Motor Unbalance Situation 7

[JBUDA54]

Hello All,

I am currently assigned to a Paper Mill and ran into a situation were a Wood Hogger No. 3 motor is going out on Phase Unbalance via a Multilin 269 Motor Protection relay. The Alarm is set at 10% 12A over FLA and the trip is set for 20% 24A over FLA. The motor installed is a 500HP 2300V FLA 112 1184RPM Westinghouse Industrial Severe Duty Motor. I was told that the motor is driving a drum that has giant hammers that are coming into contact with the bark thus breaking the bark into finer pieces. PT's are connected on the load side of contactor to motor and the PT's are all measuring a balanced output of 2.47kV. The CT's are 150/5 and connected through a shorting TB then into the relay. One observation was that in recent days the bark was water logged and caused the tripping occurence to be more frequent. Another note is that a previous 800HP motor was connected and sent out to be tested while the 500HP motor is presently handling the current load. The 500HP motor is also showing unbalance symptons. Any bones thrown at me is greatly appreciated. Thanks in advance.

 

[itsmoked]

Voltages?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JBUDA54]

Voltages? What? As stated in the e-mail the motor is 2300V and the PT are measuring 2.47kV...

 

[itsmoked]

Current imbalances can be caused by voltage imbalances. I recognize that the nominal voltage is 2300V what's the actual voltages?

A current imbalance can be caused by a high resistance connection somewhere. Often connections loosen due to thermal cycling. Has anyone re-tightened ALL the connections lately?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

As already mentioned, unbalanced voltages into the motor is the most likely cause.

 

[JBUDA54]

There were electricians suited up in their space suits performing a thermal image of the starter. They also have an ammeter connected to the 269 CT input going over to a data recorder. I just walked out there and the relay keeps flashing an unbalance alarm.

 

[JBUDA54]

The field engineer stated that the PTs connected to the motor are balanced at 2.47kV. I just left him a voicemail asking him to call me back and verify that data. I agree the voltages have to be imbalanced if the currents are imbalanced. Is it possible that the metering for a PT is not sensitive enough to pick up the imbalances?

 

[oftenlost]

Voltage imbalances often cause current imbalances of 10 times the magnitude of the voltage imbalance. I would look carefully at the voltage balance as indicated in the previous responses. High resistance connections could cause the imbalance but are unlikely to consume that amount of power without creating noticeable heat and burning of conductor insulation

 

[jraef]

It sounds like voltage imbalance to me too. Remember, just a 2% voltage imbalance will result in a 10% (or more) current imbalance. How that might relate to the wet wood issue is that with increased load, the transformer winding or connection resistance problem that is behind your voltage imbalance is exacerbated.

The field engineer stated that the PTs connected to the motor are balanced at 2.47kV.

If that's close to the exact wording of his response, I would consider that a brush-off answer. In other words, somebody is assuming that, maybe even by just reading the nameplates, not by actually measuring. I would want to know the exact voltages read phase-to-phase and phase to ground on both sides of each PT.

I have noticed lately that since the advent of stricter implementation of the space suit thing, field people are uncomfortable in them and tend to rush through tests or make generalizations so they can get out of them, then overstate their findings to avoid suiting up again. Insist on a detailed voltage report.

In the mean time can you even just go look at a switchboard meter somewhere upstream of this starter? That would not find any problems within the starter but it would confirm or eliminate any upstream issues. or does the 269 have the MTM metering module on it? If so, you could read the voltages right at the meter.

 

[Marke]

There are two issues with this type of situation.

1) Motors are wound on a laminated steel stator core. The stator laminations have a grain which results in the magnetic permeability being spacialy non linear. If you compare the inductance on two quadrature axis, you will find that the inductance in one axis does not equal the inductance in the other (unless you orient the two axis at 45 degree to the grain)
If the steel is rotated as the laminations are punched, you can get a balanced field in the gap. In many cases, the flux in the gap is not balanced due to grain orientation in the lamination steel. This results in an imbalance in the line currents into the motor when connected to a perfectly balanced supply.
2) In any supply, there are inherent imbalances. These imbalances can be due to slight variances in impedance in transformers and line impedances, or even just an imbalance in other connected load.
The important factor is not the magnitude of the voltages on each of the three phases, it is the balance of the three phase vectors. It is possible to have a situation where the three phase magnitudes are equal, but the angles are not and this will cause an imbalance in the current. If there is a vector imbalance, it will show up if you do a phase to phase voltage check, and a phase to neutral check. One of the measurements will show an imbalance. Most (all?) supplies have an inherent imbalance which may not be obvious as it may be an imbalance in supply impedances and/or voltage vectors.

