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Precharge Circuit for Medium Voltage MV VFD

It is common practice to have a precharge circuit in almost all low voltage VFDs, although this is not the case in MV VFD design. Inrush current can cause major damage to a medium voltage VFD. This blog describes how and why this damage occurs. 

Medium Voltage VFD inrush current damage.

The image to the right is what was left of a Siemens Robicon Perfect Harmony drive damaged by inrush current.  The purpose of  pre-charge circuitry is to limit the inrush current that the VFD sees when power is first applied (to avoid damage as seen in the image). Most voltage source MV VFDs have a topology similar to the Robicon cascaded H-bridge design. Robicon came up with using low voltage devices in the nineties when MV IGBTs were not readily available in the market to introduce their first MV VFD.

In this design, Robicon now acquired by Siemens, they do not use a pre-charge circuit to soft charge the capacitors and limit the inrush current to the phase shifting transformer. The inrush current to the phase shifting transformer can be as much as 12 times the rated current of the transformer for about quarter cycle (4.2 milliseconds). The magnetization current manifests itself in significant vibration of the transformer coils.  This can cause the insulation to  eventually wear; resulting in catastrophic transformer failure. The picture is from a 2500 HP Siemens Robicon Perfect Harmony drive that failed due to this phenomenon.

At EMA, we buy and refurbish MV VFDs routinely. Our refurbished MV VFDs all come with full 12 months warranty. We recently supplied a refurbished Siemens/Robicon MV VFD to one of our major customers and to avoid transformer failures like what is shown above, we designed and built a pre-charge circuit incorporated within the Siemens/Robicon MV VFD. This helped tremendously in reducing the inrush current when the VFD was powered on. It is especially important in this case, because our customer uses the VFD  as a test stand where they power it off and on frequently. In cases where the VFD is powered  on and left for years, the risk of transformer failure is reduced.

Unlike low voltage VFDs, almost all MV VFD manufacturers use their own unique topology in their MV VFDs designs; it is not uncommon to find MV VFDs that have no pre-charge circuitry/soft charge circuitry. For the benefit of the consumer, EMA provides a free MV VFD topology webinar to inform potential buyers of the advantages and disadvantages of MV VFDs  out in the market so the buyer can make a more informed decision about which product best meets their needs.

Call 770-448-4644 OR use any of the contact methods on this site to discuss this or any other medium voltage VFD needs you may have.

Abdou Barrow, EMA Inc.


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