CONTROLS AND SENSORS Ӏ EQUIPMENT FOCUS
j Ahmad has recently done exactly that to port installations in Bristol, UK. In October Alatas also completed the control system replacement on a Liebherr BOS 40/1200 offshore pedestal crane in the Dutch sector of the North Sea. New hydraulics, new PLC devices, screens and control panels and operators cabin have all been installed, and the crane has just completed its post-upgrade load test. So overall, if you fancy being
a control freak now would seem a good time to start.
Enhanced control: beginner's guide to variable fRequency drive
Listen to any manufacturer of electrical-grid-powered cranes or hoists and they are likely to extol the virtues of their VFD system with the enhanced control capabilities it can offer. Infinitely-controllable speeds are the most obvious advantage. Your motor, and your hook or overhead trolley, can run at everything from full speed, for timesaving, down to ultra-slow inching, for final precise positioning of the load. VFD stands for Variable-Frequency Drive. Fine control of AC (Alternating-current) electric motors is not achievable without it. It is indeed, in its field, the next best invention since sliced bread. So what, actually, is it? Here is a beginner's guide. Non-beginners, skip to three paragraphs down. Electric motors that run off direct current, (DC), are easy to control: increase the voltage to the motor and it goes faster; decrease the voltage
and it slows down. And the voltage is easy to alter by putting a variable resistor, or rheostat, into the circuit. Turn the knob on the rheostat, the voltage to the motor decreases, the motor slows down. None of that works with alternating current (AC) motors. Their speed is governed by the frequency of the mains grid current that supplies them, which is 50 Hz (cycles per second) in the UK and Europe, 60 Hz in the US. An AC motor with, say eight poles (sets of windings) in its core, will rotate at eight times the grid frequency. If you disconnected half of the windings you could make it turn at half that speed; but when you have done that, you have just about exhausted all your options, as far as changing its speed is concerned. So enter the Variable Frequency Drive. It does what it says on the packet: It varies the frequency of the AC electricity supply to your motor. It
takes the 50 or 60Hz alternating current from the mains grid, and turns it to direct current, DC. Then a component called the inverter turns the direct current back to alternating – but at whatever frequency you happen to fancy. If you turn your 60HZ mains current into 20 Hz, your motor will now turn at one-third of its original speed. Job done. An AC motor can now have its speed easily and infinitely varied. So why are VFDs suddenly so fashionable in the crane and lifting world? Inverters used to be large and bulky, and inefficient – they consumed power. Today they are shrinking. From being features of top-end models only they are becoming standard across all manufacturers. Manufacturers are bringing out new, more efficient models every year. An example is MacGregor, part of Cargotec. This month (November 2021) it has bought out its next generation of VFD electric cranes. MacGregor has been making electric port equipment lifting gear since 2007 – docksides are ideal for grid-powered cranes, since cabling from the mains can easily be supplied to them - but, as it says, “electric motors and drive technology has evolved significantly during this time. As a result, the new generation crane is 15 percent more efficient than the first-generation design and up to 50 percent more efficient compared to the traditional closed-loop hydraulic-powered cranes.”
The benefits, though, of Variable Frequency Drive for shipbuilders, owners and operators remain unchanged: MacGregor list savings in capital expenditure through reduced generator size, smaller electric cables and no need for hydraulic oil. Higher speeds and precision control give better cargo handling efficiency and reduce time in port. Noise levels are significantly less, which improves operator comfort; and maintenance costs are also lower. And the environmental risk of hydraulic oil leakage is removed. Electric drives are available for all merchant cranes in the MacGregor
portfolio, including port, bulker, container, multi-purpose and heavy lift cranes. In addition, all electric cranes can be connected to their OnWatch Scout condition-based monitoring and predictive maintenance service, with the option to include several automation functionalities such as Auto-Drive and Auto-Tandem.
An upgraded crane control installation by Alatas
42 CRANES TODAY
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