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.
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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
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
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.