Method for controlling braking torque of permanent magnet synchronous motor driving system

Abstract

According to the method for controlling the braking torque of the permanent magnet synchronous motor driving system, when electric braking is required, on the premise that a switching tube of an upper bridge arm of a frequency converter in the permanent magnet synchronous motor driving system is blocked, a switching tube of a lower bridge arm of the frequency converter is subjected to opening and closing control, namely PWM control; or on the premise that the switch tube of the lower bridge arm of the frequency converter is blocked, the switch tube of the upper bridge arm of the frequency converter is subjected to PWM control, and the Duty controlled by the PWM changes along with the change of the rotating speed of the PM motor so as to control the braking torque of the permanent magnet synchronous motor driving system. The problem of large current during high-speed sudden stop star sealing is solved, and the problem of overspeed caused by too small star sealing braking torque when a high-speed brake fails is also solved.

Description

Technical field:

[0001] The invention relates to an electric braking method in the technical field of elevator control, and in particular to a method for controlling the braking torque of a permanent magnet synchronous motor drive system. Background technology:

[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 slow down or limit the elevator's sliding speed, the three-phase input of the short-circuited motor is usually used, and the braking force generated by the permanent magnet of the permanent magnet synchronous motor (PM motor) is used to slow down the elevator or limit the elevator's sliding speed (that is, "sealing star technology").

[0003] One of the common methods of closing the star in elevator control systems is to use a "closing star contactor" to add a closing star contactor between the inverter and the PM motor to short-circuit the three-phase input of the PM motor. However, the closing star contactor must be selected to withstand the PM motor current and be able to connect and / or cut off this current. These contactors are often large and costly. In addition, the contactor, which includes moving parts, has a certain limited life and must be replaced after a relatively small number of operations.

[0004] The second commonly used star-blocking method is to use "electronic star-blocking", that is, while the inverter blocks the upper tubes of the three bridge arms, it controls the lower tubes of the three bridge arms to be turned on, thereby short-circuiting the three-phase input of the PM motor.

[0005] Although the star-sealing brake can brake the elevator's drive motor and reduce the elevator's moving speed, if the star-sealing brake is activated when the elevator is moving at a high speed, an instantaneous short-circuit current will be generated. The instantaneous short-circuit current will cause damage to the elevator drive system's inverter, PM motor, etc., affecting the service life of the drive system's inverter and PM motor, and more seriously, it may burn out the components.

[0006] In order to solve this problem, Chinese patent authorization publication number CN108483149B and others proposed to use delayed star-locking braking when the elevator moves at a high speed. Although delayed star-locking braking can indeed overcome the instantaneous short-circuit high current, when the high-speed emergency stop and the brake fails, overspeed will be caused due to the lack of star-locking braking torque.

[0007] Chinese patent authorization publication number CN105762766B and others proposed that the overcurrent control subunit can be used to chop the star-locking signal according to the size of the star-locking current to avoid overcurrent. However, when the vehicle makes an emergency stop at high speed and the brake fails, overspeeding is caused by the small star-locking braking torque.

[0008] In addition, after the brake fails and the star-locking brake is initiated, the device may be in the star-locking brake state for a long time, so too much heat will accumulate in the inverter and PM motor. If the heat cannot be dissipated in time, there will be a risk of overheating.

[0009] Invention scheme:

[0010] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for controlling the braking torque of a permanent magnet synchronous motor drive system.

[0011] In order to achieve the above-mentioned purpose of the invention, a method for controlling the braking torque of a permanent magnet synchronous motor drive system of the present invention is characterized in that: when electrical braking is required, under the premise that the switch tube of the upper bridge arm of the inverter in the drive system of the permanent magnet synchronous motor is blocked, the switch tube of its lower bridge arm is opened and closed, i.e., PWM control, or under the premise that the switch tube of the lower bridge arm of the inverter is blocked, the switch tube of its upper bridge arm is PWM controlled, and the Duty of the PWM control changes with the change of the speed of the PM motor, so as to control the braking torque of the permanent magnet synchronous motor drive system.

[0012] In a preferred embodiment of the present invention, the corresponding lower bridge arm switch tube is subjected to PWM control only when the phase current of the permanent magnet synchronous motor flows to the inverter, or the corresponding upper bridge arm switch tube is subjected to PWM control only when the phase current of the permanent magnet synchronous motor flows out of the inverter.

