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The structure and characteristics of common DC bus VFD system
Sep 05

The structure of common DC bus VFD system:

   The decanter centrifuge is driven by the main motor, and the differential shaft and the auxiliary motor are coaxially connected. The speed of the main and auxiliary motors is controlled by the inverter, and the DC bus of the two are connected in parallel, and the three-phase power is input to the main inverter.

   Features:

   (1) Excellent energy-saving performance When the spiral lag, the regenerated energy is sent to the DC bus of the auxiliary inverter. Since the DC bus of the main and auxiliary inverters are connected in parallel, the energy is used by the main motor through the main inverter.

For the sake of simplicity, suppose the decanter centrifuge is running at constant torque and constant differential speed in steady state (not counting the influence of acceleration torque and deceleration torque during speed regulation), then the recovered energy is: P=0.8M n/9550, In the formula: P-power (KW); M-small shaft torque (Nm); n-small shaft speed (r/min); 0.8 times before M is due to regenerative braking, even if the discharged braking resistor is not added , 20% of the internal motor is also converted into braking torque.

   (2) Fast dynamic response: Some PID regulation systems tend to have overshoots, and the transition process takes a long time. For example, the eddy current brake speed regulation system has a stable period sometimes as long as several minutes. The torque response time of the frequency conversion speed regulation system is only 150~200ms, and the dynamic characteristics are obviously improved.

(3) It is easy to deal with the accumulation of materials in the drum of decanter centrifuge caused by emergencies. The auxiliary motor runs in the first quadrant (motor state) when it reverses. At this time, the differential speed is very large: Δn=(nl+n)/i, (nl -Drum speed r/min; i-differential speed ratio). Because the inverter has a static starting torque of 2 times the rated torque, the materials accumulated in the drum are easily discharged.

(4) It is conducive to realize constant torque control. Certain materials, such as urban sewage, contain 60% to 70% of organic matter. The sediment is compressible, and the solid content is constantly changing, which makes the screw pushing torque. With the fluctuation of the feed flow rate and solid content rate, the electrical system is required to control the feed rate or differential speed in time according to the torque change, otherwise, it is easy to block the material.

written by Decanter Centrifuge


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