Case Study
Improved control and lower costs for olive oil production
Problem
Precise control over the rotation of two elements within a decanter system for separating liquids and solids at speeds of more than 3,000rpm is needed to ensure the correct level of media separation for an efficient process.
Solution
The precise motor control of the Optidrive P2 drive provides the required speed accuracy necessary for the integrity of the process.
OLIVE OIL MANUFACTURER
Decanters are horizontal centrifugal separators between solid and liquid matter. They are widely used in a wide range of industries and sectors, including oil and gas, renewable energy, biomedical, wastewater and industrial waste treatment plants, among many others.
The decanters in this case are operated by an olive oil company in Greece and are complex stainless-steel devices that require high precision engineering with motor power ranging from 22kW to132kW.
Decanter diameters range from 400mm to 1000mm with a high mass (from 1.5 to 5 tonnes), rotating at 3000rpm (47Hz-56Hz). This creates an enormous moment of inertia, frequently in excess of 3000 "g".
A differential gearbox rotates the outer drum and the inner scroll (Archimedes screw), at different speeds. The magnitude of differential speed between the drum and scroll is critical to obtaining the correct level of media separation and precise control of this is required in order to maintain an efficient process.
A relatively small speed fluctuation on drum or scroll (due to load) induces a greater fluctuation on differential speed, the accuracy of which should vary within the first decimal (eg. 12.5rpm).
Required speed-accuracy necessary
The excellent motor control of the Optidrive P2 drive, at vector speed control mode, with the "autotune" function activated, provides the required speed-accuracy necessary for the integrity of the process.
Depending on the density and the liquid content of the separated media, the internal "P.I." speed controller's values are adjusted to provide required motor speed accuracy at reasonable current peaks.
The renowned and easy to set procedures of the Optidrive P2 combined with its advanced and powerful features allow fast set-up with complete, precise control of each decanter. "Drive healthy" and "external trip" (motorPTC) features, ensure continuity of the process with the peripheral machinery.
To avoid overfeeding of the decanter, "User Relay Output" function can be selected to monitor, "motor torque" with the "high" and "low" limits set to disable the feeding pump and allow the decanter to discharge and re-engage the pump at lower feeding loads.
Feed pump is controlled by an Optidrive P2
In addition to the procedure of load/feed control, the feed pump is controlled by an Optidrive P2. The "Analog output" function of the decanter's P2 can be selected to provide an analogue signal of motor current. This signal applied to the VFD second analogue input, operated at P.I. control mode, ensures constant feed to the decanter at desirable levels, selected via its first analogue input.
Depending on size, the acceleration time can be set between 400s and 900s and the deceleration time can be set to 0s to activate the internal DC bus overload controller for the smallest possible ramp. A 520mm diameter decanter at a "coast to stop" mode needs approx. 45min to stop, whereas, at requires less than 15min.
Spin Start function
Last but not least the excellent "Spin Start" function provided by the P2 provides fast speed recovery of the decanter in case of short or longer mains power cuts, a frequent event in remote areas of the Greece country side.
When PLC and HMI are used to control the decanter and its peripheral machinery, such as feed pump, feeding screws, vibrating filters, solids discharging screws, liquid discharging pumps, the Optidrive intergrated "modbus" control mode is used to provide a precise, fast and total control of the line, with considerably lower installation costs.
