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23 September, 2013

Variable speed motor drive interferes with plastics moulding machine

The moulding machines concerned use high pressures and high temperatures to mould the plastic parts, and are about the size (and general shape) of the boiler of an old-fashioned steam railway locomotive. They use powerful AC motors to drive their pressure systems, but the pressure demand is not constant, so much of the motor power is wasted in recirculation.

To reduce electricity consumption, save money and help save the planet, the owner tried fitting a 100kW variable-speed AC motor drive to one of the machines, but whenever the drive was operated the moulding machine suffered interference to its temperature control systems. The result was that the plastic parts were deformed because either they were moulded at temperatures that were too high or too low.

The problem was solved by fitting a suitable mains filter to the drive, with the filter’s case bolted directly to the chassis (several types of mains filters were tried, but most proved useless) and also by installing a shielded flexible conduit over the motor cable, bonded directly using EMC shielding glands to both the motor frame (at the terminal box) and to the drive unit’s chassis. Neither of these techniques had any effect on their own, they both needed to be applied together, which is typical of suppressing EMI from variable-speed drives.

Originally published in "The First 500 Banana Skins", Nutwood UK Ltd 2007, Email for a copy, cost around £10 plus p&p.

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