TVSS Protection for VFDs

Does anyone have any feedback on the use of Transient Voltage Surge Suppressors (TVSS) and their effectiveness for preventing damage to VFDs due to lightning strikes and/or power surges?


High Voltage or Low Voltage VFDs?

High Voltage VFDs: Manufacturers of these drives that I have worked with include a surge supression circuit as part of the drive. These invariably are R-C networks with in-line fuses with monitoring - contacts - to the main controller, PLC or drive. If a fuse blows the main breaker is opened with an appropriate alarm. Should protect the drive.

Low Voltage VFDs: Some drives manufacturers will include, as part of the rectifier circuit, varistors across the mains input phases. These small devices will conduct on the higher voltages associated with a mains spike. The trouble is, they have a limited amount of current carrying capacity and the ones I’ve seen, don’t have any monitoring capability to indicate that they’ve blown. So they may well protect the rectifier for a while until a really big spike comes along - but then you won’t know when it’s blown and so at the next spike your rectifier can be damaged and mains fuses blow and the VFD and application is out of action.

Now, there are various manufacturers of external surge supressors with neon (or other) idicators that I have customers using that they swear by. These are fitted across the mains phases just before the VFD input. Particularly in a mining company in a region prone to lightning strikes and the maintenance guys check regularly the state of the supressors for an unlit neon to indicate a lack of protection in that phase - ie it’s blown. then they change the unit out.

Now, I haven’t any statistics from that customer to show VFD failures before and after the installation of the supressors so I don’t know if they have really been effective. But, as I said, the customer is keen on them. Fitted across the inputs of all the different drives manufacturers he has on site, he also fits on the inputs to UPSs, telephone exchanges and other equipment that I can’t remember now - so, not my equipment single out! Maybe a buckles and braces approach?

Unless the customer insists on the VFD suppliers to include these external suppressors in their offer they are unlikely to be included as standard because of the increase in price.

Voltage spikes are a problem to VFDs and I would suggest 3 more considerations;

  1. Always include input reactors to your VFDs (not necessarily required if using a dedicated transformer - many Threads on this subject!).
  2. For new installations, get your VFD transformers supplied with a shield, earthed, between the primary and secondary windings. This breaks the capacitive effect between the primary and secondary circuits and reduces the incidence of high voltage spikes appearing in the low voltage side. Contrary to what some have said to me the price of the new transformer isn’t higher in my experience. I always include this in my specs to transformer manufacturers. (A Transformer specialist may like to explain more??). But in existing plants or where others are spec-ing the transformers this shield may not be possible to get.
  3. Make sure the installation is properly earthed/grounded to the VFD manufacturers recommendations.

If you’re also asking also about a power surge where the mains volts rise these supressors are unlikely to be of any use.

Mains spikes also come from the opening of contactors and breakers underload or fuses blowing, so should be considered as a likely event that VFDs need protecting against.

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