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Motor Brnout/softstarter Failure


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#1 gaurav1981

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Posted 03 February 2008 - 09:01 AM

Dear All,

Recently faced a problem.

Some 6 months back installed a softstarter; current rating 710A on a 350 Kw motor; paper mill refiner application.

Two days back got a message that the motor has burnt & softstarter is unable to start the replacement motor of 250Kw Rating.

History of events as told to me:
During a normal running( motor was running on a bypass contactor) the motor stopped , the main circuit breaker tripped. Breaker was reset & motor checked.
Again restarted & after sometime there was smoke coming out from motor.

On seeing the motor , it was completely burnt & most probably the point where the windings were brazed has blown off.

When i tried to start the 250Kw motor, decoupled from load;

the motor shaft took 2-3 turns then stopped, again 2-3 turns & stop........
during this time the current values were oscillating from 144-185-212, 144-185-212.....
i increased the initial voltage level from 30% to 60% in steps of 5% but to no use.
I also measured resistance values betweeen input-output terminals fof S.S for each phase, readings were Rphase 2.71 Mohm, Y phase 2.72 M ohm but in case of third phase the readings never stabilized as soon as reading approached 3.7.. the tester again went into scanning mode.

During one of the trials on the 250Kw motor, the move & stop pattern of motor was going on, bypass contactor went on , motor started & eventually the main circuit breaker tripped.

Any possible explanation for this behaviour.

Thanks


#2 jraef

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Posted 03 February 2008 - 11:09 PM

Sounds as though you may have lost the firing control circuit of the soft starter. If it can no longer fire the SCRs in the correct pattern, the motor behavior become erratic. The problem with that is that if it were so, that does not explain the circumstances of the reported initial failure. If the motor were in Bypass, nothing the soft starter was doing (or not doing) would have any effect. Unless there was a complete failure of the soft starter control system, in which case it may have dropped out the Bypass Contactor as well.

But it could also be a symptom of a worse problem, such as a high resistance ground fault (earth leakage) on the motor leads somewhere. What can happen is that the soft starter is attempting to limit the current flow to ground, but only on the phae where that occurs, so the other phases get full power as determined by the firing control circuit. yet because the 3rd phase is getting less at the motor, the motor acts similar to what it would in a single phasing condition. if your soft starter does not have Ground Fault protection built-in, this could easily be the problem and given the circumstances of the failure, it may be likely.

When you say they "checked the motor" what did that entail? Did they use a megger and did they do it from the starter in order to check the leads? Anything less would be essentially useless. I would stop energizing that circuit and check the cables thoroughly. Disconnect them from the starter and use a 500V megger (assuming a 480V system or less), 1000V would be better. If it all checks out OK, then your control board (or firing board depending on the make and design) is toast.
"He's not dead, he's just pinin' for the fjords!"

#3 marke

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Posted 05 February 2008 - 09:10 AM

Hello gaurav1981

Looking through your post, it would appear that there is a strong possibility that the sequence of events is that the motor failed and the failure of the motor may have caused a problem with the soft starter.

From your description, it sounds as though the windings are all burned rather than just one or two phases. This suggests that the motor has operated under a severe overload for a significant period of time.
You do not mention the condition of the rotor windings/bars.
If the motor was damaged while it was operating (it tripped while running) then the stator would be damaged and not the rotor.
If the motor was damaged while it was not running, i.e. not able to start, then there would be signs of severe heating in the rotor.

It is possible that the rotor could also have been damaged at the aborted start attempt, but it is more likely that the stator winding had already failed and shorted turns prevented start torque being developed.

A thorough inspection by an experienced and skilled rewinder could provide some further information on the cause.

To test the soft starter, I would connect lamps connected in star across the output of the soft starter. The lamps should be rated at least 100W and the voltage rating of the lamps should be at least equal to the phase to neutral voltage of the supply.
Disable the bypass contactor.

Try a soft start into the lamps and watch the ramp up brilliance of the lamps.
All three lamps should ramp up in brightness at the same rate, and should achieve full brightness together.
If there is a variation in the lamp brightness, there is probably a problem with the soft starter. If they all ramp up together and all achieve full brightness at full voltage, then the soft starter is probably OK.

I would take a very good look at the motor protection. Is the protection built into the soft starter? is the protection still operational once the starter is bypassed? In some cases, the bypass contactor bypasses the protection as well.

Best regards,




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