limited lifespan and must be replaced after a relatively small number of operations.

[0004] Another commonly used star sealing method is to use "electronic star sealing". While the upper bridge arm drive signal of the frequency converter is blocked, the switching tubes of the lower bridge arm are controlled to conduct; or while the lower bridge arm drive signal of the frequency converter is blocked, the switching tubes of the upper bridge arm are controlled to conduct, to achieve short-circuiting of the three-phase input of the motor.

[0005] Although star sealing braking can brake the drive motor of the elevator and reduce the moving speed of the elevator, if star sealing braking is started when the elevator moving speed is relatively high, an instantaneous short-circuit large current will be generated. The instantaneous short-circuit large current will cause harm to the frequency converter, PM motor, etc. of the elevator drive system, affect the service life of the frequency converter and PM motor of the drive system, and more seriously, it may burn out the devices. To solve this problem, Chinese Patent Authorization Publication No. CN108483149B, etc. propose a scheme of delayed star sealing braking when the elevator moving speed is relatively high. Although delayed star sealing braking can indeed overcome the instantaneous short-circuit large current, when there is a high-speed emergency stop and the brake fails, overspeed will occur due to the lack of star sealing braking torque.

[0006] Chinese Patent Grant Publication No. CN105762766B proposes that through an overcurrent control subunit, when the overcurrent detection unit measures that the three-phase line current of the synchronous motor exceeds a preset value, the main controller outputs invalid motor drive pulses and a star connection signal in a chopping mode. The duration of the effective level in the star connection signal in the chopping mode is inversely proportional to the magnitude of the three-phase line current of the synchronous motor, thereby reducing the impact current between the three-phase windings of the synchronous motor in the high-speed emergency stop star connection state. However, when there is a high-speed emergency stop and the brake fails, if the preset value is unreasonable, it will result in too small a star connection braking torque and cause overspeed.

[0007] Chinese Patent Application Publication No. 108773742A proposes to maintain the star connection current within a certain range through current hysteresis control. However, the current preset value is 1.4 to 1.6 times the rated current of the elevator controller. Generally, the rated current of the elevator controller is greater than the rated current of the motor, which will result in too small a star connection braking torque for some motors and cause overspeed.

[0008] Chinese Patent Grant Publication No. CN108382934B proposes that when the elevator needs to implement star connection braking, when the current phase of the permanent magnet of the motor rotor is in the same order as the magnetic pole phase when the lower bridge arm switch tube is closed, the star connection protection signal is successively given to each lower bridge arm switch tube of the inverter, and the initial phase of the current is controlled through the protection signal to suppress the unbalanced current at the initial stage of star connection braking and avoid excessive current impact on the inverter and the motor. This strategy also cannot ensure sufficient star connection braking force.

[0009] In addition, after the brake fails and the star connection braking is started, it may be in the star connection braking state for a long time. Therefore, excessive heat will accumulate in the frequency converter and the PM motor. Once the heat cannot be dissipated in time, it will face the risk of overheating. Summary of the Invention:

[0010] To solve the above technical problems, the present invention provides an electrical braking method for a permanent magnet synchronous motor based on current control.

[0011] In the electrical braking method for a permanent magnet synchronous motor based on current control according to the present invention, when electrical braking is required, the frequency converter blocks the drive signal of the upper bridge arm, and a current controller with a current limit value is used to control the opening and closing of the lower bridge arm switch tube, or the frequency converter blocks the drive signal of the lower bridge arm, and a current controller with a current limit value is used to control the opening and closing of the upper bridge arm switch tube, thereby controlling the braking torque. The current limit value is based on the current value I0 corresponding to the maximum torque of the electronic star connection braking. I0 is obtained through experiments or simulations, or obtained according to, where Ψ r is the rotor magnetic flux, L is the inductance of the motor, R s is the phase resistance of the motor, ω ais the electrical angular velocity corresponding to the maximum torque of the star - closing braking.

[0012] In a preferred embodiment of the present invention, the current control adopts software peak current control with a control period, or hysteresis current control, or average current control with a PI regulator.

[0013] In a preferred embodiment of the present invention, through the current limit value, the braking torque is positively correlated with the current, that is, the larger the current, the larger the braking torque, and the smaller the current, the smaller the braking torque.

[0014] In a preferred embodiment of the present invention, the current control is applicable to electronic star - closing with a Duty of 100% or electric braking using PWM control.

