Separation machinery is equipment used to separate mixtures of liquids and solid particles. It mainly includes centrifuges, separators, filter presses, oil filters, and other filters. Separation machinery is generally post-processing equipment in the technological process, so it directly affects the quality of the final product. Overall, my country's separation machinery technology lags significantly behind advanced international levels. This is mainly reflected in the following aspects: The variety and specifications of separation machinery are limited, failing to fully meet domestic production needs, especially for machinery separating materials with high viscosity and fine precision. High-efficiency, high-capacity, and highly automated separation machinery largely relies on imports; research on separation mechanisms and application technologies is lagging, resulting in slow new product development; manufacturing processes are outdated, leading to low production efficiency, poor product reliability and stability, and low levels of technology and automation; and supporting equipment and materials cannot meet the needs of separation machinery production, particularly resulting in unstable product quality and reliability.
From the perspective of the development of separation machinery, digital AC frequency converters will gradually replace the original electromagnetic speed regulation, DC speed regulation, hydraulic coupling speed regulation, and multi-speed motors, becoming the main drive device for separation machinery. This article will introduce the design and application of ABB's new ACS550 frequency converter in separation machinery.
The ACS550 is ABB's latest intelligent frequency converter, designed for low-voltage AC drives ranging from 0.75kW to 355kW. It precisely controls speed and torque and is compatible with existing standard squirrel-cage induction motors. The ACS550 offers three control modes: scalar V/F control, sensorless vector control, and torque control. Therefore, this converter is suitable for both the simplest motor operations and complex applications. Its reliable overload capacity allows it to handle both normal and heavy loads.
The drive motors of separation machinery are generally divided into two types: single-motor drive and multi-motor drive. This article will mainly introduce the application of the ACS550 frequency converter in the three-legged centrifuge as a typical case of single-motor drive and the horizontal screw centrifuge as a typical case of multi-motor drive.
2. Application of frequency converters in three-legged centrifuges
The three-legged centrifuge is a simple, highly adaptable, and widely used vertical centrifuge. It is divided into two main categories: sedimentation and filtration. The filtration type is the most widely used. It is suitable for separating fine particles with a diameter of only a few micrometers, and can also be used for dehydration of packaged goods. By adjusting the duration of each operation step, it can be used to separate suspensions with varying degrees of filtration difficulty. By adjusting the washing time of the filter cake, different washing requirements can be met. This type of machine is mainly suitable for small to medium-sized production, but due to its many advantages, it is widely used in pharmaceutical, chemical, light industry, textile, food, and machinery manufacturing industries.
In this application, the frequency converter drives the centrifuge drum, ensuring smooth start-up and adjustable separation factor. It completely overcomes the shortcomings of traditional DC brush centrifuges, such as high noise, high failure rate, short lifespan, and unstable speed, making it a next-generation product for gravity sedimentation separation equipment. The AC variable frequency centrifuge has distinctive features and originality in several key indicators, including its vibration damping system and variable frequency motor. The single-unit drive power of common three-legged centrifuges ranges from 3KW to 55KW, which the ACS550 can easily handle.
The application principle of ACS550 in three-legged centrifuges: The principle of ACS550 frequency converter is mainly divided into three parts in its external circuit:
(1) Connect the DC bus UDC+ and UDC- terminals to the brake single + and - terminals, and then according to different selections (such as connecting the regenerative braking to the three-phase grid, and connecting the energy consumption braking to the braking resistor), Tk is the internal relay of the braking unit. When the unit malfunctions, Tk will activate and block the U/V/W output instantly through the inverter terminal DI4 definition.
(2) In the input and output terminals of the control loop, the definition of macro 9902 = 5 is adopted.
DI1: Manual/Automatic Start/Stop (Manual): Power-on start
DI2: Forward/Reverse (Manual): Power on reverses the direction of rotation.
DI3: EXT1/EXT2 Selection: Power-on selection automatic control
DI4: Operation Allowed: The inverter will stop once disconnected.
DI5: Forward/Reverse (Automatic): Power on reverses the direction of rotation.
DI6: Start/Stop (Automatic): Starts upon power-on
AI1: External speed setting 1: 0…10 V (manual control, potentiometer, reference voltage 10 VDC)
AI2: External speed setting 2: 0…20 mA (Automatic control, reference signal: 0…20 mA)
AI1: External speed setting 1: 0…10 V (manual control, reference voltage 10 VDC)
AI2: External speed setting 2: 0…20 mA (Automatic control, reference signal: 0…20 mA)
RO2C/2B: Relay output 2, programmable (default action: run)
RO3C/3B: Relay Output 3, Programmable (Default Action: Fault)
AO1: DC digital display (frequency or speed indicator)
When using frequency converters in centrifuges, regenerative braking, which feeds energy back to the grid, should be the preferred braking method. Regenerative braking feeds the braking energy back to the grid for reuse, making it the best method from an energy-saving perspective. It also generates no heat, making it ideal for installation in flammable environments (as regenerative braking generates a significant amount of heat). In locations where regenerative braking is not suitable, but where there are requirements regarding grid harmonics (because the harmonic coefficients of typical regenerative braking units are not ideal), a braking resistor of sufficient capacity should be installed, and heat dissipation measures should be taken. Of course, in non-flammable environments, regenerative braking is generally used to save on initial installation costs.
Typical regenerative braking units have the following basic functions:
(1) Parameter setting
Operating voltage setting: The operating voltage setting value can be set to 660V or 710V (the incoming line is 380V) via a DIP switch.
