Excessive motor starting current 3

[Brimy]

We are experiencing instantaneous magnetic breaker trips (1100 Amp setting) when transfering between normal and backup hydraulic systems. The system consists of two 60hp, 460V, 71FLA, Induction motors (Design type B). Each motor runs it's own hydraulic pump to a common hyd system. Each motor has it's own feeder breaker and reduced voltage starting autotransformer (set at 65%). When starting a motor with no hydraulic pressure, I measure a normal peak current of 240 amps, but the trips have occured when transfering from a running motor to backup motor when the meter peaks at it's maximum 600A rating. Considering inrush current and LRA I don't see how we are getting close to the 1100 Amp trip. I'm considering raising the tap to 80% for higher starting torque, any other ideas?

 

[electricpete]

Well maybe I was wrong (it wouldnt be the first time).
Here is a quote from jraef's 2nd link that I didn't read carefully:

"The more insidious aspect of starting current is the momentary "inrush" current, which persists for less than a hundredth of a second and can substantially exceed locked rotor current. Inrush current can spike as high as 13 times full-load current in standard motors and as high as 20 times full-load current in Design E and energy efficient Design B motors. Inrush current is too brief to trip thermal protection devices, but energy efficient motors powered through magnetic circuit protectors can sometimes experience nuisance stating trips."

 

[electricpete]

Well - I did some looking at NEMA documents and NEC and found some info that answers a few questions and raises a few more. You guys help me figure out what it means.

NFPA 70B (NEC) 2002

Table 430.52: Column labeled "Instantaneous Trip Breaker":

For NEMA Design E and energy Efficient B - 1100% FLA
For others - 800% FLA

430.52(C)(3) Exception 1 - allows us to go to 1700%FLA (E and energy efficient B) and to 1300% other motors if required.... presumably if tripping experienced at 1100/800.

NEMA MG1-1998, Rev 2

There is no design E. The only special requirements and discussion of energy efficient motor designation which applies only to the efficiency itself, and does not movidy the locked rotor current or other requirements.

12.35.1 gives maximum locked rotor current for designs B, C, D small and medium 3-phase motors. 60hp 460v would be 435A max (870A @ 230v * 230/460). [Side note… The original poster saw 240A…on secondary of auto I assume? which would be approx 370A LRC at full voltage]

12.36 states: "The values listed in the previous tables are rms symmetrical. i.e average of the three phases. There will be a one-half cycle instantaneous peak value which will range from 1.8 to 2.8 times the above values as a function of the motor design and switching angle. This is based upon standard ambient temperature of 25C."

OK, to me the 2.8 times factor includes sqrt(2) to convert from rms to peak times factor of 2 to allow for dc offset. 1.41*2 ~ 2.8. They are saying that the maximum peak of the total offset (inrush) waveform is twice the peak of locked rotor current. (exactly as would be expected from analysis of an L/R circuit with suddenly applied ac). If we assume that our trip device responds to the peak instantaneous value, we can expect the device will not see a peak greater than the peak associated with twice locked rotor current. At first glance sounds like agood argument for not exceeding a twice lrc setting unless by a small margin for instrument error.

Conclusions: from NEC at first glance it appears you have the leeway to go up to 1700%FLA if energy efficient and 1300%FLA for other motors. (Your settings of 1100 and 2500 are already at 1500% and 2500% of FLA=71A.) So jraef was correct that energy efficient designation makes a big difference in the allowable locked rotor current from NEC standpoint.

At least a few questions still remain:
1 - Why does NEC treat energy efficient motors like they have different locked rotor current when NEMA doesn't acknowledge that in any way in either their discussion of locked rotor current or peak instantaneous current?
2 - Why does NEMA say that peak instantaneous can’t be more than the peak of twice locked rotor current if that’s not true? Why does NEC allow settings so much higher than twice LRC if it is true?
3 – What is it about energy efficient motors that makes them act that way? Is there any spec whatsoever relating to limiting or testing this unique peak-instantaneous performance?
4 - Why does NEC 2002 talk about design E when they have been deleted from NEMA in 1998?
5 - Why is the NEC setting not as high as what jraef said? Am I missing the latest version of either of these standards?

I believe jraef has already proposed an answer to items 1-2 which would suggest that NEMA simply hasn’t updated their document to capture the unique performance of energy efficient motors, which do not differ in locked rotor current magnitude, but which do differ in the ratio of total peak (inrush) to peak locked rotor current (greater than 2).

Any more comments?

 

[jbartos]

Suggestion to the previous posting with many many eng-tips questions:
There is a big difference between NEMA and NEC, namely, NEMA standardizes motor manufacturing and NFPA 70 NEC guards a minimal safety to people and hardware they have to deal with. NEMA probably realized that there are various ways to design the high efficiency motor. One way is over reduction of R, another way is over the physical design of the motor including the better materials. Definitely, the reduction of R leads to a higher locked rotor current. Better magnetic materials lead to higher magnetizing impedance, i.e. smaller magnetizing current.
NFPA 70 is probably safeguarding personnel and hardware of the existing motors being still functioning. Certainly, any analysis of efficient-motor parameters and design would help to answer some of the above questions.

 

[jbartos]

Small encore: Visit

http://energy.copper.org/motor-text.html

for NEMA Premium Efficiency Ratings, etc.

