3.3kV motor running at greater then FLC.

[1000V]

Hi guys,

I have searched this subject with some success but the thread I'm after is now closed.

I have on site an Alsthom induction motor coupled to a sulzer HP boiler feed pump. The motor is running at 106% FLC and has been since 2006, I cannot look further back as there are no records. The plant was commissioned in 1995. Spec as follows:

2 pole 3300V 50Hz 2900kW Water at 40 deg C

2973rpm 575A 0.91pf class F insulated, White metal sleeve bearings.

The protection has been set up so that the overload curve is entered at 107% FLC. I have been told that this is working to an old CEGB standard, a rule of thumb I gather as I have seen no documents on this while trawling through old GOMs.

As the motor sometimes enters the OL curve I have been asked to further up the overload protection in order to keep the pump available. I feel uncomfortable with this as being new to the company I am unaware of this practice.

During my research I have found that the winding temperatures while stopped are 23*C. The Temperature at 107% FLC are 105-120*C.

My questions are:

As the insulation is rated F and we are well bellow the 155*C rating, are we damaging this motor by running at this increased load. By this I mean is the increased current flow in the stator likely to cause flash overs or un measurable heating affect on the rotor?

As there is no service factor, the service factor is assumed to be 1. Is there a calculation or method for calculating decrease in life expetency by exceeding the FLC continously?

I am very sorry for my limited understanding of this subject and hope that somebody could at least shove me in the right direction.

Althom sized the motor to be 15% larger than needed to drive the pump, therefore there are obviously issues with the system or pump. I do not want to put my asset at risk to nurse this problem through and distract from the real issue. I do however want to understand better the affect of running an induction machine in this way.



Thanks, T

 

[rbulsara]

As long as the winding temperature remains within the insulation's thermal rating, I wold not worry about the motor itself, especially if it is running like this without issues for a long period of time.

Also how are the voltages? You did not mention that.

On the other hand if this was working below rated current before and the anomaly is now developed, you will need a much more detailed study of the installation if you want to correct that. For example in efficiencies in the pumping system.

As for motor nameplates goes, it is not uncommon to find that the actual motor construction is capable of producing more than what the nameplate says. But only OEM can answer that.

So before you dive into something, establish first if there is indeed a problem that needs solving.

Rafiq Bulsara

http://www.srengineersct.com

 

[jraef]

Sometimes on pumps the initial system is designed for a specific flow. Then some comes along later and decides they need more flow and change something mechanically to allow that, which increases the load on the motor. A small amount of flow change results in a large change in power requirements. They could have changed a flow restriction device, changed an impeller, all sorts of things.

But if you are comfortable with your winding temperature measurements, i.e. you have something good like 6 embedded RTDs in the windings, then as Rafiq said, don't start inventing problems that don't exist.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

Class F machines can run comfortably at 120 deg C for years. Of course, there could be other modes of failure but you can bet the overload will sure to be blamed when the motor fails.

Muthu

http://www.edison.co.in

 

[1000V]

Thanks for the useful advice guys.

The Board Volts are 3301V. Sorry I ommited that information in the first instance.

Through some more research it looks as though there has been modification to allow for extra flow through boiler tube leakage etc..

Just a quick question Edison, the information I can find says that class F insulation is rated to 155*C. Am I looking in the wrong place?

 

[edison123]

155 deg C is the maximum hot spot temperature which is next to the winding copper. Since the RTD's are embedded in between two layers of coils, they measure the insulation temperature on its outer most layer. So 120 deg C is usually recommended and used to allow for the temperature gradient from the inner most insulation layer (the hottest spot) to the outer most insulation layer where the RTD's are located.

Also, lower the operating temperature of the winding, longer is its life.

Muthu

http://www.edison.co.in

 

[1000V]

Edison, You're the man.

Thanks guys, last one out turn the light off.

 

[purplepete]

We had a problem with motors running at borderline FL amps and occasionally tripping. We changed the transformer taps to take us from 575 volts to 600 volts, solving about half the problems! I'm not sure I'd do that with a 3,300 Volt motor though.

 

[edison123]

purplepete

While increasing the motor voltage may or may not reduce the FLA, it sure will increase the core loss and hence the motor temperature. And increasing the voltages at MV levels is rarely attempted.

Muthu

http://www.edison.co.in

 

[ScottyUK]

edison,

The board volts are 1 volt above nominal (5 Feb 11 2:29) - chances are the motor terminal voltage is below nominal which would cause an increase in current to maintain power output.


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If we learn from our mistakes I'm getting a great education!

 

[1000V]

Plenty of good stuff thanks. There is the opportunity to increase board volts as the board is fed via a unit transformer with off load tap changer. However, this board also feeds our static started and excitation isolation transformer therefore I am reluctant to do so.

At the risk of pushing my luck, I'm struggling to find info on the relationship between increased terminal voltage, core loss and stator temperature increase. Any chance of a link or a nudge in the right direction?

Thanks, T