1 Introduction
In the cotton spinning process, the main function of the roving frame is to draft the cotton sliver to a certain draft ratio and form it into a specific winding shape for easy storage and handling, adapting it to the feeding of the ring spinning frame. Currently, the automation system for the roving frame in the roving process mainly falls into three categories: first, using an industrial control computer, circuit boards, and frequency converters; second, a touch screen, PLC, and frequency converters; and third, a touch screen, microcontroller modules, and frequency converters. However, all three system solutions face the same problem: in the event of a power outage, multiple motors need to stop synchronously. Currently, the commonly used method is for the frequency converter to enter a braking state, generating regenerative energy to ensure the motors stop synchronously.
2 External bus capacitor energy storage synchronous braking design
2.1 Principle Design
According to general inverter principles, the DC bus capacitor is used to filter the AC component of the rectifier and stabilize the inverter's power supply voltage. For roving frames with multiple motors independently driven by inverters, the bus capacitor has the potential to provide regenerative braking excitation energy during power outages by utilizing stored electrical energy. Since the standard bus capacitor capacity is usually only sufficient for filtering and voltage regulation, it is difficult to meet the braking energy requirements during power outages. Therefore, a solution of external large-capacity bus capacitor energy storage for synchronous braking is proposed. The external capacitor will cause a huge starting current surge to the inverter's rectifier. A soft-start protection design is implemented using the roving frame's PLC, similar in principle to the internal bus capacitor connected in series with a starting protection resistor. For the bus capacitor energy storage synchronous braking design, Delta's B-series inverters have undergone customized software modifications tailored to the application characteristics of roving frames.
2. Customization of Delta B-series frequency inverter software
Delta's B-series frequency converters have undergone software modifications specifically designed for the application characteristics of roving frames.
(1) Add regenerative energy generation function selection. Parameter 06-19 is used to select the regenerative energy generation function. When 06-19=0, the frequency converter is the same as the standard frequency converter. When 06-19=1, when low voltage is detected, the frequency converter immediately closes the external terminal EF and enters the second deceleration time.
(2) Set the external terminal EF detection voltage level. 06-16 is the setting of the EF detection voltage level, as shown in Figure 1. 06-17 is the voltage detection time. The entire process is that when the inverter detects that the voltage on the DC bus is less than or equal to the level of 06-16 and the duration reaches the set time of 06-17, the inverter immediately enters the deceleration braking state. The braking time is set according to the second deceleration time 02-12.
(3) Analysis of constraints on meeting system requirements. 06-16 detection voltage level, 02-12 deceleration time, and external capacitor capacity. A higher 06-16 detection voltage level setting results in more energy retention, but too high a voltage level will not meet the operating performance requirements of the roving frame when the normal grid voltage is low; a shorter 02-12 deceleration time setting is more conducive to stopping, but too short a setting will cause mechanical vibration; a larger external capacitor capacity is more conducive to stopping, but too large a capacity will increase cost and size. Therefore, a moderate point should be chosen for these three aspects.
3. Calculation of external busbar capacity
3.1 Calculation Purpose
(1) Calculate the required external capacitor capacity to ensure that the full-load roving machine can stop synchronously when the power is lost.
(2) Design an external soft-start circuit to ensure that the power-on impact is reduced when the inverter capacitor is increased.
3.2 Calculation Steps
(1) Analysis of capacitor port voltage curve
See Figure 1 for the capacitor port voltage curve analysis. The fading of the bus voltage envelope represents the conversion and attenuation of the motor's mechanical energy during braking. The sawtooth curve of the bus voltage illustrates that intermittent braking during the braking process is achieved based on the voltage level value.
in:
— Capacitor port voltage.
— Delta B-series frequency inverter EF detection voltage level.
—The voltage across the capacitor when the energy begins to be fed back after the B-series frequency inverter enters braking mode.
— The time from the EF detection voltage to the start of energy feedback.
— The time it takes for the roving frame to stop synchronously after a power outage.
, ――― are constants in the capacitor port voltage equation.
(2) Calculate the constants
Delta's B-series frequency converters have shortened the time from EF voltage detection to entering deceleration and braking state by modifying internal software. Currently, this is tentatively assumed to be 80ms (this data is only an estimate). Setting parameter 06-16 = 495, we get 495, which is estimated to be 452. That is, when the grid voltage is AC350V , the system considers a power outage to begin and enters braking state; when the capacitor port voltage drops to DC452V, it can also be assumed that the DC bus voltage is 452V. The system then begins regenerative energy feedback. Based on this, constants can be calculated.
(The negative sign indicates that the capacitor is discharging)
(3) Relationship between capacitor port voltage and port current
(4) Capacity calculation
Based on actual conditions, the roving frame requires approximately 10-15A of current under full load. A rough estimate of i = 13A is given. Therefore:
F
Therefore, when the load is full, the external capacitor can be selected to be between 20,000 and 30,000 microfarads.
This is a practical example. By modifying the relevant parameters according to actual needs, other capacitor capacities can be selected based on the calculation formula above.
4. Soft-start protection resistor design
4.1 Soft Start Principle
Large-capacity capacitors are present in any voltage-type frequency inverter. Power-on is equivalent to applying a step signal across the capacitor. Directly connecting this signal across the capacitor results in a large charging current, severely impacting its lifespan. Therefore, most frequency inverters incorporate a soft-start circuit after the rectifier section. Based on this principle, in a roving frame, in addition to the capacitors of the three frequency inverters themselves, a large-capacity capacitor is also connected externally to the DC bus. Therefore, a soft-start circuit needs to be designed to ensure the lifespan of the frequency inverters. The main idea is as follows: An external capacitor is added to the DC bus of the frequency inverter, as shown in Figure 2. The contactor's control terminal is controlled by a PLC. Upon power-on, the PLC delays for a period of time, allowing the capacitor to charge through resistor R before the contactor closes the bypass resistor R.
4.2 Calculation of Electrical Parameters for Soft Starter
Based on the calculation results of the above example, if the external capacitor of the roving frame is 0.024F , and the charging current of the 0.024F capacitor is to be controlled within 1.2A , then: ohms; ohms; watts; charging time constant seconds.
Choose a soft-start resistor of 470 ohms with a power rating of at least 600 watts. The PLC should have a power-on delay of at least 15 seconds. Select a bypass contactor with a current handling capacity of 25A/50A.
5. Conclusion
Synchronous braking of multiple motors on inverter-driven roving frames during power outages is a unique problem in the textile industry. When power is lost, the system completely loses its external energy supply, rendering conventional inverter braking ineffective and causing hundreds of spindles to break, requiring lengthy manual rewiring to restore operation. The external bus capacitor synchronous braking scheme discussed in this paper has significant practical value, and this project has passed the research and development phase and has demonstrated excellent performance in practical applications.
