Heavy starting induction motor
Heavy starting induction motor
(OP)
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?





RE: Heavy starting induction motor
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
RE: Heavy starting induction motor
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.
RE: Heavy starting induction motor
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).
RE: Heavy starting induction motor
RE: Heavy starting induction motor
Price (induction motor)=a
Pirce (slip ring induction motor + starter)= 2a
Price (induction motor+ MVsoft starter)=1,7a
RE: Heavy starting induction motor
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Heavy starting induction motor
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
RE: Heavy starting induction motor
Do the stator faiures happen during starting or after several hours of running ?
RE: Heavy starting induction motor
or
If there is a back pressure, start opening the damper immediaely after giving the start command.
R.Thiyagarajan
RE: Heavy starting induction motor
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??
RE: Heavy starting induction motor
RE: Heavy starting induction motor
RE: Heavy starting induction motor
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
RE: Heavy starting induction motor
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.
RE: Heavy starting induction motor
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?
RE: Heavy starting induction motor
RE: Heavy starting induction motor
RE: Heavy starting induction motor
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
RE: Heavy starting induction motor
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
RE: Heavy starting induction motor
if you check in the early potings, you will find the results of your formulas. For me there is only one question. We are all engineers in plants or engineering firms. Do we or do not rely on manufacturer's data? what else do we have? If i was not lucky enough to take the starting curves before failure, how could i stand against the "inappropriate" motor case? Almost all 6kV 4pole motors 1000kw , should be able to rotate at no load (closed damper)the fan. The fan at low speed is just like an inertial (whorl?) load.
Thank you all for your advice
chronis (www.rotechen.gr)
RE: Heavy starting induction motor
I assumed 1800rpm motor.
I assumed 85% voltage.
I first assumed the fan provided 90% load at full speed.
If I assume a typical poor starting torque motor with ~75% torque from 0 - 60% speed to a breakdown peak of 180% then I get 49 sec. as the start time.
If I assume a better motor with ~110% torque from 0 to ~60% speed with a breakdown peak reaching 220% then I get 28 sec. as the start time.
I then assumed the fan provided 40% load at full speed.
With the same motor as the first example I get 36 sec. as the start time.
With the same motor as the second example I get 24 sec. as the start time.
So, it seems to me that your numbers are not correct. The fan starting time should not double when the fan changes from closed damper to open damper.
As another note, those formula's posted will give an approximation of the accel time but I have seen that answer off by as much as 10 sec. compared to an iterative calculation that goes in small speed steps using the motor and load torque at each speed step. Those formula's don't even account for the required load torque.
RE: Heavy starting induction motor
motor and fan manufacturers claim 25secs starting with closed damper.
I measured : 84amps with "closed damper" at 1500rpm (50hz)and 46secs starting time.
What i say is that what they call "closed damper" is not "no load" condition.
The motor is 22Amps at no load (uncoupled) condition.
Real numbers: Nominal torqe 5770Nm, Rel.starting torque=0,6Tn,
84Amps at 1500rpm are 4600Nm. THerefore the motor starts with 80+% of nominal load.Please check inertia figures in early postings. My calculations just indicate to me that the "closed damper" condition is the 80+% of full load, under start up. Therefore, the net accelerating torque is very small and the consequent starting time rises.
RE: Heavy starting induction motor
How are you converting amps into kW without the power factor?
What is the actual voltage drop on the system during the start? A larger voltage drop will cause a longer acceleration time but this typically has little effect on the heating of the motor.
RE: Heavy starting induction motor
i disagree. The cange of inertia changes a little the schenery. (it is just a multiplier). On contrary, the load curve that is 80 and not 40% at nominal speed, is the dominant parameter, since the startind time is proportional to the sum(d(speed)/d(Net torque)).
steady state dumpe closed: 84amps with pf 0,88 (measured), 6kV
starting voltage drop (measured)=1000V = 15% of 6,1kV nominal measured.
The point is that we find out that the load is not the intended for these motors.Do we agree on this?
RE: Heavy starting induction motor
We will have to be content disagreeing about which factor is the dominant one with respect to the starting time.
