Mining/Metals/Minerals

A case study: Comparison of MV and LV solutions for mine conveyor applications

Conveyor applications are some of the most common, and the most essential, in the mining, minerals, and metals industries. Typically motors for conveyor applications range from several kW to 1200 kW, which makes them equally compatible with LV or MV solutions. In many specifications, high power motors and their variable speed drives are immediately set in MV, even though there are no specific constraints for connecting them at long distances.

But what could be the benefits of keeping low voltage for large motor applications, especially when the motor power range can be compatible with standard offers for the motor control?

Well, there are several advantages to using LV motors in an industrial process, mainly:

  • Similar motor cost compared to MV equivalent
  • Higher locked rotor torque resulting in potentially less constraining control requirements for starting.
  • Significantly lower cost of the motor management equipment (VSD, protection, circuit-breakers and contactors, enclosures)
  • Reduced footprint of the LV equivalent
  • No specific permissions necessary for LV commissioning and maintenance
  • Shorter delivery time
  • Simpler power cable connection

Let’s consider an example. In a project for an iron ore mine, three conveyor motors of 1250kW each are specified as 6.6kV MV. The motors are driven with a VSD of the necessary power. In the customer specification the distance between VSD and motor is indicated as a maximum of 1000m, which led to specifying an MV solution. Among the reasons for that are:

  • Lower current and consecutively reduced losses in operation, losses being proportional to the square of the current
  • Lower cable cost, as generally fewer cables per phase would be necessary to carry the current to the load, and their cross section is lower
  • Cable installation cost is lower when there are fewer cables per phase

Figure 1 shows the electrical diagram for a stockyard reclaim switchroom:

Figure 1 Original electrical diagram for the MV conveyor application

Figure 1 Original electrical diagram for the MV conveyor application

For the purposes of the comparison an assumption is made that the real cable length can be much shorter, which is often the case. At a first step the comparison for cost and footprint is made only on the electrical equipment integrated in the switch room. In the best possible case the LV solution is as:

conveyors in mining

Figure 2 Optimised solution for the conveyor drives with LV equipment

The proposed LV alternative solution is ~22% more cost effective for electrical equipment integrated in the switch room. The footprint is also reduced by 16%, which additionally will decrease the cost for the integrated solution, such as in an E-House.

You may say let’s go for LV!

Yes, but there is an important cost driver of this solution which is the cost for the cables between the VSD and motor. With 10x lower voltage, the current on the cables is 10x higher.  It does not necessarily mean 10x larger cross-section but one can imagine that with cable length there will be a balance between LV and MV solution. The analyses carried out for the case show the following trend, where a positive value means that the LV solution is lower cost:

conveyors in mining

Figure 3 Comparison of the cost reduction of LV solution and MV solution

The results above assume the same cost per unit length installation and termination of LV and MV cables. It can be observed that with 100m of cable the cost reduction is ~13%, and 200m seems to be the maximum cable length where a LV solution reaches its economical limit. The numbers can vary slightly if values for LV cable installation and termination are more accurate.

So MV or LV solution, before deciding check the real cable lengths, it makes a difference!

Additionally:

Motors today consume more than 90% of the electrical energy in electro-intensive industries, according to the US Department of Energy. In Mining, Minerals and Metals industries they are used for multiple applications, such as pumps, fans, gas compressors, kilns, conveyors, crushers, etc… In these applications LV motors represent roughly 80% of the total number with power ranging typically up to 400kW when used at constant speed and up to 1250 kW when they are variable speed. MV motors are much fewer in number, typically with a power range from 200kW and up. For large motors and long distances, MV motors are often preferred to LV due to the reduced losses on conducting cables and the cable sizing itself. This is often seen as a reason to specify MV machines in motor applications where the presumed cable lengths could be important. However, this easy solution is not always the most economical and too many overestimated cable lengths could have important drawbacks in terms of footprint and weight of the necessary equipment, which both result in additional costs.

5 Responses to “A case study: Comparison of MV and LV solutions for mine conveyor applications”

  1. Shailesh Kumar Chetty

    It is worth to mention about EMC issues and often length of cable is kept within 200mts, over which special design in cable are required.

    It is also interesting to study cost economics of MV Feeder, MV Cable, MV Motor Vs Additional converterduty transformers, VSD, space in control room, higher ventilation load, LV Cable, LV motor

    Reply
    • Delcho Penkov Delcho Penkov

      Hi Shailesh,
      Thanks for the comment. I agree about EMC, that is also why LV solution could not be proposed above 200m. The post is actually comparing the MV and LV solutions including all the points you mention, as feeder, transformer, VSD, cables, motors.

      Reply
  2. Shailesh Kumar Chetty

    Noteworthy Points:
    Screened cables become mandatory where cable length is more (typically >150 to 200mtr). The screen provides potential grading and limiting of the electrical field (EMC requirements), conduction of grounding and common-mode currents as well as touch protection.

    Usually Three-phase cables with individually shielded conductors are the preferred solution. Galvanized steel armor fulfills the function of an additional common screen.

    Due to the operation principle of the converter the cable must withstand special voltage waveforms including rapid du/dt-effects and reflections. Therefore insulation for converter use has to be stronger than in sinusoidal supply.

    Maximum cable length depends on oscillation frequency which is usually kept above 80kHz, this will ensure DTC control algorithm remains un-disturbed.
    EMC filter within VSD typically damps oscillations > 100kHz.

    Reply
  3. Bok-Lim Khoo

    It is actually horses for courses. Depending on the size of motors and the lengths of the cable runs it becomes a balance between copper cost and insulation (requirement) cost.
    It would be pruduent to check for the application of 3.3kV motors where insulation costs do not rise significantly as compared to copper costs, e.g. in 3.3kV motors. In addition there is the advantage of ease of (DOL) starting of these motors compared to LV motors and there is no requirement for LV ‘soft start’ equipment.
    The author has successfully designed an economical 3.3kV 1200kW motors DOL starting system for a power plant and with ‘large’ step down transformers in service in NZ.

    Reply
    • Delcho Penkov Delcho Penkov

      A good comment. The purpose of the optimisation is to avoid using MV equipment, which will be more expensive as such. Of course, for cables it will be the opposite with power increase. Regarding DOL motors, note that in the presented case motors are driven with VSD, and most of the cost of the solution would come from VSD and indirect impacts on footprint and weight. In DOL starting the difference in the motor starters is much smaller between LV and MV équivalents. So in that case a solution in MV will be more optimised for a shorter cable length.

      Reply

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