[0013] In a preferred embodiment of the present invention, the lower bridge arm switch tubes of the frequency converter are simultaneously subjected to PWM control, or the upper bridge arm switch tubes of the frequency converter are simultaneously subjected to PWM control.

[0014] In a preferred embodiment of the present invention, the Duty of PWM control is inversely proportional to the rotation speed n of the PM motor. The higher the rotation speed, the smaller the Duty.

[0015] In a preferred embodiment of the present invention, the Duty of PWM control is proportional to the bus voltage. The higher the bus voltage Ud is, the greater the Duty is.

[0016] In a preferred embodiment of the present invention, the Duty of PWM control is inversely proportional to the back electromotive force coefficient Ke of the PM motor, that is, the larger the back electromotive force coefficient Ke is, the smaller the Duty is. The back electromotive force coefficient Ke is the rated back electromotive force of the PM motor phase divided by the rated speed.

[0017] In a preferred embodiment of the present invention, when the speed n of the PM motor is less than or equal to the threshold value n1, the Duty of the PWM control is equal to 100%.

[0018] In a preferred embodiment of the present invention, when the speed n of the PM motor is greater than or equal to the threshold n1, the Duty of the PWM control adopts (1-(n-n1)*Ke*k / Ud)*100%, wherein n is the speed of the PM motor, n1 is the switching threshold, Ke is the opposite potential coefficient, Ud is the bus voltage, k is the coefficient, generally 0≤k<4, which is related to the electrical parameters of the PM motor and the bus voltage. The optimal value can be obtained through experiments or simulations, so that the braking torque at each speed is close to the maximum braking torque.

[0019] In a preferred embodiment of the present invention, the Duty of PWM control is (1-n*Ke*k / Ud+OFFSET)*100%, and OFFSET>0.

[0020] In a preferred embodiment of the present invention, when the brake fails and the electric braking is started, after a period of stable operation according to the unbalanced load torque, and the elevator car door is closed, the elevator speed can be increased by reducing the duty of the PWM control, and the elevator speed can be reduced by increasing the duty of the PWM control, and a reasonable electric braking speed is controlled, thereby shortening the electric braking time and eliminating the risk of overheating.

[0021] In a preferred embodiment of the present invention, the elevator makes an emergency stop at high speed and the brake fails, and the electric braking is started. When the elevator car door is closed, the PWM controlled Duty first adopts (1-(n-n1)*Ke*k / Ud)*100%, and after braking to a certain speed with the maximum braking torque, the PWM controlled Duty is fixed. After the unbalanced load torque finally stably runs at the intersection of the braking curve corresponding to the Duty for a period of time, the Duty size is adjusted, and the speed of the electric braking is adjusted, thereby shortening the time of the electric braking and eliminating the risk of overheating.

[0022] In a preferred embodiment of the present invention, the permanent magnet synchronous motor drive system is a permanent magnet synchronous motor drive system with a braking unit and a braking resistor in the intermediate circuit.

[0023] One advantage of the method according to the present invention is that a separate switch is not required to realize electric braking, but electric braking can be realized purely through a frequency converter. This saves cost and space. In addition, if the number of components is reduced, the overall reliability of the system is also improved. It not only solves the problem of high current during high-speed emergency stop star locking, but also solves the problem of overspeed caused by too small star locking braking torque when the high-speed brake fails, and also solves the thermal risk caused by too long star locking braking time due to too low speed after the star locking brake is in steady state.

[0024] Although the method according to the present invention is an electrical braking method for a permanent magnet synchronous motor drive system applied to an elevator system, the method according to the present invention can also be applied to other systems besides the elevator system, such as a crane system. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the drive system of a permanent magnet synchronous motor (rectifier part omitted).

[0026] Figure 2 This is the schematic diagram of the equivalent circuit during star-sealing braking.

[0027] Figure 3 Schematic diagram of exemplary curves of the locking brake and the braking torque of the present invention relative to the speed.

[0028] Figure 4 This is a schematic flowchart of the braking method of Example 1.

[0029] Figure 5 The schematic diagram of the drive system of a permanent magnet synchronous motor with a braking unit and a braking resistor in the intermediate circuit. DETAILED DESCRIPTION

[0030] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments.

