As industry in general strives towards maximum operational flexibility, decentralised motor control architectures are escalating in popularity.
To-date, the automotive industry has led the charge to decentralised drive technology, because of its expansive linear nature.
However, the mining sector also presents application opportunities for this innovative approach.
Elements of mineral processing , reagent preparation facilities, sampling and dosing plants usually comprise a myriad of smaller drives located in a relatively compact area, and as such, are perfect candidates for decentralised control architectures.
Decentralised motor control systems involve the relocation of power components, plus monitoring and speed control units, to strategic positions near or on the motor. Such architectures offer a deal more installation and future-proofing flexibility than traditional motor control centres, plus the option of distributed intelligence across the facility.
Although the initial capital cost of the system may be higher, the cost reductions associated with the planning, installation and maintenance of such systems lead to overall savings on the total system cost of 10 to 30%.
The key components of a decentralised drive system are the motor and localised motor control electronics. Generally comprising either a motor starter or a variable speed frequency inverter, the motor control unit is housed on or close to the motor, and features standard communications and power supply interfaces.
Distributed motor control architectures most often involve integration of the motor control electronics onboard the motor. For basic installations, an actuator/sensor interface (AS-i) allows connection of a compatible AS-i communications cable carrying both signal and control voltage for the system; the individual inverter motors are then daisy-chained by a power supply.
For more demanding applications, field distributors can be used. These distribution units provide localised marshalling and distribution points for power supply, control voltage and a communications fieldbus.
The field distributor’s fieldbus interface permits drive control through fieldbus systems (AS-i, Profibus, Interbus, DeviceNet and CAN), while providing extensive drive diagnostic and visualisation options. Sensor and actuator signals are also connected to the field distributor’s communications interface.
The illustration shows three possible decentralised motor control system architectures incorporating field distributors.
In reality, it is not always convenient or practical to accommodate the field distributor onboard the motor itself.
Prefabricated hybrid cables, supporting power, control voltage and data communications within the same cable, provide the field distributor-to-motor link.
The use of hybrid cables, connected via special plug connectors on the motor, dramatically simplifies installation, commissioning and maintenance activities. Drives can be unplugged and replaced in a simple procedure. The interchange-ability of machinery and subsequent adaptability of the production line is a major benefit of the use of hybrid cables.
While most applications permit the motor control electronics to be mounted onboard the motor, it is sometimes necessary to physically remove them. For example, where high temperatures, chemical or gaseous reagents, potential mechanical interference, or merely space restrictions are involved.
Under these circumstances, both the motor control electronics and the field distributor can be located together, separate from the motor. Supply and control voltage, plus data communication cables, connect to the integrated frequency inverter/field distributor assembly, which is again connected to the motor via no-fuss hybrid cables.
At an operational level, the deployment of decentralised drive systems offers many other production advantages.
These include the ability to accommodate localised shutdowns, shorter set-up and line changeover periods, and an inherent flexibility that underpins total line future-proofing.
A high level of serviceability results from the ease of sectional isolation, which can often avert a worst-case total plant shutdown scenario.
In production processes with considerable labour power, such as mineral processing facilities, the subsequent reduction in downtime can be a key factor.
Darren Klonowski is applications engineer at SEW-Eurodrive .
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