 

[jraef]

electricpete
What you have caught onto here is a little political game that went on in the motor industry. NEMA was about to release the "Design E" standards, so NFPA made reference to it in the NEC. Then at the last minute, NEMA backed off. As you are no doubt aware, NEMA is made up of equipment manufacturers, and I suspect that all of the variations between motor manufacturers as to what they called "Design E" got in the way of publishing that standard. What it appears has happened recently is that NEMA released a "Premium Efficieny" designation as jbartos pointed out. I have not yet read it (I'm waiting for my copy), but from what I understand it is a performance criteria, not a design standard. In other words, it doesn't matter how the motor manufacturer achievs the goal, it just matters that it does. What this means to the end user is, you need to know a lot more about your motor in order to properly protect it.

As to your question on 1300% or 1700% etc., I admit that my statement about the NEC allowing up to 2000% for EE motors came from a summary paper on upcoming NEC changes from last year. If you have the latest NEC and it says 1700%, it must be so. I loaned my new copy out and have yet to get it back. I appologize for ASSuming. :-(

This entire subject has been covered very well in articles and discussions over the past 2-3 years in Electrical Apparatus Magazine, the trade journal for EASA. Unfortunately, they do not yet publish on-line. If anyone is serious about staying in touch with motor design issues I highly recommend this rag, even though it's not a freebie.

jbartos, thanks for that link. That is a very good paper. A little biased towards copper, but usefull nonetheless. Your bookmark file must be enormous! Subvert the dominant paradigm... Think first, then act!

 

[electricpete]

I see that a 67 page condensed version of NEMA MG1-2002 can be downloaded for free at:

http://www.nema.org/index_nema.cfm/1427/8672E77E-D3BC-494F-A0D0CE6E55B99A74/

I also see a summary of NEMA Premium Efficient Motor Program at:

http://www.nema.org/index_nema.cfm/1018

I haven't digested these yet. One thing I note is that the phrase "Premium Efficient Motor" is not mentioned in NEC-2002 or MG-1-1998. Both of these standards use the same words: "Energy Efficient Motors". That phrase specifically defined in MG1-98 SECTION 12.60, and includes only an efficiency specification (no mention of special locked rotor requirement for energy efficient motor anywhere in mg1-1998). We'll see if these newer documents shed any light.

 

[electricpete]

My review of MG1-2000 indicates the only change related to this discussion is they have added a definition of Premium Efficient which again includes only an efficiency definitions. So now there are two energy efficient classifications, neither of which carries any special locked rotor current characteristics.

NEMA Design B still will have a locked rotor current limited to 600-700%.

Section 9.6.3 still talks about peak instantaneous (total inrush) current being no more than 2.8 times locked rotor current.

I was pleased to see the addition of table 11 which concisely defines NEMA designs A, B, C, D along with key torque-speed parameters, locked rotor range, slip, applications, relative efficiency. I don't know how many times I have seen individual authors try to extract this info from the detailed MG-1 tables and it has always come out slightly different. Now there is an authoratative source. One good thing.

In the separate premium efficiency page, I went to the specification link and saw only efficiency info, nothing about locked rotor.

As far as I can tell the most absolutely recent publications of NEMA has not given one single clue that energy efficient motors will have any allowance for either:
higher locked rotor current
or
higher ratio of peak total inrush to locked rotor current.

I am somewhat amazed that NEC and MG-1 can be so far apart on this issue.

 

[electricpete]

correction to first line of previous message: "My review of MG1-2002..."

 

[electricpete]

One clarification would be in order:
The document available at

http://www.nema.org/index_nema.cfm/1427/8672E77E-D3BC-494F-A0D0CE6E55B99A74/

is identified as "Condensed MG 1-2002" on the download page, but section 1 of the document itself indicates it is a summary of nema standards on small/medium induction motors NEMA MG1-1998 through revision 2.

(the description on the download page and description in the document are not the same).

 

[electricpete]

I'd like to revisit the whole issue of instantaneous current for efficient motors.

I read EASA's "Principles of Large AC Motors" copyright 2000, page 2-32, which provides discussion leading up to the changes in NEC for energy efficient motor. There are mentioned two properties of energy efficient motors:

#1 - they tend to have locked rotor current
#2 - they tend to have higher L/R ratio's.

That is the extent of any explanation I can discern from this publication concerning the reason for increased instantaneous settings on high-efficiency motors.

Here is my interpretation of the significance of these statements:

A - #1 has an obvious effect on instantaneous trip, although for design B my understanding is the limits on LRC still hold.

B - #2 has the effect that the dc component will decay less in the first half-cycle so the peak inrush current can get closer to the max theoretically possible inrush current of 2*sqrt(2)*LRC.

So in energy efficient motors the increase of peak inrush above LRC will likely be higher than in others (due to higher X/R), but still not above 2.0*sqrt(2)*LRC..... at least that's my understanding from what I've read.

But I certainly still could be missing something. Everything in the two links above seems to fit together except for the statement at

http://www.esmagazine.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2503,62165,00.html

that: "The peak value of inrush current, which can be as high as 20 times rated current, will be different each time the equipment is energized due to residual magnetism in the core."

I am skeptical of this statement. There is certainly variability on inrush current due to the closing angle with respect to voltage which is not even mentioned. While significant residual magnetism theoretically has the ability to push the core farther into saturation upon application of voltage and therefore could theoreticallyincrease peak inrush current, I have never heard that mentioned elsewhere.

Any thoughts?