Upcoming Webinar:  Energy Savings Using Medium Voltage VFDs   Register Now →

Buyer’s Guide: How to Choose a Medium Voltage VFD

Do you find yourself in the market for a new medium voltage variable frequency drive? Where do you begin? Well, you’ve come to the right place. EMA is the industry leader when it comes to variable frequency drives, and we’re happy to point you in the right direction. 

This is a big investment, one that should last you 20 years or more, and there are several factors to consider before purchasing; not all variable frequency drives are created equally. Some of the more important factors are: capacitor type, pre-charge circuitry, overall maintenance time and cost, and output topology. 

Let’s examine each element in more detail: 

A medium voltage drive that is shut

Capacitor Type:  

For voltage-source medium voltage drives, capacitors make up the primary power source for the VFD and smooth out the power coming from the utility. There are 3 main types of capacitors used by VFD manufacturers: electrolytic, film, and oil filled, and each have different useable lives which contributes to the total cost of ownership. 

Electrolytic capacitors are the least expensive and most common type of capacitors within medium voltage VFDs and they’re used almost exclusively on low voltage drives. There are multiple factors that affect electrolytic capacitor life, but one should plan on around 7-10 years before they need to be replaced. Because they’re so common, electrolytic capacitors are relatively easy to find and can be purchased from multiple vendors (including EMA) when it is time to change them.  One other thing to consider is that even if the DC Bus is not comprised of electrolytic capacitors, many of the printed circuit boards within the VFD may use smaller ones for filtering, which could still require replacing with 7-10 years.  

Film Type capacitors have a life span of around 15 years but are a little more expensive to produce. Because of the stigma around electrolytic capacitors, film type capacitors are becoming more and more popular among MV drive manufacturers. Siemens now offers these on their GH180 drives.  

Oil filled capacitors have a shelf life of 20-30 years, but are heavier and much more expensive than electrolytic and film type capacitors. Oil filled capacitors are not as readily available if/when they need to be changed. 

Pre-Charge Circuitry 

In low voltage drive topology, a method of “softly” charging the capacitors is universally accepted, but in the medium voltage drive world, not all drives use pre-charge circuitry. For more on what the purpose of pre-charge circuitry is, see here. Because voltage-source medium voltage drives all use some sort of phase shifting transformer to mitigate harmonics, the impedance of the transformer is often used to reduce the current drawn by the capacitors on initial power up, which can cause significant stress on the transformer and can damage them eventually.  

Several manufactures utilize a pre-charge circuit to significantly reduce this potential issue. Adding this additional circuitry often adds cost and puts another element within the drive that can fail. There are arguments to be made on both sides as to which method is most economical, but it is a critical factor to consider when purchasing a medium voltage VFD. 

Maintenance Time and Cost 

Another major factor to consider is post-warranty maintenance time and cost. For most medium voltage applications, time and cost are one in the same, so the longer it takes to fix a problem, the more the drive costs the owner. Many medium voltage drives have the output phases in modular form (called power cells), which are designed to be fairly quickly removed and replaced with a spare to get back up and running. The design of the medium voltage variable frequency drive will determine how many and what type of power cells there are.  

Spare part availability is critically important as well. Waiting for days or even weeks for spare parts can end up costing you well more than the drive itself. Simply put: some manufactures are better than others at shelving spare parts. It is not uncommon for there to be a lead time on parts for a drive still in production! The simplest way to avoid this is to stock critical spare parts at your facility instead of relying on the manufacturer, but there are times the budget won’t allow that. EMA stocks many critical and hard to find spare parts, please contact us if interested. 

Output Topology 

A medium drive voltage that is open

Variable frequency drives create an output to the motor by switching the DC bus into a variable frequency and variable voltage output. For a better description of how this works, see here. “Topology” refers to the switching scheme employed by the VFD, and some are more “motor friendly” than others. Simply put: the closer the output waveform of the VFD is to a pure sine wave, the better it is for the motor. This especially comes into play the farther the motor is from the drive as dv/dt issues become more prevalent. There are filters that can be used to clean up the output of a VFD but those add additional cost and points of failure.  

The downside to a better output switching topology is that more output steps = more components and more components = more cost and more potential points of failure. Replacing a medium voltage motor is an expensive undertaking and another factor to considering what type of output topology to go with on a medium voltage VFD. 

Ultimately, it’s easy to get distracted by the initial investment in of a medium voltage VFD, but savvy shoppers know how to factor in the multiple elements that need to be considered. In many cases, initial upfront savings could cost you in the long run.  

Still unsure what you need out of a variable frequency drive? Contact us today! Our unbiased, 3rd party approach can help you make the right decision. No One ANYWHERE Is Better At Drives Than We Are! 


Leave a Reply

Your email address will not be published. Required fields are marked *

Get the EMA blog straight to your email

Get the latest and freshest content on managing your drives.
Related Articles