Jouni Ikaheimo describes the development and benefits of a permanent
magnet (PM) motor for low speed applications, such as those encountered
in the paper industry.
Used for decades in fast running applications due to its favourable
weight-to-performance ratio, the permanent magnet motor has now been
adapted by ABB to provide high accuracy and reliability at low speed
without the use of gearboxes. It is a synchronous motor, which, as there
is no rotor slip, provides better accuracy than standard, asynchronous
units, where the slip varies according to speed and load. With a
synchronous motor, it is easy to optimise the speed, while the
elimination of slip compensation improves the dynamic motor control.
The construction of the traditional synchronous motor is more complicated
than that of the asynchronous motor, requiring more maintenance. However,
permanent magnets (in this case made from neodymium boron iron) simplify
the construction by creating a constant flux in the air gap, thereby
eliminating the need for the rotor windings and the brushes normally used
for excitation. The motor is energized directly on the stator by the
variable speed drive.
The synchronous motor can also deliver more power from a smaller unit.
For example, to drive the in-drives of a paper machine directly at 300 to
850rpm with a conventional asynchronous motor would require a motor frame
substantially larger than that of a 1,500 rpm motor. The new motor type
is, in most cases, the same size or even smaller than the existing
induction motor.
Standard induction motors, normally designed to run at 750-3000 rpm, are
not particularly well suited for low speed operation as their efficiency
drops with the reduction in speed. They may also be unable to deliver
sufficiently smooth torque across the speed range. This is normally
overcome by using a gearbox - a complicated piece of machinery that takes
up space and needs maintenance as well as considerable quantities of oil.
The new solution provides a high torque drive directly coupled to the
in-drive of the paper machine section. Eliminating the gearbox saves
space and installation costs, as the user only needs to prepare the
foundations for one piece of driving machinery. This also gives more
freedom for the design of the machinery layout. Getting rid of the motor
brushes and gearbox not only reduces maintenance; eliminating gearbox
losses also saves energy. The new motor is at the heart of a system known
as DriveIT Direct Drive. This consists of a DriveIT permanent magnet
motor, controlled by a DriveIT low voltage ac drive, based on the ACS 600
inverter, connected directly to the driven machine, without gearboxes or
pulse encoders.
Familiar motor type in new role
The permanent magnet motor has been around for decades but it is only now
that the technology is being applied to large motors. Their small size
and high accuracy have, in the past, made permanent magnet motors the
preferred type for wristwatches and computer hard drives. Now, the
largest permanent magnet motor weighs in at seven tonnes. This air or
water-cooled unit is of radial flux construction, with a permanent magnet
rotor. Power ranges from 27 to 1,800 kW and the motor voltage is
typically 400/690Vac. Like all synchronous motors, it can only be
controlled by a variable speed drive, and motor control must be
specifically developed for permanent magnet flux control. ABB's Direct
Torque Control method has now been further developed to achieve this.
Although the new motor will find uses in many industries, it will
initially find its greatest application in the paper industry, as paper
machines require large numbers of high accuracy, low speed drives. High
performance at low speed is sometimes achieved by using a dc drive, but
compared with this, the new development will save on motor maintenance,
as the permanent magnet mo