The medium voltage drive industry is growing, giving consumers more brand options than every before. For years, a small number of manufacturers dominated the market, but the amount of manufacturers producing medium voltage drives has increased dramatically. The ingenious patent implemented by Robicon expired in 2014, staging the way for other manufacturers to create similar drives (often called “Robiclones”). While this has given consumers more options and better pricing, it also leads to confusion; without understanding a few key details, you could end up with a very expensive mistake.
Footprint:
Size is especially important when retrofitting an existing medium voltage variable frequency drive. There are certain compact designs that simply fit in smaller spaces. Toshiba has long been known for their extremely compact design, while Siemens’ newer models are comparable as well. Yaskawa and Schneider MV drives tend to have a larger footprint, especially in the 2000 HP and below frames. Compact does not always mean better though. From a maintenance perspective, a large footprint, where components are spaced out a bit more, tends to be easier to work with and allows heat to dissipate easier. Smaller is not always better.
“Optional” Disconnect Means:
To be more cost effective, many manufacturers do not include input disconnect means as part of their standard package, it is an add-on option. However, for safety and controls reasons, it truly isn’t an option. When a MV Drive faults, it is good safety practice to completely isolate the medium voltage via a contactor or fused disconnect to protect important components within. Existing input disconnect means can absolutely be used with new equipment, but the integration is often not as easy as it seems. The cost and labor of cabling to connect the existing switch can quickly become as expensive as the add-on option would have been. Saving money by not going with these “options” may end up costing you in the long run. Quick plug: EMA SELLS AND DESIGNS SWITCHGEAR SOLUTIONS! If you need help implementing existing switchgear or need new switchgear, reach out to us!
Capacitor Type:
In medium voltage voltage-source variable frequency drives, there are three main types of capacitors—electrolytic, film, and oil-filled—each with different lifespans and costs.
- Electrolytic Capacitors:
Common and cost-effective, electrolytic capacitors are used exclusively in low voltage drives and often used in medium voltage drives. Electrolytic capacitors have a 7-10 year useable life so it is important to consider this for overall cost of ownership. - Film Capacitors:
With a 15-year lifespan, film capacitors are gaining popularity due to reliability and longevity concerns with electrolytics. Though more expensive, they’re now featured in drives like Siemens’ GH180 series and WEG’s MVW3000 - Oil-Filled Capacitors:
These offer the longest life—20–30 years—but come at a higher cost and weight. They’re less commonly stocked, making replacements more challenging.
Choosing the right capacitor type affects the total cost of ownership. Understanding these differences helps in planning maintenance and long-term investment.
Distance Between Drive and Motor:
In an ideal world, the medium voltage variable frequency drive would be directly next to the motor it is driving, but we rarely operate in an ideal world. If your application or plant design has the motor over 300 feet from the drive, you need to check the output topology of the drive you’re thinking of choosing. Medium Voltage Drive manufacturers utilize different output “steps” or “levels”, and some are more “motor friendly” than others. A 3-level topology is much less motor friendly than a 7 or 9-level topology. As additional levels or steps are added, less voltage is switched and a more sinusoidal output is created, thus allowing you to use longer lead lengths. Yaskawa has a nice video explaining this (start at time mark 10:37). The lower level topology drives maybe be a bit cheaper but additional output filters may be required, especially if the lead length is long. If the lead length is incredibly long, like a pump on an oil rig, a sine wave filter is necessary, regardless of the MV drive you choose.
Application/Rating:
In the variable frequency drive world, there are 4 main types of driven loads: constant torque, variable torque, impact, and constant horsepower. Knowing the type of load your particular drive will be running is critical to knowing if the drive is rated properly. Most medium voltage applications are variable torque loads, like fans, pumps, and centrifugal compressors. In variable torque loads, torque required varies with the speed of the motor (hence the name). As you increase the speed of the motor, the torque requirements increase exponentially, and very little torque is needed at low speeds. This characteristic is how VFDs save money on variable torque applications; for a better explanation, see here. Medium Voltage drives are sized for variable torque loads. Constant torque loads, like conveyors and positive displacement pumps and compressors require more torque at low speeds, so the drive needs to be rated appropriately to handle that. If a constant torque rating is required, please make sure someone (like EMA) helps you rate it properly. The last thing you want to do is buy an expensive drive that you cannot use.
Amps, Not Horsepower:
Speaking of rating: the full load amp (FLA) rating of the drive needs to meet or exceed the FLA of the motor it is driving. Many people get confused by the horsepower rating of the drive but that will vary by manufacturer. Below are two examples: The Siemens “1000 HP” drive is rated at 140 amps at 4160V and the Toshiba “1000 HP” drive is rated at 124 amps at 4160V. You might have a 1250 HP motor but if the FLA is below 140 amps, the 1000 HP Siemens will run it fine. This is a common mistake, make sure you size variable frequency drives by amps, not horsepower.
Need Some Advice?
If you need some advice about which drive to choose or if you’re looking for a refurbished Medium Voltage VFD, let us help you! Email us, Call us, or use the chat feature on our site. Let us prove to you that No One, ANYWHERE Is Better At Drives Than We Are!