RE: Heavy starting induction motor
As regarding to the starting time is fairly easy using an excel sheet to decide whether the damper or the inertia have more effect to the motor starting severity.
Now, isn't it correct that 84amps at 6kV with a pf 0,88 at 1500rpm is a 4,7kNm of torque? That is reality.
Isn't there a diference between the claimed nominal figure of 0,5Tn=2800Nm for the closed damper?
RE: Heavy starting induction motor
As regarding to the starting time is fairly easy using an excel sheet to decide whether the damper or the inertia have more effect to the motor starting severity.
Now, isn't it correct that 84amps at 6kV with a pf 0,88 at 1500rpm is a 4,7kNm of torque? That is reality.
Isn't there a diference between the claimed nominal figure of 0,5Tn=2800Nm for the closed damper?
RE: Heavy starting induction motor
RE: Heavy starting induction motor
I used some custom software written to calculate the acceleration in 0.01 second time steps. With the limited data you have given, my opinion is that it is very unlikely that the accel time will change from 48 sec to 24 sec by unloading the fan further. I've literally seen 100's of motor curves so by creating some "typical" motor curves based on the data you provided as well as using a fan load and the inertia you gave I just can't make it happen. The closest I could come is a time change of 1.5 times.
So, to answer you initial question - What's wrong?
#1 - There is an issue with the closed damper fan load.
#2 - I'm suspect of the 24 sec vs 48 sec accel times.
RE: Heavy starting induction motor
Are this motors new , surplace or remanufactured ??
Taking account the manufacturer data ¿where the 25 seconds is coming from? From the manufacturers calculation only for your application, or is the maximum allowable for their motor during a DOL start?
the difference could be there , the manufacturer could be citing the maximum for the motor, rather than what the load does.
Regards
Petronila
RE: Heavy starting induction motor
Most likely the dampers are not closed. However, the wk^2 of the fan may be inaccurate or it may have been calculated for the wrong RPM. How is the voltage drop, and the voltage balance? Somewhere in the data is a parameter that is being accepted as valid that is not valid.
However, given the excess running current with the dampers supposedly closed, and the high ratio of "Damper closed" current to "Damper open" current, check the dampers, the air temp and, by the way, the voltage drop and balance. You may also want to check for a phase shift on the supply power. That will lead to high motor current.
If this is a retrofit of an existing fan, the customer may have replaced a double squirrel cage, or other high slip motor with a low slip motor in the pursuit of energy savings.
Still, the 84 amps points to either a partly open damper, voltage or phase shift problems or possibly an air temperature problem.
respectfully
RE: Heavy starting induction motor
Point being, the dampers are getting a lot of attention and to me they are highly suspect, but I believe the topic needs to be broadened to any significant (or lots of insigificant) source of air leakage in the suction side of the fan.
rmw
RE: Heavy starting induction motor
A slip ring motor was employed, 950kw, 1500rpm, with an oil cooled starter, is operating excellent for more than 3 weeks now, with a very smooth starting 30 secs(acc to the owner). As i see it, seems to me like a starting with " not closed" damper
regards, chronis
RE: Heavy starting induction motor
What is the no-load current of this slip ring motor under "damper closed" condition ?
RE: Heavy starting induction motor
RE: Heavy starting induction motor
Good ol' WRIM to the rescue again! There's a reason for their existence. Low starting torque in squirrel cage motors, some voltage drop in supply and cables, limited rotor heat capacity and a high load inertia are some of the reasons why they are needed.
A soft starter could have helped, but it produces just as much rotor heat. Reduced current drain and a "well behaved" source and cable voltage drop may have helped. But a WRIM with external starter always does it. Mainly because you can have a high torque during all of the acceleration and not a very weak starting torque like most squirrel cage motors have.
Your 30 seconds acceleration time with external starting resistors vs calculated 24 seconds at .85xUn makes me wonder if your inertia value is correct and if that wasn't the "root problem"? Using rated torque and 413 kgm2 results in 11.3 seconds. Adding motor inertia and some windage makes 15 seconds a probable acceleration time with WRIM and starting at rated torque. So. I wonder if there isn't a problem with the old GD2 and inertia getting mixed up?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...