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PRESS RELEASE Maximum Availability In addition to wasting time, production interruptions are also very expensive. This example of an application implemented at a factory that makes plastic products shows how intelligent frequency inverter drives and modular controllers can be used to create solutions that are both reliable and economical. In the plastics industry, conventional drive systems are still the norm – DC motors are still used on a good 2/3 of all extruders. In contrast to this, mechanical engineering manufacturers are now already fitting half the machines they produce with asynchronous motors and frequency inverter drives, because the dynamics and control performance of this combination are comparable to those of high-end DC and servo drives. Using 3-phase AC motors’ sensorless regulation capability can also eliminate the expense of tachometers. In addition to this, users generally don’t need to have any detailed knowledge of the connected motor because the frequency inverter can register the motor data automatically. The latest development by GF-I (www.gf-i.de), a well-established and successful manufacturer of specialised machines based in Amberg, provides a good illustration of the advantages of this for manufacturers in the plastics sector. This project is a drive and control system for industrial extruders using FR-A500 frequency inverter drives and modular controllers from Mitsubishi Electric, connected by a Profibus-DP bus. It controls three extruder screws in a new three-layer blown film plant for the production of stretch covers at the factory of GF-I’s client, Forum Plast. The stretch covers are made of PE (polyethylene) film and are used for industrial packaging, almost exclusively for products on pallets. They have a circumference of approximately five metres, are between 20 and 250µm thick and can be produced in a range of different colours. There are several reasons why the engineers chose to use 3-phase AC motors with frequency inverters and without a feedback loop for this project: In addition to being inexpensive, this combination also significantly reduces the motor servicing overheads Mitsubishi Electric Europe B.V. Gothaer Str. 8 40880 Ratingen 1/5 because they don’t have any carbon brushes that need to be replaced. Modern frequency inverter technology is also getting much better at coping with the problem of starting torque at low speeds. “The dynamics aren’t the only important factor in extruder drives, however, speed constancy is also crucial,” explains GF-I’s CEO Marcus Gut. “With Mitsubishi Electric’s frequency inverters this is possible without a feedback loop, achieving excellent performance even at low speeds.” Automatic motor data registration with Online Autotuning The 120-kilowatt FR-A 540 frequency inverters chosen for the extruders are high-end products that are available with outputs of up to 450kW. What makes them special is their combination of sensorless vector control (SLV), Online Autotuning for automatic motor data registration and a special system called Advanced Magnetic Flux Vector Control, which ensures that maximum torque is generated even at low output frequencies. In addition to this, they also feature automatic slip compensation and an improved version of the Soft PWM system for noise reduction. Three additional internal slots make it possible to add more I/Os and interfaces for modern field bus systems like Profibus, CANopen, CC-Link and DeviceNet. The SLV sensorless vector control system stores all the values needed for its calculations in a “machine model”, eliminating the need to read the motor speed from a tachometer. The motor characteristics are registered automatically by the frequency inverter when the motor is connected, making configuration very much easier for the user. It is possible to connect any asynchronous motor; the only parameters that must still be entered are specifications like voltage, power, frequency and the number of pole pairs. This is also one of the reasons why drives without a speed feedback loop are so popular. Now that users are demanding more and more in terms of torque and speed accuracy it is reasonable to ask to what extent the winding resistance of the rotor, which changes with temperature, is included in the calculation model. This is precisely why Mitsubishi Electric developed its Online Autotuning system. Instead of reading the parameters while the motor is standing still, as many other systems do, the Online Autotuning system works while the motor is actually running. This makes it Mitsubishi Electric Europe B.V. Gothaer Str. 8 40880 Ratingen 2/5 possible to adjust the parameter values continuously as temperatures rise, thus achieving significantly better speed constancy. This system can achieve high torques at low output frequencies. For example, at a frequency of 0.5Hz, 150% of the rated torque is generated on the motor shaft. With open-loop control, the speed adjustment range is 1:120 (and 