3786 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
these 4-node modes to the overall vibration signature of
the ring motor and thereby ensures the dynamic stability
of the air gap. An analysis of the full dynamic response of
the motor is also conducted to assess the overall vibration
level in the motor. The finite element model which is used
to calculate all frequency responses of the stator is verified
in the workshop with a tapping test on each stator (Orser,
Svalbonas and van de Vijfeijken, 2011 and Bordi &Green,
2023). The FEA also confirms that the air gap and vibra-
tions remain below the alarm limit in case of seismic events
and short circuits.
Later during project execution, the FEA is extended to
the overall system which combines the coupled dynamic
behavior of the ring motor, mill, anchoring, foundations,
and soil. The overall system analysis is an advanced assess-
ment methodology used to validate the mill system per-
formance during operating load cases, it comprises of two
parts:
1. Verification that the system has sufficient stiffness
to withstand the forces generated by the motor,
such that air gap is maintained within acceptable
limits pre-defined for the specific project.
2. Dynamic study evaluating system vibration perfor-
mance considering both charge and drive excita-
tion sources.
A future stand-alone paper will be presented on the topic
of system analysis. The criticality of system analysis is high-
lighted here as an essential element associated with mitigat-
ing risk related to taking the next step in mill and GMD
sizes. System analysis is currently being run proactively for
44 ft SAG and 30 ft ball mill designs, with results to be
presented in the planned paper.
Manufacturing and Transport
The ring motors are manufactured in ABB’s factory in
Bilbao, Spain, near the harbor on the Atlantic Ocean.
The stator is the largest and heaviest part of the GMD
and is usually split in four quarters to allow transport to
site. Figure 11 shows the transport dimensions of a ring
motor for a 44 ft mill. In case of project specific weight and
dimension limitations due to tunnels or bridges, the stator
can be split into more pieces. This has already successfully
been done, for example the Mt Milligan motor for the 40 ft
SAG mill and the Andina motor for the 25 ft ball mill were
both split in 5 pieces, installed and commissioned more
than a decade ago and in operation since. A survey of the
route from the ABB factory to the harbor of Bilbao was
conducted and confirmed that a stator quarter of a 44 ft
ring motor can be transported by truck. ABB assembles the
stator in the factory prior to shipment to check the sta-
tor interfaces and to ensure that the quarters will match
perfectly during site assembly. Two complete 44 ft GMD
stators can be assembled simultaneously in the space of the
workshop.
INTERFACE BETWEEN THE MILL AND
RING MOTOR
Successful design and delivery of any mill supplied with
a gearless motor requires close collaboration between mill
and motor vendors. As the motor rotor is mounted directly
on the mill, the mill body essentially becomes the motor
shaft. As a result, the motor needs to be designed to accom-
modate the expected mill body deflection. Likewise, the
mill needs to be designed to withstand significant drive
forces experiences through the range of operating states and
eccentricity conditions.
Historically this interface was managed primarily
from a geometric perspective. The mill vendor prescribes
mill dimensions, including GMD flange details, which the
GMD vendor is required to consider when developing their
rotor flange design. Likewise, the GMD vendor defines sta-
tor dimensions that the mill vendor needs to accommodate
within the mill foundation design.
In more recent years some gearless motor vendors have
prescribed stiffness criteria upon the mill vendor, to be
adhered to when designing the rotor mounting flange. This
is a mechanism for controlling mill flange deformation and
subsequent changes in rotor to stator air gap. As an added
benefit, controlling stiffness at the motor flange generally
improves mill performance from a fatigue life perspective.
In this way, mill and GMD suppliers have successfully
worked together for decades on designing and supplying
gearless mill systems. Processes and systems have been well-
defined to correctly handle and properly document this
important interface between the mill and the motor. The
same proven process will continue to be used for larger
mills.
In addition to the usual interface checks and consider-
ations, modern computing allows more detailed evaluation
of the mill to motor interface. Specifically, the rotor geom-
etry can be included in the mill body finite element model
as a three-dimensional solid body. This enables explicit
evaluation of GMD connection performance, includ-
ing bolt stress and contact utilization. Likewise, it enables
quantification of rotor deflection during mill operation.
Combined, this approach facilitates significant risk mitiga-
tion when increasing mill size and motor power.
