VFDs Provide Process & Speed Control in Centrifuge Operations

Today’s industrial marketplace uses many different machines—such as pumps, fans, compressors and conveyors. One machine that is often overlooked is the centrifuge. The centrifuge is not well-known but is commonly used in industrial applications and frequently affects operations. A centrifuge is often used in simple operations in the milk and sugar industries. Centrifuges are also used in wastewater treatment plants. This article includes a case study that examines how VFDs were applied to a Philadelphia wastewater dewatering process.

Centrifuges & VFD Technology

The centrifuge’s design takes advantage of variable frequency drive (VFD) technology to regulate the process speed. To apply a VFD to a centrifuge correctly, some factors must be understood. Centrifuge applications have the same basic considerations as many machines—such as voltage, enclosure type, horsepower (the full load amps of the motor current), and ambient environment (temperature and humidity, for example)—that are found in many applications. However, centrifuge applications require close examination. Centrifuges have long acceleration times (a heavy-duty, constant-torque drive is required). In the past, wye-delta starters were used and had acceleration times of 3 to 4 minutes, drawing a tremendous amount electrical current. This method has been replaced with VFDs. VFDs allow the user to slowly bring the centrifuge up to speed, greatly reducing the mechanical stress on the motor, bearings and bowl. They provide better process and speed control. When starting a very large mass with short acceleration time, the VFD most likely goes into over-current and trips. This is true of any high-inertia application (fans, presses, loaded conveyors). (Note: If over-current faults occur in any application during startup, try increasing the acceleration time to resolve the problem.) A similar deceleration time is also a good idea. If shorter deceleration times are required, an active, front-end or braking resistor module with the proper resistor should be applied. Other factors to consider with a centrifuge are the available power grid current during peak summer demand and high ambient temperatures. Both affect VFD operation.

Case Study: Synagro & the City of Philadelphia

During the last year, a drive manufacturer has worked closely with Philadelphia Renewable Bio Fuels, a division of Synagro Inc. Synagro operates the dewatering facility under contract for the City of Philadelphia water department. The centrifuges at the facility weigh more than 15,000 pounds and remove 30,000 gallons of liquid (the weight of water is 8 pounds per gallon) per hour for each machine. They operate as many as seven of the 10 machines each day. This plant processes wastewater 24 hours per day, seven days per week. The final process of the solid waste removal converts this to useful fertilizer. The drive manufacturer and Synagro’s Electrical Maintenance Lead Joe Marchesani installed replacement drives for three 315-kilowatt drives. The drive manufacturer had assisted two other Synagro locations that experienced problems because of flooding and other natural disasters with quick delivery and onsite technical assistance. “I had been told by other facilities that this drive manufacturer delivered what they quoted and as promised and that its product’s quality and performance was solid,” Marchesani said. He explained that he currently needed a replacement drive, and because of the intra-company testimonials, he was willing to try the drive manufacturer as long as the price was competitive and the company could meet his small window of time for the installation. Marchesani also expressed a major concern in this retrofit, and Synagro wanted to ensure that the VFD was sized properly to prevent tripping caused by the overhauling characteristic of the load in the bowl pushing the bowl faster. Other considerations needed to be addressed—including the potential for reduced power grid current during peak summer demand and high ambient temperatures. The plant has no air conditioning to cool the drives during the hot summer months. Synagro was interested in the drive manufacturer’s active front-end to recover energy and place it back on the power grid and its ability to handle the plant’s short deceleration time requirement. Marchesani indicated that he had read about centrifuges in sugar processing that had energy savings up to 15 percent, along with improved processing time. Unfortunately, in this case, the process was continuous, and the centrifuge ran for weeks at a time without being shut down. The savings would not be exactly the same. He also wanted to install a VFD in the existing enclosure to reduce the retrofit cost and decrease downtime that had been experienced with the existing drive. Marchesani’s final requirement was that the drive be capable of running the larger 300-horsepower motor on other centrifuges. The other installed drives were old and lacked flexibility. A 335-horsepower, heavy-duty (constant torque) VFD was selected. The drives were installed into the existing drive cabinet. Installing the VFD in the existing enclosure allowed Synagro to save thousands in cabinet costs, labor and eliminated downtime. The drive manufacturer’s team handled the startup, working closely with Marchesani and his team, checking all the motor and communication connections. The VFD was 25 percent smaller than the original, failing drive. Therefore, it fit much easier into the existing space than anticipated, making this a quick turnaround. The motor in place was a rewound 250-horsepower 460 motor with no accurate motor winding data. Because of this lack of data, the drive manufacturer operated the drive in volts-per-hertz mode. The drive and centrifuge came up to speed smoothly in manual speed mode. The control system was reconnected to the existing programmable logic controller (PLC), and it communicated flawlessly from the beginning. The keypad was quickly adjusted to show current as a percent of motor full load amps, which is more meaningful to operators. This was accomplished with a few simple key strokes and allowed the centrifuge operators to take action when a centrifuge’s capacity was reached. Synchronizing the current information with the PLC display involved a simple output from the new VFD, which was accomplished again with a few key strokes. This provided the PLC with accurate current information from the VFD. After the setup process, the VFD was operated at 50 percent of full load amps at the processing speed required. The operators also noticed motor noise reduction when compared to the previous drive. Marchesani noted that the drives were easy to program and control. Because of the cooling system, the drives offered 50 C ambient temperature, so Marchesani no longer needed to worry about the summer heat. The VFDs have run effectively since August 2012. They have operated up to full load during internal testing and when the speed was reduced at times if the centrifuge reached its maximum speed and capacity. No nuisance tripping or other problems have been experienced. Philadelphia Bio Fuels’ satisfaction with the performance of the drive can best be illustrated by the fact that two additional drives have been ordered and installed since January 2013. The new drives were installed by Marchesani and his crew into existing cabinets and connected to existing PLC software with no assistance from the drive manufacturer. A 125-horsepower VFD drive has also been ordered for a progressing cavity pump. Understanding the basic application requirements and the expectations of the VFD are the keys to successful application.