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 Post subject: Variable Frequency Drives
PostPosted: Mon Aug 11, 2014 8:58 am 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
Hey guys,

I have a question about variable frequency drives. This is not really an area I am familiar with.
I was told that at our company if a motor is fed from a VFD it cannot feed back on the grid during a short circuit elsewhere. I do not know if this is true for all VFD’s or only for the ones we use.
Are their VFD’s that enable the motor to feed into the short circuit elsewhere on the grid?
And is this true for AC/AC, AC/DC and AC/DC/AC drives?

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 Post subject: Re: Variable Frequency Drives
PostPosted: Mon Aug 11, 2014 12:27 pm 

Joined: Tue Jul 29, 2014 4:02 pm
Posts: 5
Yes, there are VFDs with bypass capability, meaning a contactor is closed connecting the motor terminals to the incoming feeders once it has been started or does no longer need to be throttled.
Check the VFD literature to see if the drive has the capability.

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 Post subject: Re: Variable Frequency Drives
PostPosted: Mon Aug 11, 2014 1:38 pm 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
A VFD certainly has the capability to "backfeed" a certain amount of energy. It depends on the drive though.

First off, lets be clear of one thing. Though any drive can theoretically short circuit for a short period of time, the time is short (microseconds to milliseconds) before the semiconductors in the drive fail, sometimes explosively, which severely limits fault energy. The protection means is high speed fuses or breakers. If the protection isn't there, then the semiconductors become the fuses and open up on their own.

Now getting further into details. First we come to the front end. This can be a free wheeling diode array. These drives have zero regeneration capability so as such, as stated, there is ZERO fault energy conducted back onto the line side.

Second, there are cycloconverters. These are pretty rare but this allows you to make a slightly cheaper drive by pulsing an array of 9 IGBT's in such a way that it is a true AC/AC conversion with no DC link including regeneration capability, as opposed to the more typical 12 IGBT (or more) drive which has a DC link.

Third, there are SCR drives for DC. These essentially amount to the exact same drive as an AC inverter drive except without an inverter. Still usually have some kind of DC link but the regeneration capability (and limitations) are there.

Finally we come to the most common inverter drive which uses an AC active front end (GTO's, IGBT's, GCT's, etc.,) to convert AC into DC and feed it into the DC link which can be a capacitor (most common) or an inductor. Then the back end inverter costs of another set of converters (GTO's, IGBT's, GCT's, etc.) that convert the DC energy into AC. Typically these components are all rated in terms of RMS currents. Thus their maximum output (DC) would be 1.732 times the rated RMS current as a maximum, or 200% for convenience. Almost every spec sheet lists the capability of outputing up to 150-200% of rated current for up to about 2 seconds, which is long enough from an arc flash point of view. This is the limit in both the forward (motoring) and backward (regeneration) direction.

Thus unless you know that you have a free wheeling diode front end, you can basically assume that the drive can output up to 200% of rated current in a fault condition before the drive either acts to shut itself down to avoid damage. As we are dealing with a steep exponential curve, that's about it. Otherwise, the semiconductors themselves will self-limit the current via thermal decomposition (blow apart).. Some folks recommend 150% but the spec sheets I've read mostly seem to indicate a short time rating of 200% these days.

This does not consider a bypass contactor. If you have one, then the drive can THEORETICALLY deliver 100% of fault energy back to the line when it is run in bypass mode. And there's the rub. A long time ago (1980's), VFD's were horrendously unreliable so you always put in bypass contactors. However these days it is very rare to actually run a drive in bypass mode and they cost a lot of extra money for a feature that rarely, if ever, gets used. I can't tell you the last time any of them in my plants ever actually even got inspected to verify that they even work. So my personal advice is that this is a degenerate case that represents a rare exception and not a rule, so it probably should not be used for arc flash study purposes as it distorts the results by providing much higher fault currents than are likely to occur in almost all cases, which can adversely affect your results producing both higher and lower results than would be expected. So I just plug in the 200% factor on regenerative drives and 0% on non-regenerative drives.

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 Post subject: Re: Variable Frequency Drives
PostPosted: Tue Aug 12, 2014 7:46 am 
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Joined: Tue Apr 17, 2012 8:19 am
Posts: 241
Location: Charlotte, NC
It really does depend on which inverter you have.

We have a inverters that are designed to regenerate back to the grid. We do make Grid-Tie inverters, but we also have Active Front Ends for inverters that don't use a DB resister to keep the Bus Voltage from going over a preset point. But the power bridge is comprised of IGBTs. We do use semiconductor fuses which have very fast tA's as well.

But as Paul said, even without fuses, IGBTs do not tolerate currents much over the rated current for very long at all.

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