How to choose a medium voltage VFD: Life cycle cost

What determines the life cycle cost? The factors are:

– maintenance intervals

– required service expertise

– maintenance effort

– lifetime of components

– prices of spare parts

– control upgrades

– design lifetime

– service support

Maintenance intervals define how frequently have the inspections to be done. Important is the duration of such a check and also if the inspection can be done while the equipment is running or if you need to shut down (loss of production).

Service expertise basically means if your personnel can perform the required maintenance tasks (at least some of them) or if you need to engage a service specialist from the manufacturer with expensive daily rates and possibly also limited availability. Does the manufacturer offer a maintenance training? That is a good investment in the long run.

Maintenance effort tells how much time and money will the individual inspections cost you. It could be anything starting from short visual inspection up to complex measurement campaign. Is the VFD designed in a way that it is service friendly, i.e. all main components are easily accessible and replaceable? If not, the maintenance tasks will require much more time not even mentioning a failure case and time to repair…

Lifetime of components is important. Lower quality components or imperfect design require more frequent replacements, higher cost, more shutdowns and less productivity. Look for the maintenance manual and study the replacement intervals of key components.

Pricing strategy for spare parts is crucial. Can you get the spare on the market or do you need to buy original components exclusively from the manufacturer? At least some of them you most likely don’t find on the market. If from manufacturer, is the pricing reasonable? As already mentioned, some manufacturers base their strategy on selling equipment for very attractive price, but making their profit on costly spare parts.

As the control hardware evolves much faster than the power hardware, you will very likely need a control upgrade during the lifetime (Let’s not confuse upgrade of control hardware with a new software release). Does the manufacturer offer such service? If not you shall be very careful! There are many customers that replace the VFD after some years. Not because the power hardware does not work anymore, but because the control system is outdated and they get no support from manufacturer… It can get very costly as the new VFD is potentially not compatible with existing transformer so you need to purchase also a new transformer. Motor compatibility will govern your VFD selection so the choice will be limited. Your personnel will have to learn how to operate the new VFD. Replacement of old VFD and commissioning of the new one will take more time than a control upgrade of existing VFD would take. It all can be avoided if control upgrade is available.

Design lifetime determines the expected period when the VFD is operational. Real lifetime depends on the way it is operated (continuous duty vs frequent On/Off, stable load vs heavy duty overload), ambient conditions (within or outside of design specification), regular maintenance etc. Inductive components (reactors/chokes) last very long, often more than 30 years. Capacitors might need more frequent replacement, but generally self-healing foil capacitors provide about 12-15 years of lifetime. Some semiconductors like classical thyristors or IGCTs practically do not age as confirmed by rigorous analysis when returned back from field after many years of operation.

Service support basically sums up all previous items. Is the manufacturer willing to support you throughout the lifetime of the VFD or is he only interested to sell you a product and leave you alone with all challenges that will come? Both types of manufacturers exist. And just because all of them mention excellent service capabilities on their website does not necessarily mean that they really do it. Ask for references. Check how they support the existing installed base of older VFDs. Contact the end users for their feedback.

If you want to get an idea about life cycle cost (LCC) you can request a firm offer for 10-20 years maintenance contract as option. You might not purchase this maintenance package in the end, but it gives you a fairly realistic picture about the future expenditures. When comparing product A and B you shall maybe compare the sum of equipment price and price for maintenance contract. Don’t forget to check the control hardware upgrades and manufacturer’s experience in that field. And simply look at what the manufacturer have done in the past and is doing today. What kind of support do you get for a VFD manufactured 15 years ago? Is it supported at all?

Hopefully more users will base their purchase decision based on life cycle cost and services rather than just initial VFD price. The cheapest VFD might actually have the highest life cycle cost. In worst case the manufacturer will not be around in few years anymore.