Overheating Of Induction Motor 55kw

HI

I guess there's an error in the re-winding process of your motor!!! make sure of the number of poles, all connections inside the motor "are they well-connected?" are the three phases well-separated into the motor?

awaiting for response!!!!!!!!!!!!

I have checked for the Loose connections, but connection found satisfactory. And the three phases are well-separated which is same as usually provided by manufacturer..

Good post.

Rest of post erased as it is covered by other subsequent posts.

do an insulation test on the windings, also resistance test to establish if windings have a fault in them.

can shaft be rotated easily ie is it binding or catching the winding cage.

Yes i have checked the rotation of Motor.. it is quite free.. its not catching.. winding resistance is about 0.5 ohms for U1-U2, V1-V2 while w1-w2 is slightly less 0.3 ohms..

i am not fully familiar with what the readings should be but are those readings a bit low almost a short

could you not contact the manufacture of the motor and get data as to what they should be ?

or tell us who made the motor and its specifications so we can look up the specs.

Its ABB 55 Kw Motor.. As far as i knw readings are satisfactory...

There are two possibilities for the problem. 1, as said before error in re winding 2, motor shaft alignment. I think the second issue will be the reason as you said it is started after the . Check your shaft alignment. check your pulley alignment. Check the Belt alignment. Did you check the previous motor and how the housing damaged?

Alignment?

Pully tension? (I have seen rotor getting heated up due to belt friction)

The electrical portion looks to be OK, at least not too bad,

The resistance looks to be on a bit higher side (almost 5KW is I²R loss which seems a bit too high for a 55KW motor) and that makes the motor almost 79% efficient at rated full load.

This may be a bit problem, since motor may not be able to dissipate the heat.

And also check the cooling fan, which by mistake is not reversed during the repair (if it is bidirectional then OK) and the cooling slots (if there) are not choked.

_{(To confirm, check the resistance of other motor though result of simple I²R formulae is not too appealing, and that is when I calculate the value at FLA and then equate the iron loss to copper loss as maximum efficiency criterion)}

dear frnd, U r quite correct.. they have changed the cooling fan during the repair.. the new fan is of small size.. (quite smaller than the previous one).. hence the cooling efficiency of new fan is very low.. Cooling slots are not choked.. motor is quite clean.. But i have doubt on vibration also.. May i know for 55kw of motor how much vibration is allowable..

Recommended motor vibration at 120 RMS range, above this range need to be check and rectified.

Nelson

Your problem may not be electrical..maybe mechanical? Overheating of bearings perhaps? Do you have thermistor protection? Maybe they are tripping due to overheat, inadequate ventilation, whatever?

hello Mr. sridhar.. Yeah we have thermistor protection and they are only tripping due to overheat.. without this protection our motor would have burned by now.. regarding Mechanical problem of motor.. i have checked for free rotation of motor.. its quite free.. there is no play in shaft.. Motor bearing seems to b quite ok..

Okay.. no mechanical problems.. any airvents blocked by some stuff you can't see much?

Assuming no.. unbalanced currents in the three phases ? Please note, a 1% unbalaced voltage can lead to a 6-8% unbalanced current... and these relays of today are extraordinarily sensitive to unbalanced tripping...

Sorry if i am unable to help much... but i know exactly how you feel .. been under fire myself many times.

just a thought have you tried to read the actual temperature of the motor using say an infra red thermometer

http://www.rapidonline.com/Tools-Fasteners-Production-Equipment/Test-Equipment/Thermometers/Fluke-66-Infrared-thermometer/83215/kw/85-3611?source=googleps&utm_source=googleps

reason i ask could it be thermistor is too sensitive ?

did you do insulation test ? ie windings to ground using 1000 volts ?

and if yes was the reading within the spec on the link i sent ?

Hello Mr. Peter.. i have checked physically everytime the motor tripped. I found Motor very Hot to touch (>80 degree centigrade) which is not while its normal operation. I have also done its insulation test. i have found 20 Mega ohms while it was hot and >100 Mega ohms while its cool (<40 degree centi). [resistance is Line-ground using 1000V DC]

I didn't read anyone asking you if you had done a complete no load test of the motor. If so or when you do does the draw change at all? Does it still overheat?

Good Idea if the motor does not overheat when not connected perhaps the problem lies with the other part that the motors connected to ?

This is what I mentioned in #6. It might be mechanical problem.

Another factor to consider is the motor speed. If you are using an inverter duty rated motor, you should be fine down to around 20 Hz. If you are not using an inverter rated motor, then any speed below probably 45 Hz will be a problem as the fan (which you said is smaller than the original already) will not be turning fast enough to cool the motor. A relatively simple fix would be to add an 'external' fan to increase the volume of cooling air acrossed the motor.

Also, are you sure that the inverter program is correct? I would at least check the basics, voltage, Kw, FLA.

The suggestions so far have been very practical & if you are sure that your supply is balanced & clean then a decent thermal imaging camera has been most useful to us with pinpointing or tracing the source of the temperature rises on bearings or electrical housings with the hottest areas glowing white at times on the thermal. If you suspect electrical fault then a quality of supply, etc, recorder could give you useful info for the period leading up to each trip.

The decrease in time between trips ,depending on your protection setup, is an indicator that the winding insulation is possibly degrading as well. So possibly extending the delay between startups may become necessary & fitting a smaller fan will not help unless it is a highly efficient propellar arrangement, this should be confirmed with ABB manufacturers.

