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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
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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
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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”
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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.
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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
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