[0031] refer to Figure 1 The "electronic star sealing" of the prior art can be achieved by blocking the driving signals of the controllable switch tubes T1, T2, T3 (such as IGBT) of the upper bridge arm of the inverter, while the driving signals of the controllable switch tubes T4, T5, T6 (such as IGBT) of the lower bridge arm are always turned on, or by blocking the driving signals of the controllable switch tubes T4, T5, T6 (such as IGBT) of the lower bridge arm of the inverter, while the driving signals of the controllable switch tubes T1, T2, T3 (such as IGBT) of the upper bridge arm are always turned on.

[0032] Because one of the bridge arms of the inverter is always blocked, Figure 1 The bus capacitors C1 and C2 (and the grid power supply) in the intermediate circuit provide the PM motor with driving energy, so "electronic star-off" is a safe electrical brake. The equivalent circuit at this time is as follows: Figure 2 As shown in FIG. 1 , the resistances R1, R2 and R3 of each phase winding of the PM motor, the inductances 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 increasing first and then decreasing as the speed of the PM motor increases, as shown in FIG. Figure 3As shown by curve 31 in . This is because as the speed of the motor increases, the back electromotive force e generated by it increases, the current flowing through the brake circuit increases, and the generated braking torque also increases. However, because the inductive reactance of the PM motor also increases with the increase of speed, the ratio of the inductive reactance to the resistance in the brake circuit also increases with the increase of speed, so that the current in the brake circuit develops in the direction of 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-closing current of some PM motors can reach 3 to 4 times the rated current, but its braking torque is less than 50% of the rated torque.

[0033] In elevator applications, Figure 3 The curve 36 in the figure represents the unbalanced torque caused by the mass difference between the car and the counterweight. When the car is stopped at high speed and the brake fails, the braking torque of the brake circuit shown in the curve 31 is always smaller than the unbalanced torque, the speed of the PM motor cannot be controlled, and the speed of the PM motor becomes faster and faster, and the sliding speed of the elevator cannot be limited within the specified range.

[0034] The present invention is further described below by blocking the driving signals of the controllable switch tubes T1, T2, T3 of the upper bridge arm and controlling the opening and closing of the controllable switch tubes T4, T5, T6 of the lower bridge arm (i.e., PWM control). The principle is also applicable to blocking the driving signals of the controllable switch tubes T4, T5, T6 of the lower bridge arm and controlling the opening and closing of the controllable switch tubes T1, T2, T3 of the upper bridge arm. That is, it prevents Figure 1 The bus capacitors C1 and C2 of the intermediate circuit (as well as the grid supply) provide the energy for driving the PM motor. In this case, the PM motor cannot generate torque except in conjunction with braking.

[0035] Embodiment 1:

[0036] refer to Figure 1 Only when the phase current of the PM motor is negative (the phase current of the PM motor is positive when it flows to the PM motor), that is, when the phase current flows from the PM motor to the inverter, the controllable switch tubes T4, T5, and T6 of the lower bridge arm will be controlled by the phase drive signal of the corresponding phase current; when the phase current is positive, even if the phase drive signal corresponding to the phase current is valid, T4, T5, and T6 will not be turned on, and their current will flow to the motor through the diodes D4, D5, and D6 connected in anti-parallel to T4, T5, and T6. In order to simplify the control, T4, T5, and T6 can be controlled at the same time, that is, the duty (the duty of PWM control is the proportion of the conduction time to the cycle) of the drive signal of T4, T5, and T6 is consistent, and such control logic does not require a separate inference about the flow direction of the motor current.

[0037] like Figure 4As shown, an electrical braking method for a PM motor is provided, and the method is applied to an elevator control system. The method includes:

[0038] Step 410, when electrical braking is required, such as when the elevator stops, or when brake failure of the PM motor is detected.

[0039] Step 420: determine whether the speed n of the PM motor is less than or equal to a threshold value n1. The threshold value n1 can generally be less than or equal to the speed corresponding to the maximum star-locking torque during star-locking braking.

[0040] Step 430, when the speed n of the PM motor is less than or equal to the threshold value n1, the Duty is equal to 100%, that is, T4, T5, and T6 are all turned on, and the conventional technology of star-lock braking is performed.

[0041] Step 440: When the speed n of the PM motor is greater than the threshold value n1, the Duty is inversely proportional to the speed of the PM motor, that is, the greater the speed of the PM motor, the smaller the Duty.

[0042] like Figure 3 As shown, curve 31 represents the braking curve when the Duty is equal to 100%. As the Duty decreases, the braking curve moves to the right, as shown by curves 32, 33, 34, and 35, but the maximum braking torque is not affected.