20 Meg Ohms isn't very much (300 Megs is more normal), but it's not the problem.
I think the cooling is the issue.

What kind of cooling fan is in the motor?

Are we TEFC? ODP? Perhaps the motor is designed to be in the airstream of a fan it powers. All possibilities.

If the motor runs slowly under load, even though it is not overamped, it can overheat because its fan is insufficient.

Most likely it would be mechanical in my opinion. But if you are completely satisfied you have eliminated all mechanical sources of transmitted heat due to friction, I would look at unbalanced currents as previously mentioned. A current imbalance in the motor can lead to vibration, in fact there are articles available about how to use vibration analysis to detect current imbalance problems. Unbalanced current will increase the motor temperature 10% for very 1% of unbalance (if I remember correctly), so if you have just a 5% unbalance, you will increase the motor temperature by 50%!

Isolate the problem;

It's often difficult to properly read the output current to a VFD. I suggest temporarily bypassing the VFD with a DOL starter (make sure to physically disconnect the wires from the VFD output terminals). Run the motor DOL and read the separate phase currents. This will tell you if the motor has an issue creating the imbalance.

Assuming that checks out, then if you don't have the proper instrumentation to interpret the PWM output (and filter the harmonics), you can hook up any other motor to the VFD, i.e. a small one, and see if that motor overheats as well. If it does, then something in your VFD output has gone bad and the drive needs repairing. Maybe even do this first!

Check the Motor Bearing Housing (Both DE and NDE). There might be misalignment betwen motor shaft due to bearing housing and coupling.

I think that all the answers you got have been covering all the possibilities and you already found the fan was replaced with a wrong small one. Replace it with a suitable size as per original.

Remains the Vibrations: Apart from all the suggestions so far, there remains One: Unbalanced rotor. If the rotor has been rubbing before or any signs of wear etc Or even sometimes, a balancing weight has been lost during repairs ??

Check the rotor balance over the speed range you are working with.

Failing all others...maybe! (Unbalanced windings since one of the phase winding is 0.3 Ohms compared to 0.5 Ohms...can cause vibrations on the rotor)

Vibration is due to vertical conveyor structure ( how many bearings are there?) What VFD used for in your case? What frequency your motor is running at?

we have 6 bearings for that vertical screw conveyor.. the screw is around 20m long. We are using ABB SAMI GS Variable frequency Drive. (ACS600). At normal operation Motor is running at 49.8 Hz (around 50Hz).

fmalura,

It is good to see your quick responses to the posts. Thanks for staying involved. Another thought is that when the motor is rewound, the first step is a "burn-out oven" to remove the varnish and allow the old winding to be taken out. If this oven's temperature wasn't properly monitored or controlled, a predictable result is the poorer magnetic performance of the motor's steel laminations. This means that for the same kW output, the rewound motor runs hotter and has a shorter life span. Motor rewind shops can be sensitive and unhappy if customers ask questions about how well they monitor and control such problems. However, if they are worth returning to for future business, they will answer honestly and openly. Since you already appear to have had a too-small cooling fan installed on the motor, this raises the level of concern about the burn-out temperature control.

When you reported the winding resistance of 0.5, 0.5, and 0.3 ohms, what was the precision on the meter? Was it +/- 0.1 ohm or was it a low-resistance meter. I suggest looking at this closer. Find a decent low-resistance meter with accuracy limits of better than +/- 0.01 ohm and double-check these data.

--JMM

Hello JMM, I have checked this readings with FLUKE 110 multimeter.. It sets its range automatically... I guess precision is ok.. I think you are correct about the Burn-out oven Process.. May be that is the cause..

fmalura,

On the resistance setting, your meter has a precision of 0.1 ohm and an accuracy of 0.9%. For comparison of winding resistances in larger motors, this precision is not good enough. Many times, I have found a significant error (magnitude of error was 0.0 to +0.2 ohms, when reading low resistances) just with how the meter's probe tips are contacting the wires in the circuit. My Fluke meter has the same precision as yours!

--JMM

This thread can be marked as an example of what CR4 is all about. Fmalura had a problem, and many contributed their views, and he read and responded to every one very promptly. Now the issue has been solved and he has properly closed it.

i have seen many people post questions as guests, and never bother to visit the thread again

Dear Mr. fmalura,

Please clarify the following since you have not mentioned the following in your description.

1. Is the motor is suitable for VFD application, and if not the over-heating will occur.

2. What was the low speed operation and for what duration it is run in Low speed. a conventional motor should not be run at low RPM with VFD system.

DHAYANANDHAN.S

Mr. dhayanandhan,

I hope your comments are not taken as absolute rules. With loads such as centrifugal pumps or fans, using a VFD to reduce the speed will reduce the power requirements much more, so a motor will actually run cooler. The primary design difference in an inverter-duty motor is the use of winding insulation with a much higher voltage rating so it can withstand the voltage doubling commonly occurring at the start of the winding because of the impedance mismatch between the winding and the supply conductors. I have had no problems with applying a VFD to a standard motor IF the installation included an appropriate load reactor to minimize the possibility of voltage spikes. Your caution about low speed operation is important. External cooling fans are one way to avoid the overheating that can occur with periods of low-speed operation.

--John M.

Dear Mr.Jmueller,

Quite true. Thanks for your comment.

DHAYANANDHAN.S