In many cases where there is an imbalance in motor current, it can be difficult to see the reason why, but if you rotate the connections to the motor, you will find that one combination will maximize the imbalance, and one combination will minimize the the imbalance, essentially using the imbalance in the motor to cancel the imbalance in the supply.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[JBUDA54]

The field engineer stated that the Motor tripped 3 times on the 3-11 shift yesterday and he did mention a voltage imbalance of 10-20 volts phase to phase. The Contractor had a data logger connected and they are coming today to review the readings.

I wrote down the driven equipment info to get an understanding of the driven equipment and found the following website.

http://www.jeffreycorp.com/size_reduction/Wood_Hogs/index.cfm

They are not too far from my neck of the woods.

Thanks for everyones help. I will let you know what transpires.

Jason

 

[waross]

Has anyone considered that the motor may be failing and meggered the motor? Has anyone watched the currents simultaneously on three meters? Hog motors have a very rough life. They can go from no load to a considerable overload in the time that it takes a large amount of waste to drop into the machine. You may have mechanical deflections under heavy loading that are affecting the air gap and indirectly the current balance. Don't forget that this motor is replacing a motor that was 60% larger. Is the unbalance present all the time or just under heavy loading. Have the secondary connections and wiring of the instrument transformers been checked for high resistance problems, particularly the CT secondaries?
respectfully

 

[itsmoked]

Good points waross.

Thanks for the link JBUDA54.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

"I agree the voltages have to be imbalanced if the currents are imbalanced."

Voltage unbalance is a likely cause, but not the only possible cause. Some other causes mentioned above.

"...motor is going out on Phase Unbalance via a Multilin 269 Motor Protection relay. The Alarm is set at 10% 12A over FLA and the trip is set for 20% 24A over FLA"

What does that mean? Vector sum of three curretns exceeds FLA+20? That would be a very high unbalance setpoint. Does the relay record the actual currents at the time of trip?

Is it running or starting when it trips?



=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[JBUDA54]

The Multilin 269 Phase Unbalance alarm is set at 10% of FLA which is 112A or 11.2A and the Trip setting is set at 20% or 22.4A. This seems high to me as well, but I am simply providing what the Mill field engineer has fed to me. I believe this setting was recently cranked up to keep the process moving. It may have been 5 alarm and 10 trip previously. The Motor runs for a while before it finally trips on unbalance. The motor starts up fine, I believe. The other day I stood in front of the starter and the Multilin was triggering several current unbalance alarms. I was there for a good 10 minutes and I thought the starter would trip in front of me. There is an electrical contractor that set up a data recorder for this motor, monitoring the voltages and currents and they were supposed to report back sometime yesterday and any additional clues to this situation. I believe that ruled out loose connections in the starter with a thermal imaging of the starter. I will update as soon as I hear word.

Jason

 

[JBUDA54]

Field engineer wrote this:

We found out yesterday that the unbalanced trips were nuisance trips. The data logger showed at the time of the trip, the phase currents were only 2% different. The Multilin 269 has an entire page in the book on this type of trip, especially with this type of application. We changed the Multilin out with a 369 which looks at unbalance in a different way. In addition to this, we activated the thermal circuitry for measuring unbalanced trip. This is used to make an additional check using the RTD's in the motor. Only time will tell, however the Hog ran at 20 Ton/Hr last night. The system is down today for a Field Day. We will continue to monitor. By the way, the voltages were around 2% different from phase to phase.

Since I am not as familiar with the 269 vs the 369 can anyone elaborate on this?

 

[jraef]

Sounds as though this was a known problem with the 269. Years ago they upgraded the 269 to the 269 Plus, which may have solved this issue (among others, such as being affected by harmonics). In reality now though, the 369 is a better relay and costs the same if not a little less than the 269 Plus.