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Sadashiv Vaidya
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Join Date: Dec 2010
Posts: 13

3.3kV Soft-Starter Motor Persistant Problem

01/09/2011 11:17 PM

Motor is 600kW/3.3kV rated (Full Load-121Amp).

Motor start was given and it was observed that, after 10 sec motor found tripped on Instantaneous over current (IOC) and during this time current was recorded as 350 amps on soft starter and no voltage variation observed on Turbine Gen. control panel. Direction of rotation of motor was found reversed.

Instantaneous over current (IOC) is set as 700% and instantaneous over current start delay is set as 1 cycle.

The cables of two phases were interchanged on the soft starter to reverse the direction of rotation as required.

In the next attempt to start, the motor tripped within 10 secs of starting on Instantaneous Over-current. The current on the soft starter was observed to be 320 Amps. The direction of rotation was found to be correct.

In the second attempt, the motor tripped this time on Locked Rotor Current I2T after running for almost approximately 30 secs. The current on the soft starter was observed to be 190 amps at the time of tripping.

The rotor continued to rotate for more than 5 minutes after the tripping of the motor.

All current observations were made on the analog ammeter on the Soft Starter.

Q. What could be possible reason for motor tripping. Should LRC be increased from existing setting of 300% to 500%? Existing step timer is set to 7sec - should this be modified?
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TonyS
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#1

Re: 3.3kV Soft-Starter motor persistant problem

01/10/2011 6:26 AM

What has the soft start manufacturer had to say about the fault? You've had 2 weeks to contact them.

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JRaef
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United States - Member - New Member Engineering Fields - Power Engineering - New Member

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#2

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/10/2011 10:55 AM

I have had a lot of experience with MV soft starters, I used to work for a mfr.

2 immediate possibilities jump out at me.

  1. You said "Should LRC be increased from existing setting of 300% to 500%? Existing step timer is set to 7sec - should this be modified?" Are you sure you are reading the LRC setting, or the Current Limit setting? They would be completely different. If you have the LRC set at 300%, you are causing your own problem. The LRC setting should be set for the motor's LRC rating, usually 500-600%, never less otherwise you risk nuisance tripping. It's quite possible that in the course of accelerating, the current needs to go well beyond 300%. If you meant 300% Current Limit setting, then that would have no direct bearing on the tripping. But if, at 300% CL, the motor fails to fully accelerate oast 90% speed, then when the ramp time is done the bypass contactor will close and it's quite possible that you would still get a current transient equal to the LRC anyway. If that's the case, you will need to increase the CL setting, or decrease the ramp time, or a combination of both.
  2. You have an insulation breakdown somewhere in the circuit. It has high resistance, so it does not show up until the ramping voltage increases to the point of overcoming the resistance to where the current rises too fast. Most likely the transient is much much to fast to be seen by an analog meter, however a scope would see it . You may also have an internal flash over happening somewhere inside the soft starter. Same basic issue, different location.

Did you not have this commissioned by the soft starter mfr?

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Sadashiv Vaidya
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Join Date: Dec 2010
Posts: 13
#3
In reply to #2

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/11/2011 12:12 AM

Thanks JReaf, for your valuable input. The points clarified by you are under active consideration. Our Plant technicians tried to start the motor themselves with original settings, but ultimately, we are asking the manufacturer to modify the settings and resolve the issues.

Will you let know when we successed to run the motor w/out trips. Thanks once again!
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Anonymous Poster
#4

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/11/2011 4:06 PM

You better do not base your action on advises of people who have never seen your system. You might loose your reputation and blown the entire system. It is dangerous to full around this power. Why don't you use a consultant? Better safe than sorry, didn't mama thought you that?
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iyersv
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Join Date: Dec 2010
Posts: 3
#5

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/11/2011 5:28 PM

kindly check the acceleration of the motor. The soft starter may not be developing enough torque.

Regards,

iyersv
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Anonymous Poster
#6

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/12/2011 1:46 AM

Several point to consider during commisioning of MV starter:

1. Megger motor value as per manufacturer recommendation.

2. MV soft starter inrush current at 3 to 3.5 of motor RLA - pls. verify soft starter setting.

3. Soft starter starting current setting from manufacturer normally at 7 to 10seconds acceleration time - Pls. verify.

4. Exceeded starting inrush and acceleration time above manufacturer limits should be avoided to protect the motor winding from overheating/failure - which happen in your case.

5. Main ckt. breaker setting to be check to insure that motor will not trip at Overload and acceleration time should hacve been avoided to reach 30seconds acceleration time.
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WAWAUS
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Location: Melbourne, Australia
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#7

Re: 3.3kV Soft-Starter Motor Persistant Problem

01/12/2011 5:56 PM

Excessive current could be related to excessive load during startup - electrical or mechanical - anything from bearings on up.

Regards

Bill
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Anonymous Poster (2); iyersv (1); JRaef (1); Sadashiv Vaidya (1); TonyS (1); WAWAUS (1)

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