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 ?

 

[waross]

Is there any other method or site test by which we can diagnose the actual problem ?

You can analyse the inertia of the motor-pump combination and add the friction load as the speed increases. If your analysis is accurate it will show that the pump will not start. You all ready know that. Go to the 80% taps on the autotransformer. This is assuming that the motor is good, there are no problems with the pump, there are no problems with the conductors and the autotransformer is properly sized and in good condition.

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

 

[itsmoked]

And don't violate the starts per hour!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Have you checked the actual input & output voltages of the auto ?

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.

 

[burnt2x]

Please take a closer look at your pump starting procedures. Maybe you could change how big your initial discharge valve opening is, or if you have a recirculation valve, that valve should be arranged to recirculate more at start up to allow the motor to come up to speed earlier.
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.

 

[arbil]

Thanks all for your replies
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 !!!

 

[NM13]

Pratically speaking tnay motor in prolong condition during starting the driven equipment/system should be checked first.

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.

 

[burnt2x]

Recirculation line returns the discharged fluid from the pump to the suction point or somewhere near. The pump is therefore loaded light as the head is much lower than when the discharged fluid is allowed to flow the total length of the discharge line or pumping fluid to a higher elevation.
I have experienced this kind of high starting current/ long starting times before when the recirculation valve control went wrong!

 

[RRaghunath]

Could it be possible the actual voltage at motor terminals (during starting) is lower than 60%!!

 

[arbil]

raghun : we don't have any way of measuring the actual voltage at motor terminal .The voltage at the bus is OK .
What we are thinking next is to modify the control circuit a bit to take trial run on DOL .

 

[edison123]

How often do you start this motor ? May be you can increase the t2 time delay a little bit more to say 17 secs ?

 

[slavag]

Hi.
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

 

[edison123]

I^2t . Good point, Slava.

 

[EEBubba]

Another possible solution would be to use a zero speed switch and have it interrupt the stop function on "t2". Consult with the motor supplier, but one of the most common "thermal withstand" times given is the "Locked rotor / Stall Time". As this is only valid during a stall, and by definition an accelerating motor is not stalled, the settings may be giving you a false positive.

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!

 

[arbil]

This motor is normally started once every day and runs for approximately 3 hrs .
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 ?

 

[edison123]

If you are sure that the motor, the pump and the cable terminations are not the problem, and if you still face this starting problem often, I think you can risk increasing the t2 time a bit. Your start duty is not that severe to do thermal damage.

 

[LionelHutz]

Have you monitored the current during the start?
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.

 

[arbil]

LionelHutz : Yes, we have monitored the current during starting .As per the datasheet it should be around 280 Amps .The actual current is more than that (approx. 320 -330 Amps) .We have also tried to monitor the speed during starting .Although the measurement has not been very accurate , the speed has been less than 2700 rpm (rated is 2968) at the time of tripping .
Right now our Mechanical maint. section is rechecking the recirculation part.Maybe I can update after that.

 

[edison123]

The starting current seems to be about 2.2 times the rated current. If your auto has a higher setting like 80%, the problem should go away. While the current will be higher at about 3 times the rated, run-up time will surely come down with a much better starting torque (70% more of torque @ 60% voltage setting). Of course, you need to set your relays accordingly.

 

[LionelHutz]

The motor is reaching a "plateau" at 330A and 2700rpm where the torque it produces is equal to the torque the pump is requiring to turn. It will not accelerate any further unless you increase the terminal voltage.

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.