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Now the transmission control of paper machine basically adopts AC frequency conversion transmission control system, and there are many brands of frequency converters on the market with different functions. Faced with so many inverter products, how to choose the inverter and the corresponding control system has become a difficult problem for each manufacturer. The following briefly introduces the basic requirements of the paper machine for the inverter and how to choose the inverter.

1. The transmission characteristics of the paper machine and the mechanical characteristics of the frequency converter

The drive of the paper machine is a constant torque load, so the motor is required to adopt constant torque speed regulation. The frequency converter is required to work in the state of constant torque speed regulation control, so a constant torque mechanical load frequency converter should be selected.
Some companies use different types of inverters for constant torque speed regulation and constant power speed regulation, such as Siemens' ECO fan and water pump control inverters and MDV and 6SE70 constant torque inverters. Some companies use the same type of inverter for constant torque speed regulation and constant power speed regulation. For the same inverter with different control methods, the motors connected to it are different, and the internal parameters of the inverter are set differently. Such as ABB company ACS400, ACS600 series inverters. For example, the inverter model ACS401-0030 is connected to a 22Kw motor for constant torque speed control, and a 30Kw motor for fan and water pump constant power control. So users should understand the matching of mechanical characteristics of the frequency converter.

2. The influence of frequency resolution on the transmission performance of the paper machine

Frequency resolution is an important index to measure the frequency converter. Frequency resolution includes frequency given resolution and frequency control resolution.

1. Frequency given resolution refers to the resolution of the given channel of the frequency converter to the input signal, and generally refers to the number of digits of the analog input channel AD. For the communication channel, the given accuracy is generally much higher than that of the analog channel, so if the analog given channel can meet the requirements, the communication channel is absolutely satisfied.
Generally, the control accuracy of the transmission speed regulation of a paper machine is required to be 0.1%. For a certain paper machine, from the perspective of frequency resolution, if it can meet the given control accuracy at the lowest speed, then there is no problem in high-speed operation. . Let us illustrate with an example:
Assuming that this paper machine is making paper at low speed, and the frequency converter is running at 10Hz, the given accuracy is required to be: 10*0.1%=0.01Hz
Therefore, it is required that the frequency resolution of the inverter must be less than 0.01Hz.
Assuming that the paper machine is making paper at high speed and the frequency converter is running at 50Hz, the given accuracy is required to be:
50*0.1%=0.05Hz
Therefore, it is required that the frequency resolution of the inverter must be less than 0.05Hz.
Therefore, when selecting the drive system, we pay attention to whether the frequency converter can meet the requirements of control accuracy when the paper machine is at low speed. Without worrying about the given accuracy at high speed. The given accuracy can also be adjusted through the inverter parameters, but the disadvantage is that the parameters need to be readjusted when changing the working state.
For example, a sizing pump inverter used for headbox control, its operating frequency is generally between 30~50Hz, and the control adopts SR73A regulator, but the liquid level of the headbox cannot be stabilized, the amount of sizing varies, and the inverter always The adjustment cannot be stabilized. Cause Inverter setting resolution is not enough. The given is big and small, 1mA corresponds to 2.5Hz. We can adjust the parameters. The minimum setting of the frequency converter is 30Hz, that is, a given 0mA corresponds to 30Hz, a given 20mA corresponds to 50Hz, so that 1mA corresponds to 1Hz, and the resolution of the given frequency is increased by 2.5 times.

2. Frequency control resolution refers to the minimum resolution of the output of the frequency converter. Usually, it is relative to say that the frequency converter is stepless speed regulation. The frequency converter has its minimum resolution, and the minimum change of each speed regulation frequency. Now the minimum resolution of the frequency converter is generally 0.01Hz. Users should pay attention to whether the frequency resolution can meet the needs of the equipment when selecting the frequency converter.

3. The influence of the inverter with slip compensation on the steady-state accuracy of the system

All AC asynchronous motors have a slip frequency, and the output torque and power of the motor are proportional to the slip frequency. In order to improve the speed control accuracy, the frequency converter adopts slip compensation for the motor control.
According to the mechanical characteristics of the asynchronous motor, when the load torque increases, the slip frequency increases, and the rotor speed drops. Slip compensation is to change the synchronous frequency of the stator according to the load to compensate the speed drop of the rotor due to the load and eliminate the static difference of speed regulation, so as to ensure that the speed of the rotor remains constant.
The slip compensation of the frequency converter is carried out according to the mechanical characteristics of the motor. The frequency converter requires input of the motor nameplate parameters, and the control mathematical model of the motor is determined according to the nameplate parameters. The slip compensation amount should be:
Because the impedance of motors from different companies is different during the process of motor frequency control and speed regulation, slip compensation is sometimes undercompensation and sometimes overcompensation. Therefore, it will have a certain influence on the steady state and dynamic accuracy of the system.

Therefore, the reasonable use of slip compensation in the process of using the frequency converter will eliminate the static error 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 and operation of board machines and corrugating machines.


4. Basic requirements for inverter functions
The requirements of the paper machine for the functions of the frequency converter are related to the selected upper control system, and different control methods have different requirements for the functions of the frequency converter. Paper machines generally require that the inverter be able to perform control operations such as start and stop, crawling/running, and speed fine-tuning on the console; it can display functions such as branch line speed and motor current; it can display functions such as the working status of the inverter.
The following lists the functional requirements for the inverter under different control modes.

1. Analog control and speed chain controller control:
The inverter is required to have more than two analog input and output functions; more than two digital input functions; two digital output functions.
Two analog inputs are used for speed setting and feedback input, and the third analog input is used for speed additional given control. One of the two analog outputs is used to display the line speed of the branch, and the other is used to display the output current of the inverter. More than two digital inputs are used for start-stop control and crawl/run conversion control. Two digital outputs are used to display the working status of the inverter.

2. Motorized potentiometer control mode:
The inverter is required to have two analog output functions; more than four digital input functions; two digital output functions.
There are two analog outputs, one is used to display the branch line speed, and the other is used to display the output current of the inverter. Two-way digital input is used for start-stop control and crawling/running conversion control; two-way digital quantity is used for speed fine-tuning control. Two digital outputs are used to display the working status of the inverter.

3. PLC communication control mode
The PLC communication control method is divided into two control methods according to the operation method, one is the traditional button operation method, and the other is the touch screen operation method. These two control methods require the inverter to have a communication control function and a corresponding communication adapter. For example, the Modbus network requires a Modbus communication board; the Profibus-DP bus requires a Profibus-DP communication board.
(1) The button control method requires the inverter to have two analog output functions, two digital input functions, and two digital output functions. One of the two analog outputs is used to display the line speed of the branch, and the other is used to display the output current of the inverter. Two digital inputs are used for start-stop control and crawling/running conversion control; two digital outputs are used to display the working status of the inverter.
(2) The touch screen control method requires the inverter to have a digital input for emergency stop function. All other display controls are performed on the touch screen.

5 Conclusion

Whether the special paper machine can operate normally and the products produced can meet the requirements of customers is not only determined by the process, but also by the electric drive control system. Therefore, how to choose a good transmission system (that is, to choose a core device of this system - frequency converter) is particularly important.