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06/12/2010
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Some solutions to problems in controlling motor drives are assessed by Tom Shelley.
 Control and automation equipment becomes cleverer, but so do the demands to improve safety and reliability; reduce energy and costs, and ward off cyber attacks.
On this last matter, the arrival of the Stuxnet worm shows that computers controlling industrial systems are as vulnerable as the normal varieties. When asked about the infection in Iran and elsewhere, a spokesman from Siemens responded by pointing out that it was 'very specifically targeted'. This was certainly the case, since it was apparently designed to only affect inverter drives made by Fararo Paya in Iran or Vacon in Finland, and only if they were able to operate at between 807 Hz and 1210 Hz, speeds appropriate to uranium enrichment gas centrifuges.
But a 'Rubicon' has now been crossed and the Siemens spokesman admitted: "People are developing more and more sophisticated threats." A solution to this kind of problem is now available from Norman Data, in the form of a 'SandBox', which unlike most anti-virus software, reacts to previously unknown threats by allowing executable code to run on a simulated virtual computer to see if it does anything potentially bad, before letting it through to the computer or that part of the computer that is actually executing the control task.
Most industrial system breakdowns, of course, are not caused by malicious writers of software, but by more mundane problems such as electric motor overloads leading to winding burnouts and less often, by electronic failures in inverters and controllers. Most winding failures, according to Steve Ruddell of ABB, result from bearing failures, followed by insulation breakdowns resulting from prolonged overheating. He argues that if an old motor does burn out, it is almost certainly more economical to replace it with a new and more efficient one than to pay for a rewind. And if the purchase of the new motor is coupled with the purchase of a matching variable speed drive, manual starter,or soft starter and overload relay, a fast acting trip will prevent the new motor burning out in the event of an overload.
To guard against electronic failure, keep processes going, or just to save money, Control Techniques has brought out a device called an 'SPM Power Selector'. Combined with modular Unidrives, this enables automatic re-routing of connections between SPM control and power stages. In the case of a system where serious consequences result if it comes to a stop, it can either switch in a normally redundant spare drive, or keep the system going at half power by switching out a failed member of a pair of drives and reducing the speed of the remaining one.
In a complex system, such as a rubber-tyred gantry crane, there are normally separate drives powering motors for lifting, traversing and travelling longitudinally. Since there is no normal circumstance in which the crane will be lifting and travelling longitudinally at the same time, it is possible to take out one complete 55kW drive by allowing the Power Selector to switch between motors, so that the same pair of 110kW drives powers either the 215kW hoist motor or the four 30kW long travel motors.
And in a rolling road test rig for heavy goods vehicles, it is possible to automatically switch parallel drives in and out to match output current to test tasks. Since torque is most easily measured by monitoring motor current, rather than using torque sensors, running smaller numbers of drives at full power rather than running more drives at less power, improves the accuracy of the torque measurements.
Design Pointers
• The advent of viruses for industrial control systems is worrying, but a solution has been found in terms of a virtual computer within the system, that can allow new viruses to execute their code in a protected environment
• Old motors that burn out should in many cases be replaced with new and more efficient motors with modern control gear to protect them against overloads, rather than pay for rewinds of obsolete equipment
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Author
Tom Shelley
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