Getting Technical
to less than half the size of an ordinary shoe box. Smaller
controllers can now be designed to be mounted on inside
the casings of the rotating machines themselves thereby
eliminating the need for any separate enclosures. In fact, it is
now possible to buy magnetic bearings with the controllers
completely integrated into the actual bearings which
eliminates the need for separate controllers.
In the past, engineers encountering magnetic bearing
systems for the first time were often surprised by the quantity
and complexity of all the cables and connectors involved.
Electromagnets usually required dozens of electrical cables
carrying high currents and high switching frequencies.
Sensors also required more than a dozen wires carrying high
frequency signals. These cables and wires were often routed
to remote control rooms more than 100 meters away where
the magnetic bearing controllers were located. These lengthy
cables were prone to causing undesirable emissions leading to
electromagnetic interference and electrical noise pickup from
the sensor wires.
Newer compact separate controllers significantly simplify
the systems. Remote power which is typically 48-300 VDC
is supplied to the controllers by short cables between the
controllers and the machines, simplifying connections, reducing
emissions and eliminating the need for special sensor tuning.
ROTATING MACHINE VIBRATION MONITORING
In the past, monitoring rotating machines required dedicated
vibration monitoring installations. These large, expensive
systems consisted of proximity probes, conditioning electronics,
high-speed data acquisition systems, digital processors, and
alarming hardware. However, machines already equipped
with magnetic bearing systems can now also perform their
own monitoring without additional attachments through their
existing high-resolution position sensors, digital processing,
and communications which can also be networked to external
computer control.
Figure 2 shows a 400kW, 20 000rpm drive train with
magnetic bearings. The drive train incorporates a high-
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RACA Journal I January 2020
efficiency motor/generator. As such, the machine may be
used either as an electrical motor or a generator. Because of
the small size of the magnetic bearing controller it integrates
into the housing of the drive train so that the only required
connection to the controller is DC power. The magnetic
bearing controller can also control other aspects of the
machine such as the position of inlet guide vanes. The built-
in intelligence of the drive train eliminates the need for an
external controller.
Figure 2: 400 kW,
20 000rpm high speed
drive train on magnetic
bearings including
controller.
Electrically driven fans and centrifugal refrigerant compressors
in chiller plants using magnetic bearings benefit not only
from reduced friction but also avoid potential problems from
chemical interactions between refrigerants and lubricants in
refrigeration circuits.
In this age of environmental consciousness, equipment
manufacturers around the world are competing to build
better, safer, and greener machines. In striving toward such
lofty goals, many industries are rediscovering the advantages
of a fundamental principle — magnetics. Magnetic bearings
improve reliability, reduce friction, minimise vibration and
offer advanced monitoring and diagnostics — all without the
disadvantages of lubricants.
Figure 3: A standard
industrial motor supported
on magnetic bearings with
integrated electronics.
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