Hi,
I have several VFD's that have fried SCR's. Allen bradley says this is from a power surge or Lightning strike. We have a 138 kV line coming in that is stepped down to 13.8 then 4160 then 480 etc. etc.

Is the best solution a Isolation transformer to protect the VFD's or is there another solution?

Thanks

Windmiller

Hello Windmiller,
I recommend you install a sqare D surgelogic TVSS EMA series on the 480volt side of you service. They are very expensive depending on the level of protection and if you want an integral diconnect. Intermatic makes great TVSS units also. I have installed 6 3500 series TVSS and just installed a square D surgelogic EMA series. They also have surge counter and dry contacts on the from for remote monitoring. Just a suggestion and worth checking out the links below

http://ecatalog.squared.com/pubs/El...ge%20Protective%20Devices/1300BR0201.pdf


http://www.intermatic.com/Default.asp?action=subcat&sid=124&cid=63&did=6

Thanks i will look into this.

windmiller

You already have three isolation transformers front of your drive to "protect" it from lightning or surges external to your power grid, why do you think one more will help? You may want to look at your internal power grid and see if your facility is creating the power surges.

In the mean time the TVSS suggestion is probably your best solution.

VFD's do not have SCR's in them. If the fried section of the drive is on the input side, you have a bad rectifier bridge . If it is on the output side it will be the IGBT's.

Isolation transformers are used primarily on DC drives, as DC drives care little regarding what is feeding them, provided the voltage is in the correct range and all three phases are present. You already have three general purpose XFMR's in front of the drive. Or are they isolation XFMR's?

Input surges can be tamed to a certain degree with the use of a line reactor. If your drives do not have line reactors built in, then you must install them yourself.

Also make sure that the fuses that supply the drive are of the SCR type. Same type used on DC. If it's a breaker you will have to live with it.

There is nothing you can do if it is a direct lightning strike. You can purchase surge protectors, but lightning will blow them out too. However, there are some products available that say they can protect from lightning strikes. I don't believe it.

CHECK ON THE LINE REACTORS.

GET POCO INVOLVED. IT VERY WELL MAY BE THEIR PROBLEM NOT YOURS.

Hi,
Thanks for the reply. I will pass on your comments to Allen Bradley who has inspected the drives and responded that the SCR's were undoubtedly fried by lightning.

On the rest of your comments I have forwarded them on to our engineer for review.

I will check the fuses.

thanks

Windmiller

Hi,
I found this post:

Bipolar Transistor technology began superceding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology, which will form the basis for our discussion.

Thanks

Windmiller

You are confusing the output of the drive with the input. IGBTs are used on the output only.

Most VFDs use diode bridge input sections, however I believe the A-B still offers SCRs as an option.

If lightning was causing your drives to fail, then you should have other components in your facility also affected. It is unlikely that the A_B factory could differentiate between a lightning induced over voltage and one caused internally to your facility.

Hi,
ok thanks. I apprecitate your comments.

It sounds like you know a thing or two about it.

Windmiller

JBD,
Good point regarding other equipment failure resulting from lightning. If other equipment did not see catastrophic failure I doubt that it was lightning.
I once had several inverters that had defective input bridges when we received them. Of course they did not fail until the drives had run for several days or weeks. The warranty covered the repairs.

Not to knock Allen Bradley, but my drive repair guy hates the control. Just the drives, not the PLC's ect....

Hi,
Anybody else have any experience with AB Powerflex 700 Drives?

Thanks

Windmiller

A-B 1336 VFDs do indded have SCRs in the front end. They use them instead of diodes because they don't use a pre-charge current limiting resistor (and associated contactor) to protect the DC bus from inrush when you first power them on. Instead, they ramp the incoming voltage into the DC bus using phase control firing of the SCRs configured for rectification. It's a convoluted way of attaining the desired effect, but it saves them from having to design a pre-charge circuit. Several VFD manufacturers do this on larger HP drives now, especially those like A-B who also make DC drives and Soft Starters where their buying power for SCRs makes them competitive to diodes.

Still, IMHO the firing circuit necessary for them just makes for another thing to fail and if it does, they misfire, causing the SCRs to short. I have had dozens of 1336 front ends go bad on me in areas where line power is less than perfect, i.e. generator power in Alaska. The generator AVR can interact with the SCR firing circuit and cause both of them to start oscillating, which ends up in the SCRs failing. Once I figured out what A-B was doing, I gutted the SCR front ends and built my own diode front end and a pre-charge circuit, problems went away. Incidentally, A-B insisted all along that I was getting hit by lightning strikes, yet THEIR DRIVES were the only thing failing!