[0015] In a preferred embodiment of the present invention, the current limit value is based on the d - axis part I d0 of the current value I0 corresponding to the maximum torque of the star - closing braking, and this value is approximately equal to, and the feedback current of the current control is the d - axis feedback current.

[0016] In a preferred embodiment of the present invention, speed control can be performed on the basis of current control, so that after the brake fails and the electric braking is started, the elevator can run controllably to the end stage at a certain speed, reducing the long - time state of star - closing braking.

[0017] In a preferred embodiment of the present invention, speed control can be performed on the basis of current control. When the elevator makes an emergency stop during high - speed operation, with a certain speed as the target, it decelerates from the current elevator speed at a certain deceleration rate.

[0018] In a preferred embodiment of the present invention, speed control can be performed on the basis of current control. By setting different deceleration curves, it can be determined whether the electric braking accelerates the braking of the elevator. When it is not required to participate, the deceleration rate can be smaller than the braking deceleration rate when the brake is normal.

[0019] Due to the adoption of the above - mentioned technical solutions, one advantage of the method according to the present invention is that: realizing electric braking does not require a separate switch, but can be purely achieved through the frequency converter. This saves costs and space. In addition, if the number of components is reduced, the overall reliability of the system is also improved. It can solve the problem of large current during high - speed emergency stop and star - closing, as well as the problem of overspeed caused by too small star - closing braking torque when the high - speed brake fails. At the same time, it can also solve the problem of thermal risk caused by too long star - closing braking time due to too low speed after the steady state of star - closing braking. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic diagram of the drive system (the rectifier part is omitted) of a permanent - magnet synchronous motor.

[0021] Figure 2Schematic diagram of the equivalent circuit during star-sealing braking.

[0022] Figure 3 Schematic diagram of exemplary curves of the star-sealing braking and the braking torque of the present invention versus speed.

[0023] Figure 4 Schematic diagram of exemplary curves of the star-sealing braking and the motor current of the present invention versus speed.

[0024] Figure 5 Schematic block diagram of the speed closed-loop and current closed-loop in Embodiment 2. Detailed implementation manners

[0025] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners.

[0026] Reference Figure 1 , by blocking the drive signals of the controllable switch tubes T1, T2, T3 (such as IGBTs) of the upper bridge arm of the frequency converter, and keeping the drive signals of the controllable switch tubes T4, T5, T6 (such as IGBTs) of the lower bridge arm conducting all the time, or by blocking the drive signals of the controllable switch tubes T4, T5, T6 (such as IGBTs) of the lower bridge arm of the frequency converter, and keeping the drive signals of the controllable switch tubes T1, T2, T3 (such as IGBTs) of the upper bridge arm conducting all the time, the "electronic star-sealing" of the prior art can be achieved.

[0027] Since a certain bridge arm of the frequency converter is blocked all the time, it prevents Figure 1 the capacitors C1, C2 (and the power grid power supply) in the intermediate circuit from providing driving energy to the PM motor. Therefore, the "electronic star-sealing" is a safe electrical braking. The equivalent circuit at this time is as Figure 2 shown. The resistance R1, R2, and R3 of each phase winding of the PM motor, the inductance L1, L2, and L3 of each phase winding, and the back electromotive force e1, e2, and e3 generated by the rotation of the motor form a braking circuit. For this braking circuit, its braking torque shows a characteristic of first increasing and then decreasing as the speed of the PM motor increases, as shown by the curve 31 in Figure 3 . This is because as the speed of the motor increases, the generated back electromotive force e increases, and the current flowing through this braking circuit increases, as shown by the curve 41 in Figure 4 , and the generated braking torque also increases. However, since the inductive reactance of the inductance of the PM motor also increases as the speed increases, the ratio of the inductive reactance to the resistance in this braking circuit also increases as the speed increases, causing the current in this braking circuit to develop in a direction leading the back electromotive force e by 90°, resulting in a decrease in the braking torque after a certain speed of the PM motor. At the rated speed, the star-sealing current of some PM motors can reach 1.5 to 2 times the rated current, but its braking torque is less than 50% of the rated torque.

[0028] In an elevator application, curve 36 represents the unbalanced torque generated due to the mass difference between the car and the counterweight. When making an emergency stop at high speed (e.g., 80% of the rated speed) and the brake fails, the braking torque of this braking circuit shown by curve 31 is always less than the unbalanced torque. The rotational speed of the PM motor cannot be controlled and it rotates faster and faster, and the overrun speed of the elevator cannot be limited within the specified range.

