A variable frequency control method based on encoderless low-frequency speed

Abstract

The present invention relates to a variable frequency control method based on encoderless speed and low frequency, comprising taking moment of inertia compensation as an additional given value of torque, setting a static torque set value within the range of 0 to 3 Hz of encoderless vector control, and simultaneously modifying the switching point between a current control AMP mode and an encoderless vector control SLVC mode, so that a potential load operates in the encoderless vector control SLVC mode when the potential load is stopped. Taking moment of inertia compensation as an additional given value of torque not only obtains a stable additional given value and increases the lifting torque of the potential load, but also reduces dynamic overshoot, meets the requirement of low-frequency gun lifting, avoids the accident of gun slipping due to insufficient torque, realizes stable torque output at low frequency, can achieve accurate parking when stopping, ensures the stability of the potential load operation, ensures the smooth progress of production, and has broad promotion value in the field of AC control.

Description

Technical Field

[0001] The present invention relates to the technical field of converters, and in particular to a frequency conversion control method based on encoder-free low-frequency speed. Background Art

[0002] The oxygen lance system in a steel mill is a typical potential load, carrying multiple gases and cooling water during operation, and is extremely heavy. The oxygen lance's operating speed is a gradual, continuous curve. During startup and shutdown, it inevitably undergoes a low-frequency operation phase, and the oxygen lance system operates in encoderless vector control mode. As we all know, the electromagnetic torque output by the inverter at low frequencies is relatively low, which deteriorates dynamic characteristics, increases starting current, and sometimes causes stall faults. When the torque is insufficient to overcome the gravity and friction of the oxygen lance, the lance can slip and become unable to be lifted. This can cause the oxygen lance nozzle to burn, cooling water to enter the converter, and even cause serious explosions. The challenge we faced was how to control the low-frequency characteristics of the inverter.

[0003] The oxygen lance transmission control system in steel mills mostly uses vector control with encoders. For vector control systems without encoders, the frequency converter runs at low frequency in V / F control mode. The existing technology mostly adopts the method of voltage boost to improve starting torque. In the transcription oxygen lance transmission control system, since the five-encoder control low frequency needs to consider the control accuracy and stability issues, the existing technology mostly uses vector control with encoder feedback. Summary of the Invention

[0004] The present invention provides a variable frequency control method based on encoderless speed and low frequency, which realizes stable torque output at low frequency, can achieve accurate parking when stopping, ensures the stability of potential load operation, ensures smooth production, and has broad promotion value in the field of AC control.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A variable frequency control method based on encoderless low-frequency speed includes the following contents:

[0007] 1) Take the moment of inertia compensation as an additional given value of the torque;

[0008] 2) Set the static torque setting value in the low-frequency 0-3Hz range of encoderless vector control, and modify the switching point between the current control AMP mode and the encoderless vector control SLVC mode at the same time, so that the potential load is parked in the encoderless vector control SLVC mode.

[0009] Furthermore, the moment of inertia calculation formula is as follows:

[0010] Me-Mz=J×dω / dt (1)

[0011] Among them, Me is the variable frequency output electromagnetic torque, unit is Nm; Mz is the load torque, unit is Nm; J is the total moment of inertia, unit is kg.m 2 ;ω is the mechanical angular velocity, unit radian / s;ω=2πn / 60;n is the motor speed, unit Rpm;

[0012] Limit the output torque limit value to 5% to 20% of the rated torque, record the time required for the motor speed to change from zero to the rated speed, calculate the moment of inertia J1, change the output torque limit value, calculate the corresponding moment of inertia J1 to Jn, calculate the average moment of inertia J, and calculate the additional torque under different speeds and accelerations according to formula (1) as the additional torque setting.

[0013] Furthermore, the encoderless vector control SLVC mode for stopping the potential load specifically includes the following contents:

[0014] (1) Set the motor model MotMod characteristic to current control AMP mode when starting at a low frequency of 0 to 3 Hz;

[0015] (2) Specify the frequency threshold for switching from current control AMP mode to encoderless vector control SLVC mode;

[0016] (3) Specify the frequency hysteresis for switching from SLVC to AMP mode.

