(This blog is part of our series on motor management which covers various aspects of motor integration in an electrical network and the industrial process. See end of blog for other topics)
I am often asked whether LV or MV is a better solution for connecting motors.
As it turns out, answering this question is not just a comparison of direct costs related to equipment investment. It also concerns indirect costs such as for personnel training, habits of using LV or MV for a certain range of motor power, availability of suitable voltage levels, as well as the impact on parallel loads and overall electrical system. In the 100-800 kW range both LV and MV connection levels are feasible, and this is a range where established practices are just as important as some of the technical and economic arguments.
A few comparisons
Below are the results of several recent analyses I made comparing LV and MV equivalents for motors in the 100-800 kW range. More detailed information on the analysis behind these results can be found in our new motor management guide.
Figure 1 compares generally-established practices for voltage level selection and their reasonable limits. It shows that switching to MV sometimes occurs sooner than necessary, potentially creating hidden costs and unnecessarily increasing CAPEX.
In the decision process several aspects need to be considered:
- Electrical distribution architecture and availability of adequate voltage levels
- Capital expenditure (CAPEX) related to the necessary investment in equipment, software, etc.
- Operation expenditure (OPEX) related to operational performance, maintenance costs, and energy efficiency
- Total cost of ownership (TCO) of the installation
Obviously, if a certain voltage level is not available in an installation (usually MV), then the question of which connection voltage to choose is not relevant. But when there is a choice, it is quite difficult in the early phases of a project to predict and optimize OPEX. You need to have the process running before deciding if there is any room for improvement. CAPEX is much more straightforward and can be done at the very beginning stages, thereby getting your process up faster and more efficiently.
Using CAPEX as the primary driver
The design of the feeder for large motor applications contains several main components (see 5 Steps for Efficient Motor Management Design):
- Motor starting and operation control
- Power quality impact and additional cost for solutions
- Cable length
- Monitoring and control
All of these components can be specified as either LV and MV, however, the cost of MV equipment is typically higher for MV versus LV (except for monitoring and control). So then why is there even a question as to which way to go? Why isn’t LV normally the right choice?
Cabling as the deciding factor
In fact, there is one element to the solution whose cost is highly variable and which can be more expensive in LV than MV, and that is the cost for cabling. The reasons for this involve:
- Length of the cable
- Number of cables needed per phase for the same motor power at each voltage level
- Cost of labor for installation
In particular, the last two in the list above differ significantly between LV and MV. High power motors from 400kW and up may need only one cable per phase in MV, but two or three in LV. Therefore, although the unit cost of cable is lower in LV, the final total cabling costs and overall project CAPEX could be much higher. It’s estimated that LV cabling costs can be as much as 50% of the total cost of the electrical installation, which significantly impacts the otherwise lower cost of the rest of the equipment and gives rise to the question of which is better: LV or MV?
Some examples in pumping
To compare the different variants, we will consider three large pump applications in mining, each one related to a specific motor starting and control requirement:
The application for clear water pumping using a direct on-line started pump shows that MV is quickly a preferable solution, especially for motors at 400V:
The results above show a grey range where the LV to MV ratio is close to each other. In this typical situation, the decision is determined by indirect factors such as habits, training, local practices, servicing, space available, etc.
For the second and third examples (Figures 3 and 4) using a soft-starter or variable speed drive, the LV option remains attractive even for longer cables due to the higher cost of MV equipment:
It’s not one-size-fits-all when trying to optimize your motor management projects, and cabling may be the deciding factor when designing and planning your next electrical system for motor management.
Want to know more?
To get a more complete understanding of this topic, download our new guide on motor management: “Solving the LV vs MV dilemma when optimizing costs for motor management.”
Or visit our website on motor management.
Other blogs on motor management
“A case study: Comparison of MV and LV solutions for mine conveyor applications.”
“Large Motor Starting 101: Discover the Constraints”
“Large Motor starting 102: Motor applications and their characteristics – for a better motor management”
“How the right motor management saves +20% OPEX in LNG ships”