Oversizing Drive due to Thermal Considerations

[rupej]

I sort of follow your logic.

What would it hurt to put 10A of protection on a 15HP drive that has been de-rated to only pull 10 amps? I do not see a danger here.

Nothing is wrong with undersizing the fuses- that's not in dispute. Your comment about under-sizing the conductors is the problem.

 

[Tharon]

NEC really gets on my nerve sometimes. I understand safety but sometimes the rules makes no sense or are overkill.

I use a cooling motor in a vacuum furnace.
The motor runs in a pure argon atmosphere enviroment.
Because it is in Argon it must be rated at 230V by NFPA 86. )I believe this is related to the fact that argon gas can be "charged" to carry current, so higher voltages MAY cause the windings to "flash over".)
Now NFPA is fire protection code......in a pure argon environment, how is a motor going to catch fire?
I have several jobs overseas that are running 415V motors for years with no issues..... in an argon enviroment.

While the NEC is under the NFPA, a Fire Protection authority, it is not designed only to prevent fires. That just happens to be the authority that took control of that regulation.

NFPA also has regulations regarding guarding and operation of industrial equipment.

So I don't think it's right to dismiss them or question them just because something they are regulating is outside the scope of fire protection.

 

[kamenges]

Originally posted by rupej:

Nothing is wrong with undersizing the fuses- that's not in dispute. Your comment about under-sizing the conductors is the problem.

You are correct from a standards standpoint. However, from a pure engineering standpoint this becomes something of an oddity. The general rationale of overcurrent protection is that you are protecting the conductors from carrying more current than they are capable of. If you have limited the current available to the conductors by selecting a specific size fuse why can't the conductor size follow the overcurrent protection? The answer is there is no good engineering reason that they can't. However, there is a standards reason.

Originally posted by Tharon:

NFPA also has regulations regarding guarding and operation of industrial equipment.

I have not personally seen a copy of this but I feel pretty confident in saying that the original issue of NFPA 70 from 1897 says nothing about guarding or operation of industrial equipment. Like most standards, NFPA 70 is a compilation of personal pet issues and unchallenged power grabs that happen to not violate the standards charter. The spirit of the 1897 standard was to guide the application of a fledgling technology to prevent accidentally burning a city to the ground. Many of the sections in NFPA 70 go beyond that by a good margin.

Keith

 

[Tharon]

I may be misunderstanding some things. But in the past when I've discussed things like Mechanical presses, two hand safety operation, and emergency stop circuits with people I've been referred to NFPA 79 (but never actually read or possessed a copy). So I thought NFPA 79 discussed safe operation of machine controls.

 

[NetNathan]

I may be misunderstanding some things. But in the past when I've discussed things like Mechanical presses, two hand safety operation, and emergency stop circuits with people I've been referred to NFPA 79 (but never actually read or possessed a copy). So I thought NFPA 79 discussed safe operation of machine controls.

You are sort of correct...but the NEC is NFPA 70 not 79.
Here are a few I reference...more depending on what you are working on...

NFPA 70 = National Electrical Code
NFPA 79 = Electrical Standard for Industrial Machinery
NFPA 86 = Standard for Ovens and Furnaces

Here is an NFPA directory, also written in "standard form" (these guys are paper bound and love writing in "gobbly goop").
The standards are listed about 2/3 down.
https://community.nfpa.org/community/nfpa-today/blog/2015/04/09/download-a-free-copy-of-the-2015-nfpa-standards-directory

 

[Tharon]

You are sort of correct...but the NEC is NFPA 70 not 79.
Here are a few important ones to reference...
NFPA 79 = Electrical Standard for Industrial Machinery
NFPA 70 = National Electrical Code
NFPA 86 = Standard for Ovens and Furnaces

Understand, I never meant to imply that NEC is NFPA 79. My comment was that NFPA has guidelines outside the scope of fire protection. I didn't mean to say that the NEC had standards for Industrial machinery.

I can only imagine this came from the NFPA not wanting people to burn down things using electricity, and then from there it being on them because it's all related back to that electrical source.

 

[jraef]

If a drive has to be oversized (125A -> 156A) due to the ambient temperature around it. Does the upstream components require the same oversizing?

I assumed the over-sizing requirement was so the components in the drive could operate at the 125A range at the desired temperature. From a mechanical standpoint I don't see how the system could use over 125A so why oversize the drive upstream components?

Does this even make sense?

Back to the original concept, one thing to remember is that consuming devices like VFDs can be designed to a maximum temperature that is typically higher than normal components. For example a typical VFD is designed for 40C ambient, but can be de-rated to operate at 50C. With a circuit breaker however, they are calibrated for 30C operation in switchboards and although they can operate at 50C as well, it requires different packaging, not de-rating.

 

[rQx]

I think I rest my case here since the american standard obviously differs alot from european standards

 

[rdrast]

Also to get back to the original point...
A drive is a different load from its attached motor.
A larger drive has a larger precharge current for one thing, and also a larger drive, with more capacitance, has larger peak current demands from the line for the front end. It doesn't matter if a 1HP or 100HP motor is hooked up to a 100HP drive, the drive has to keep converter section up to full buss voltage.

 

[Mark Snodgrass]

Besides the wiring for the input, I think we need to address why he is increasing the VFD size. He says its due to high ambient temperature, so I feel he needs to address the cooling aspect of the drive. Using the PF700 numbers as a guide, up-sizing the drive will actually add an additional 300W of waste heat generated by the drive. If you are not removing it any better then it may actually cause it to be higher. He is increasing the frame size (if AB) so it would have a bigger heat sink, but that still may not help with the problem.

einnh, can you tell us how the drive is installed? We may be able to recommend a less expensive and more efficient way of keeping the drive cool.

 

[jraef]

...Using the PF700 numbers as a guide, up-sizing the drive will actually add an additional 300W of waste heat generated by the drive. ...

Well, not really. Yes, the total watts loss number is higher, but there are two values that make that up; internal watts loss and external watts loss. The EXTERNAL value is based on the actual MAXIMUM motor current based on the current rating of the VFD as the "worst case scenario". The true watts loss will however be based on the actual connected motor current under load, which if the motor is smaller, will be that smaller value. So really, if the motor is 100HP it will not pull more than the 125A regardless of the size of the drive running it, so the actual external watts loss will be the same regardless of the VFD size.

The INTERNAL watts loss between those two sizes of drives is slightly different, but in this case the larger drive has a LOWER internal watts loss; 282W for the 156A drive vs 290W for the 125A drive, because there is a frame size change between these two ratings, so different boards.