Auto transformer start motor tripping on prolonged starting time
Auto transformer start motor tripping on prolonged starting time
(OP)
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 ?
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 ?





RE: Auto transformer start motor tripping on prolonged starting time
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Auto transformer start motor tripping on prolonged starting time
Keith Cress
kcress - http://www.flaminsystems.com
RE: Auto transformer start motor tripping on prolonged starting time
Is this a new installation or an existing one suddenly having this problem ?
Since the motor starts decoupled, the problem seems to be the pump. Can you check the pump for free rotation ?
Another point. What about the direction of rotation ?
Yeah, I know. More questions than answers here.
RE: Auto transformer start motor tripping on prolonged starting time
Pumping duty is very much dependent on total dynamic head being placed on the drive motor. The bigger the opening, the bigger the load the motor has to carry while starting.
RE: Auto transformer start motor tripping on prolonged starting time
edison123
It is an existing installation .Previously also similar problem has occured ,but we have always been able to trace it back to either pump or discharge valve passing .Only this time everything seems to be OK .Pump was also resent to our workshop for checking .But no problem was observed .
Direction of rotation is OK .Also pump has been checked for free rotation.
burnt2x
Normally we are starting the pump with discharge valve closed.Yes,recirculation line is there.We haven't tried to recirculate more at startup .Infact we have tried the opposite .we have tried to reduce the recirculation line .Can you please elaborate on that .Maybe we are doing that wrong !!!
RE: Auto transformer start motor tripping on prolonged starting time
But here as you have opted all the do's, i think if Motor protection relay is having thermal meter you can record the thermal contents ( can be compare with old record also if available) & then for the sitution you can decide upon the increasing the time setting.
RE: Auto transformer start motor tripping on prolonged starting time
I have experienced this kind of high starting current/ long starting times before when the recirculation valve control went wrong!
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
What we are thinking next is to modify the control circuit a bit to take trial run on DOL .
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
Additional point.
Thermal withstand is I^2t function, why you build your logic only on the t2 time?
I agree with Edison, try increase the t2 delay.
What is current limeters?
I recommend use relay with integration function I^2t, and not definite time limiter
Best Regards and Good Luck.
Slava
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
When you contact the supplier, also have available the load information, and inertias, so that the suppler can give as good an analysis of the starting situation of the motor as possible and give any recommendations they feel are needed, including starting frequency and cooling times etc.....
Best of luck!
RE: Auto transformer start motor tripping on prolonged starting time
We have the thermal withstand curve for the motor which has a plot between stator current and time .For 60% tap , the current is 1.83 times FLC and the time against that current comes out to be approx. 17 seconds .If we increase the time t2 then effectively we are allowing the current to flow for more than its withstand time.Is it OK ?
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
Have you monitored the motor speed during the start?
The solution depends on the motor acceleration.
If the motor has stopped accelerating before it trips then you have reached the maximum speed for the voltage you are applying. You either need to increase the voltage or decrease the load if you want it to accelerate further.
If the motor is still accelerating when it trips then you have not given it enough time to accelerate.
Another option is to turn up the setting on the current relay a bit. Maybe someone was messing with it.
I think burnt2x is talking about using the recirculating valve to pump fluid in a circle instead of pumping the fluid down the pipe. If you keep the discharge valve closed then this shouldn't apply. Recirculating fluid will probably take more power than dead-heading the pump.
RE: Auto transformer start motor tripping on prolonged starting time
Right now our Mechanical maint. section is rechecking the recirculation part.Maybe I can update after that.
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
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.
If you don't want to or can't find the reason it quit working then either bump up the transformer taps or increase the transition current level. Those are the only 2 ways you'll get the motor to start in it's present condition.
RE: Auto transformer start motor tripping on prolonged starting time
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:
CODE
Time = -----------------------------------------------
308 X (Ave. mot. torque - Ave. process load torque
RE: Auto transformer start motor tripping on prolonged starting time
Also the fluid which is being pumped is water .
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
Arbil,please take in account, possible case.
Best Regards.
Slava
RE: Auto transformer start motor tripping on prolonged starting time
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!
RE: Auto transformer start motor tripping on prolonged starting time
RE: Auto transformer start motor tripping on prolonged starting time
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
RE: Auto transformer start motor tripping on prolonged starting time
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:
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And there are more others.
Best Regards
RE: Auto transformer start motor tripping on prolonged starting time