Analysis of frequency converter in centrifuge

1. Introduction

Overall, there is a big gap between the technical level of separation machinery in China and the advanced level abroad. This is mainly manifested in: there are few varieties and specifications of separation machinery, which cannot fully meet the domestic production needs, especially for machinery with high viscosity and fine precision of separation materials. Most of the separation machinery with high efficiency, large production capacity and high degree of automation rely on imports. From the development of separation machinery, digital AC inverter will replace the original electromagnetic speed regulation, DC speed regulation, hydraulic coupling speed regulation and multi-speed motor, and gradually become the main driving device of separation machinery.

The ACS550 used in this article is the latest intelligent inverter launched by ABB. This series of inverters is used for 0.75KW~355KW low-voltage AC transmission. It can accurately control speed and torque and can match the existing standard squirrel cage asynchronous motors. ACS550 has three control modes, namely scalar V/F control, sensorless vector control, and torque control, so this inverter is not only suitable for the simplest motor operation, but also can be used in complex working occasions. Its reliable overload capacity design can also meet both ordinary load and heavy load work. The drive motors of separation machinery are generally divided into single motor drive and multi-motor drive. This article will mainly introduce the application of ACS550 inverter in the three-legged centrifuge as a typical case of single motor drive and the application of horizontal screw centrifuge as a typical case of multi-motor drive.

2. Application of frequency converter in three-column centrifuge

The three-column centrifuge is a vertical centrifuge with a simple structure, strong adaptability to materials, and the widest application. It is divided into two categories: sedimentation type and filtration type. In this application, the frequency converter drives the drum of the centrifuge, which starts smoothly and has an adjustable separation factor; it completely overcomes the shortcomings of traditional DC carbon brush centrifuges such as high noise, high failure rate, short service life, and unstable speed, and is an updated product of gravity sedimentation separation equipment. The AC variable frequency centrifuge has distinct characteristics and originality in several important indicators such as the shock absorption system and variable frequency motor. The common single-unit drive power of the three-column centrifuge is between 3KW and 55KW, and the ACS550 is fully capable of it.

The principle of ACS550 application in three-column centrifuge The principle of ACS550 inverter, in its peripheral circuit, is mainly divided into three parts:

(1) The DC bus UDC+ and UDC- terminals are connected to the + and - terminals of the brake unit, and then according to different options (such as regenerative braking connected to the three-phase grid, and dynamic braking connected to the brake resistor), Tk is the internal relay of the brake unit. When this unit fails, Tk will act and define the terminal DI4 of the inverter to instantly block the U/V/W output.

(2) In the control loop input and output terminals, the definition of macro 9902=5 is adopted

DI1: Manual/Automatic Start/Stop (Manual): Power on to start

DI2: Forward/Reverse (manual): When energized, the direction of rotation is reverse

DI3: EXT1/EXT2 selection: Automatic control when powered on

DI4: Run permission: Once disconnected, the inverter will stop

DI5: Forward/Reverse (automatic): When energized, the direction of rotation is reverse

DI6: Start/Stop (automatic): Start when powered on

AI1: External speed reference 1: 0…10 V (manual control, potentiometer, reference voltage 10 VDC)

AI2: External speed reference 2: 0…20 mA (automatic control, reference signal: 0…20 mA)

AI1: External speed reference 1: 0…10 V (manual control, reference voltage 10 VDC)

AI2: External speed reference 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 indication)

When the centrifuge uses a frequency converter, the braking method should give priority to regenerative braking that feeds back energy to the power grid. Electric energy feedback braking is to feed back the braking energy to the power grid for reuse. From the perspective of energy saving, it is the best way, and no heat is generated, which is very suitable for installation in flammable places (because energy consumption braking will generate a lot of heat). In places where this regenerative braking device is not suitable, if there are grid harmonic requirements (because the harmonic coefficient of general energy feedback braking units is not ideal), a braking resistor of sufficient capacity should be installed and heat dissipation measures should be taken; of course, in non-flammable places, energy consumption braking is mostly used to save one-time installation costs.

The general dynamic braking unit has the following basic functions:

(1) Parameter setting

Action voltage setting: The action voltage setting value can be set to 660V or 710V (the incoming line is 380V level) through the DIP switch.

Braking utilization rate: The working utilization rate of the braking unit can be set by the DIP switch. The default value of the general energy-consuming braking unit is 10%, but due to different braking requirements of the system, different braking rates can be set, up to 100%.

(2) Status Node

Module abnormality: When the DC circuit is short-circuited, overloaded or the IGBT module is damaged, the brake unit alarms and the fault relay Tk operates. Radiator overheating: The brake unit radiator overheats and the fault relay Tk operates.

When the brake unit operates normally, Tk is closed, and the inverter is in the enabled state and can work normally; when a fault occurs, Tk is opened, and by defining the inverter input terminal DI4, the U/V/W output can be instantly blocked to play a protective role.

For direct feedback braking, there are mature products, but the cost is generally several times higher than that of energy consumption braking. In order to achieve bidirectional energy transfer between the DC circuit of the inverter and the incoming grid power supply, one of the most effective ways is to use active inverter technology: that is, to invert the regenerated electric energy into AC power with the same frequency and phase as the grid and send it back to the grid to achieve braking. It uses a current tracking PWM rectifier, which makes it easy to achieve bidirectional power flow and has a very fast dynamic response speed. At the same time, such a topological structure enables us to fully control the exchange of reactive and active power between the AC side and the DC side. Of course, different products have different control methods, and we must also pay attention to the power factor, harmonic components, output phase, etc.

3. Application of ABB inverter in horizontal screw centrifuge

In this scheme, the reactive excitation current required by the auxiliary drive motor and the active current when the auxiliary motor occasionally operates as a motor (such as the startup phase and the acceleration and deceleration transition process) are provided by the main inverter. Therefore, the capacity of the incoming rectifier bridge should be considered when selecting the power of the main inverter. It must be ensured that the current passing through is the sum of the electric currents of the two motors. The characteristics of this design scheme are simple circuit, no need for debugging, and extremely high operation reliability. The characteristics of this centrifuge control: (1) The vector control method of the centrifuge speed ensures the correctness of the speed control; (2) The common DC bus can be used; (3) Speed ​​difference control, the encoder is eliminated, and the open-loop sensorless vector control can solve the expensive feedback control scheme with encoder; (4) Load compensation of the barrel shaft drive.

In general, the frequency conversion application of the horizontal screw centrifuge has the following characteristics:

(1) Energy saving: Common bus dual motor dual inverter drive is widely used in horizontal screw centrifuges, that is, the main and auxiliary motors are each driven by an ordinary inverter, and their DC buses are connected in parallel in an appropriate way, which solves this problem better. It is of particular significance today when energy is increasingly scarce. The speed difference between the screw and the drum of a modern centrifuge can be automatically adjusted according to the change of feed. Compared with eddy current braking and hydraulic coupling braking, variable frequency speed regulation has a high utilization rate of electric energy and is more energy-saving.

(2) Fast dynamic response: The speed differential adjustment process is reduced from several minutes for PID regulator to several seconds for variable frequency sensorless vector control. The speed differential adjustment is precise (even up to ±0.05 rpm), which greatly increases the dry matter content in the discharge material (about 2% for sludge treatment).

(3) Torque control function: The torque control of ACS550 can easily switch between speed and torque, and handle the accumulation of materials in the drum caused by emergencies, thereby improving work efficiency. The centrifuge using ACS550 has a larger effective torque, with a minimum continuous torque of 5,000Nm and an instantaneous load torque of up to 27,000Nm.

The above are the analysis results of the frequency converter in the centrifuge.