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Excessive motor starting current
3

Excessive motor starting current

Excessive motor starting current

(OP)
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?

RE: Excessive motor starting current

Questions:
Q1. Is this a new/recently modified installation... i.e. has this system worked properly before? or always had that problem? If something was changed... what was it?
Q2. If this is a new installation, do you have all the check valves, pressure regulating valves, relief valves properly adjusted and installed (i.e. not backwards)
Q3. Is the problem symmetrical? i.e. Let's say motor1 is now lead and motor2 is lag...what happens if motor2 is lead and motor1 is lag?
I am trying to rule out any system setup problems before going into the electrical.
HTH
Saludos.
a.
 

RE: Excessive motor starting current

(OP)
I should have mentioned that the problem has only recently occured and is intermittent.
The installation has been in service for approximately 3 years without the reported problem.
Regarding Q3, it gets more complex, motor1 trip is set at 1100 Amps, and motor2 trip is set at 2500 Amps! Needless to say, #2 has never tripped, but I have measured the >600 Amps on both motors.
I do appreciate the mechanical viewpoint on this problem, but if we assume worst case situation of the motor not even turning, I don't understand how it's getting close to the 1100 Amp trip?
NEC does not allow greater than 1300% for the instantaneous setting (another issue), so even at the minimum setting of 1100 Amps, it is higher than allowed.

RE: Excessive motor starting current

Do your breakers have thermal overload?  Are they motor starters or just breakers?  

RE: Excessive motor starting current

Is it possible that the pump is already running in the wrong way, moved by the flow caused by the stopping pump?

The electromagnetic transient you described seems to be an opposite phase feeding: the reserve motor is already running and the residual magnetic flux generates a voltage, you close the breaker with a voltage with a different phase. This causes a very big torque, active power and consequently current. And this explains also why the trip happens only sometimes.


What do you think, peolple?

RE: Excessive motor starting current

Good point alex68, to have the pump running the wrong way it should have the discharge check valve leaking back, shouldn't it? Although if these are positive displacement pumps that may be difficult.

brimy,
Do you have a press gage or xmitter at the discharge of the std by pump, before the check valve?
do you see pressure in the stand-by pump?

...From what you said, that the installation is 3 yrs old, my gut feeling is that you have some kind of mechanical problem and not electrical.
If possible... do you have a spare motor to exchange with the one that trips?
...or if your service is not essential (i.e. can be taken off-line) interchange both motors.
to rule out electrical problems on the motor... same thing with the pumps.

Have you tried starting the tripping motor un-coupled? If you have a problem with the motor that should rule it out, because if the motor trips only when coupled... there is something mechanical creating the situation.

May be somebody more savvy on the electrical side has an idea on that side... but from your description seems like a developing mechanical problem... trying to fix it by increasing the electrical settings sounds like using "a bigger wrench", and eventually something will fail.

What I would do is thoroughly check all the mechanical components, valves, pumps, piping, vibration, alignment, bearings.

Guys, do you have other ideas?
Saludos.
a.

RE: Excessive motor starting current

I can't think of any good explanation why locked rotor current would ever go up.  

Alex's explanation seems pretty far out. Note if there were no residual rotr magnetism than it really shouldn't make much difference at all, but perhaps rotor residual magnetism creates an out of phase voltage that can increase the starting current.  It seems far out, but of all the scenario's I can think of it sounds the most credible. It should be easy to check whether one pump windmills in reverse direction when the other pump is running.

You should be able to determine the locked rotor current from the nameplate code letter which corresponds to a kva per horsepower rating (NEMA motors).   Then you'll have to adjust it for the reduced voltage.

I believe that the dc offset can make the first quarter cycle go up to a peak which corresponds to twice the normal locked rotor current (peak value).  (actually slightly less since the decay L/R decay has slightly reduced by the time the first peak occurs at one quarter cycle after closure).  

The fact that your settings are so high must mean something. Either someone has experienced previous trips and jacked up the setting, or they are aware of some factor that we are not.

Measurement of inrush current on a meter is always suspect in my mind, although it is interesting that you see substantial variations.  A waveform trace of the offset ac waveform is best.

Is it possible that the instantaneous trip is occuring during the switch from starting auto-transformer to full-line voltage? If this is a timed transition, then it is easy to see that the difference in loading may mean the difference between whether the motor is still drawing high starting current at the time of transision (fighting against another pump) or the motor current has decreased to normal full-load value (starting from zero pressure).  I don' t really know whether or why current spikes might occur during the transition…. just thinking out loud.

Perhaps it's worth noting that the majority of mechanical factors which would increase the torque load during start have the effect of increasing the time duration of the start, but NOT increasing the locked rotor current (reverse rotation seems like one possible exception).

Bottom line - first step should be to check for reverse rotation.

RE: Excessive motor starting current

Question to the original posting. What kind of current measuring set-up do you have?

RE: Excessive motor starting current

(OP)
Thank you all for your input.
-jbartos, I'm using an AEMC MX54 DMM, with clamp-on probe MR461 rated to 600A.
I have verified that the pumps are not being motored prior to starting.
An interesting specification on the Siemens magnetic trip breakers, their range is -20/+30%!  So, at 1100A setting it is possible to trip at 880 Amps.
I am still considering raising the starting tap from 65 to 80%.

RE: Excessive motor starting current

what benefit do you expect from raising the starting tap?

RE: Excessive motor starting current

(OP)
-electricpete, if I increase the starting torque, the motor should reach running speed sooner.  As you stated in a previous post; perhaps the trip is occuring during the transition to full line voltage. (It is a timed transition).

