| These disturbances
pose a particular problem for machines with unwinders, and
therefore also for those with braking generators. They produce
an angular velocity at constant paper web velocity which increases
as the diameter of the paper reel gets smaller. If one of
the winders cuts out because of a dip in the voltage, the
entire plant is brought to a standstill. Considerable losses
in production result. The same goes for all other machines
with unwinding equipment.
"Condat has also had to face this problem”, explains
Mangier. The paper mill has implemented several measures to
protect its machinery, for example, it has installed an interruptible
power supply at the controls and adopted a policy of decelerating
the converters to a safe state and then gently restarting
them after an interruption. But the problem of voltage dips
on unwinders had not yet been solved.
Compensating for voltage fluctuations
As a way of significantly reducing the losses in production
that result from voltage dips, Siemens – the supplier
of coordinated multi-motor drives – has suggested installing
Simovert Masterdrives with a self-commutated infeed/regenerative
feedback unit; the so-called Active Front End (AFE). Unlike
line-commutated diode and thyristor rectifier units, an Active
Front End comprises a self-commutated inverter with IGBT modules,
a line-side clean-power filter and an intelligent control
unit. The AFE converter corrects the power dips which so often
result in downtime by acting as a step-up converter with a
DC link voltage that is higher than the peak value of the
supply voltage. In this way, brief voltage fluctuations can
be adjusted up to 65 percent of the supply voltage. For special
applications, it is possible to provide correction of even
greater voltage fluctuations.
In regenerative feedback operation – of the braking
generators, for example – a power failure does not result
in a commutation failure and tripped fuse, allowing processes
to run almost without interruption.
Knowing this, Condat decided to equip two of its new machines
with AFE; a coating machine with 91 drives and a screening
machine with six drives. The machine speed is 1200 rpm for
a base weight of 85 to 165 g/m2 and a web width of 4600 mm.
Low failure rate
After more than a year of operation, it is now possible to
make initial comparisons between the screening machine equipped
with AFE and the other machines which are either equipped
with direct-current generators or fed from thyristor rectifiers.
All machines are fed from the same supply system and at 550
to 650 kW their powers are comparable. Whereas the machines
using conventional technology failed about four to six times
last year, the machine supplied from the AFE failed only once
In addition to minimizing the number of failures caused by
voltage fluctuations, the AFE has improved operation in other
ways, too. For instance, it is possible to optimize the power
factor. One feature of the Active Front End is that it is
possible to select the power factor best suited to the application.
It is adjusted either directly by parameterizing the AFE,
or dynamically via field bus systems. Any value between 1
and 0.8 can be set.
Sinusoidal current with almost no harmonics
The AFE contributes to energy savings by supplying electric
current that is virtually free of harmonics and therefore
perfectly sinusoidal. Heavy energy losses that result when
the transformers of the multi-motor drive heat up, a characteristic
of conventional circuits with a high harmonic content, are
eliminated. This current is kind to networks. The combination
of a selective and high-speed IGBT and a line-side clean power
filter is responsible for the sinusoidal output current of
the AFE. As system perturbations never actually arise with
this technology, no elaborate efforts are needed on the line
side to correct them. Reactive power is also reduced.
The AFE technology is based on the Siemens standard frequency
converter Simovert Masterdrives. This cuts down on spare parts
inventory costs as well as the training of operating personnel.
So it no surprise that Condat has already made definite plans
to install their next Simovert Masterdrives converter with
Active Front End. Jacques Mangier: "The favorable results
we have had with our coating machine mean that we will also
be equipping our new roll slitter with the AFE”.
Drive, Switch & Control 1 / 2002 : (More featured topics
from Siemens website...>>)
<<
Back to Industry News
|
