Transformerless drives - Truth and myths
Most medium voltage VFDs are supplied from the grid through an input isolation transformer. This transformer provides several key functions, such as voltage adaptation, galvanic isolation, limitation of fault current, often also harmonic reduction by phase shifting etc (for more details see [1]). However, there are also specific VFDs that allow direct connection to the grid without any input transformer. These VFD configurations are called transformerless drives, Direct-To-Line™ (DTL) or Direct-To-Drive™ drives. Transformerless VFD topology is shown in Figure 1 in blue rectangle.
Transfomerless drives are marketed to have multiple advantages over conventional solution with input transformer. Most commonly articulated arguments are:
- Lower hardware cost (no transformer)
- Higher efficiency (no transformer losses)
- Lower overall footprint
While the above 3 points are principally correct, the benefits are often smaller than they look at first. Moreover, the transformerless solution comes with certain limitations. They are typically not highlighted in the marketing brochures and catalogs but the user shall be aware of them before purchasing such a drive system. Later rectification may be very costly.
What you should know before deciding for transformerless VFD
Is the transformerless solution the right fit for you?
Purchase our publication and learn:
♦ What are the benefits of transformerless medium voltage drives
♦ Explanation why the marketing arguments may be too optimistic
♦ Fact-based realistic assessment of the benefits
♦ What limitations transformerless drives have
♦ What you shall absolutely know before purchasing such a VFD
♦ Applications where transformerless drives are a good fit
Avoid costly errors and specify the right technology for your application. With our inputs you can double check whether VFD with direct connection to the grid is a good fit for your application/your installation.
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References
[1] VFD Transformers: Introduction, MB Drive Services, November 2019, available online, https://mb-drive-services.com/vfd-transformers-introduction/
[2] When to go for an active front end drive, MB Drive Services, September 2024, available online, https://mb-drive-services.com/when-to-go-for-an-active-front-end-drive/
[3] What has a transformer and a gear in common, MB Drive Services, June 2025, available online, https://mb-drive-services.com/what-has-a-transformer-and-a-gear-in-common/
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