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:
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:
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:
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!
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.