ASHRAE Journal - April 2020 - 55

APRI L 2020

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ASHRAE JOURNAL

PHOTO: DANFOSS

case of oil-free centrifugal
FIGURE 1 Magnetic bearings/control.
compressors, that technology was the magnetic
Bearing-MotorPosition Feedback
Position Feedback
Compressor
bearing. This technology
Controller (BMCC)
was the key, along with
Channel Assignments
Axi s
Bea ri ng
Cha nnel
the associated positioning
Position Command
X
Front Ra di a l
Fx
Signals
sensors and permanent
Y
Front Ra di a l
Fy
Y
X
Rea r Ra di a l
Rx
Shaft
X
magnet motor, because
Bearing
Y
Rea r Ra di a l
Ry
Z
PWM
250 VDC
Z
Axi a l
Axi
they enabled the highAmplifier
speed, contact-free cenShaft axes monitored
trifugal compression,
by position sensors
Y-axis
Position
which in turn enabled
Y-axis
Channels
Position
Target Sensor
Channels
Fx, Fy
Touchdown
Sensor
Sleeve
Rx, Ry
Channel Axi
small, quiet, efﬁcient
Bearings
and oil-free. At that time
magnetic bearings, which
X-axis
X-axis
Touchdown
Position
Position
were being experimented
Bearings
Sensor
Sensor
with on applications
such as large pumping
Impellers
Z-axis
Front
Rear
Motor
Position
Axial
systems, were extremely
Radial
Radial
Sensor
Bearing
Bearing
Bearing
complicated and costly.
Sensor Ring
Sensor Ring
The exponential computing power and cost
reduction curve of the time gave
The electromagnetic ﬁeld is conFIGURE 2 First prototype compressor.
them conﬁdence that, even if it was trolled by a high-speed processing
not economically feasible then, it
system and the location of the shaft
should be by the time they get to
is sensed by proximity sensors. They
market.
are placed radially around the shaft
The oil-free centrifugal magnetic
on both sides of the motor stator
bearings invented with this develand the axial position is sensed by
opment consist of radial and axial
an additional proximity behind the
components. The bearings control
second stage impeller. The shaft
the radial position of the shaft
position is fed into a real-time digiwithin its orbit as well as countertal processor which adjusts the elec- bearings. Versions of these technoloacting the axial forces placed on
tromagnetic ﬁeld to perfectly center gies existed previously but did not
the shaft by the thrust forces of the
the shaft within its orbit and desired ﬁt to the application or compressor
impeller. The axial force is based
axial position.
goals.
on the differential pressures across
The ﬁrst compressor prototype was
From there, the ﬁrst commercialthe impellers which push the shaft
commissioned in 1995. The enabling ization with compressors on chillers
in the direction of the comprestechnologies were not the only
in the ﬁeld occurred in early 2001.
sor suction. The magnetic bearing
signiﬁcant technology component
The compressors featured two-stage
levitates the shaft and positions it
barriers. Achieving the compressor
centrifugal, direct-drive, oil-free,
axially using electromagnets. The
prototype also involved inventing
magnetic bearing, permanent magbearings use a combination of perseveral other enabling componet motors, with fully integrated
manent and electromagnets. The
nent technologies, including the
power electronics. The capacity was
permanent magnets levitate the
soft start, IGBT, DC-DC converter,
300k W (~90 tons) and they achieved
shaft and the electromagnets are
high-speed motor, position sensors
the goals of being a package which
used to ﬁne tune the shaft position.
and digitally controlled magnetic
was more efﬁcient, signiﬁcantly

IMAGE: DANFOSS TURBOCOR APPLICATIONS MANUAL

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ASHRAE Journal - April 2020

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