Allowing a motor to be slightly overloaded (105%)

[digitrex]

I have a difficult situation at this moment whereby a 6.6kV 3.1MW(FLC=320A) is needed to operate at a current just beyond 320A and upto 325A. This motor is driving a gas compressor. Unusual fluctuation of gas composition (increase in nitrogen content) lead to this higher load.

This motor has RTD in its windings and an overtemperature alarm and trip relay at the motor starter, but there is no digital display of the temperature. We manually connect a portable temperature meter to the RTD and found the temperature is around 105deg.C.

The overload relay setting had been re-adjusted to 328A.

We have to read the ammeter every 30min interval in order to be assured there is no sudden overload but this exercise may worn us down.

I now have the following plans and asking for your opinions whether I can safely do these and any other suggestions:

1. There 3 spare RTDs in the motor windings which I plan to install temperature transducers and have the temperature readings display in the Control Room computer screen such that it will generate alarm at 120deg.C (Class F insulation) and it will allow temperatures to be continuously trended.

2. With the ability of continuously monitoring the motor winding temperatures on the computer screen, I will increase the overload protection setting to 105% (or 336A).

 

[mpparent]

Do you know what the service factor of the motor is?

Mike

 

[edison123]

If it is a class F insulated stator winding and if the RTD's are embedded between two layers of the coils, (assuming an inlet / ambient air temp of 40 deg C)120 deg C limit is acceptable as per standards.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[digitrex]

To mpparent,
The motor is loaded 100%..102%(who knows, it may increase again) continuously for 3 years until next plant turnaround maintenance. No standby unit.

 

[hottip]

The slightly overload of a HV motor it's common practice in a gas plant. The O/L protection behavior should be described in the Protection relay setting study of the electrical system of the gas plant where you are. Have a look in this book and check about overloading of the motor info.
For example one case (I've been there): gas plant, 5 MW gas compressors, the O/L trip will commence at load above 107% of FLC. And they are running overloaded 102-103% from a long time. The downside is the life time of the motor will be reduced, but not that much.
Regarding the temp. monit: for a HV motor the temp. trip for the motor windings is about 145 grdC. Check if the temp. sensors are conected to DCS system in control room. Sometimes you can have this situation.
Good luck!

 

[GregLocock]

Make sure the cooling vents are free from obstruction, and consider going to a forced or cooled air supply for it.

To this /mechanical/ engineer a 5% overload sounds OK, if you take precautions.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jraef]

Hottip's experience matches my own with regards to MV gas compressors. The 3% continuous overload condition will result in an increased temperature rise of 6%. That will result in a motor running at 100deg C to run at 106deg C, which, assuming Class F insulation, will reduce the thermal life from 1,000,000 hrs to about 800,000 hrs. So as he said, not a problem for 3 years of operation.

The problem I see here is that it appears as though someone has gone cheap on this and is NOT using a Motor Protection Relay. A basic overload relay is going to be poor protection for this application, you will be sacrificing top end OL protection in order to avoid nuisance tripping from that low level OL. You need to get a really good MPR, one that can be tuned to this application as hottip suggested, and has RTD inputs with it. That will also allow the use of a decent communication system back to your control room so that you don't have to be "worn down" with checking things manually. They aren't cheap, but they are cheaper than a new motor and the down time.

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[digitrex]

Thank you all for your comments.

Manual trending of motor winding temperature(by max/min function of a multimeter) in the past one day shows that the max winding temperature is about 113deg.C which occurs at around 5pm.

To Hottip,
Our plan is to connect temp sensors to DCS (I mentioned computer screen earlier).

