Srat Delta starter

[bob]

Hi,

 

I ve got two star delta starters driving two 185 kW electric motors 400 V 2850 rpm . The motors drive two feed water pumps on a boiler station and should always be started on load that is with the feed valve almost in the opened position. One of the motors always achieves to reach 80 % nominal speed before transiting to delta but the other one, similar name plate characteristics , never achieves to near nominal speed with the result that the switching to delta always results in full locked rotor current and eventual tripping of the circuit breaker or overload relay of the starter.

I am planning to use a soft start but have never installed one so far. How will a soft start behave in such an application ?

 

Thanks.

 

Bob.

 

[marke]

Hello Bob

 

Sounds to me like you have the classic case of two motors that appear to be similar, but have very different starting characteristics.

 

If you can get the data sheets, have a look at the LRT and the LRC. I suspect that you will se a major difference between the LRT of the two motors. (Assuming the start torque requirements of the two pumps are very similar.)

 

If the LRT on both pumps is very similar and the start torque requirement is also similar, then I would look at the rotor bars in the bad motor. It is possible that you have a rotor bar problem which is reducing the start torque that it is producing.

One easy rotor bar check is to run the motor under very light load and watch the current with an analog meter. If you see the current swinging a lot at a low frequency, and the frequency of that swing increases with load, then you could have a bar problem. This is not uncommon on motors that have been started with a star/delta starter for a period of time.

 

If you use a soft starter, the start current wil be higher than the current in star, but the motors will get to full speed without the step to DOL current and there will not be any transients.

The start current required is dependent on the LRT, the LRC and the start torque of the driven load. It is also dependent on the actual soft starter used and the set up employed.

 

Best regards,

[joe cambel]

Hi,

 

I ve got two star delta starters driving two 185 kW electric motors 400 V 2850 rpm . The motors drive two feed water pumps on a boiler station and should always be started on load that is with the feed valve almost in the opened position. One of the motors always achieves to reach 80 % nominal speed before transiting to delta but the other one, similar name plate characteristics , never achieves to near nominal speed with the result that the switching to delta always results in full locked rotor current and eventual tripping of the circuit breaker or overload relay of the starter.

I am planning to use a soft start but have never installed one so far. How will a soft start behave in such an application ?

 

Thanks.

 

Bob.

hi,

I have an answer to you , you said"The motors drive two feed water pumps on a boiler station and should always be started on load that is with the feed valve almost in the opened position".

so my qustion : do you have timers to control the opening of the feed valve, and if that the case dose the opening time for the second motor is enough to get the feed valve fully open by the time the starter turn to delta connection.

Thanks.

[kens]

Hi Bob, have you considered using VSDs in this application? There could be some worthwhile energy savings to be found.

Ken