VFD Industry Requirements: Mining and Minerals
In this post we will explore VFDs working in the mining and minerals market segment. What are the services, drive requirements and technical challenges? Let’s have a brief look.
Drives in the mining industry
VFDs in mining industry are used to crush the ore, mill the material, transport it inside the processing plant etc. Therefore we have applications such as ball mills and SAG mills, high pressure grinding rolls, conveyors, positive displacement pumps and ventilation fans.
In this section we mention the most typical representatives along with their common characteristics.
i. Excavation
— Typical bucket wheel type of excavator
–High starting torque and high overload
— Power range 70-150 kW (small) / 300 – 2’000 kW (medium/large)
— Domain of LV drives
ii. Primary crushing
— First stage of material processing
— High starting torque and high overload
— Typical power range 200 – 2’000 kW
— Domain of LV drives
iii. Main processing and refining
— Ball mills, semi-autogenous (SAG) mills, high pressure grinding rolls (HPGR), ring-geared mill drives (RMD) and gearless mill drives (GMD)
— Ball/SAG mills with up to 8’000 kW per motor, typically geared solutions (‘high-speed’) using asynchronous motors with 6-8 poles and rated speed 1’000 – 1’800 rpm
— HPGR with rated power 1’500 – 3’000 kW
— RMD with up to 9’000 kW as single pinion and up to 18’000 kW as dual pinion
— GMD with typical power over 5’000 kW, using special low speed ring motors
— Generally high overloads during start-up
— Domain of MV drives
iv. Material handling and transport
— Conveyors for ‘horizontal transport’ (open pit mines)
— Uphill conveyors
— Downhill conveyors
— Typical power range 500 – 3’000 kW
— Braking capability required
— Multi-drive concept maybe an advantage
— Master/Follower control
— Hoists for vertical transport (underground mines)
— Typical power range 500 – 5’000 kW
— Wide speed range 10% – 100%
— High starting torque and high overload
— Braking capability required
— Slurry pumps, cyclone feed pumps
v. Auxiliary drives
— Fan drives for ventilation
— Pump drives for dewatering
Specific challenges
Mining applications set specific challenges for the drive systems that serve there. In this section some of them are mentioned.
i. Harsh environment
The environmental conditions are often very harsh. Most relevant factors are dust, extreme ambient temperatures and presence of vibration and shock. They apply mainly for the motors. However, the VFD may be partly exposed as well. Therefore, robust design and proper maintenance are desired. Liquid cooled drives may be more suitable due to higher ingress protection (IP class) and easier integration into a container (if applicable).
ii. Sudden load changes
Load torque is not always smooth. Besides short time overloads there are sudden load changes, especially in belt conveyors, crushers and certain types of mills. They affect the mechanical components and transmit to the electric motor as well. In addition, the control system must be able to cope with such dynamic torque requirements.
iii. High starting torque
Many applications in mining and minerals industry have constant torque character. Moreover, there are additional overloads appearing at low speed during start up. The torque required to rotate the mill depends on the angle with reference to steady state. Highest torque may be reached at approx. 60° to 70° of inclination. The overload peak may be challenging for VFD dimensioning as some VFDs have reduced current capability at low output frequency. High torque operation at low speed also brings challenges for the control system, particularly due to highly non-linear motor behaviour.
iv. Installations at high altitude
Some mines are located at high altitude, often exceeding 4’000 meter above sea level. The impact of high altitude must therefore be considered in system design and drive dimensioning [2]. High altitude affects the effectiveness of air cooling due to reduced air density. It generally affects all drive system components (e.g. increased BIL level for transformer). This is of course less relevant for liquid cooled drives. However, high altitude also impacts the dielectric insulation property of air that may lead to reduction of system voltage.
Third factor is increased concentration of cosmic ray which directly influences the reliability of power semiconductors. It is expressed as so-called FIT rate (FIT = failure in time) which tells about the number of device failures in 109 device hours.
v. Weak grid
Mines are mostly located in remote areas or even extreme places. Consequently, the supplying grid may often be rather weak (i.e. low short circuit capacity). It affects the drive system at least in two ways. First, the VFD topology shall be ‘grid friendly’ causing low harmonic distortion. Second, weak grid tends to have larger voltage variation (fluctuation) that may be observed during larger load changes. At the same time, weak grid is another good argument for using VFD driven motors rather than DOL machines.
Essential requirements
Each industry segment has its specific set of requirements on VFDs and power drive systems. Although there are some project-specific items and certain things also depend on customer philosophy, we can identify typical requirements that are valid for majority of projects in mining and minerals. Let’s have a closer look at some of them.
VFD design for mining and minerals
There are few design requirements frequently specified for drives utilized in mining. Some of them are listed in this chapter.
– Constant torque profile
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– Overload at low speed
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– Master/Follower mode
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– Exciter type
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– Remote support and diagnostics
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– Application specific functions
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Benefits of VFD driven motors in mining and minerals
Use of VFDs in mining and minerals industry brings multiple benefits. In this section the most important ones are listed.
– Smooth start
– Soft starting capability
– Decoupling from network transients
– Special service/maintenance modes
– Plant automation
– Advanced control functions
Summary
VFDs are used in the mining and minerals industrial sector for several decades. They help to reduce the energy consumption making mining a bit more sustainable. At the same time, VFDs inherently provide soft starting capability hence being more grid and load friendly compared to alternative starting methods. VFD fed motors have better controlability, especially in dynamic situations, such as sudden load changes. VFDs support plant automation that makes mines more profitable.
References
[1] KR400Nk compact bucket wheel excavator made by NOEN, https://www.noen.cz/en/reference/kr400lk-bucket-wheel-excavator-k-100/
[2] VFD dimensioning: High altitude, https://mb-drive-services.com/vfd-dimensioning-high-altitude/
