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?

 

[abeltio]

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.

 

[Brimy]

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.

 

[Laplacian]

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

 

[Alex68]

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?

 

[abeltio]

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.

 

[electricpete]

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.

 

[jbartos]

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

 

[Brimy]

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%.

 

[electricpete]

what benefit do you expect from raising the starting tap?

 

[Brimy]

-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).

 

[jraef]

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!

 

[electricpete]

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.

 

[Brimy]

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

 

[dpc]

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?

 

[jraef]

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!

 

[jraef]

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!

 

[electricpete]

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.

 

[jbartos]

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?\\

 

[tmahan]

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...