[0017] Furthermore, it also includes adding a static torque Td to the input end of the magnetic field current Id regulator, Td = the input end of the maximum load torque. When the static torque Td = 0, only the magnetic field current Id flows into the motor. When Td = 100%, the motor rated current Ie flows into the motor.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1) Taking the moment of inertia compensation as an additional reference for torque can not only obtain a stable additional reference and increase the lifting torque of potential loads, but also reduce dynamic overshoot;

[0020] 2) The low-speed range of encoderless vector control sets the static torque set value, so that the potential load is parked in SLVC mode, ensuring the accurate parking of the potential load;

[0021] 3) The two control methods adopted for low-frequency frequency converters have very good effects on electronic control systems that require high torque, such as oxygen guns. They can not only improve the starting torque to meet the requirements of low-frequency gun lifting, without the accident of gun slipping due to insufficient torque, but also suppress system overshoot, thereby improving the dynamic performance of system startup and achieving stable torque output at low frequency. Through speed control, the system can be accurately positioned and can be accurately stopped when stopping, ensuring the stability of potential load operation and the smooth progress of production. It has broad promotion value in the field of AC control. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the present invention using rotational inertia compensation as an additional given value of torque.

[0023] Figure 2 It is a curve diagram of the static torque and mode switching of the present invention. DETAILED DESCRIPTION

[0024] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0025] The present invention provides a variable frequency control method based on encoder-free low-frequency speed, comprising the following contents:

[0026] 1) For torque current Iq: take the moment of inertia compensation as an additional given value of the torque;

[0027] See Figure 1 The traditional approach to moment of inertia compensation is to use it as a feedforward control for the speed regulator, that is, it should be added to the front end of the n regulator. However, due to network characteristics, the speed feedforward control has a pulse trend in the dynamic process, which makes the torque in the dynamic process pulsating. The instability of the torque will have an impact on the mechanical system. We abandon the traditional approach and use the measured moment of inertia to calculate the torque compensation through the PLC program. As an additional torque setting, it is added after the n regulator and before the torque limiter. M-suppl is the torque compensation calculated by the first-level PLC program. This not only obtains a stable additional setting and increases the lifting torque of the oxygen gun, but also reduces dynamic overshoot.

[0028] According to the formula Me-Mz=J×dω / dt(2)

[0029] Among them, Me is the variable frequency output electromagnetic torque, unit is Nm; Mz is the load torque, unit is Nm; J is the total moment of inertia, unit is kg.m 2 ;ω is the mechanical angular velocity, unit radian / s;ω=2πn / 60;n is the motor speed, unit Rpm;

[0030] Limit the output torque limit value to 5% to 20% of the rated torque, record the time required for the motor speed to change from zero to the rated speed, calculate the moment of inertia J1, change the output torque limit value, calculate the corresponding moment of inertia J1 to Jn, calculate the average moment of inertia J, and calculate the additional torque under different speeds and accelerations according to formula (2) as the additional torque setting.

[0031] 2) For the magnetic field current Id: in the low frequency range of 0-3Hz for encoderless vector control, set the static torque setting value, and modify the switching point between the current control AMP mode and the encoderless vector control SLVC mode, so that the oxygen gun is in the encoderless vector control SLVC mode when it stops. See the attached Figure 2 , below the AMP / SLVC switching point, the AMP mode is used, that is, the current mode sets the static torque current, increases the starting torque, and ensures that the oxygen lance does not fall after the gate is opened; above the AMP / SLVC switching point, the SLVC mode is used, that is, the speed control mode, to accurately control the operating speed of the oxygen lance and accurately stop;

[0032] Transmission systems with encoder vector control have encoder feedback in the low-frequency range, so vector control with encoder feedback can be used to obtain stable and reliable torque and speed control. However, for systems without encoder feedback, the speed and torque feedback of the system are calculated by the motor model of the control system. The characteristic of this encoderless vector control is that the low-frequency feedback calculation accuracy is low, so it is usually switched to encoderless vector control SLVC control at low frequencies. However, for loads such as oxygen guns that require control accuracy at low frequencies, it is necessary to select a suitable low-frequency operating speed and make it work in the vector control state.

