AC drives claim to boost dewatering substantially over hydraulic units

The use of alternating current (AC) motors to drive dewatering and sludge removal systems, formerly the province of hydraulic units, is claimed to boost the throughput of these units by tenfold. Yaskawa says its GPD 515 AC drives have already been tested in major applications.

Significant savings are achieved by removing as much water as possible from drilling sludge before moving the solids and storing them. The manufacturer states that centrifugal dewatering systems using hydraulic motors incorporate drawbacks, such as high-maintenance components, complex plumbing and control systems, and continuing oil and seal leakage.

With the new system, a "decanter" centrifuge operates in a continuous-feed mode using three GPD 515 AC drive/motor combinations in open loop vector mode to power the system's bowl, pump, and auger. A PC-based control system automatically adjusts all speed rates to keep motors running within programmed load and speed limits. Each drive is tied to the control system using DeviceNet serial communications.

GPD 515 drives in closed loop configuration are commonplace on decanter-style centrifuges and have a long, successful history. The use of an open loop PC-based control scheme with serial communication to the GPD drives, however, does away with the expensive and troublesome encoders required for closed loop operation, providing substantial cost savings.

A 40 hp GPD 515 Drive controls the rotational speed of the main bowl. In addition, this drive provides the torque levels necessary to overcome the high inertia of the bowl. Potentially excessive current draw during starting is eliminated as the drive provides smooth acceleration to bring the bowl up to operating speed at low current levels. Inside the bowl, a full-diameter auger turns slightly slower than the bowl to force the heavier-than-water sludge to the outside of the bowl and out through a solids discharge. Simultaneously, water separated by centrifugal force is disposed of through a water discharge.

Decanter centrifuges use a planetary gear box with a fixed differential speed to control auger speed. The gear box pinion must be changed to adjust differential speed. Sustaining optimum performance through changing conditions, including weight and texture of sludge, could require frequent pinion changes.

Because changing a pinion was a complicated, time-consuming operation, machine operators usually left the speed unchanged even though the centrifuge wasn't operating at optimum levels. Utilization of the AC drive, however, solves this problem by permitting adjustment of pinion through the drive controlling the motor, allowing for infinite variations in auger speed. The auger is controlled by a 20 hp drive programmed to accelerate the motor up to speed, then set a differential auger speed for effective removal of solids from the inside of the bowl and their transport to the discharge. The optimal differential speed varies with bowl and feed pump speed.

Regenerative mode

The centrifugal force generated by the bowl is so strong that the auger would follow at the same speed if no retarding torque was available to maintain a differential speed. This condition would not be acceptable since no solids would be discharged. The auger must operate at a slightly slower speed than the bowl to act as a plow and push the solids out though a discharge hole.

To accomplish the differential speed, the auger drive motor actually is in regenerative mode for most applications - running as a generator. This regenerated power is fed back into the main bowl drive by tying the auger drive and main bowl drive together via a common bus. The GPD 515 drives allow the operator to vary the speed at will, matching drum, pump, and auger speeds exactly to prevailing needs.

A 15 hp drive controls the speed of the pump feeding sludge into the bowl. The machine operator, using a PC display panel, can control and adjust drum, and pump and differential speeds. The PC-based system automatically adjusts all speed rates to keep motors running within programmed load and speed limits. Current and speed are constantly monitored using DeviceNet and adjustments are made as needed.

All motors used in the new centrifugal sludge removal system are explosion-proof Class I Group D rated to meet industry safety standards and are capable of operating in a constant torque 1,000:1 speed range. All control components are located in a positive pressure purged air conditioned cabinet. The drives also incorporate conformal coated circuit boards to seal against damage from salt air and water and to withstand heavy in-process vibration levels. The GPD drive products can communicate via all the popular communication protocols, including DeviceNet, Ethernet, Interbus-S, Modbus, Profibus, and Sysbus.