This detailed interface evaluation has been performed
proactively for a typical 44 ft, 32 MW SAG mill and a
these 4-node modes to the overall vibration signature of
the ring motor and thereby ensures the dynamic stability
of the air gap. An analysis of the full dynamic response of
the motor is also conducted to assess the overall vibration
level in the motor. The finite element model which is used
to calculate all frequency responses of the stator is verified
in the workshop with a tapping test on each stator (Orser,
Svalbonas and van de Vijfeijken, 2011 and Bordi &Green,
2023). The FEA also confirms that the air gap and vibra-
tions remain below the alarm limit in case of seismic events
and short circuits.
Later during project execution, the FEA is extended to
the overall system which combines the coupled dynamic
behavior of the ring motor, mill, anchoring, foundations,
and soil. The overall system analysis is an advanced assess-
ment methodology used to validate the mill system per-
formance during operating load cases, it comprises of two
parts:
1. Verification that the system has sufficient stiffness
to withstand the forces generated by the motor,
such that air gap is maintained within acceptable
limits pre-defined for the specific project.
2. Dynamic study evaluating system vibration perfor-
mance considering both charge and drive excita-
tion sources.
A future stand-alone paper will be presented on the topic
of system analysis. The criticality of system analysis is high-
lighted here as an essential element associated with mitigat-
ing risk related to taking the next step in mill and GMD
sizes. System analysis is currently being run proactively for
44 ft SAG and 30 ft ball mill designs, with results to be
presented in the planned paper.
Manufacturing and Transport
The ring motors are manufactured in ABB’s factory in
Bilbao, Spain, near the harbor on the Atlantic Ocean.
The stator is the largest and heaviest part of the GMD
and is usually split in four quarters to allow transport to
site. Figure 11 shows the transport dimensions of a ring
motor for a 44 ft mill. In case of project specific weight and
dimension limitations due to tunnels or bridges, the stator
can be split into more pieces. This has already successfully
been done, for example the Mt Milligan motor for the 40 ft
SAG mill and the Andina motor for the 25 ft ball mill were
both split in 5 pieces, installed and commissioned more
than a decade ago and in operation since. A survey of the
route from the ABB factory to the harbor of Bilbao was
conducted and confirmed that a stator quarter of a 44 ft
ring motor can be transported by truck. ABB assembles the
stator in the factory prior to shipment to check the sta-
tor interfaces and to ensure that the quarters will match
perfectly during site assembly. Two complete 44 ft GMD
stators can be assembled simultaneously in the space of the
workshop.
INTERFACE BETWEEN THE MILL AND
RING MOTOR
Successful design and delivery of any mill supplied with
a gearless motor requires close collaboration between mill
and motor vendors. As the motor rotor is mounted directly
on the mill, the mill body essentially becomes the motor
shaft. As a result, the motor needs to be designed to accom-
modate the expected mill body deflection. Likewise, the
mill needs to be designed to withstand significant drive
forces experiences through the range of operating states and
eccentricity conditions.
Historically this interface was managed primarily
from a geometric perspective. The mill vendor prescribes
mill dimensions, including GMD flange details, which the
GMD vendor is required to consider when developing their
rotor flange design. Likewise, the GMD vendor defines sta-
tor dimensions that the mill vendor needs to accommodate
within the mill foundation design.
In more recent years some gearless motor vendors have
prescribed stiffness criteria upon the mill vendor, to be
adhered to when designing the rotor mounting flange. This
is a mechanism for controlling mill flange deformation and
subsequent changes in rotor to stator air gap. As an added
benefit, controlling stiffness at the motor flange generally
improves mill performance from a fatigue life perspective.
In this way, mill and GMD suppliers have successfully
worked together for decades on designing and supplying
gearless mill systems. Processes and systems have been well-
defined to correctly handle and properly document this
important interface between the mill and the motor. The
same proven process will continue to be used for larger
mills.
In addition to the usual interface checks and consider-
ations, modern computing allows more detailed evaluation
of the mill to motor interface. Specifically, the rotor geom-
etry can be included in the mill body finite element model
as a three-dimensional solid body. This enables explicit
evaluation of GMD connection performance, includ-
ing bolt stress and contact utilization. Likewise, it enables
quantification of rotor deflection during mill operation.
Combined, this approach facilitates significant risk mitiga-
tion when increasing mill size and motor power.
This detailed interface evaluation has been performed
proactively for a typical 44 ft, 32 MW SAG mill and a