To jraef,
Our motor starter is 15years old and it is fixed with a motor protection relay and another temperature relay for winding. My plan 1 is the choice that will be consistent with other part of the plant. The temperature relay is set to alarm at 120deg.C and trip at 140deg.C

 

[ScottyUK]

Hi digitrex,

jraef's comment about an MPR is a very valid one. The motor is a major capital item, and if it fails it will be a big job just to swap it, not to mention time taken for a rewind or repair. Have a look at the GE Multilin MPR's. They are very typical of what a good MPR is capable of - it would be a wise investment if you are taking the motor up to or just beyond its design limits. Your RTD's are better employed feeding in to the MPR where it will afford you protection rather than just display of temperature - almost all MPR's can run an RS-485 link back to your DCS which will give you all the data the MPR sees, so you could actually gain additional information and free up some DCS I/O at the same time.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[digitrex]

Hi ScottyUK,

The protective functions (overload and winding temperatures) are complete eventhough they are in 2 different relays and we can't read the temperature. We are also considering to upgrade to a better MPR (Areva Micom is in our mind) but this will not take place until plant shutdown 2 years later. Meanwhile what we can do is addition of temperature transducers for DCS and we can do it online.

Integration with DCS(through MPR or field transducers) will follow our plant standard, and we have more than 50 HV motors, so it will be a big exercise.

 

[ScottyUK]

Hi Digitrex -

Our plant sounds comparable - from our experience it is relatively simple to get the MPR's talking on a multidrop RS-485 link back to DCS when that time comes. The biggest mistake we made was using a copper twisted pair to carry the data back from the remote switchgear. We should have used fibre - the material cost is a little more but the extra bandwidth allows all manner of things to be run, and the installation cost is very similar. You hopefully can learn from our decision! Other option was to site the RS-485 / DCs interface in the switch room, but this wasn't possible for us (legacy of the original design).



----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[swgrmfg]

Whereas winding temperature is in the core of an ultimate trip, the two separate relays are not communicating with each other. Granted, this has been common practise for a long time. But, a state-of-the-art MPR, such as the one mentioned above, will have a thermal mapping feature that shifts the O/L curve based on thermal capacity of the motor, overloads, current unbalance, starts per hour, cooldown time, etc, etc... One drawback though, the proper parameters must be on hand when programming the device. And, good luck finding them for an aging motor.
Beware of frequent starts, for hot spots may not be detected by RTD's - they can be 'accounted' for by the MPR!

Bottom line: it's best to find a relay that you can set/program thoroughly. i.e. the fine print is not what the relay can do for you, it's what information you have about the motor that you can give the relay. So I would start backwards reading the programming manual first, not the sales brochures. At best, the motor manufacture might give you the data - only for a brand new motor.

Good luck.

 

[jraef]

Excellent points from swgrmfg and Scotty.

digitrex,
I understand your point about having existing components that do the job separately and not having the budget to replace them now, but consider this. What you are trying to accomplish is to push the motor into the safety factors that may or may not have been designed into it. Having those 2 seprate systems that don't coordinate is increasing the risk of failure many many times. Biasing of the thermal register with the RTDs is a very important feature, especially in your situation. What you are planning is to try to rely upon human observation to accomplish this. That may work in the short term, but do you want to be the guy who went to the bathroom and missed seeing the temperature rise faster than normal as a winding heated up, which the OL relay failed to notice yet? When the accountants get done tabulating the lost revenue and cost of emergency motor replacement, the price of that new MPR will look incredibly insignificant.

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[jraef]

I appologize for being preachy, but my experience with my own short-sightedness has finally taught me to see things this way. It was a very expensive class, not offered at the University.

 

[Marke]

Operating the motor at a small overload will shorten the motor life due to the increase in the insulation temperature.
If you can cool down the temperature of the air that is being used to cool the motor, you could reduce that winding temperature so that it is not degraded by the increased load.
While it is usually the temperature rise of the windings that is quoted, the real constraint is the operating temperature. Reducing the ambient temperature (cooling medium) you will reduce the winding temperature. - just a thought.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[digitrex]

Thank you for all your advises.

I also read the other discussion thread on "Motor Ratings". Can I get a advise whether maintaining the motor winding temperature to not more than 120deg.C, will it cause degradation to the winding insulation?

We have a pre-alarm set at 120deg.C and trip set at 135deg.C.

 

[jraef]

You never posted the insulation class rating of your motor. Can't tell without that.

 

[digitrex]

Oops, sorry, may be I was not able to show this info clearly in my first long post above. (Thank you edison123 for your comment)

The motor winding insulation is Class F.

Appreciate comments from all of you on my last questions above.