1:200 in the FR-A700 series, which is the successor to the FR-A500 series). Excellent speed constancy is maintained irrespective of load fluctuations, even in the low speed range. Mitsubishi’s improved Soft PWM system also makes life more pleasant for users by reducing the unpleasant whistling noises that can be generated at low frequencies. This is achieved by introducing a certain amount of fluctuation (jitter) around the setpoint frequency, instead of operating the inverter at a constant frequency. Mitsubishi Electric also uses its own power stages in all its frequency inverters, which means that they are always perfectly matched to the inverters. This may also be the reason why it was possible to specify 50°C as the standard operating temperature. “The key benefits of Mitsubishi’s frequency inverters are their high availability thanks to their cooled capacitors and their stable control capabilities at low speeds without a tachometer,” says Marcus Gut. “There are very few vendors who can combine that kind of performance with excellent service-friendliness. This is clearly moving into the domain of servo technology.” Combined with modular controller technology for a complete solution The individual components of the blown film plant are decentralised and connected via a Profibus network. The control gear for each extruder is installed locally in a switchgear cabinet. All the components, including the frequency inverters, are connected via the bus, which makes it easy to read all the necessary parameters. Of course, the frequency inverters used in this project could also be connected via other networks like DeviceNet and CC-Link, and they also have standard interfaces like RS-422 and RS-485. The entire extrusion plant is operated from a central display console with rotary controls, built around a Mitsubishi Electric PLC. The console, which is manufactured by Resotec, is custom-designed specially for use with extruders. The rotary controls are necessary because a touch screen is not so well suited for “manual” Mitsubishi Electric Europe B.V. Gothaer Str. 8 40880 Ratingen 3/5 initiation of the complex processes. Another important element for the manufacturing system are the Resotec/Mitsubishi temperature modules, which enable remote temperature control in 10 separate zones via the Profibus network, using the Autotuning function. The Mitsubishi Electric controllers installed at Forum Plast’s facility are modular, consisting of a module rack on which the power supply unit, CPU, I/O modules and all the network hardware are installed. Mitsubishi offers a choice of six different CPUs for this system. However, for Marcus Gut the real advantage of these controllers is their communications capabilities: Every module has its own processor, which ensures that the From/To processes involved in the connections do not reduce the system cycle times. The communications system consists of the gravimetric throughput control system, which determines the weight-per-length, the Profibus, the frequency inverter drive, the display console and the Ethernet network. The Mitsubishi Electric controllers can be upgraded to a total of between 16 and 1,024 I/Os each, depending on the type (or 4,096 in the case of the Melsec System Q automation platform). They have cycle times as fast as 0.075ms per logical instruction and their integrated RAM memory can store up to 124k PLC program steps. A PCMCIA card can be added to extend the memory up to a maximum of 4MB. At this year’s Hannover Messe trade show Mitsubishi Electric launched the new FR-A700 series of frequency inverter drives, which is the successor to the FR-A500 series. The new line covers an output range from 0.4 to 630KW and the units have their own integrated PLC, which provides direct access to all drive parameters. The control systems have been further enhanced for even higher torques at low speeds. Intelligent maintenance and diagnostics functions guarantee maximum availability. The FR-F700 version features new technology for achieving even lower power consumption, making it ideal for pump and fan applications. It also outputs precise information on the current power consumption and the load factor in percent. Mitsubishi Electric Europe B.V. Gothaer Str. 8 40880 Ratingen 4/5 It is also worth mentioning that the latest customer satisfaction survey by IMS Research rates Mitsubishi Electric’s inverters among the best in their class. This is crucial for applications in manufacturing facilities with “difficult” environmental conditions, because interruptions waste time and generate additional costs, not just for extruder systems. The results of the study are confirmed by Forum Plast’s experience with the system: The company now has 70 frequency inverters and 120 controllers from Mitsubishi Electric installed at their new stretch cover plant, and to date they have been operating around the clock without a single failure. Author: Dr Peter Stipp Journalist Mitsubishi Electric Europe B.V. Gothaer Str. 8 40880 Ratingen 5/5