There is a growing trend among drives suppliers to provide what is
becoming widely known as the 'decentralised' drive - a robustly housed,
well-sealed product that can be located out in the plant, near to the
motor it controls. Malcolm Staff takes a closer look
The concept of accurate, dynamic shaft performance from ac motors in
industrial applications has been developed to near perfection through the
last decade. Microprocessors and new circuit board techniques allied with
more effective vector control strategies have made decentralised drive
solutions not only possible but highly practical. Even sophisticated
frequency converters are now squeezed into tiny enclosures and can be
mounted directly on the motor - or close to it - allowing the drives to
become integrated parts of their applications.
The evolution of decentralised drives started in the early '90s, with the
introduction (by Danfoss, Siemens and others) of the 'variable speed
motor', effectively a motor with an inverter mounted on it. For some
applications these proved popular but where the assembly was installed in
relatively inaccessible locations, the reduced reliability of this
motor/electronics combination gave rise to maintenance problems.
Furthermore, at a time when fieldbus was less widely used, all that could
be offered was standard analogue control, involving long runs of
multi-core control cabling. These inconveniences limited the growth of
what we now know was to become the next generation of variable speed
drives.
The option of having the drive and motor integrated - or at least located
very close to one another - offers significant benefits. Cables
connecting the drive to the motor can be kept very short, effectively
eliminating EMC problems. Looping the power cabling inside the drive also
reduces the length of cable required, while fieldbus connectivity greatly
simplifies the control cabling - all of which has a positive impact on
installation costs.
The mounting flexibility, moreover, enables the whole drive assembly to
be relocated when required, with quick and simple re-connection to mains
and fieldbus cabling for rapid re-commissioning. Fieldbus is actually
quite critical to achieving the full advantages of decentralised drives
installations. In addition to reducing cabling requirements, networks
such as Profibus and DeviceNet provide the facility for remote control
and monitoring, as well as enabling drives set-up from a remote PC
running special configuration software.
Placing the drive in, on or near by the motor exposes it to contamination
such as water, dirt, oil, cleansing agents, high pressure hosing, and
even to mechanical wear. The motor-drive combination, therefore, has to
be extremely robust and designed to resist these adverse conditions while
preserving the benefits of simple installation, ease of commissioning and
minimum maintenance requirements. IP66 sealed enclosures are a must, and
to protect against adverse site conditions and aggressive cleaning
agents, these enclosures are often treated with an acid and alkali
resistant paint finish.
Easy installation is clearly desirable, and this is an important aspect
of the hardware design Decentralised drive products from the author's
company, for example, gather all the connections together in the bottom
of an IP66 rated, cast aluminium shell. Spring operated cable clamps
provide fast, maintenance free connections and looping of both power and
fieldbus cables inside the shell, independent from the control
electronics. These are housed in the top part of the shell, and are
easily removed or replaced without interfering with the cabling. The top
control section connects with plugs to the bottom power section,
minimising commissioning and downtime to create a plug-and-play
decentralised drives solution.
Malcolm Staff is a director of Danfoss