Tenth International Symposium on Magnetic Bearings, August 21-23, 2006, Martigny, Switzerland
required force from the axial magnetic bearing. This
passive support is also present when the magnetic bearings
are not active, thereby reducing the axial load on the
backup bearings.
As described by Hawkins [1], the thrust backup bearing
is located in the lower, or bottom, end of the flywheel
module, as shown in Fig 2. The passive lifting element is
on the opposite, or upper, end of the flywheel module.
While subject to differential thermal growth effects that
can change its operating clearance, the axial air gap is
relatively large and the flux is provided by a permanent
magnet, thus significantly reducing force changes as a
result of gap changes. This allows for a fairly consistent
passive lifting force during all operating conditions.
Fig 2. Thrust Backup Bearing Arrangement
When the primary magnetic bearing system does not
sustain rotor levitation, the rotor drops onto the backup
bearings. To prevent continued operation during this event,
a fault signal from the magnetic bearing controller is sent
to the FESS controller, whereby the FESS goes into
shutdown mode. The duration of operation on the backup
bearings is then dependent on the type of shutdown present.
A typical verification test for a backup bearing system is
to deactivate the magnetic bearings at operating speed,
causing the rotor to drop onto the backup bearings and spin
down to rest. For the flywheel the duration of spin-down
can vary as it can be loaded or unloaded during spin-down.
There is a substantial body of work in the open literature
that investigates AMB rotors on backup bearings. Hawkins
[2] developed a rotordynamic simulation that included a
nonlinear backup bearing clearance effects to analyze
shock response in a magnetic bearing system. Several
authors have also described full five axis drop tests for test
rigs or machines for industrial service. Kirk [3] and
Swanson [4] have presented numerous test results and
analysis from a full scale, AMB rotor drop test stand.
Caprio [5] presented results for drop testing on a large,
vertical energy storage flywheel. However, all of these
drop tests are for machines considerably heavier and
slower than the flywheel described here, and all but [5] are
for EM bias magnetic bearings. Thus the available
literature was not able to provide much insight to aid in
guiding the design of the backup bearing system and its
expected results.
Three key magnetic bearing failure/fault types that
initiate shutdown mode were identified and used as the
basis of development and testing to validate backup
bearing performance.
1) MBC fault: During this type of failure, the FESS
commands a powered shutdown, with the FESS
discharging power to cause the rotor to spin down within
10 minutes. The rotor is assumed to be on the backup
bearings (though not necessarily the case in most
instances), with a 10 minute operating time on the backup
bearings. For this to function the FESS must be connected
to a UPS and the UPS under load, thus allowing the
flywheel to push power to the load. Most UPS’s will allow
such a condition, whereby the FESS raises the dc voltage
on the UPS bus to be the dominate power supply to the
UPS load. During this condition the flywheel energy is
discharged at a constant current, either at a level the UPS
load can support or at a maximum level set in the FESS
controller. This level is maintained all the way to 5,000
rpm, where then the FESS isolates itself from the UPS and
coasts down in speed to zero rpm.
2) Loss of primary AC power: This fault will activate an
auxiliary power system - the critical power supply (CPS).
The CPS takes in the Bemf voltage from the flywheel,
goes through an AC-DC converter, and powers the MBC.
The MBC is normally power by the auxiliary AC power
available in the FESS. The MBC is designed though to
accept both AC and DC power, thus allowing the DC
power from the CPS to support the MBC when AC is not
available. This use of flywheel power allows the rotor to
maintain levitation until approximately 8,000 rpm. At this
speed the flywheel rotor drops to the backup bearings for
the remainder of the spin down. Typical operating time on
the backup bearings is 30-40 minutes.
3) Multipoint failure: For example, failure of the
CPS/MBC and the FESS controller at the same time. This
failure would a spin-down on the backup bearings with no
assistance to minimize time on the backup bearings. This
spin-down takes between 2.5 to 3 hours on the backup
bearings from full speed to zero speed. This case, while
the most remote, is the most extreme in terms of backup
bearing wear. While in the other cases the most of the
bearing configurations proved successful, in this case
many did not survive.
One further failure type is the failure of the backup
bearings themselves during case 3. While not detailed in
this paper, this destructive test was prepared by cutting the
phenolic backup bearing cages in multiple locations prior
to assembly in the flywheel. The flywheel rotor was
dropped at full speed for an unassisted spin down. The
damaged bearing set failed during the first 2 minutes of