VFD overload capacity 2

[thinker]

Question about current overload capacity of the VFD inverter: If the OL capacity is only stated as 150% for 60 sec., can one assume that 200% for 5 sec. will be also available as default? Or that capacity should be specifically confirmed by manufacturer? I realize that OL value vs duration is a non-linear function, but there might be typical curves for intermediate values?

 

[ScusaMe]

Each manufacturer's Design Criterion must be known before a blanket statement could even be considered to be postulated.

Some things to 'think' about :
- ambient temperature— 25 deg. C ......40 deg. C .... 50 deg. C
- altitude
- environment (clean vs dirty) ... and impact upon cooling of heatsink(s)
- how often applied ... i.e. duty cycle..
- performance characteristics/limitations of power devices

Where this all leads ..... is to the junction temperature of the power devices.... ergo, heat rise in the heatsink is of major significance.

And not the last ..... how close to margins was the unit designed to....

Always best to ask the people that designed and built the unit under consideration than to seek assurances from members of a forum, regardless of how good their intentions are.

After all, if ya go to a Proctologist to cure a brain aneurism; I think you can guess what kind of result to expect.
<g>

 

[Skogsgurra]

Be very careful! No VFD that I know of is designed for 150 % overload. For a 100 HP VFD, that would mean outputting 250 HP.

I know that you realize that 150 % overload actually means 50 % overload. But some people and some specifiers may be more "by the book" than "by the brain". It is well worth being careful with the words here.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScusaMe]

U-u-u-m-m-m-m Skogsgura ..... you sure about that ?????

No VFD that I know of is designed for 150 % overload. For a 100 HP VFD, that would mean outputting 250 HP.

Think you'll find that the universally accepted definition for overload .... is 100% load + 50% more = 150%

What you said would constitute 100% rated + 100% more + 50% more = 250% and fer sure... that don't happen as a rule.

Design rule for Constant Torque is 100% rated load + 50% more (overload) for sixty-seconds, or 1-minute, which ever comes first <g>

Suggestion: Google .. VFD Overload Rating ....

Here's one reference from such a search:

VFD Buying Guide
www.vfds.com/vfd-buying-guide
So you'd like to pick up a new Variable Frequency Drive (VFD)? ... A constant torque drive should have an overload current capacity of 150% or more for one ...

... and another
Yaskawa VFD Drives | Adjustable Speed V7N DeviceNet Micro ...
www.clrwtr.com/Yaskawa-V7-Drives.htm
The Yaskawa VFD V7 Drive is intended for constant torque applications, with current overload rating of 150% for 60 seconds.

 

[thinker]

Dear ScusaMe,

I agree with your technical reasoning. I share this opinion, just wanted to get more ammunution in argument with supplier. Yet I think the last part of your post is not quite appropriate, I was not seeking for assurance, but rather asked for opinion. And your medical reference does not sound polite to all forum participants. Thanks for your response, anyway.

 

[LionelHutz]

Broadly speaking, the device heatings is caused by I^2t. You would likely be OK expecting 200% for 5 seconds assuming that 200% is not above the hardware or hard coded current limit. Most VFD's have some kind of thermal overload protection function for the IGBT's, a software hard coded current limit and a fixed hardware current trip.

 

[jraef]

LionelHutz beat me to it. You cannot ASSume anything. A number of VFDs will simply trip off line or artificially lower the commanded speed in order to avoid delivering more than the Mfrs stated OL capacity. Others will not.

Those that will not tend to be the more expensive Vector drives, and one indirect way to tell is to look at the peak torque output capabilities. In many vector drives, you will see that it tells you that it can make the motor develop peak torques of something like 220 to 250%, and may qualify that by giving a time limit, ie 5 seconds or 2 seconds, etc. What they are saying is that they can drive the motor into Break Down Torque (BDT) and in order to do that, it will need to deliver a comensurate amount of current. The subtle subtext here is that they are referring to the MOTOR current, not the DRIVE current, there is usually a difference. For example you may buy a drive for a 100HP 460V motor with a FLC of 124A, but the VFD current rating is 132A. The VFD OL capacity, stated under the old "Constant Torque" rules will be 132 x 1.5 for 60 sec.. But if you look at the torque specs and it says 220% for 5 sec., that is going to be 124 x 2.2, not necessarily 132 x 2.2.

And Gunnar, whether or not it is technically correct, that is the conventional way OL capacity is specified in all VFDs I am aware of. 150% = 100% + 50% more.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

You may have noticed that my last sentence went: "I know that you realize that 150 % overload actually means 50 % overload. But some people and some specifiers may be more "by the book" than "by the brain". It is well worth being careful with the words here."

It may not have happened to you experienced and seasoned people. But, given that we see more and more MDAs and accountants doing "engineering" these days, I thought that I should mention the linguistic anomaly and the risk involved.

Accepted or not, it is a problem to use words loosely if you haven't agreed with the everyone in the Universe that this is what it means - in this case. I have seen this mistake happen and it did cause a serious delay in starting up a process plant.

Sorry if you got offended by my unnecessary remark.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScusaMe]

Well Thinker,

I think the last part of your post is not quite appropriate, I was not seeking for assurance, but rather asked for opinion

If that be so, then consider that your O.P. (Original Post) lacked the clarity necessary to discern your intent.
The information I imparted to you was based upon FACT... not opinion.

Do you really believe that OPINION... that is not grounded by FACT... has any useful value ?

As you are new to this forum, you have not had the exposure to responders who will render an opinion even though they lack the credentials from whence to offer facts as opposed to conjecture. They usually start their post with statements such as....." don't know much about that BUT ............" and continue with conjecture.
Are you sure you really want to go to the bank—so to speak— with such opinions ?

THINK about it.

your medical reference does not sound polite to all forum participants.

My "medical reference", as you put it, was meant as ..... tongue-in-cheek H U M O R ..... Perhaps you missed the <g> or didn't understand the <g> at the end of my remark, which is shorthand for grin .... and infers that the preceding statement was a humorous remark. .... see: [link]

http://www.internetslang.com/*G*-meaning-definition.asp

[/url]
There certainly was no intent to offend the sensibilities of the membership of this august forum.

 

[thinker]

OK, ScusaMe.
Thank you for clarification. I apparently missed your <g>. I might have a kind of excuse since that was Internet slang, and the English is my second language.

About opinions: There are several reputable forum members whose opinions I consider highly credible and, yes, based on facts. Those opinions I could take to the bank (of course, after detailed analysis).