Heavy starting induction motor

[polvalrot]

It was the 3rd failure in 900kw 6kV 4 pole induction motor, turning a fan in a steel plant. Originaly the closed damper starting dictates that the load starting characteristic is all below the 0.3 of nominal torque at nominal speed. Neverthless, the measured current at closed damper condition and nominal speed was at 84Amps. THe starting acc to manufacturers data sould be 24 secs at 0,85Un, while for all 3 failed motors was 48 secs. The damper is claimed to be realy closed. Motor inertia is 33 and load inertia 413kgm2. Acc to my calculations, with 84Amps at closed damper (considered as no-load(!)) is really 47secs. What's wrong?

 

[Marke]

Hello polvalrot

I tis difficult to give you an answer based on the information that you have provided.

To calculate the start time, you would need to have the speed torque curve and the speed current curve of the motor, and the speed torque curve of the driven load. You would also need the inertia of the driven load and the speed of the drven load. You would also need to know the starting method applied to the motor.
One of the problems is, that there are very large differences between the starting characterisitcs of motors of the same size. While the manufacturer may have correctly calculated the start time for a given motor and start method, if you change the motor or start method, you can have significantly different results.
Additionally, if you apply a reduced voltage starter, it is imperative that the voltage is not reduced too low as this will restrict the free acceleration of the motor and load and can cause a failure.
If you can provide the full information, someone can calculate the start time and see how it compares.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

900 kW, 6 kV, SCIM will take around 450 amps at 85% Voltage start.

If it takes 84 amps something is wrong. The stator winding is not correct or the rotor cage is open.

The motor is not developing enough torque to accelerate in spite of the load reduction by closing the dampers. Test the motor parameters in a good equiped motor test facility.

 

[polvalrot]

Thanks for the answers.
Motor speed =fan speed. Motor inertia=33kgm2, fan inertia=413kgm2. 84 amps is steady state with still closed damper , following starting transient. Staring current is 700Ampa SCIM(6,5X In).

 

[edison123]

If 84 amps is the no-load, damper-closed current, what is the normal load current with damper open ?

 

[polvalrot]

exactly edison123! that is m point. 84 Amps is 750kw. the almost open damper is 110 amps (1050kw). the manufacturer claims 0,3Rated torque with closed damper. This is much more! Therefore, is it correct design for Direct on Line Starting the fan? I would certainly prefer the poor old slip ring motor. After all
Price (induction motor)=a
Pirce (slip ring induction motor + starter)= 2a
Price (induction motor+ MVsoft starter)=1,7a

 

[Skogsgurra]

Is it the rotor that fails? Or stator windings? Do you have a cooling interval between starts? Was this motor delivered together with the fan or was it at all recommended by the fan manufacturer? Twice as long starting time as calculated, using manufacturer data, makes one wonder if it is an I vs GD2 thing? Three failures in same application says something is fundamentally wrong - or too many starts/hour.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rmw]

Have you verified that the closed damper is "closed"? What type of damper is it. Is it louver or inlet vortex vane (IVI)? If it is a multiblade louver type, which type, parallel blade or opposed blade.

If Louver type, are the blades individually adjustable, or ganged together by a fixed linkage? Same question if it is IVI. Also, if Louver, is it inlet or outlet louver?

Louvers dampers are not very tight shut off to begin with and just a small amount of maladjustment of the blades or linkage(s) or material build up can cause a lot of leakage.

Ever now and then the occasional piece of tramp iron from construction or duct work failure can jam an inlet damper of any type.

Also, the damper is sized for operating conditions, which means that if this is a hot gas fan, then a little leakage of cold air upon start up can amount to a lot of fan load.

Check your damper linkages for totally closed at cold conditions. Someone said in another thread that "gremlins don't change wiring", but temperatures do change linkages.

Bottom line, totally eliminate any or all damper issues before attacking motor problems, notwithstanding the good advice the electrical guys will give you in this forum.

rmw

 

[edison123]

It looks to me the motor is undersized for the 100 % damper opening (1050 KW vs 900 KW). Do you run the motor continously at this load of 1050 KW ?

Do the stator faiures happen during starting or after several hours of running ?

 

[rajant2001]

Do not fully close the damper. Keep it at minimum opening position if there is no back presure.
or
If there is a back pressure, start opening the damper immediaely after giving the start command.

