Blow moudler had the bottle to switch DC for AC
August 1998

The advent of sensorless flux vector inverters have enabled the use of AC motors on plastics blow moulding machines and now 45kW Hitachi J300 drives are coupled with standard AC motors on the machines specified by international moulding company Lawson Mardon Fibrenyle.

Plastic moulding machines have traditionally used DC motors because of the high starting torque required on start up. Standard voltage/frequency inverters had been tried in conjunction with AC induction motors in the past, but the lack of control over the flux vector meant that starting torque was insufficient to overcome initial resistance and the drive would nuisance trip.

However, as international blow moulding company Lawson Mardon Fibrenyle explained, the DC machines suffered from excessive brush wear. "The PVC we mould to make plastic bottles gives off slightly acidic fumes", said Mike Udall, production engineer at the company's Beccles plant in Norfolk. "Brushes in DC motors wear out anyway, but we have been spending up to 60,000 a year" he added.

Mike Udall pointed out that motor rewinding for the arduous duty cycles the moulding machines motors endure requires specialist repairers. Sets of brushes cost up to 500 each and a company such as Lawson Mardon Fibrenyle that has large numbers of machines, may stock several sets of a dozen or more sizes at any time. Downtime for repairs is expensive - the blow moulding machines operate round the clock seven days a week producing hundreds of millions of bottles a year. Moreover when a motor broke down, the PVC being moulded burns creating even more problems for the plant engineers.

Several inverter suppliers had suggested methods of replacing the DC drives, but according to Mike Udall, only HID with its Hitachi drives satisfied all the needs of the machines.

The inverters control the two screw motors of the machines which determines the rate at which material is delivered as the two parazons (the blown "bags" of material) which is then encased by the mould tool. Air is passed into each parazon while it is in the mould to create the final shape. The rate of feed from the screws, and hence the speeds of the motors is determined by a photocell which detects the material as it emerged from each nozzle. The signal from the sensor is relayed to the inverter via a PLC, and the drive adjusts the motor speed as required.

"HID has given us excellent service before, during and after commissioning," said Mike Udall, "we now have six machines fitted with pairs of Hitachi inverters". In this application, HID supplied the drives, software, training and the force cooled Leroy Somer 45kW motors which are used.

The Hitachi J300 sensorless flux vector inverters were the first AC drives to be introduced in the UK which have the capability of generating 150% torque at the lowest practical speeds. The drives have solved problems in all sorts of diverse applications across a broad spectrum of industries.

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