[0043] When the current of a phase of the inverter is negative and the switch tube of the corresponding lower bridge arm is turned on, it is in the star-locking braking state. The back electromotive force of the PM motor stores energy in the energy storage element (inductor) of the motor. When the switch tube of the lower bridge arm is turned off, the energy of the energy storage element is released into the intermediate circuit, causing the voltage of the intermediate circuit to increase. Figure 5 The drive system of the permanent magnet synchronous motor with a brake switch tube Sb and a brake resistor Rb in the intermediate circuit can consume the excess energy as heat through the brake resistor Rb and the brake switch tube Sb. Generally, hysteresis voltage control is adopted, that is, when the bus voltage Ud is greater than the threshold value A, the brake switch tube Sb is turned on, and when it is less than the threshold value B, the brake switch tube Sb is turned off. The threshold values ​​A and B are related to the grid voltage, the bus capacitor C1 / C2 and the withstand voltage of the brake switch tube Sb, that is, it can prevent the brake resistor Rb from being overloaded due to the conduction of the brake switch tube Sb when the normal grid voltage fluctuates, and also prevent the bus capacitor C1 / C2 and the brake switch tube Sb from being over-voltage. For example, in a three-phase 380V system, the threshold value A can be 640V and the threshold value B can be 620V.

[0044] Furthermore, in step 440, the Duty of the driving signals of T4, T5, and T6 is proportional to the bus voltage Ud. The higher the bus voltage is, the greater the Duty is.

[0045] Furthermore, in step 440, the Duty of the drive signals of T4, T5, and T6 is inversely proportional to the back potential coefficient Ke of the PM motor (the rated back potential of the PM motor divided by the rated speed), that is, the larger the back potential coefficient Ke is, the smaller the Duty is.

[0046] Furthermore, in step 440, Duty adopts (1-(n-n1)*Ke*k / Ud)*100%, where n is the speed of the PM motor, n1 is the switching threshold, and k is a coefficient, generally 0≤k<4, which is related to the electrical parameters of the PM motor and the bus voltage. The optimal value can be obtained through experiments or simulations, so that the braking torque at each speed is close to the maximum braking torque. When electrical braking is required for high-speed emergency stop, a suitable voltage is output through the Duty, the current is controlled, and the braking torque is close to the maximum braking torque, so that the elevator decelerates, so that the braking working point is at Figure 3 The maximum braking torque of the curves 35, 34, 33, 32, and 31 shown in FIG. If the brake fails, the elevator finally stably runs at the intersection with the curve 31 according to the unbalanced load torque 36. If the brake does not fail, the braking torque and braking current at high speed can be reduced by selecting a suitable k value to prevent the deceleration of the light load downward or the deceleration of the heavy load upward from exceeding the requirement.

[0047] Furthermore, steps 420, 430, and 440 can also be simplified to Duty using (1-n*Ke*k / Ud+OFFSET)*100%. Since Duty is the ratio of the conduction time of the drive signals of T4, T5, and T6 to the cycle, the range of Duty is limited to [0,100%]. At this time, OFFSET is greater than or equal to 0, so that Duty is 100% below a certain speed. Above this speed, Duty is inversely proportional to the speed n of the PM motor and the PM motor reverse potential coefficient Ke, and is proportional to the bus voltage Ud.

[0048] If the brake fails after the elevator stops normally, because the speed n of the PM motor is lower than the switching threshold n1, the duty is 100%, and its braking curve is shown as curve 31. The elevator finally runs stably at the intersection with curve 31 according to the unbalanced load torque 36.

[0049] Embodiment 2:

[0050] Usually the brake fails and the star-locking brake is started. Because the steady-state speed of the star-locking brake is relatively small, generally less than 0.3m / s, for high-rise elevators, they will be in the star-locking brake state for a long time, so too much heat will accumulate in the inverter and PM motor. If the heat cannot be dissipated in time, there will be a risk of overheating.

[0051] like Figure 3As shown, the present invention can start electric braking when the brake fails, and when the unbalanced load torque 36 finally stably runs at the intersection with the curve 31 for a period of time, and the elevator car door is closed, by reducing the Duty, the elevator speed can be increased, and by increasing the Duty, the elevator speed can be reduced, so that the elevator can work at the intersection of the unbalanced load torque 36 and the curves 32, 33, 34, 35, etc. That is, by adjusting the Duty size, the speed of electric braking can be adjusted, thereby shortening the time of electric braking and eliminating the risk of overheating.