R.Thiyagarajan

 

[polvalrot]

Dear All
when the customer says "damper is closed", means they even let people work on the bag filter. Therefore: damper is closed. The network is ungrounded. 3 motor failures, only one developed 3phase to ground. the other 2 were 1 phase interrupted from the other 2. My question is: In the manufacturer datasheets, the motor is to start at 25 secs. Why is it starting at 46? I have calculated the starting time really to be 25s if manufacturers data are reliable. Also i calculated 47secs, if we have a quadratic load torque that at nominal speed and closed damper goes to 750kw or 84 Amps or 5000Nm. Then what is missing??

 

[polvalrot]

And yes, i forgot to mention, that up to know they have opened the 1st failed motor, copper squirel cage and lifted bars..... For me 46 secs means that 150 deg Celcius are reached in stator and maybe 300 in rotor. The other 2 failed motors have aluminium rotor and i suspect aluminium decoration in the stator....

 

[polvalrot]

i could post a .mdi file of starting. How could I upload it?

 

[rmw]

You didn't answer the question regarding the damper type.

What country or state is this in and what are the 'man safe' standards for baghouse entry? In my experience in most advanced parts of my country 'man safe' entry requires a minimum of sealed guilliotine and often double bladed sealed guilliotine dampers. Is this what you have?

Even then guilliotine seals fail and tramp iron or material build up gets under guilliotine blades preventing full closure. Some of the crap I have seen in steel mill ductwork makes me mighty suspicious.

Until you go put your eyeballs on the damper and tell me that it is closed and I mean absolutely closed at start up, no amount of "custmer says" will convince me of anything. Been there and been burned too many times.

rmw

 

[polvalrot]

Dear rmw
i am electrical engineer and definatelly not been in the air circuit. I will though, because, as you do, from my calculations, the only option is that the so called "closed damper" is not closed. A fan is drawing 750kw when delivers work and not when rolling.

 

[jraef]

I recently worked on one (new installation) where they had limit switches to detect the damper position. They were complaining of similar issues, it turned out the electrician had looked at the drawings and decided for himself that the logic of the limit switches was backwards because he believed that if the damper were closed it would mean more load on the motor and not allow it to start. Then when it didn't start, the people who knew that it should start easier with the damper closed had no idea that it was open because the limit switches were backwards.

Probably not the case here, but I brought it up to reinforce the idea that people often ASS-U-Me things based on casual observations and/or see what they EXPECT to see.

Another remote possibility: Is it a damper or a diverter?

 

[macmil]

Also, as rmw said, check the air temperatures , both at start up and operating temperature, and compare them with the design temperatures for the fan and hence the motor. Leakage of cold air may well overload your motor. You should also check if there is instrumentation on the other side of the damper which will help you monitor possible leakage ~ manometer or pressure differential measurement or flowmeter. If not, consider installing something.

 

[polvalrot]

another maybe useful note is that acc to my calculations, the impact of a larger fan inertia (if we doubt the manufacturer data) is mch less of that of an inadequate damper.

 

[rmw]

Material build up on the blades would add to the WR^2. Normally, however there are balance problems associated with that problem that manifest themselves long before starting problems do.

The design inertia should be easy enough to verify, either from the manufacturer or by calculation. If this is the problem, it should have been present from the beginning and should be present in duplicate fans if you have any or there are any at sister plants.

rmw

 

[petronila]

Hello Polvalrot,

I think something is wrong with calculations, if the first failure involved the squirrell cage damage indicate a Very high locked rotor amperage (Due to High Inertia). Is possible due to high inertia your Motor´s Design is wrong so check in name plate.Recheck too the WK2 load Calculations and like marks sayds use
speed torque and the speed current curve of the motor.

This formulas could help you too :

WK2 = Inertia of rotor + (Inertia of load x Load RPM2/Motor RPM2)

Time for motor Reach Operating Speed (In Seconds)=

WK2(Lb-Ft2) x Speed Change (RPM)/308 x Avg.Accelerating Torque(Lb-Ft)

Avg.Accelerating Torque= ((FLT + BDT)/2) + BDT + LRT)/3

Where: BDT = Breakdown Torque FLT= Full-Load Torque and LRT = Locked-Rotor Torque.

Regards

Petronila