Modern variable speed drives can do much more than vary the speed of a
motor. With onboard microprocessors running special algorithms, they form
an increasingly important part of a process control system. Many
incorporate quite sophisticated control features that allow them to take
the place of external controllers. Steve Ruddell reports
The programming flexibility of the newer drives allows them to be
supplied with control software tailored for the many applications drives
are called on to perform - pump and fans, crane and hoist control as well
as special control requirements in equipment such as washing machines.
Some drive manufacturers give users the tools to do the programming
themselves, like ABB has done with its Adaptive Programming, which allows
the user to freely programme the drive with a set of software blocks.
PLC-like functions integrated into the drive's firmware can eliminate
separate PLC components. The increased processing power and memory of
drives is changing the architecture of industrial control systems,
enabling configurations that are better adapted to the application,
sharing the work between the drives and overriding control systems.
Some ranges of drives can be supplied with special control algorithms
designed to suit particular applications. These can include algorithms
for control of centrifuges, decanter control for accurate speed
difference control of two shafts, extruders, pumps and fans, spinning and
traverse control in textile machines and control of cranes and hoists.
One such control algorithm developed by ABB is the software available for
electrical overhead travelling (EOT) cranes, an application to which
variable speed drives are well suited. Drives have good torque response,
vital to the control of the load. When travelling at speed, a great deal
of power is applied in the acceleration and deceleration of these cranes.
With a suspended load, if torque is not controlled, the load will swing,
with the danger of causing damage through uncontrolled impacts.
Web control in textiles and papermaking is a major application area for
drives. Drives using ABB's Direct Torque Control (DTC) motor control
platform can calculate the speed of the motor with a high degree of
accuracy. Motors are kept running correctly and the correct tension in
the material is maintained, taking account of the changing diameter of
the reel. The result is a straightforward, cost effective solution for
web handling applications.
Drives using DTC have software presenting a number of control options,
allowing the user to choose the control scheme that best suits the
application. The software can also incorporate a roll diameter
calculator. An internal calculation routine works out the roll diameter
as the product is wound around a centre core, while the external
calculation routine uses an external sensor input of the actual roll
diameter.
The most useful and cost effective way of modifying a drive today is to
change the software. A drive controlling a conveyor belt in a biscuit
factory, for instance, can be programmed to start and stop at certain
intervals, advance a certain distance, stop again for a number of
seconds, and so on. The same drive used in a ventilation system can be
programmed to keep constant air pressure in a ventilation duct.
PID (Proportional-Integral-Derivative) control, which is a standard
feature of many drives, allows the process controller to accurately
maintain a set point by adjusting the control outputs. Proportioning
control continuously adjusts the output dependent on the relative
positions of the process values and the set point. An example is
controlling the rate of flow of liquids or gases into an industrial
process. A variable speed drive with PID control capability would receive
as feedback the actual rate of flow of the fluids. The PID controller
automatically adjusts the pump or fan to