Could Negative Sequence be causing my over amping motor problems?

[MOENGINEERING]

Could Negative Sequence be causing my motor problems?

I have a project I am working on in Missouri with a city water utility.
The water utility just finished replacing an old 150 HP motor and pump with a new 150 HP motor and pump.
The new pump on this motor is configured to be nonoverloading, meaning it will never pull more than 140 HP.

The reason I was called is the motor is drawing more power than it should and tripping the old 150 HP soft-start.
The electrician working on the project told the city the soft-starter was bad and needed to be replaced.
The water department manager questioned this, as the soft-starter was working fine with the old motor and is only 5 years old.
I have confirmed with flow tests and pump curves that the correct pump and impeller are installed and thus the pump is a true nonoverloading pump.
I have also confirmed that the soft-starter is functioning correctly.
The soft-start goes through a 60-second ramp up during starting and a 60-second ramp down when stopping.
After the 60-second ramp up the starter goes into a bypass mode, where the SCR’s are bypassed and the motor continues to run off straight power.
To make a long story short, the starter is protecting the motor just as it was designed to do.
The motor has the following name plate rating (460 V, 164.0 FLA, 1.15 SF, 94.5 NEMA Efficiency, 90.8 NOM PF, 3555 RPM)
Voltage prior to start up
A – B = 484v
B – C = 488v
C – A = 488v
Using a RMS digital meter, the readings I have been getting, as measured on the motor leads (after the soft-start is bypassed and motor is at full rpms) are the following:

Running Voltage
A – B = 476v
B – C = 482v
C – A = 483v
Running Current
A = 184amps
B = 195amps
C = 199amps

The Voltage reading seems to be normal to me. There is a slight imbalance between phase voltages and a slight voltage drop at startup but I would consider both of these to be normal, or at least within normal limits.


Here is what I am unable to explain.
I don’t understand why the full load running amps are much greater than the name plate rating. The motor is running well beyond even the service factor.
With the pump and present flow rates the motor should be running around 130 HP not 183 HP (which is what I calculate).

I just cannot believe that this new motor could be bad and yet still operate this close to its design ratings.
I’ve used a megger to check the motor and the windings test out that there are no shorts.
I have also checked all of the leads and all are connected correctly.
Could the motor have been manufactured with the incorrect number of windings to through off the name plate information?

My only other thought besides a bad motor is that there could be a problem with the incoming power, but I don’t even know what to look for.
Is it possible to have a phase shift on the incoming service such that the three phase voltages are no longer 120 out of phase (negative sequencing)?

One of the water utility employees said he worked on a 10 HP submersible well pump many years ago that kept burning out.
The symptoms were similar in that they had voltage on all three phases yet were also drawing more amps than the nameplate.
They replaced the pump three times, each time with the motor burning out.
Finally, the problem was narrowed down to the power transformer.
The transformer was replaced and the problem was fixed.
He could not remember what the problem was with the transformer.
Do you know of any problems besides phase loss in a transformer that could cause a motor to burn out or operate above nameplate ratings?

The service for this pump is fed from a pad mounted 3-phase transformer that is fed from a 3-phase overhead line. I’m not sure where the substation is (near or far) in relation to this site and I do not know if there are any cap banks on the line.


Any ideas would be greatly appreciated.
I am at a loss to explain this and would appreciate any help you could offer or any ideas on what to test next.

Thanks again.

Jim

 

[aolalde]

Negative sequence is the result of phase Voltage unbalance. The reported operating voltages are not that bad 0.9% imbalance (476v- 482v- 483v). The currents are imbalanced but into expected percentage, 4.5% imbalance (184amps- 195amps - 199amps).
You have two problems causing over current, 4.4% over voltage but more critical is “overload” since the 192.66 average current is the result of increased load on the motor rotor. In spite of your “true no overloading pump“, the load is larger than the original. Look for mechanical rubbing that should be generating heat if the power is not in the volume and pressure of the pumped liquid.

 

[rbulsara]

What is the power factor? Measured vs. nameplate?

 

[jraef]

Forget problems with the soft starter, if the current runs high when in bypass, there is nothing the soft starter is doing to cause that. This is just another example of someone wanting to blame the box they don't understand.