[0052] Furthermore, if the elevator stops suddenly at high speed and the brake fails, the electrical braking is started. When the elevator car door is closed, the method of Example 1 can be used to first brake to a certain speed with the maximum braking torque, and then fix the Duty. After the unbalanced load torque 36 finally stably runs at the intersection of the braking curve corresponding to the Duty for a period of time, the Duty size is adjusted and the speed of the electrical braking is adjusted, thereby shortening the electrical braking time and eliminating the risk of overheating.

Claims

1. A method for controlling the braking torque of a permanent magnet synchronous motor drive system, characterized in that: When electrical braking is required, under the premise that the switch tube of the upper bridge arm of the inverter in the permanent magnet synchronous motor drive system is blocked, the switch tube of the lower bridge arm is opened and closed, that is, PWM control, or under the premise that the switch tube of the lower bridge arm of the inverter is blocked, the switch tube of the upper bridge arm is PWM controlled, and the Duty of PWM control changes with the speed of the PM motor to control the braking torque of the permanent magnet synchronous motor drive system.

2. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 1, characterized in that: When only the phase current of the permanent magnet synchronous motor flows to the inverter, the corresponding lower bridge arm switch tube performs PWM control, or when only the phase current of the permanent magnet synchronous motor flows out of the inverter, the corresponding upper bridge arm switch tube performs PWM control.

3. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 1, characterized in that: The lower bridge arm switch tubes of the inverter are simultaneously PWM controlled, or the upper bridge arm switch tubes of the inverter are simultaneously PWM controlled.

4. A method for controlling the braking torque of a permanent magnet synchronous motor drive system according to any one of claims 1 to 3, characterized in that: The Duty of PWM control is inversely proportional to the speed n of the PM motor. The higher the speed, the smaller the Duty; or the Duty of PWM control is proportional to the bus voltage. The higher the bus voltage Ud, the larger the Duty; or the Duty of PWM control is inversely proportional to the reverse electromotive force coefficient Ke of the PM motor. That is, the larger the reverse electromotive force coefficient Ke, the smaller the Duty. The reverse electromotive force coefficient Ke is the rated reverse electromotive force of the PM motor phase divided by the rated speed.

5. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 4, characterized in that: When the speed n of the PM motor is less than or equal to the threshold n1, the Duty of the PWM control is equal to 100%.

6. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 5, characterized in that: When the speed n of the PM motor is greater than or equal to the threshold n1, the Duty of PWM control adopts (1-(n-n1)*Ke*k / Ud)*100%, where n is the speed of the PM motor, n1 is the switching threshold, Ke is the opposite potential coefficient, Ud is the bus voltage, k is the coefficient, generally 0≤k<4, which is related to the electrical parameters of the PM motor and the bus voltage. The optimal value can be obtained through experiments or simulations, so that the braking torque at each speed is close to the maximum braking torque.

7. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 4, characterized in that: The Duty of PWM control is (1-n*Ke*k / Ud+OFFSET)*100%, OFFSET>0.

8. A method for controlling the braking torque of a permanent magnet synchronous motor drive system according to any one of claims 1 to 3, characterized in that: When the brake fails and electrical braking is started, after a period of stable operation according to the unbalanced load torque, and the elevator car door is closed, the elevator speed can be increased by reducing the duty of PWM control, and the elevator speed can be reduced by increasing the duty of PWM control. The reasonable electrical braking speed is controlled, thereby shortening the electrical braking time and eliminating the risk of overheating.

9. The method for controlling the braking torque of a permanent magnet synchronous motor drive system according to claim 8, characterized in that: The elevator stops suddenly at high speed and the brake fails, so the electric braking is started. When the elevator car door is closed, the PWM controlled Duty is first (1-(n-n1)*Ke*k / Ud)*100%. After braking to a certain speed with the maximum braking torque, the PWM controlled Duty is fixed. After the unbalanced load torque finally stabilizes and runs at the intersection of the braking curve corresponding to the Duty for a period of time, the Duty size is adjusted, and the speed of the electric braking is adjusted, thereby shortening the electric braking time and eliminating the risk of overheating.

10. A method for controlling the braking torque of a permanent magnet synchronous motor drive system according to any one of claims 1 to 3, characterized in that: The permanent magnet synchronous motor drive system is a permanent magnet synchronous motor drive system with a brake unit and a brake resistor in the intermediate circuit.

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