[0029] The following further describes the present invention by blocking the drive signals of the controllable switching tubes T1, T2, T3 of the upper bridge arm and controlling the disconnection and closing of the controllable switching tubes T4, T5, T6 of the lower bridge arm. The principle also equally applies to blocking the drive signals of the controllable switching tubes T4, T5, T6 of the lower bridge arm and controlling the disconnection and closing of the controllable switching tubes T1, T2, T3 of the upper bridge arm. That is, it prevents Figure 1 C1 and C2 (as well as the grid power supply) of the intermediate circuit from providing driving energy to the PM motor. In this case, the PM motor cannot generate a torque other than for braking.

[0030] Embodiment 1:

[0031] Reference Figure 1 , only when the phase current of the PM motor is negative (the phase current of the PM motor is positive when flowing into the PM motor), that is, when the phase current flows from the PM motor to the frequency converter, the controllable switching tubes T4, T5, T6 of the lower bridge arm will be controlled by the phase drive signals corresponding to the phase current; when the phase current is positive, even if the phase drive signal corresponding to the phase current is valid, T4, T5, T6 will not turn on, and its current flows through the diodes D4, D5, D6 anti-parallel to T4, T5, T6 to the motor. For the sake of simplified control, T4, T5, T6 can be controlled simultaneously, that is, the Duty (i.e., the ratio of the conduction time to the period) of the drive signals of T4, T5, T6 is the same. Such a control logic does not require a separate estimation of the flow direction of the motor current.

[0032] When electrical braking is required, specifically, such as when the elevator stops or when it is detected that the brake of the PM motor fails, the drive signals of the controllable switching tubes T1, T2, T3 of the upper bridge arm are blocked, and a current controller with a current limit value is used to control the braking torque. The current limit value is based on the current value I0 corresponding to the maximum torque during star connection braking, which can be obtained through experiments or simulations, or according to the motor parameters Ψ r and L, such as, or obtained according to, where Ψ r is the rotor magnetic flux, L is the inductance of the motor, R S is the phase resistance of the motor, and ω0 is the electrical angular velocity corresponding to the maximum torque during star connection braking.

[0033] By giving different current limit values, at different motor speeds, the effective value of the PM motor current is controlled asFigure 4 The curves 40, 42, 43, 44 shown, that is, when the rotational speed is low and the PM motor current is less than the current limit value, the current is the current during traditional electronic star connection sealing (i.e., the Duty of the driving signals of T4, T5, and T6 is 100%); when the speed increases, the current is controlled to the given current limit value. Curve 40 represents the current value corresponding to when the current limit value is less than the maximum torque of star connection braking; curve 42 represents the current value corresponding to when the current limit value is equal to the maximum torque of star connection braking; curves 43 and 44 represent the current values corresponding to when the current limit value is greater than the maximum torque of star connection braking.

[0034] Figure 3 The braking torque curves 30, 31, 32, 33, 34 corresponding to the curves 40, 41, 42, 43, 44 are shown. That is, when the current is less than the current limit value, the braking torque is the braking torque during traditional electronic star connection sealing and varies with the speed; when the current is controlled to the current limit value, the braking torque is a constant value, and this constant value is the electronic star connection braking torque corresponding to when the current just enters the limit.

[0035] Furthermore, when the current limit value is equal to the current value corresponding to the maximum torque of star connection braking, and when the speed is greater than the speed n1 corresponding to the maximum torque of star connection braking, the braking torque is the maximum torque of star connection braking.

[0036] Furthermore, when the current limit value is less than the current value corresponding to the maximum torque of star connection braking, the braking torque is less than the maximum torque of star connection braking and is positively correlated with the current, that is, the greater the current limit value, the greater the braking torque, and the smaller the current limit value, the smaller the braking torque.

[0037] Furthermore, when the current limit value is greater than the current value corresponding to the maximum torque of star connection braking, there is a maximum braking torque, which is equal to the maximum torque of star connection braking, and at this time it corresponds to the speed n1. When the speed is greater than n1, the braking torque is negatively correlated with the current limit value, that is, the greater the current limit value, the smaller the braking torque.

