introduction
Currently, almost all papermaking machine drive control systems utilize AC variable frequency drive control systems. However, the market offers a wide variety of variable frequency drive (VFD) brands with diverse functions. Faced with such a multitude of VFD products, selecting the right VFD and corresponding control system has become a challenge for manufacturers. Below is a brief introduction to the basic requirements of papermaking machines for VFDs, and how to choose the right VFD.
1. Paper machine drive characteristics and frequency converter mechanical characteristics
Paper machine drives are constant torque loads, so the motor must use constant torque speed regulation. The frequency converter must operate in constant torque speed regulation control mode, therefore a constant torque mechanical load frequency converter should be selected.
Some companies use different models of frequency converters for constant torque speed control and constant power speed control, such as Siemens' ECO frequency converters for fan and pump control, and MDV and 6SE70 constant torque frequency converters. Other companies use the same model of frequency converter for both constant torque and constant power speed control, but connect different motors to the same frequency converter using different control methods, and the internal parameter settings of the frequency converter differ. For example, ABB's ACS400 and ACS600 series frequency converters. For instance, the ACS401-0030 frequency converter is used with a 22kW motor for constant torque speed control, but with a 30kW motor for constant power control of fans and pumps. Therefore, users should understand the mechanical characteristics matching of the frequency converter.
2. The impact of frequency resolution on paper machine drive performance
Frequency resolution is an important indicator for evaluating frequency inverters. Frequency resolution includes frequency setpoint resolution and frequency control resolution.
1. Frequency setpoint resolution refers to the resolution of the input signal for the setpoint channel of the frequency converter, generally referring to the number of bits in the analog input channel's AD converter. The setpoint accuracy for communication channels is generally much higher than that for analog channels, so if the analog setpoint channel meets the requirements, the communication channel will absolutely meet them.
Generally, paper machines require a speed control accuracy of 0.1% . From a frequency resolution perspective, if the given control accuracy can be met at the lowest speed, then high-speed operation will not be a problem. Let's illustrate this with an example:
Assuming this paper machine is operating at low speed with a frequency converter running at 10Hz, the required accuracy is: 10 * 0.1 % = 0.01Hz.
Therefore, the frequency resolution of the inverter must be less than 0.01Hz .
Assuming this paper machine is operating at high speed and the frequency converter is running at 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 selecting a drive system, we should pay attention to whether the frequency converter can meet the control accuracy requirements at low speeds of the paper machine, without worrying about the accuracy at high speeds. The accuracy can also be adjusted through the frequency converter parameters, but the disadvantage is that the parameters need to be readjusted when changing the operating state.
For example, a frequency converter used for headbox control of a feed pump typically operates between 30 and 50 Hz, controlled by an SR73A regulator. However, the headbox level is unstable, and the feed rate fluctuates wildly, with the frequency converter unable to maintain a stable setting. The reason is insufficient frequency converter setting resolution. The setpoint is inconsistent, with 1 mA corresponding to 2.5 Hz. We can adjust the parameters; the minimum setpoint of the frequency converter should be 30 Hz, meaning a setpoint of 0 mA corresponds to 30 Hz, and a setpoint of 20 mA corresponds to 50 Hz. This way, 1 mA corresponds to 1 Hz, improving the setpoint frequency resolution by 2.5 times.
2. Frequency control resolution refers to the minimum output resolution of the frequency inverter. The statement that a frequency inverter is infinitely variable is relative. A frequency inverter has its minimum resolution, which is the minimum frequency change for each speed adjustment. Currently, the minimum resolution of frequency inverters is generally 0.01Hz . Users should pay attention to whether the frequency resolution meets the equipment requirements when selecting a frequency inverter.
3. The impact of slip-compensated frequency converters on system steady-state accuracy
AC asynchronous motors all have a 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 frequency converter uses slip compensation for motor control.
Based on the mechanical characteristics of asynchronous motors, when the load torque increases, the slip frequency increases, and the rotor experiences a speed drop. Slip compensation utilizes the change in stator synchronous frequency according to the load to compensate for the rotor's speed drop caused by the load, eliminating steady-state error in speed regulation, thereby ensuring that the rotor speed remains constant.
The slip compensation of the frequency converter is based on the mechanical characteristics of the motor. The frequency converter requires input of the motor nameplate parameters and determines the control mathematical model of the motor based on the nameplate parameters. The slip compensation amount should be:
Because motors from different companies have different impedances during variable frequency speed control, slip compensation may be under-compensated or over-compensated at times. This can have a certain impact on the steady-state and dynamic accuracy of the system.
Therefore, proper use of slip compensation during the operation of frequency converters can eliminate steady-state error and improve the steady-state accuracy of the system. Adjusting slip compensation can obtain special mechanical characteristics, which is very important in the commissioning and operation of board paper machines and corrugated paper machines.
4. Basic requirements for frequency converter functions
The requirements of a paper machine for the functions of its frequency converter are related to the selected host control system. Different control methods have different requirements for the functions of the frequency converter. Paper machines generally require the ability to control the frequency converter on the control panel, including starting, stopping, crawling/running, and fine-tuning the speed; displaying the section linear speed, motor current, and other functions; and displaying the operating status of the frequency converter.
The following lists the functional requirements of frequency converters under different control methods.
1. Analog control and speed chain controller control:
The frequency converter is required to have at least two analog input and output channels; at least two digital input channels; and two digital output channels.
Two analog inputs are used for speed setpoint and feedback input, and a third analog input is used for additional speed setpoint control. Two analog outputs are used: one to display the partial linear speed, and the other to display the inverter output current. Two or more digital inputs are used for start-stop control and creep/run switching control. Two digital outputs are used to display the inverter's operating status.
2. Electric potentiometer control method:
The inverter is required to have two analog outputs, four or more digital inputs, and two digital outputs.
Two analog outputs are provided: one for displaying the linear velocity and the other for displaying the inverter's output current. Two digital inputs are used for start/stop control and crawl/run switching control; two digital inputs are used for speed fine-tuning control. Two digital outputs are used to display the inverter's operating status.
3. PLC communication control method
PLC communication control methods can be categorized into two types based on operation: traditional button operation and touchscreen operation. Both methods require the inverter to have communication control functionality and a corresponding communication adapter. For example, a Modbus network requires a Modbus communication board; a Profibus-DP bus requires a Profibus-DP communication board.
(1) The push-button control method requires the frequency converter 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 partial linear velocity, and the other is used to display the output current of the frequency converter. The two digital inputs are used for start/stop control and crawl/run conversion control; the two digital outputs are used to display the operating status of the frequency converter.
(2) The touch screen control method requires the frequency converter to have one digital input for emergency stop function. All other display and control are performed on the touch screen.
5. Conclusion
Whether a papermaking machine can operate normally and produce products that meet customer requirements depends not only on the manufacturing process but also crucially on its electrical drive and control system. Therefore, selecting a good drive system (i.e., choosing a core component of this system—the frequency converter) is extremely important. This article, based on the author's many years of experience in papermaking drive system design, is provided for your reference.
