Could Negative Sequence be causing my motor problems?

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jdove

Member
Could Negative Sequence be causing my motor problems?
I have a project I am working on in Missouri with a city water department.
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
The readings I have been getting, as measured on the motor leads (after the soft-start is bypassed and motor is at full rpms) with a RMS digital meter 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 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 or some other flaw 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.
Could my problem possibly be 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 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 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.


I am at a loss to explain this and have run out of ideas on what to test or measuer. Any ideas or explanations of similar experiences would be greatly appreciated.

Thanks again.

Jim

{Moderator's Note: Edited to remove personal contact information. If you wish to contact this person, please send a Private Message.}

[ July 11, 2005, 08:31 AM: Message edited by: charlie b ]
 

Ed MacLaren

Senior Member
Re: Could Negative Sequence be causing my motor problems?

It sounds as if the new motor is overloaded. Can the motor be un-coupled from the pump and checked for proper operation at no-load.

It reminds me of a case I saw involving a refrigeration compressor. The old motor failed and a new one of the same horsepower rating was installed. The new motor drew current in excess of it's rating and burned out.
They had to install a motor with a higher horsepower rating to carry the load.

Seems as if some of the newer motors are "optomistically" rated.

Ed
 

jdove

Member
Re: Could Negative Sequence be causing my motor problems?

I think that is a good suggestion about decoupling the pump from the motor. that might eliminate some of the tail chasing.

The motor and the pump are both new.
The pump was matched to this motor.
By that I mean it should be a non overloading pump for this motor.

So if I decouple the pump from the motor and the moror still draws more than the listed 32 no load amps does that mean I have a bad motor or is there a possibility of there still being something else from the power source causing this problem?
 
O

oliver100

Guest
Re: Could Negative Sequence be causing my motor problems?

Electrically the motor is OK. Unless, (not very likely) the rotor cage is deffective or the lamination is shortened. If this is the case no load test will give you some indication. Before doing that try to reduce the flow.

Most likely the pump overloads the motor. How are they matched - flow rate, pressure, RPM? Is it pure water or mud?

In your calculations, did you take into consideration the efficiency of the pump?
 

rcwilson

Senior Member
Location
Redmond, WA
Re: Could Negative Sequence be causing my motor problems?

The voltages are within 1% balance, the currents are within 5%. If there was a negative sequence problem, there would probably be more of an imbalance.

The motor is drawing 17% over nameplate FLA. The pump or a bad bearing are overloading the motor. My experience says it is a pump problem. An uncoupled run will check the motor bearings.

Pump curves are estimates unless you paid extra to have a test block run done on the pump. The test block results should give you the actual kW or HP consumed by the pump at the operating conditions, after adjusting for speed differences between the test block speed and actual operating speed and any difference in specific gravity of the pumped fluid.

I would concentrate on the pump, since that is the major thing that has changed. If there was a utility voltage problem, it would probably also show up on other circuits in the plant.

Good luck!
 

james wuebker

Senior Member
Location
Iowa
Re: Could Negative Sequence be causing my motor problems?

Had the same problem a while back. Took the motor down at the local motor repair place to have it checked. It was OK. Found out that the pump was bad. It was also a 150 hp motor. Some time I could get it to run and again I couldn't.
Jim
 

electricman2

Senior Member
Location
North Carolina
Occupation
Retired Electrical Contractor
Re: Could Negative Sequence be causing my motor problems?

It sounds to me like for some reason the pump is overloading the motor. Verify that the hydraulic conditions are the same as with the old pump, that is GPM and TDH. The pump manufacturer can determine if the replacement pump has the same characteristics.
 

rattus

Senior Member
Re: Could Negative Sequence be causing my motor problems?

Is the RPM of the new motor correct? Possibly could be wired wrong.
 

stew

Senior Member
Re: Could Negative Sequence be causing my motor problems?

I am not at all comfortable with the voltage unbalance. This kind of unbalance causes winding currents to increase dramatically more than you might think. If you can determine the cause of the voltage unbalance and correct it and it may just be a problem with the soft start then your current will decrease. A 5% unbalance between phases can cause a 10% or MORE increase in currentdue to unbalancences internally in the windings. The motor become much les efficient and currents bgin to buck themselves the more you get out of voltage balance. The older motor may not have noticed the unbalance but the new unit with very possibley a different winding design may be much more prone to unbalance problems. It has to do with unbalance cuasing the magnetic fields to be sort of not insync with each other. The other fellows have also made valid suggestions about mechanical problems as well. I just think the design of the newer motor is just really prone to voltage inbalance amperage increases.
 
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