XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 3781
the GMDs compared to the RMDs and 3 million USD if
the RMDs have a gearbox. The reduced CO2 emissions due
to the higher efficiency of the GMDs depend on the type
of energy supply (renewable or fossil) and cannot be readily
quantified. A plant relying predominantly on fossil energy
will improve its environmental footprint by using GMDs.
The availability depends on the individual components
of the drive train. Some GMDs suffered major failures in the
early year 2000, as mentioned in the introduction, however
they are no longer representative of the modern installed
base. In a first approximation availability correlates with the
number of components that can fail. Due to the absence
of ring gear and pinions GMDs have an average of 1.5 day
more uptime yearly compared to RMDs and even 2 days
in case a gearbox is used between the motor and the pin-
ion. Given the throughput considered in Figure 1 and 2 the
increased availability of the GMDs leads to several million
USD more production depending on mineral type, grade
and recovery. Despite the assumptions and approximations
made here above, fewer larger gearless mills lead to a faster
return on investment of large mining project.
THE CHALLENGES OF LARGER GMDS
GMD System Overview
The motor of a GMD is a synchronous motor. Unlike
with RMDs where the motor is driving the mill through
a pinion and a ring gear, the poles of the ring motor of the
GMD are directly mounted on the mill. A cycloconverter
converts the fixed network input frequency into an adjust-
able lower output frequency for the motor. This allows the
speed to vary and achieve the low mill speed. Figure 3 gives
an overview of the complete drive system, consisting of the
ring motor, a containerized electrical room, and the cyclo-
converter and excitation transformers. The containerized
electrical room houses the cycloconverter, and the control
and power of the mill auxiliaries. The GMD is connected to
the on-site control room and, if required, remotely to any
off-site operation center.
Cycloconverters use proven power electronics compo-
nents and the 18-pulse configuration currently in operation
can supply power up to 40 MW. Likewise, the technology
for high power transformers is mature. Therefore, the focus
here will be on the ring motor. Increasing the size and
power of a ring motor impacts the electrical, mechanical,
and thermal design of the motor. These three disciplines
are interdependent and tackled iteratively to obtain a final
overall optimized design. The larger mill diameter and
higher power requirement obviously lead to larger electrical
key components of the motor, such as stator laminations,
windings, poles. This increases the weight, dimensions and
the drive forces thus modifying the mechanical require-
ments of the motor. Moreover, several mining sites under
development are at altitudes higher than 4500 meters above
Figure 3. GMD consisting of the ring motor (stator and rotor poles) an electrical room housing the
cycloconverter, control and power of the mill auxiliaries and transformers
the GMDs compared to the RMDs and 3 million USD if
the RMDs have a gearbox. The reduced CO2 emissions due
to the higher efficiency of the GMDs depend on the type
of energy supply (renewable or fossil) and cannot be readily
quantified. A plant relying predominantly on fossil energy
will improve its environmental footprint by using GMDs.
The availability depends on the individual components
of the drive train. Some GMDs suffered major failures in the
early year 2000, as mentioned in the introduction, however
they are no longer representative of the modern installed
base. In a first approximation availability correlates with the
number of components that can fail. Due to the absence
of ring gear and pinions GMDs have an average of 1.5 day
more uptime yearly compared to RMDs and even 2 days
in case a gearbox is used between the motor and the pin-
ion. Given the throughput considered in Figure 1 and 2 the
increased availability of the GMDs leads to several million
USD more production depending on mineral type, grade
and recovery. Despite the assumptions and approximations
made here above, fewer larger gearless mills lead to a faster
return on investment of large mining project.
THE CHALLENGES OF LARGER GMDS
GMD System Overview
The motor of a GMD is a synchronous motor. Unlike
with RMDs where the motor is driving the mill through
a pinion and a ring gear, the poles of the ring motor of the
GMD are directly mounted on the mill. A cycloconverter
converts the fixed network input frequency into an adjust-
able lower output frequency for the motor. This allows the
speed to vary and achieve the low mill speed. Figure 3 gives
an overview of the complete drive system, consisting of the
ring motor, a containerized electrical room, and the cyclo-
converter and excitation transformers. The containerized
electrical room houses the cycloconverter, and the control
and power of the mill auxiliaries. The GMD is connected to
the on-site control room and, if required, remotely to any
off-site operation center.
Cycloconverters use proven power electronics compo-
nents and the 18-pulse configuration currently in operation
can supply power up to 40 MW. Likewise, the technology
for high power transformers is mature. Therefore, the focus
here will be on the ring motor. Increasing the size and
power of a ring motor impacts the electrical, mechanical,
and thermal design of the motor. These three disciplines
are interdependent and tackled iteratively to obtain a final
overall optimized design. The larger mill diameter and
higher power requirement obviously lead to larger electrical
key components of the motor, such as stator laminations,
windings, poles. This increases the weight, dimensions and
the drive forces thus modifying the mechanical require-
ments of the motor. Moreover, several mining sites under
development are at altitudes higher than 4500 meters above
Figure 3. GMD consisting of the ring motor (stator and rotor poles) an electrical room housing the
cycloconverter, control and power of the mill auxiliaries and transformers