Braking utilization rate: The operating utilization rate of the braking unit can be set via a DIP switch. The default value for a typical energy-efficient braking unit is 10%, but different braking rates can be set depending on the system's braking requirements, up to a maximum of 100%.
(2) State nodes
Module malfunction: When a short circuit, overload, or IGBT module failure occurs in the DC circuit, the braking unit alarms, and the fault relay Tk trips. Heatsink overheating: The braking unit heatsink overheats, triggering an alarm and the fault relay Tk trips.
When the braking unit is operating normally, Tk is closed, and the frequency converter is in the enabled state and can work normally; when a fault occurs, Tk is opened, and by defining the frequency converter input terminal DI4, the U/V/W output can be blocked instantly, thus playing a protective role.
For direct regenerative braking, mature products already exist, but their cost is generally several times higher than that of regenerative braking. One of the most effective methods for achieving bidirectional energy transfer between the inverter's DC circuit and the incoming grid power supply is to use active inverter technology: converting regenerated electrical energy into AC power with the same frequency and phase as the grid and feeding it back to the grid, thus achieving braking. This method uses a current-tracking PWM rectifier, which easily enables bidirectional power flow and has a very fast dynamic response speed. Simultaneously, this topology allows for complete control over the reactive and active power exchange between the AC and DC sides. Of course, different products have different control methods, and we must simultaneously consider power factor, harmonic components, output phase, etc.
3. Application of ABB frequency converters in horizontal screw centrifuges
Horizontal screw conveyor centrifuges (referred to as horizontal screw centrifuges) are widely used in petroleum, chemical, metallurgical, pharmaceutical, food, and light industries. They can be used for solid dewatering and classification, as well as liquid clarification, holding an important position in the centrifuge field. Due to their advantages such as large single-unit processing capacity, convenient operation, continuous automatic operation, low labor intensity, small footprint, and low maintenance costs, screw centrifuges have been widely used in sludge dewatering since the 1950s, gradually replacing other dewatering machinery and becoming the preferred equipment for sludge dewatering in large-scale urban wastewater treatment plants. The suspension enters the high-speed rotating drum through the feed pipe in the center of the screw conveyor. Due to centrifugal force, heavier or larger particles are thrown against the inner wall of the rotating drum and ejected from the drum through the spray holes at the small end of the screw, while lighter solid particles and liquid phase overflow from the drum through the overflow holes at the large end. The suspension is fed by a screw pump with stepless speed regulation.
From a development perspective, the drive system of horizontal decanter centrifuges has evolved from energy-intensive eddy current braking to the currently mainstream dual-motor, dual-frequency conversion drive. A typical horizontal decanter centrifuge has two drive shafts: one drives the cylinder to rotate, and the other drives the cylinder shaft; the speeds of these two shafts need to be precisely coordinated. Variable frequency vector control automatically adapts to load changes based on different liquid concentrations, efficiently and reliably completing the centrifugal separation process. Specific separation control functions are integrated into the PLC centrifuge control software. In terms of wiring, the AC power grid is connected to the input terminal of the main frequency converter, and the DC buses of the two frequency converters are directly connected in parallel. The auxiliary drive frequency converter is not directly connected to the 380V input line, thus enabling convenient and reliable energy sharing. Under normal operating conditions, the auxiliary machine is in generator mode, while the main motor is in motor mode. The auxiliary machine's generator causes the bus voltage to rise, and then, through bus interconnection, the risen voltage is consumed by the main machine, thereby reducing the energy absorbed from the grid and achieving energy saving.
In this scheme, since the reactive excitation current required by the auxiliary drive motor and the active current when the auxiliary motor occasionally operates as a motor (e.g., during the start-up phase and acceleration/deceleration transition process) are both provided by the main frequency converter, the capacity of the incoming rectifier bridge should be considered when selecting the power of the main frequency converter, and it must be ensured that the current passing through it is the sum of the motor currents of the two motors. The characteristics of this design scheme are simple circuit, no need for debugging, and extremely high reliability of operation. The characteristics of this centrifuge control are: (1) the vector control method of centrifuge speed ensures the correctness of speed control; (2) a common DC bus can be used; (3) speed difference control eliminates the encoder, and the use of open-loop sensorless vector control can solve the expensive feedback control scheme with encoder; (4) load compensation for the drum shaft drive.
In summary, the variable frequency application of horizontal screw centrifuges has the following characteristics:
(1) Energy saving: The dual-motor dual-frequency converter drive with a common bus is widely used in horizontal screw centrifuges. That is, the main and auxiliary motors are each driven by a common frequency converter, and their DC buses are connected in parallel in an appropriate way, which solves the problem well. In today's increasingly energy-scarce world, it is of particular importance. The speed difference between the screw and the drum of a modern centrifuge can be automatically adjusted according to the changes in the feed. Variable frequency speed regulation has a higher energy utilization rate than eddy current braking and hydraulic coupling braking, and is more energy-saving in terms of energy consumption.
(2) Fast dynamic response: The differential speed adjustment process is reduced from several minutes by the PID controller to several seconds by the frequency converter sensorless vector control. The speed difference adjustment is precise (it can even reach ±0.05 rpm), which greatly improves the dry matter content in the unloaded material (about 2% for sludge treatment).
(3) Torque control function: The torque control of ACS550 can easily realize the switching between speed and torque, and handle the accumulation of material in the drum caused by sudden events, thereby improving work efficiency. The centrifuge using ACS550 has a larger effective torque, with a minimum continuous torque of 5,000 Nm and an instantaneous load torque of up to 27,000 Nm.
