Burning of motor

[rajant2001]

Dear Members,
We are a Boiler manufacturer. We have supplied 2 Nos Low Tension motor - 415V; 50Hz 0f 110 Kw for fan application. Each fan is having full load capacity. The first fan is with VFD and the second is with star/delta starting and inlet regulating vane flow control. The boiler was commissioned and operating for some days with first fan. Then they have started second fan. After running far a day with some load, the motor winding got burned. The motor was opened out for analising the cause of burning. It was noted that there was rubbing of stator and rotor winding on the NDE side. The motor manufacturer rewound the same motor and sent to the project. The motor was meggered which was 5 Mohm plus. Run on no load for a day and then coupled to the fan. After running for just 90min., the motor burnt once again. Opening of motor indicates same rubbing of stator and rotor winding on the NDE side.
There was no specific additional noise noted during both failures.
The coupling of fan and motor is standard flexible coupling.
The control circuit is provided with thermal over load relay for over load protection and fuses for short circuit/earth fault protection. We have also provided single phase preventor across the power fuses.
During both occassions, the voltage was normal and current taken were within limits.
Customer is now asking for a new motor and not once again rewing the motor. The motor manufacturer was telling the failure of motor was due to fan problem only which we have disputed as the rubbing was noted in the nondrving end.
I request the fourum can discuss on cause and remedy for the above problem.
Regards,

R.Thiyagarajan.

 

[itsmoked]

Probably it is the fan but can you tell us more about the setup?
How is the motor mounted and on what?
What kind of fan?

It is the VFD driven motor?

I recommend you provide us with pictures.
In these cases experienced folks here often see problems right away with pictures after getting nowhere with just words.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Which motor failed, the VFD driven or the wye/delta driven ?

Did they find out why the rubbing happened in the first place ? Insufficient air-gap ? Eccentric rotor mounting ? Loose fit of bearings ? Bearing inner race rolling on the shaft journal ?

 

[rajant2001]

Application Details of burned motor:

Method of starting: star/delta
Type of Fan: Axial - Overhung impeller
Application: Induced draft fan handling flue gas at 130 deg. C approx.
Mounting : Common base frame for fan and motor and in turn base mounted on aconcrete structure.

Rubbing of rotor with stator was found in both casses.

Right now drawings/pictures not available.

Hope the above further details are adequate.

R.Thiyagarajan

 

[waross]

The problem is mechanical, not electrical. The rotor rubbing the stator will generate enough friction heat to burn the motor but the extra current may not be enough to affect the motor overload protection.
Check your fan alignment. An extremely poor alignment may cause an existing problem to get worse.
Look for a failing bearing if the bearing was not changed. Check the end bell, bearing recess and the back end of the stator for eccentricity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

The shop which did the rewind should have done the mechanical checks I mentioned. Check with them.

The problem is mechanical, not electrical (unless you're exceeding the nameplate current).

 

[electricpete]

Good questions above.

In addition, were any vibration readings taken while running prior to the failure(s)? The magnitude and characteristics of vibration could provide a clue what kind of problem was present that led to rub.

Was the rub all the way around the rotor and part way around the stator (characteristic of off-center rotor), or part way around rotor and all the way around the rotor (characteristic of bowed rotor or possibly excess bearing clearances although not as definitive for excess clearance). If possible describe the arc of contact on both rotor and stator in degrees . How close was the rub to the NDE? Right at the end of the cores or toward the center displaced slightly toward NDE.

I assume it is simple deep groove bearings for the motor?
How many bearings does the fan have and what orientation (between bearings or overhung). Was the alignment motor to fan checked ? Is there possible change in alignment from off-line to on-line based on thermal growth, movement from torque or from fluid forces?

Certainly no matter what the repair shop should do a careful analysis of as-found condition of the motor including pictures, bearing ID and OD measurement, shaft OD and bearing housing ID. Check the internal clearance of the as-found bearing (while still mounted to rotor but removed from housing push/pull and monitor with dial indicator.... tells the bearing internal clearance assuming no looseness at bearing ID to shaft). Check rotor TIR. Perhaps do rotor checks such as loop test with thermography and injected current with thermography looking for possible rotor problem that may lead to bow. Also do stator core test. Check as-found airgap at 4 locations around the stator if ODP motor. Rotate rotor 90 degrees three times and repeat the four measurments around the stator. For TEFC I don't think there is any way to check the as-found airgap(any comments edison or starkopete or others?). In absence of airgap checks, very careful measurements for concentricity of bearing housings to stator bore and characteristics of stator bore.

Even though it is very very likely mechanical there is perhaps a small possibility that the problem arose from overload/overheating which caused distortion or bow AND the overloads were not set properly (have you looked at the overload protection settings or did you measure current under load?). Shop should review winding details carefully.

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[electricpete]

Was the noise from the rub heard? Did it occur during start or right before the trip? Pullover from magnetic forces (which is not necessarily what happened) is more common during start.

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