VFD overcurrent protection 2

[toygasm4u]

Hello all,
I have a question about properly fusing VFDs. An example that has me perplexed at the moment seems to be falling in a grey area for me, as my ability to interpret NEC is lacking.

I have three 230V 1.5 HP motors with a 5.6A FLA each, to be driven from a single VFD. This would typically call for a 5HP VFD which requires 35A fuses or circuit breaker. The twist is, for this system, a high carrier is required for noise reasons. Therefore, I'm using a 10 HP drive at high carrier, which derates down to 5HP at a high carrier. This 10 HP drive calls for 60A fuses or Circuit breaker.

Is there any exception to the rules as I understand them, that says I can size my feeder circuit based on load + a percentage, vs. just sizing based on manufacturer's recommended 60A? This is a huge circuit for three 1.5 HP motors.

Thanks in advance,
~Mark

 

[DRWeig]

toygasm4u,

I'm looking at the 2005 NEC, 430.122. Branch or feeder circuit conductors to your VFD "...shall have an ampacity not less than 125 percent of the rated input to the power conversion equipment."

You can use a smaller breaker if you want, but you can't use smaller conductors based on my understanding...

Good on ya,

Goober Dave

 

[davidbeach]

The drive needs to be installed per its installation instructions. If those instructions call for 60A OCP then you need to install a 60A OCP. Part of the cost of your over sized drive is the connection of an over sized drive.

 

[toygasm4u]

Thanks guys. I didn't figure there were any magic loopholes anywhere, but I asked anyway.

 

[controlsdude]

Make sure you do overload protection for each motor, the drive can not protect the different size motors by its parameters.

Look at the overall length of the wire for all vfds you might need a line reactor in line side of the vfd. Look at the users manual pdf for the manu of the vfd.

 

[itsmoked]

That just seems so wrong...

If the loads cannot ever draw close to 10HP why would you need to provide 10HP wiring? I just don't see a good reason. If a fuse exists that would open if you even tried to get to the drives full rating how would there be any fire hazard? Seems like insisting on this greater power supply you would actually be increasing the hazard.

Anybody got an explanation?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Keith,

The concern is probably that at some point in the future someone sees a 10 hp drive and decides they can increase the motor size, then the fuse blows and they just put a larger fuse in since the fuse was "obviously" too small to begin with.

Also, you get into all the usual UL listing Catch-22s. The UL testing for the VFD was based on a certain size OCP. If you deviate from that, then it is not installed per manufacturer's requirements, blah, blah, blah.

Logic takes a back seat to the NEC and UL, unfortunately.

 

[itsmoked]

Yep thanks dpc for illuminating those points.

Makes sense now.. In a, um, NEC sort of way.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Yes, there is an explanation. The VFD has an internal DC link with capacitors designed for rated current.

The capacitors charge from zero volts to close to sqrt(2) times grid RMS voltage when the drive starts. The inrush current is usually limited, but only to values that does not trip rated OCP. It will probably/certainly trip OCP with lower rated current.

Not fair? Perhaps. But that's the way it is. Unless you use a sine filter at the output to get rid of noise.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Now there is non-NEC answer that I can more easily embrace!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Gunnar,

Hey, don't confuse me with facts - I was on a roll

 

[Skogsgurra]

Sorry - no harm meant. Roll on!

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

I had never understood that issue from an engineering standpoint either, I was in the "grin and bear it" camp. But I figured it out on an field problem I once had where fuses cleared too easily on an over sized drive (because of a 1 phase input source). They had sized the fuses for the motor load current x 1.732, but would occasionally pop the fuses after a power failure (on restoration of power). Turns out that because the drive ended up more than 2X the motor current (because of standard sizes available), their sizing method ended up with fuses that were significantly too small for the drive. Everyone thought it would be fine because of the actual load being so much smaller than the drive, and they checked the drive input current to see that it was what they expected it to be. I told them to fuse it per the NEC and the problem went away, but at the time I couldn't adequately explain it. I figured it out about a week later when discussing it with someone else, but of course by then nobody else cared.

 

[itsmoked]

We don't care Jeff.

We kid .. because we love.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Ouch...

 

[Skogsgurra]

Careful, smoked. I wouldn't have luvved that either.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Hey BTW Jeff when's your next sortie down here.

Gunnar you're welcome to come too. I'll buy!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

It could be sooner than you think! Beers for me - remember?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Yuck.. But yes, as a gentleman I would buy you one(1).

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

One crate is good enuff for me. Remember that Jeff wants one, too!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...