VFD overcurrent protection
VFD overcurrent protection
(OP)
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
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





RE: VFD overcurrent protection
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
RE: VFD overcurrent protection
RE: VFD overcurrent protection
RE: VFD overcurrent protection
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.
RE: VFD overcurrent protection
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
RE: VFD overcurrent protection
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.
RE: VFD overcurrent protection
Makes sense now.. In a, um, NEC sort of way.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
Hey, don't confuse me with facts - I was on a roll
RE: VFD overcurrent protection
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
RE: VFD overcurrent protection
We kid .. because we love.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
RE: VFD overcurrent protection
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
Gunnar you're welcome to come too. I'll buy!
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
RE: VFD overcurrent protection
One of my favorite restaurants is in Scotts Valley.
Curious, what type of product does your SV customer build?
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
Which one is your fav? Didn't seem like there were too many choices; 2 sushi places, a falafel shop and a bar and grill, aside from the chain operations. We went to Malone's Bar and Grill for lunch. Seemed like great food, but I can't taste so it was kind of a waste. Nice texture though...
RE: VFD overcurrent protection
Norman Malone's IS good food. Damned pricey though.
I was referring to Bruno's Barbecue. Really really good prices for very good food. As a family we drive clear up there regularly as we can eat for a reasonable number of $$.
Oh yeah! The very best Chinese food in the entire county is Mei Garden, just about across the street, to the right,(facing out), from NMalone's.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD overcurrent protection
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD overcurrent protection
RE: VFD overcurrent protection
Keith Cress
kcress - http://www.flaminsystems.com