VSD Failure

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Jul 6, 2012
Reaction score
South Ayrshire, Scotland
Hi everyone

Fitted some big agricultural fans this week and one of the guys when back to adjust the louvres to direct the air. Both fans are controlled by VSD’s and one controller, instead of turning them off at the controller he just switch them off whilst running, which to me is always a bad idea…anyway…the vsd’s are now dead, breaker trips as soon as you try and start the fans.

This has happened a couple of times before on other pieces of equipment running of different types of VSD’s where they’ve been running and then turned off at the isolator or breaker and then they’re just dead.

Why does this happen and will the same thing occur during a power cut?

Thanks ☺️
20 years ago power fail regularly damaged VFD's, nowadays nearly all drives are designed to survive a complete unexpected power outage. Usually somewhere in the 600 page, 14 language manual it will state a limit to the frequency of power outages the drive is capable of handing and where I am we have an unstable power grid with planned and scheduled 'loadshedding' several times a day so we need to be mindful of this when specifying VFD's. I recently installed a Danfoss drive that was rated to handle 'occasional' complete power outages and they didnt define exactly what occasional meant. Another drive I recently installed was a small Yasakawa (maybe 18kw) and it was rated to withstand no more than 1 power failure per hour.

Far more likely to cause failure would be rapid cycling of power supply either where the isolation isn't fast and clean or where isolation was followed closely by reapplication of power. Also I'd check for the usual suspects for drive failure such as poor wiring terminations.

I'd be interested to know what the suppliers find when you hand it in for warranty claim.
Spoke to the fan supplier today and they’re coming out Monday to find out what’s happened. I’ll see if there’s any info in said 14 language manual 😂 I wouldn’t like to think that they were switched off and on several times but it might have been.

Our other guy who turned them off has been told now to shut them down properly in future 😂