The first thing I would check is the speed. If the old motor was slower than the new one, that would definitely alter the current draw, even if it is a little bit, i.e. 3450 vs 3550. I doubt that your motor's nameplate has the incorrect number of poles shown. You should have 2 poles, and you can't have fewer than that, so if you had 4 poles, the motor would run at 1/2 the speed it should, and you would draw LESS current because you would have less flow.

Along the lines of what aolalde has said, it could also be that the old motor was wound as a true 480V motor, and the new one is wound for 460V, in which case you are over exciting the motor a bit with 5% overvoltage. Newer more energy efficient motors have closer voltage tolerances than the older ones did. 5% technically shouldn't make much of a difference, but it is possible. If you still have access to the old motor, check that out. Also check to see if you have taps on the transformer feeding this new motor that can be used to drop the voltage a bit. Look for something that says "5% FCBN" on the transformer nameplate. That means Full Capacity Below Normal".

Side note, 60 seconds is WAAAAAAY too long of an acceleration and deceleration ramp time on a water pump, I don't care what kind of pump it is.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

Hello MOENGINEERING

I agree with the above and would add that operating the motor above it's design voltage could result in iron saturation. This will icrease the magnetising current but would not increase the line current under load by the increase that you are indicating.
It sounds to me as though you have too much shaft load for one reason or another.
Increasing the line voltage will cause the work current to fall and the magnetising current to increase. At full load, you will not see the effect of the increase in magnetising current until it is very major and the motor would be running red hot very quickly!!
I would epect that the motor should be drawing no more than 157 amps at 480 volts (based on 164A at 460V) The way I see it is that you have a serious overload that will damage the motor if you do not correct it.

I would suggest, if possible, uncouple the motor from the pump and run the motor open shaft. The current will then bew essentailly the magnetising current and should fall in the range of 20 - 40% of the motor rating. I would expect a mag current of around 50 Amps. If the magnetising current is realistic, then you have a mechanical/load in excess of that calculated.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[MOENGINEERING]

When I first looked at this, I thought for sure they had just forgot to trim the impeller in the pump. I had a mechanical engineer evaluate the pump flow but according to the pump curves this is the correct pump and impeller. When the motor is off the pump can freely be spun by hand such that it is not binding and there is no excessive friction. I understand your point that more power is being used than what is going into pumping the water, but as far as I can tell there are no excessive frictional losses.

I have started grasping at what if's, as I have no explanation for what is going on but I do not believe it is due to frictional losses.

 

[MOENGINEERING]

Unfortunately I do not have a meter that measures power factor. What is your thought on this?

I agree the soft-start is not the problem. The motor runs at 3553 RPM's.

I don't think the number of poles was labeled incorrectly.
As you pointed out that is directly related to the speed.
What I thought might have happened is that the wrong number of wraps were put on each of the poles, creating a lower impedance and higher amp draw.
Just a thought.

I don't quite understand your comparisons of the old motor and the new motor. I am just trying to understand way this motor is over amping.

The 60-second startup is to help reduce water hammer in the water main. As you pointed out, this would be excessive for a typical pump startup.

Although it is a major job, I like the idea of decoupling the pump!
I know from the motor performance sheets that the no load amp draw should be 32 amps.
This could help narrow down the problem by tell me if the problem is in the motor or the pump.
We did deadhead the pump to test the amp draw but come to find out the gate valve was not fully closed and thus the pump was not truly deadheaded.

Please keep the suggestions coming as I still have no explanation as to why the motor is drawing higher than normal amps.

 

[rbulsara]

It is entirely possible that the new pump-motor set is woroking harder than the old pump and indeed drawing more power. You must find out the power factor to know actual kW (HP).

The old pump may have worn out bearings, impeller etc. requiring greater opening of the valves etc to get the desired flow. A new set with same old valve opening setting will force more fluid thru it and draw more power..

I think it is just a matter of adjusting flow within capacity of the pump set (motor amp) or need to correct the pf for the same amperage. A PFC with a VFD btw is not a good idea..

 

[jraef]

I don't quite understand your comparisons of the old motor and the new motor. I am just trying to understand way this motor is over amping.

My mistake. I was thinking that you only replaced the motor on the same pump, but after re-reading your original post (enough times), I see that both were replaced. OK, sorry.

Motors don't "over amp" by themselves. The current drawn by a given design is determined by 2 things. Load and efficiency. So start from there.

I seriously doubt that the motor manufacturer made a mistake such as not putting enough wraps on the winding. That would affect the current draw I suppose, but the FLA on the nameplate is tested, not theoretical. So that would not explain why it is drawing more current than it shows on its own nameplate.

