There is a good deal more to pump applications than most drives suppliers
realise, as one major pump supplier to the sector claims in this article.
At least one drive manufacturer has picked up the challenge with a
tailor-made solution for pumping applications in the water and waste
treatment industries. Tony Pickering reports
It is paradoxical, that while inverter drives have gradually become so
comprehensive that modern designs can provide functionality right into
the realms of servo performance, that there have also been parallel
developments of 'application-specific' drives designed specially to suit
the burgeoning market for fans and pumps. The HVAC market is case in
point; here, more competitive drives could be provided for fan operation,
taking advantage of the quadratic torque characteristics of fans. This
enabled a one frame size cost saving to be leveraged by providing a less
generous overload characteristic (recognising that fans can be
accelerated and driven with no need for greater than 120% overload).
The author's company adopted this strategy and became a market leader in
the HVAC sector. And recognising that in a similar way, drives specially
tailored to the needs of pumps in the water and wastewater sector could
improve competitiveness while also offering a range of features that
would simplify drive application for a range of process characteristics
peculiar to the water industry, Danfoss developed the VLT 8000 'Aqua'
drive.
While pumps represent a broadly quadratic load, somewhat similar to fans,
they need a wider range of control capability to simplify drive
application. Aqua offers both quadratic and constant torque performance
capabilities making it suitable for both centrifugal and constant torque
applications. Despite the perception that all centrifugal pumps exhibit
quadratic torque characteristics, constant torque drives are important on
waste water pumping stations, where maximum energy is required by the
impeller at lower speeds in order to reduce the chance of the pump
clogging on solids within the waste being handled. Moreover, the drive
enables the motor to be run at full load/speed without any motor
de-rating requirement.
Traditionally, this has required a 15% motor de-rating factor, making the
pump selection 'non standard' and therefore more expensive. At the same
time, improved energy efficiency is achieved by automatic energy
optimisation software, which actively controls the motor flux in response
to load demand, enhancing overall motor and drive efficiency and
minimising losses. Taking advantage of advances in power device
technology and particularly improvements in choke design and heat
management techniques also enabled a reduction in the enclosure size of
the high power Aqua models by more than 35%.
Reliability issues
In every survey of drives users, reliability is highlighted as the most
critical requirement. In many pump applications, drives are frequently
installed at remote locations, controlled via an Ethernet link and
effectively left to fend for themselves. This is where the Aqua's
automatic de-rating feature comes into play. Even under conditions of
phase-loss and supply voltage imbalance, which would significantly
shorten the life of a submersible borehole motor, the drive will
self-protect by reducing the output frequency and hence power, for the
duration of the condition. In the same way, this feature protects the
drive against damage from increases in ambient temperature beyond the
design point, perhaps not a common UK problem but one that has to be
addressed in some overseas markets.
Duty/stand-by and duty/assist/standby operation, a frequent requirement
of pumps, is catered for by pump rotation cycling alternation controls
which allow the drive to control either of two motors (to each of which
it is tuned). An embedded timer allows the user to sele