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PDL's Microvector drive offers precision control
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Bill Telford, Project Engineer, Plix Packaging with Richard Murton, PDL Electronics Applications Engineer.
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PDL Electronics' R & D department in Napier have once again trumped
the volatile motor drive market with their newest development in AC control
technology, the Microvector series of inverters.
"We perceived a need for a vector drive targeted at applications
which demand more control than the standard scalar inverter can meet,"
explains PDL's resident drive developer, Dr Simon Walton.
With the vector-based inverter you get very precise control over torque
and speed, which makes this type of drive particularly valuable in applications
requiring accurate feed and measurement control - say in a cut-off-to-length
conveyor situation - or where smooth zero speed and reversing ability is
required as in cranes and hoists."
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PDL's Microvector solution has only recently become possible due to advancements
in induction motor control theory and the arrival of a new generation of
high performance inverters and more powerful microprocessors. The result
is a drive specifically targeted at the most demanding end of the drive
market where precision and superior performance are demanded - feeders,
conveyors, positioning tables, indexing systems, stirrers and mixers, pumps
and compressors, cranes and hoists, winches and elevators, tension control
systems, wire drawers, coilers and winders, dosing and metering pumps.
There's nothing experimental about the final package. The new motor drive
has been developed on an already tried and true platform - PDL's Microdrive
series, which has a strong presence on the scalar drive market both here
and overseas. The new inverter's hardware remains essentially unchanged
from the now standard and proven UDi.
"Basically the Microvector achieves all the controllability of DC
drives using a standard AC motor and drive," explains PDL applications
engineer Richard Murton. "However, the drawbacks of the DC motor systems
are obviated and the principle benefits of the AC solution are maintained.
Obviously there's a substantial cost benefit in favour of the vector drive."
PDL are finding a ready response to the new series from industries where
accuracy and precision are in high demand.
Plix Packaging, a member of the Carter Holt Harvey plastic products group
sited near Hastings on the Main South Road, already had a suitable problem
for the Microvector solution.
"We've always had difficulties with one of our thermoformers used
in making plastic biscuit and chocolate packaging," explained Bill
Telford, Plix's project engineer. "This machine functions by pulling
a continuous plastic sheet through over a set distance and then heat forming
the required shapes in a repeating run/stop cycle. The main criteria for
this system are: (1) Accurate positioning for length and (2) Fast acceleration
and deceleration times in pulling the plastic sheet from its roll.
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"The DC drive system that came with the machine gave intermittent
problems over the four years it was operating. With the repeated breakdowns
we found there were no spare parts or servicing agents in New Zealand or
Australia - or even, for that matter, on the western seaboard of the USA,
its country of origin."
"At the point when we were facing a bill of $30,000 to import an
upgraded drive from the states, PDL came along and suggested we look at
the Microvector and replace the DC motor with a standard 11kW AC motor.
Quite frankly, the initial appeal lay in the timing factor - PDL could deliver
straight away, against several months for the Americans. And then there
was the additional fact that the Microvector cost less than half the American
upgrade and, what is more, PDL could offer immediate installation, backup
and servicing.
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A machine operator checks the shapes on one of Plix Packaging's
Thermoformers.
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"But the first benefit from installing the Microvector is not only
that it goes every time, but that it gives very accurate control over the
stepped movement of the plastic sheet through the thermoforming process.
The sheet has to be accelerated forward and then brought to a dead stop
over a 500 millisecond cycle to allow the actual heat moulding process to
take place in the forming section of the machine. With the old controller
and its DC motor this process created a substantial backlash in the chain
drives which had to be controlled by a torque-hold facility in order to
avoid transmission wear and damage."
"The advantage we've found with the Microvector lies in its different
control strategy. Instead of coming to a shock stop, it actually slows and
creeps to the final position, but all within the established 500 millisecond
cycle. So you simply don't have a backlash problem and the whole drive train
is set for the next cycle."
Machine operators at Plix get a lot more control over production parameters
such as acceleration and deceleration rates, cycle time rates and speed
with the Microvector, which is instantly adjustable by the operator from
a digital keyboard and display.
The thermoformer plays a vital part in the Plix plant, forming PET plastic
sheet into various shaped forms for the food and confectionery industries.
Because the plastic moulds are themselves fed into totally automated packing
systems, each must be identical with the other.
"We need a machine that is highly reliable, producing identical,
repeatable product over each cycle," stresses Bill Telford. "It
was crucial that the new drive be able to offer accuracy along with reliability
and Microvector has certainly provided that. We've very pleased with its
performance."
PDL Electronics are themselves more than satisfied with the Plix Packaging
result.
"The development of the Microvector series means that we can now
offer a complete lineup of drives to cover every industrial AC motor control
situation over the full power range from 0.75kW to 600kW," says PDL
Electronics general manager David O'Donoghue. "This is important news
for manufacturers like Plix Packaging who require precision control and
on-the-spot service. It also sends a clear message to the international
motor drive market that New Zealand is amongst the leaders in state-of-the-art
technology."
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