Is something rubbing? I know what you said about "spinning the pump by hand", but read this thread, especially the end. It was a very similar situation! thread237-123565

Is the motor of a much lower efficiency rating than what the pump was designed to work with? 150HP is not necessarily a hard and fast specification. Maybe on THAT motor, with THAT pump load curve, it may be that a lower efficiency is translating into more losses. If the pump and motor were tested together by the pump mfgr however, that would negate that theory.

The "phase shift" issue isn't really valid by the way. You could have an open delta system, and that could make the motor draw more current, but that would show up as a lot more of a voltage imbalance than what you describe.

How long is it running before it trips off? Could you still be filling the pipe and therefore running at essentially open head? That technically shouldn't be the case since you replaced another pump that was working in the same system. If however the flow rate from this one is significantly lower, maybe it takes too long to fill the pipe and you overload the motor before it can finish. In that scenario, it didn't happen on the old pump because it filled the pipe before it ran into the OL trip curve of the old motor. I just ran into that on some Medium Voltage VT pumps. They worked fine when connected to the system, but when bypassed into an empty pipe, they tripped on OL before the pipe filled.

Speaking of that, did you remember to re-adjust the OL settings on the starter for the new motor nameplate info?

Whatever it is, it is most likely a load problem that is ever so slightly different than the old system.


"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[electricpete]

Lots of possibilities. Most covered already. Ill throw some out in order from most likely to least likely (my opinion)

** Could it be a motor connection problem? If it were designed for 480v delta connection, then 480v wye connection would give approx sqrt(3) times FLA at full load.

On the mechanical side, as was mentioned, check speed of the motor compared to speed shown on the pump curve. Power is proportional to speed^3 so small differences in speed make a big difference in power. While you're at it, if you can measure speed with hand-held strobe (or using spectrum of vibration), you can get an estimate of slip which should be roughly proportional to power... compare against nameplate speed/slip may help piece together the puzzle.

Also look at the assumptions for fluid density. Denser fluid draws more power.

Impeller installed backwards (or phasing backwards)? I would think backwards rotation would have shown up on your curve but you never know... maybe it only causes tremendous decrease in efficiency (increase in current)

Is it possible when you switch from soft starter to bypass that the phase rotation gets reversed?

Almost dead last on the list of things to look at is the motor itself. Winding resistnace test and insulation resistance test. Additionally, sometimes surge test can identify internal miswiring in the motor when winding resistance can't. Last on the list of things to look at with the motor would be a rotor problem which can rob the motor of torque and increase current draw. But not worth worrying about until you have scrubbed the mechanical and electrical systems outside the motor very carefully.

=====================================
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[Birk]

This is just a field guy type thought and might not be helpful:
Can you describe the termination in the j-box in the motor?
It occurs to me that there might be leads mismarked, or perhaps even stranger: There could be a conflict between the supply system and the motor coil voltage.
Either could cause over amping.

 

[subtech]

I'm certainly not a motor expert, but let me suggest something basic here. Go take a look at the three phase pad mounted transformer that you said feeds the faciility.
Check the nameplate on the transformer carefully to make sure of its ratings. Now, is this transformer equipped with a tap changer? Many have them. It's standard policy at my company to always order padmounts with a tapchanger that can accomdate voltage inputs of plus 2.5 or 5% and of course -2.5 or 5% from the nominal input rated voltage.
PLEASE keep in mind that it is very likely that this tap changer is NOT to be operated with the transformer energised!
If this transformer is equipped with said tap changer, make arrangements to have it de-energised, the tap changed to allow for either a 2.5 or 5% reduction in its output voltage (your choice)and then re-energised. If you get your voltage at the motor closer to the nameplate rated 460 volts, I'll bet you'll see more reasonable numbers. If you are working for a relatively small town water company with its own electrical utility, you should have no problems getting this done. Again, I'm not a motor expert, but when I look at your numbers it seems to me that you are pushing the knee of saturation just a little here. Remember, above all things, when you get ready to change that tap on the padmount transformer, COMPLETELY DE-ENERGISE FIRST! This is not an option, it is a must! Changing taps should only take about 2 linemen and 30 minutes time at the most. When this is weighed financially against changing out a new motor getting the change made should be no problem.
Best of luck to you.