RE: Excessive motor starting current

Brimy,
All of the above responses are worth checking out thoroughly BEFORE you change to 80% taps, which may in fact exacerbate your problem. I have 3 more things to add:

You never answered the question from Laplacian with regards to the nature of the breakers. You mention in the original post that you are experiencing magnetic trips, but how do you know? You do not specifically state that these are magnetic-only Motor Circuit Protector (MCP) breakers. I bring this up because you may be assuming they are since you have a motor starter, but be aware that many manufacturers do not have their RVAT starters UL listed with MCPs since it requires additional testing and they do not sell enough units to justify the expense. If you want this explained further, respond in the forum. It does not however appear that you have a thermal trip problem from your description, just be aware that it might be.

Another overlooked potential problem may be in the motor. If as you say, this is only 3 years old, it is entirely possible that you have Energy Efficient motors. Some designs of EE motors have instantaneous peak Inrush Currents of 2000% - 2200% FLA. Look at the latest code and you will see a new(er) exception allowing up to 2000% instantaneous trip settings on breakers to accomodate this. This is not LRA, it is instantaneous Inrush Current, often confused with LRA. They are not the same. This is the phenomenon that the 1300% exception was put in to cover on older motors. In this case, your motors would momentarilly draw 1480A, far above the 1100A setting. Typically mechanical breakers have been too slow to capture this phenomenon, but newer electronic trip breakers can. The +20% to -35% setting accuracy that you described could easilly explain the intermittant nature of the trip.

If you do not have an EE motor, megger it with a real megoghm meter that applies at least 1000VDC to the windings. People often think that using a VOM to check winding resistance will detect any problems, but think about it. You are expecting a 9V battery to try to punch through weak insulation. It's not going to happen unless you have a complete failure which would be otherwise obvious. You could have a devloping phase or ground fault in the motor that, for now, only shows up when the conditions are just right. If this problem has been increasing in frequency over time, I'd look here thoroughly.

Subvert the dominant paradigm... Think first, then act!

RE: Excessive motor starting current

jraef - those are good comments.

One question I have is about the higher instanataneous currents on the new energy efficient motors. I was under the impression that the highest instantaneous current which can be reached including dc offset is twice the peak of the LRA (adjusted for voltage) under any circumstances.  

RE: Excessive motor starting current

(OP)
-jraef, the motor is design type B. Could this still be energy efficient?
A megger check would be wise.
(breaker style is magnetic only).

RE: Excessive motor starting current

If the breaker is tripping on instantaneous, I don't see how changing the tap from 65% to 80% is going to help.
Have there been any changes in the motor starters or have you added any capacitors lately?  

I find it interesting that this only occurs when starting one motor with the other one already running?  Or am I reading too much into your original statement.  Does this occur if you start the motor when the other motor is not running?  

Have you tested the circuit breaker in a test set to see if it is operating correctly?  

RE: Excessive motor starting current

Brimy
It could still be "design B" and be energy eff., since NEMA never officially recognized "design E" as it was to be. Without the new designation, some still refer to them as design B since that best describes the starting and running torque-speed curve.

Electricpete,
That's what I was taught also, but I was edumacated in the pre-energy conscious world of the 70s (and much of that went "up in smoke" so to speak). I have since been further instructed as to the difference between Starting Current and Inrush Current, the latter being primarilly magnetization current and completely independent of load.

The phenom in EE motors has to do with grain orientation of the steel laminations, slot geometry and smaller air gaps used to reduce eddy current and magnetic losses. They also create a very flat flux curve that pulls extra high instantaneous current to get to maximum (then decays within 3 cycles). The amplitude can vary depending on residual magnetism and phase angle at the time of energization, but 20x is quite common. The best paper I've read on the subject came with a package of info from the Dept. of Energy in their "Motor Challenge" program some time ago, but their website is down right now so I can't identify it.

Subvert the dominant paradigm... Think first, then act!

RE: Excessive motor starting current

Follow-up. The DOE website is back up, but I can find no reference to the paper on energy efficient motor design issues that I saw some years ago, perhapse it was never scanned for internet availability. I'll call them tomorrow. In the mean time, here is a decent, albeit simplistic, article that mentions this issue.

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

Here is another, look at question 13 specifically:

http://www.eimotors.com/QA.html

Subvert the dominant paradigm... Think first, then act!

RE: Excessive motor starting current

jraef - Thanks for that response to my question. If the energy efficient design E allows LRC up to 10x FLA as indicated in your link, then a peak equivalent to 20x FLA would be the expected maximum total including dc offset contribution.

I'm not sure that's exactly what's going on but it sounds to me like a reasonable explanation for the 20x figure.  If that's the case, it wouldn't apply to design B where LRC is limited much lower than 10x FLA

Either way it's good info. Looking forward to more good discussion.

RE: Excessive motor starting current

Comment on Brimy (Electrical) Aug 12, 2002 marked ///\\\
Thank you all for your input.
-jbartos, I'm using an AEMC MX54 DMM, with clamp-on probe MR461 rated to 600A.
///How do you obtain those values over 600A?\\\

RE: Excessive motor starting current

Jreaf and others for the DOE websites. It may be helpful to do a search on "Motor Challenge" The DOE subcontracted out most of the information distribution activities so it is very hit and miss but the folks that have taken over will bend over backwards to help...

RE: Excessive motor starting current

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."

RE: Excessive motor starting current

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?

RE: Excessive motor starting current

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.

RE: Excessive motor starting current

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!

RE: Excessive motor starting current

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.

RE: Excessive motor starting current

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.

RE: Excessive motor starting current

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

RE: Excessive motor starting current

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).

RE: Excessive motor starting current

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?

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