introduction
At present, the paper machine drive control basically adopts the AC variable frequency drive control system, and the inverters on the market have many brands and different functions. Faced with such a large number of inverter products, how to choose the inverter and the corresponding control system has become a problem for all manufacturers. The following briefly introduces the basic requirements of the paper machine for the inverter and how to select the inverter.
1. Paper machine transmission characteristics and mechanical characteristics of the inverter
The paper machine drive is a constant torque load, so the motor is required to use constant torque speed regulation. The inverter is required to work in the constant torque speed control state, so the constant torque mechanical load inverter should be selected.
Some companies use different types of frequency converters for constant torque speed regulation and constant power speed regulation, such as Siemens ECO fan, water pump control inverter and MDV, 6SE70 constant torque inverter. Some companies use the same type of frequency converter for constant torque speed regulation and constant power speed regulation. Different motors are used for different inverters with different control modes. The internal parameters of the inverter are different. Such as ABB ACS400, ACS600 series inverter. For example, the inverter with model ACS401-0030 is equipped with 22Kw in constant torque speed control and 30Kw in constant power control of fan and water pump. Therefore, the user should understand the mechanical characteristics of the inverter.
2. Influence of frequency resolution on the transmission performance of paper machine
Frequency resolution is an important indicator of the frequency converter. The frequency resolution includes the frequency given resolution and the frequency control resolution.
1. Frequency given resolution refers to the resolution of the input channel of the inverter to the input signal, generally refers to the number of bits of the analog input channel AD. Generally, the accuracy of the communication channel is much larger than that of the analog channel. Therefore, if the analog channel is satisfied, the communication channel is absolutely satisfactory.
Generally, the control precision of the paper machine for the speed control of the drive is 0.1%. For a certain paper machine, from the perspective of frequency resolution, if the minimum control speed can meet the given control accuracy, there is no problem in high-speed operation. . Below we give an example:
Set the paper machine at low speed papermaking, the inverter running frequency is 10Hz, the required precision is: 10*0.1%=0.01Hz
Therefore, the frequency resolution of the inverter must be less than 0.01 Hz.
If the paper machine is used for high-speed papermaking and the inverter frequency is 50Hz, the required accuracy is:
50*0.1%=0.05Hz
Therefore, the frequency resolution of the inverter must be less than 0.05Hz.
Therefore, when choosing the transmission system, we pay attention to whether the inverter can meet the control accuracy requirements when the paper machine is running at low speed. You don't have to worry about the high speed given accuracy. The given accuracy can also be adjusted by the inverter parameters, but the disadvantage is that the parameters need to be re-adjusted when changing the working state.
For example, a sizing pump inverter used for headbox control, its operating frequency is generally between 30~50Hz, the control adopts SR73A regulator, but the headbox liquid level cannot be stabilized, the sizing amount is suddenly large and small, the inverter always The adjustment cannot be stabilized. Cause The inverter setting resolution is not enough. Given a big and small, 1 mA corresponds to 2.5 Hz. We can adjust the parameters, the minimum setting of the inverter is 30Hz, that is, given 0mA corresponds to 30Hz, given 20mA corresponds to 50Hz, so 1mA corresponds to 1Hz, and the given frequency resolution is increased by 2.5 times.
2. The frequency control resolution refers to the minimum resolution of the output of the inverter. Generally speaking, the inverter is a stepless speed regulation. The frequency converter has its minimum resolution, the smallest change in the frequency of each speed regulation. The minimum resolution of the frequency converter is now generally 0.01 Hz. When selecting the inverter, the user should pay attention to whether the frequency resolution can meet the needs of the equipment.
3. Influence of the inverter with the slip compensation on the steady-state accuracy of the system
AC asynchronous motors all have slip frequency. The output torque and power of the motor are proportional to the slip frequency. In order to improve the speed control accuracy, the inverter adopts slip compensation for motor control.
According to the mechanical characteristics of the asynchronous motor, as the load torque increases, the slip frequency increases and the rotor produces a speed drop. Slip compensation is to use the stator to change the synchronous frequency according to the load to compensate the rotor for the speed drop of the load and eliminate the speed difference, so as to ensure the rotor speed is kept constant.
The slip compensation of the inverter is based on the mechanical characteristics of the motor. The inverter requires input of the nameplate parameters of the motor. The mathematical model of the control of the motor is determined according to the nameplate parameters. The amount of slip compensation should be:
Since the motor has different impedances of the motor in the variable frequency control speed regulation process, the slip compensation is sometimes under-compensated and sometimes over-compensated. Therefore, it will have a certain impact on the steady state and dynamic accuracy of the system.
Therefore, the reasonable use of slip compensation in the process of using the inverter will eliminate the static difference and improve the steady-state accuracy of the system. Adjusting the slip compensation can obtain special mechanical characteristics, which is very important in the commissioning operation of the paper machine and corrugating machine.
4, the basic requirements of the inverter function
The requirements of the paper machine for the function of the inverter are related to the selected upper control system. Different control methods have different requirements for the function of the inverter. The paper machine generally requires control operations such as start-stop, crawl/run, and fine-tuning of the inverter on the console; it can display functions such as partial line speed and motor current; it can display functions such as the working status of the inverter.
The functional requirements for the frequency converter in different control modes are listed below.
1. Analog control and speed chain controller control:
The inverter is required to have more than two analog input and output functions; two or more digital input functions; two digital output functions.
Two analog inputs are used for speed reference and feedback input, and a third analog input is used for speed additional reference control. The two analog outputs are used to display the partial line speed, and the other is used to display the inverter output current. Two or more digital inputs are used for start and stop control and creep/run transition control. Two digital outputs are used to display the operating status of the drive.
2, electric potentiometer control mode:
The inverter is required to have two analog output functions; four digital input functions; two digital output functions.
The two analog outputs are used to display the partial line speed, and the other is used to display the inverter output current. Two digital inputs are used for start-stop control and creep/run conversion control; two digital quantities are used for speed fine-tuning control. Two digital outputs are used to display the operating status of the drive.
3, PLC communication control method
PLC communication control mode has two control modes according to the operation mode, one is the traditional button operation mode, and the other is the touch screen operation mode. These two control methods require the inverter to have communication control functions and corresponding communication adapters. For example, the Modbus network requires a Modbus communication board; the Profibus-DP bus requires a Profibus-DP communication board.
(1), the button control mode requires the inverter to have two analog output functions; two digital input functions; two digital output functions. The two analog outputs are used to display the partial line speed, and the other is used to display the inverter output current. Two digital inputs are used for start-stop control and creep/run conversion control; two digital outputs are used to display the operating status of the drive.
(2) The touch screen control mode requires the inverter to have a digital input for the emergency stop function. Other display controls are performed on the touch screen.
5 Conclusion
The special paper machine can operate normally and can make the products meet the requirements of customers. It is not only the process that can be decided on one hand, but also the electric drive control system plays an important role. Therefore, how to choose a good transmission system (that is, choose a core device of this system - frequency converter) is particularly important. The author is the author's income for many years of paper-making transmission system design work for your reference.
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