[0038] Furthermore, when the unbalanced torque generated due to the mass difference between the car and the counterweight is Figure 3 the curve 36 shown, if the effective value of the PM motor current is controlled as Figure 4 the curves 40, 42, 43 shown, when making an emergency stop at high speed (for example, 80% of the rated speed) and the brake fails, due to Figure 3 the braking torques shown by the curves 30, 32, 33 being greater than the unbalanced torque, the rotational speed of the PM motor is controlled and finally decelerates to the intersection point A of the star connection braking torque curve 31 and the unbalanced torque curve 36. If the effective value of the PM motor current is controlled as Figure 4For the curve 44 shown, when making an emergency stop at high speed (e.g., at 80% of the rated speed) and the brake fails, since the braking torque shown by curve 34 is always less than the unbalanced torque, the speed of the PM motor cannot be controlled and it rotates faster and faster, and the overrunning speed of the elevator cannot be limited within the specified range.

[0039] It should be noted that for current control, software peak current control with a control period can be adopted. That is, in the control period, compare the magnitude of the current limit value with the feedback current of the PM motor. When the feedback current is greater than the current limit value, then turn off the drive signals of T4, T5, and T6, as Figure 1 shown. At this time, the motor current releases the energy of the inductor to the intermediate circuit through D1 or D2 or D3 of the upper bridge arm, causing the voltage Ud of the intermediate circuit to increase, thereby quickly reducing the current; when the feedback current is less than or equal to the current limit value, then turn on the drive signals of T4, T5, and T6 to increase the current. The selection of the control period is related to the required current ripple magnitude, and generally can be taken as 50 us to 1 ms.

[0040] It should be noted that current control can also be achieved by using regulators such as PI to control the Duty of the drive signals of T4, T5, and T6. That is, when the feedback current is greater than the current limit value, reduce the Duty; when the feedback current is less than the current limit value, increase the Duty, so that the feedback current is equal to the current limit value or the feedback current is less than the current limit value (when the unbalanced torque is small and the Duty has been adjusted to the maximum of 100%). Since the carrier frequency is fixed and the Duty can be continuously adjusted, this method has a smaller current ripple than software peak current control.

[0041] It should be noted that current control can also be achieved by hysteresis control. That is, when the feedback current is greater than the current limit value, then turn off the drive signals of T4, T5, and T6; when the feedback current is less than a preset value, such as 80% of the current limit value, then turn on the drive signals of T4, T5, and T6. Under the same control period, the ripple of hysteresis control is larger than that of software peak current control.

[0042] In addition, it should be noted that the above-mentioned feedback current can obtain the instantaneous phase current through the PM motor current sampling technology of the existing technology, and then obtain the magnitude of the synthesized current vector according to the existing technology. When software peak current control or hysteresis control is adopted, the feedback current can also directly adopt the instantaneous phase current.

[0043] Furthermore, software peak current control can also be combined with the electric braking of PWM control with Duty varying with speed. That is, when the feedback current is greater than the current limit value, then turn off the drive signals of T4, T5, and T6; when the feedback current is less than or equal to the current limit value, then turn on the drive signals of T4, T5, and T6.

[0044] Furthermore, the current limit value can be I0*k, where k ≤ 1. When the maximum torque of the star-connected braking of some motors is 2 times or more of the rated torque of the motor, the excessive current can be limited by multiplying by k, thereby also reducing the braking torque.

[0045] Furthermore, the current limit value can also be based on the d-axis part I of the current value I0 corresponding to the maximum torque of the star-connected braking d0 , and this value is approximately equal to. At this time, the above-mentioned feedback current is the corresponding d-axis feedback current, which can obtain the instantaneous phase current through the PM motor current sampling technology of the existing technology, and then obtain it through coordinate transformation according to the rotor position of the PM motor.

[0046] Embodiment 2

[0047] When the brake fails after the elevator stops normally, since the elevator speed n is lower than n1, the feedback current is less than the current limit value, and the Duty is 100%. Its braking curve is as shown by curve 31, and the elevator finally operates stably at the intersection point A with curve 31 according to the unbalanced torque 36.

[0048] Usually, when the brake fails and the star-connected braking is started, due to the small steady-state speed of the star-connected braking, generally less than 0.3 m / s, for high-rise elevators, it will be in the star-connected braking state for a long time. Therefore, excessive heat will accumulate in the frequency converter and PM motor. Once the heat cannot be dissipated in time, there will be a risk of overheating.