[0033] Normally, in the low-frequency 0-3Hz range, the encoderless variable frequency control system uses the current control AMP mode (i.e., current control mode) to reduce variable frequency power consumption. To ensure the torque at the start of the oxygen gun, a static torque Td must be added to the input of the magnetic field current Id regulator. Td = maximum load torque. When Td = 0, only the magnetic field current Id flows into the motor. When Td = 100, the rated current Ie flows into the motor.

[0034] The current control AMP mode is similar to open-loop control and cannot accurately calculate the actual speed of the motor. To ensure accurate parking of the oxygen gun during parking, the motor model MotMod must switch from AMP mode to encoderless vector control SLVC mode. The switching point between the two modes must be adjusted to ensure the duration of static torque during startup and to ensure that the encoderless vector control SLVC mode is entered as quickly as possible during parking.

[0035] The oxygen gun is stopped in the encoderless vector control SLVC mode, which specifically includes the following contents:

[0036] (1) Set the motor model MotMod characteristic to current control AMP mode when starting at a low frequency of 0 to 3 Hz;

[0037] (2) Specify the frequency threshold for switching from current control AMP mode to encoderless vector control SLVC mode;

[0038] (3) Specify the frequency hysteresis for switching from SLVC to AMP mode.

[0039] After taking the above measures, the starting torque is guaranteed from the two aspects of torque current Iq and magnetic field current Id, avoiding the phenomenon of oxygen gun slipping. At the same time, when stopping to low speed, the motor is in the encoderless vector control SLVC speed mode, which provides a guarantee for the accurate parking of the oxygen gun and increases the stability of the system.

[0040] The above embodiments are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the above embodiments. The methods used in the above embodiments are conventional methods unless otherwise specified.

Claims

1. A variable frequency control method based on encoderless low-frequency speed, characterized in that: Includes the following: 1) Take the moment of inertia compensation as an additional given value of the torque; The calculation formula for the moment of inertia is as follows: Me-Mz=J×dω / dt (1) Among them, Me is the variable frequency output electromagnetic torque, unit is N·m; Mz is the load torque, unit is N·m; J is the total moment of inertia, unit is kg·m 2 ;ω is the mechanical angular velocity, unit radian / s;ω=2πn / 60;n is the motor speed, unit Rpm; Limit the output torque limit value to 5% to 20% of the rated torque, record the time required for the motor speed to change from zero to the rated speed, calculate the moment of inertia J1, change the output torque limit value, calculate the corresponding moment of inertia J1 to Jn, calculate the average moment of inertia J, and calculate the additional torque at different speeds and accelerations according to formula (1) as the additional torque setting; 2) Set the static torque setting value in the low-frequency 0-3Hz range of encoderless vector control, and modify the switching point between the current control AMP mode and the encoderless vector control SLVC mode at the same time, so that the potential load is parked in the encoderless vector control SLVC mode.

2. The frequency conversion control method based on encoderless low-frequency speed according to claim 1, characterized in that: The encoderless vector control SLVC mode for stopping a potential load specifically includes the following: (1) Set the motor model MotMod characteristic to current control AMP mode when starting at a low frequency of 0 to 3 Hz; (2) Specify the frequency threshold for switching from current control AMP mode to encoderless vector control SLVC mode; (3) Specify the frequency hysteresis for switching from SLVC to AMP mode.

3. The frequency conversion control method based on encoderless low-frequency speed according to claim 1, characterized in that: It also includes adding a static torque Td to the input end of the magnetic field current Id regulator, where Td = the input end of the maximum load torque. When the static torque Td = 0, only the magnetic field current Id flows into the motor. When Td = 100%, the motor rated current Ie flows into the motor.

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