Auto transformer start motor tripping on prolonged starting time 1

[arbil]

We are having a 6.6 KV,1500 KW motor with auto transformer starting .The tap setting of auto transformer is at 60% .As per the motor data sheet/curves,at 60% tap, the motor starting time is 11 seconds and thermal withstand time is 17 seconds.
As per the control scheme , the changeover from auto transformer start to normal operation is dependent on timer 't1','t2' and current relay , i.e the set minimum time 't1'(set at 6 sec) should elapse AND starting curent should come down.If current does not fall within time 't2' , the main circuit breaker will trip.
The timer setting 't2' is kept at 15 sec , i.e. 2 sec less than the thermal withstand time .
Now the problem is that we are not able to run the motor as the starting surrent is not coming down within 15 seconds .Normally this would have indicated problem with the pump or discharge valve .However both have been checked and no problem has been observed .Bearing conditions are also OK .We cannot increase the timer 't2' setting also , as it will cross the thermal withstand time .Motor starting is OK in decoupled condition .System voltage is also stable .We are not able to ascertain where the actual problem is .
Is there any other method or site test by which we can diagnose the actual problem ?

 

[burnt2x]

"I read again and missed that this is an existing installation that had been working. In that case, something has changed and you are still missing it."

That's a good point from Mr. Hutz! If you have a handy motor formula, the only reason for your pump to exhibit longer starting time is for the following to have occured:
1. The average process load torque during run-up has gone up (process changes upstream of the pump discharge, etc.)
2. The total system rotational inertia has increased (impeller change, tight packings, rubbing shafts, etc)

Please remember that the time to reach operating speed is:

             Tot. System Rot. Inertia X pump speed
 Time = -----------------------------------------------
         308  X (Ave. mot. torque - Ave. process load torque

 

[arbil]

One more thing I may not have added : This is an existing installation which is in operation for the last 25 years.
Also the fluid which is being pumped is water .

 

[7anoter4]

I think the rotor cannot reach the rated speed due to a reduced torque .This could be caused be one or more broken rotor bars.

 

[arbil]

7anoter : Infact , its standby motor was once detected with a broken rotor bar .But in that case the symptoms were different .The motor used to start but during running there was a continuous hunting observed in the load current .Moreover in the vibration signature , sidebands were observed at slip frequency .We got its current signature analysis done in which it got confirmed that the motor indeed had broken rotor bars.It was subsequently repaired.

 

[slavag]

7Another4, intresting idea.
Arbil,please take in account, possible case.
Best Regards.
Slava

 

[burnt2x]

7anoter4,
In that case, it is the average motor torque that reduced (referring to the time to reach operating speed formula)! A big possibility. Star to you!

 

[arbil]

7anoter:But how to confirm/rule out the possibility of a broken rotor bar without actually opening up the motor .We have run the motor in decoupled condition .The current seems to be steady.

 

[slavag]

Hi Arbil.
From protective relay ( Broken Bar Detection) point of view...it's a big problem.
It's some very special algorithm on the high level of harmonics and from expirience of other dosen't work.
Only open the motor.
Best Regards.
Slava

 

[7anoter4]

Broken rotor bars rarely cause immediate failures, especially in large multi-pole (slow-speed) motors.
However, if there are enough broken rotor bars, the motor may not start as it may not be able to develop sufficient accelerating torque.
For a symmetrical rotor with no broken bar the resultant of backward rotating fields is zero. When there is a broken bar, no current flows through the broken rotor bar, thus no magnetic flux is generated around the broken rotor bar.
This generates an asymmetry in the rotor magnetic field by yielding a non-zero backward rotating field. This non-zero backward-rotating field rotates at slip frequency speed with respect to the rotor, and induces harmonic currents in the stator windings, which are superimposed on the stator winding currents.
These superimposed harmonics are used as signatures of broken rotor bar fault in motor current signature analysis (MCSA) techniques.
Motor current signature analysis technology has existed for many years to help diagnose problems in induction motors related to broken rotor bars, air gap eccentricity, drive-train wear analysis, and shaft misalignment.
The technology relies on the fact that each of these problems produces recognizable frequency patterns in the motor load current that can be predicted by using empirical formulae and measured.
There are a lot of articles on the Web treating these methods, for instance:

http://www.lib.ncsu.edu/theses/available/etd-12072005-232928/unrestricted/etd.pdf

http://www.maintenanceworld.com/Articles/maintenancetech/Detecting-Broken-Rotor.htm

http://redalyc.uaemex.mx/redalyc/pdf/810/81040221.pdf

And there are more others.
Best Regards

 

[arbil]

But for carrying out the motor current signature analysis , the motor should run .As per our ommunications with agencies carrying out the tests , the requirement is for the motor to run with at least 50% load .