VFD catalog rating
You have surely noticed that most manufacturers have catalogs and brochures with tables listing their VFD types along with basic information such as rated output current, motor shaft power, cooling requirements, enclosure type, dimensions, weight etc. What is behind this VFD catalog rating and how to understand those values? And why is your project rating different than the rating stated in catalog?
Factors influencing loadability
As explained in our previous articles, the VFD loadability depends on multiple factors. The most important ones are:
– environmental conditions (coolant maximum temperature, minimum ambient temperature, altitude etc)
– motor parameters (rated voltage, rated frequency, power factor, equivalent circuit parameters etc)
– load profile (quadratic torque versus constant torque, short time overloads, field weakening range)
Table 1: Factors influencing VFD rating
Cooling obviously plays a vital role in VFD loadability [2]. Temperature of cooling medium affects the maximum ampacity of the product. There is a standard temperature range in which the VFD can be loaded with its nominal (catalog) value. Higher temperature of cooling medium leads to a current (and therefore power) derating. Sometimes lower temperature (below standard range) may allow certain uprating. It is not always applicable. For instance, a low cooling water temperature does not help for air cooled components. The VFD manufacturer knows the limiting factors and uses set of rules to handle all kind of non-standard operation conditions.
Note that liquid-cooled VFDs may be available with several variants of enclosures (ingress protection IP) that consequently have different impact of ambient conditions on power rating.
Installation at high altitude (typically above 1’000 m.a.s.l. or 2’000 m.a.s.l.) may cause reduction of system voltage and current reduction due to less efficient cooling. For more details and practical experience refer to article [3] dedicated to high altitude topic.
Standard VFD design usually covers variation of grid voltage in the range -10% to +10%. Wider range, particularly transient overvoltages, cause system voltage to be reduced that in turn means power reduction.
From above it is rather clear that values stated in technical catalogs present “typical” VFD catalog rating. In fact, below the rating tables there are usually notes and remarks explaining the assumptions. Therefore, do not be surprised if the VFD rating in your project slightly differs from the catalog value. It does not necessarily mean a mistake. More likely, it is the impact of project specific conditions that are not identical to the VFD catalog values.
High starting torque or low power factor of the machine are common factors that cause the drive project rating to be somewhat lower compared to values stated in a catalog.
VFD catalog rating versus project rating
The difference between catalog and project specific rating may go in both directions. In most cases your project rating will be somewhat smaller. For instance, the VFD catalog rating usually considers no glycol content for liquid cooled VFDs. However, your project ambient conditions require certain percentage of glycol as anti-freeze protection. Since glycol has a lower value of specific thermal capacity than water, the water-glycol mixture is a worse coolant compared to a pure water. As a result, there is certain power derating.
Table 2: Rating factors as function of glycol content
Project conditions might also work for you. Having a very cold cooling water helps to neutralize the glycol derating and eventually even allow slight uprating. It obviously depends on the fact how the catalog values are defined and what is the limiting factor in VFD loadability.
kVA versus kW
One good thing to remember is that most VFDs are rated based on the apparent power, i.e. kVA. The maximum motor shaft power is indicative and depends on motor parameters. Although the load-ability of the inverter also depends on the power factor of the load (e.g. internal loss distribution), you generally get higher shaft power with a motor having better power factor. This is also the reason why you might get a smaller frame size of the VFD feeding a synchronous machine compared to asynchronous machine of same shaft power and same nominal voltage.
How to dimension the VFD correctly
Renowned VFD manufacturers properly consider all factors having impact on the loadability of the VFD. This is an essential point to ensure high reliability and long lifetime. It is also necessary to keep high reputation of the manufacturer and maintain the brand name.
Smaller VFDs and VFDs for general purpose can often be configured by the user himself. The manufacturers provide sizing tools. A standardized VFD can be configured within shortest time. All the relevant factors having impact on the product loadability shall be user inputs in the tool.
Larger VFDs or VFDs for special purpose tend to be more tailor made products. There might be a sizing tool available, but more likely a drive specialist configures the VFD for you based on your specification.
Technical catalogs and brochures are often available at manufacturers’ homepage. It is perfectly fine to use the data, but don’t forget to read the assumptions and/or limitations. If your project conditions are rather special it is recommended to contact the manufacturer.
Summary
VFD catalog rating is useful to get a quick indicative information about VFD capability, its dimensions, weight etc. Small VFDs are usually treated as “standard”, “off-the-shelf” products that may really be sold based on catalog values. In medium voltage class, and especially in higher power range, the VFDs are much more customized. The customization is a loose wording. In practice, it ranges from almost standardized VFDs with few engineered options up to fully customized units dimensioned exactly for the specific project. In the latter case the catalog values provide just very rough inputs in early stage. In fact, for the most customized VFDs a catalog in this sense might not even exist.
References
[1] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[2] VFD dimensioning: Derating and uprating, https://mb-drive-services.com/vfd_dim_derating_uprating/
[3] VFD dimensioning: High altitude, https://mb-drive-services.com/vfd-dimensioning-high-altitude/
