Drives can now offer a performance approaching that of servo motion
control systems. However, not every application demands this level of
sophistication, so a user must choose wisely, based on a thorough
knowledge of their application's needs. Mark Daniels offers some guidance
When selecting a drive for a particular application, the first
consideration should be the expected characteristics of the motor load.
Following on from the theme developed in Rob Easthope's article this
month, most loads fall into one or other of three main categories:
variable torque, constant torque/no overload, and constant torque with
overload. In applications requiring constant torque, the continuous and
peak torque - and hence the steady and overload motor currents - need to
be determined so that a drive of the correct rating is selected.
Matching the right type of drive to the application is essential for
reliable, cost effective operation. More control does not always mean
better control, and a drive that is overly complex is more difficult to
set up and operate. Also to be considered nowadays is the drive's
fieldbus connection capabilities - a requirement that cuts across all
types of drive, regardless of their level of sophistication. Most ac
drives fall into one of three categories: Volts/Hertz (V/Hz), sensorless
vector and field-oriented control. The performance of general purpose
sensorless vector drives continues to improve, and these products are
increasingly used in applications requiring approximate positioning,
rapid acceleration, high torque at or near zero speed and wide speed
ranges.
Conveying applications usually require constant torque, though they may
also need an overload capacity if shock loads are likely or the drive
needs to start under heavy load. Inclined conveyors also require constant
torque but the drive's continuous rating needs to be higher and starting
torque may also be similarly increased. As speed control is unlikely to
be critical, conveying applications are well suited to basic V/Hz drives,
although conveyors with very high starting loads or close control of
speed may require a sensorless vector drive. Drives for conveyor control
have historically been stand alone, but communications are becoming more
important in distributed control applications. Networking a drive
provides rapid diagnostics and configuration, allowing more flexibility
and reducing downtime. If the drive is to be mounted on or close to the
conveyor motor, the IP rating may also be an important consideration.
Blowers, pumps and fans are generally variable torque applications and,
with the exception of positive displacement pumps, they do not usually
require a high starting torque or a large overload capacity. These
applications are ideal for basic V/Hz drives, sized for the continuous
load. However, the ability to connect to a network for remote diagnostics
and control is becoming increasingly important, even at this basics level
of drive.
Mixers require high starting torque, and in applications such as dough
mixing, a high running torque too. Therefore, a mid-performance
sensorless vector drive is required, carefully sized to cope with the
peak starting torque and occasional shock loads without being over-sized
for the continuous operating load. Mixing applications are usually stand
alone though, again, they occasionally require the ability to network the
drive.
Extruders, often found in the steel, aluminium and plastics industries,
require high starting and running torque, and a high performance
sensorless vector drive with encoder feedback is the minimum requirement
for this type of application. Some extruders require close control of the
tension of the material, and this requires the precise torque regulation
of a field-oriented control drive. Winding applications, such as paper or
wire making machinery, also require close control of torque and sp