[0049] As Figure 5 shown, speed control can be carried out on the basis of current control, where Imax = I0*k, and I0 is the current value corresponding to the maximum torque of the star-connected braking, k ≤ 1. That is, when the feedback speed n fdb is greater than the command speed n ref , the i ref adjusted by PI increases. Since i ref is less than or equal to the maximum current limit value Imax, the braking torque increases and the motor decelerates; when the feedback speed n fdb is less than the command speed n ref , the i ref adjusted by PI decreases. Since i ref is less than the maximum current limit value Imax, the braking torque decreases and the motor accelerates, so that the motor stably operates at n ref . Thus, after the brake fails and the electric braking is started, after finally operating stably at the intersection point A with curve 31 according to the unbalanced torque 36 for a period of time, and when the car door of the elevator is closed, by setting a reasonable speed curve, it can run to the end step at a controllable speed, reducing the time in the star-connected braking state for a long time.

[0050] When the elevator makes an emergency stop due to a failure during high-speed operation and electric braking is required, speed closed-loop control is performed. By setting different deceleration curves, it is possible to make the electric braking accelerate or decelerate the elevator. If the brake has not failed, aiming at a certain speed, decelerate from the current elevator speed at a certain deceleration. When the deceleration is less than the braking deceleration when the brake is normal, at this time the speed command is greater than the actual speed of the elevator. As can be known from the speed closed-loop regulator, i ref decreases to 0, that is, it does not participate in braking. When the deceleration is greater than the braking deceleration when the brake is normal, or when the brake fails, the speed command is less than the actual speed of the elevator, i ref increases, but is limited by the maximum current limit value Imax, and the electric braking participates in the braking of the elevator to prevent overspeed.

[0051] Furthermore, if the elevator makes an emergency stop at high speed and the brake fails, the electric braking is started. When the car door of the elevator is already closed, since the present invention adopts speed closed-loop and current closed-loop control, by setting a reasonable deceleration curve, when the brake is normal, it does not need to participate in braking, and it will not cause the deceleration of light load going down or the deceleration of heavy load going up to exceed the requirements, nor will it generate excessive current. It can participate in braking only when the brake fails. At this time, due to current closed-loop control, instantaneous short-circuit large current will not be generated.

Claims

1. An electric braking method for a permanent magnet synchronous motor based on current control, characterized in that, When electric braking is required, the inverter blocks the drive signal of the upper bridge arm and uses a current controller with a current limit value to control the opening and closing of the switch tube of the lower bridge arm, or the inverter blocks the drive signal of the lower bridge arm and uses a current controller with a current limit value to control the opening and closing of the switch tube of the upper bridge arm, so as to control the braking torque. The current limit value is based on the current value I0 corresponding to the maximum torque of the electronic star-blocking braking, which is obtained through experiments or simulations, or obtained according to... where Ψ r is the rotor magnetic flux, L is the inductance of the motor, and R s is the phase resistance of the motor, and ω0 is the electrical angular velocity corresponding to the maximum torque of the star-blocking braking.

2. The electric braking method of a permanent magnet synchronous motor based on current control according to claim 1, characterized in that , only control the corresponding lower-arm switching tubes when the phase current of the permanent magnet synchronous motor flows into the frequency converter, or only control the corresponding upper-arm switching tubes when the phase current of the permanent magnet synchronous motor flows out of the frequency converter.

3. An electric braking method for a permanent magnet synchronous motor based on current control according to claim 1, characterized in that , control the lower-arm switching tubes simultaneously, or control the upper-arm switching tubes simultaneously.

4. An electric braking method for a permanent magnet synchronous motor based on current control according to claim 1, characterized in that, The current limit value is based on the d-axis component I of the current value I0 corresponding to the maximum torque during the star connection braking d0 , and this value is approximately equal to the feedback current of the current control being the d-axis feedback current.

5. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, The current control adopts software peak current control with a control period, or hysteresis current control, or average current control with a PI regulator.

6. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, Through this current limit value, the braking torque is positively correlated with the current, that is, the larger the current, the larger the braking torque, and the smaller the current, the smaller the braking torque.

7. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, The current control is applicable to electronic star connection with a lower-tube Duty of 100% or electric braking using PWM control.

8. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, Speed control can be performed on the basis of current control, so that after the brake fails and electric braking is started, the speed can be controlled to run to the end step, reducing the long-term star connection braking state.

9. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, Speed control can be performed on the basis of current control. When the elevator runs at high speed and makes an emergency stop, it decelerates from the current elevator speed at a certain deceleration rate with a certain speed as the target.

10. An electric braking method for a permanent magnet synchronous motor based on current control according to any one of claims 1 to 4, characterized in that, Speed control can be performed on the basis of current control. By setting different deceleration curves, it can be determined whether the electric braking accelerates the braking of the elevator. When it is not required to participate, the deceleration rate can be smaller than the braking deceleration rate when the brake is normal.

Images