Star Delta Starter Problem

[AB2005]

Dear All,

 

We have a very old shredding machine which has a main motor rated at 40HP, 400V, 50HZ, 52A. It has a conventional Star Delta starter with a simple delay on timer for star to delta conversion. During my period of job in this plant, I faced a problem two times that its Main and Delta contactors stuck out during timer operation for shifting Star to Delta. Contactors were over-rated i.e. Main & Delta = 60A and Star = 50A. Now I have replaced the timer with Star Delta timer and problem solved. But question is in my mined that why that fault occurred even when the Main, Star and Delta contactors were interlocked with each other i.e. if star contactor was on delta contactor couldn’t energise.

Any comments?

 

[jraef]

Could be a number of reasons.

1) The mechanical interlock mechanism between the contactors was defective or not adjusted correctly (if it was an older style that needed adjustment).

2) The mechanical interlock worked, but the actual contact components were welded shut, so even though the armature dropped out (which is what the mechanical interlock would be connected to), the contacts themselves were left behind in a welded state.

3) You had no mechanical interlock and were relying solely on electrical contacts as interlocks, in which case they failed.

4) Again, no mechanical interlock and your old timer was not a specific Start-Delta timer that has a dwell time in between changeover to allow for contact dropout time, then your electrical interlocks failed due to being chattered repeatedly.

[AB2005]

Dear Mr. Jraef,

 

Thanks for replying. Actually contactors were interlocked through Auxiliary contacts coupled on the top of TELEMECHANIQUE contactors. These Auxiliary contacts had no adjustment. First time, when contactors were stuck, I changed the both contactors and there Auxilary contacts. Timer was not a star delta timer just a simple OMRON brand timer with one set of NO & NC contacts. I think that such that problems were caused by Engineers to design a star delta timer in which timer provides a delay in between star to delta transition. Because many of old machines, which I have seen, have motor starters with a simple delay on timer. But question is that what happened during star to delta transition which was caused such problems?

 

[GGOSS]

The timer used in a star delta starter is a purpose built timer. Internally there are 2 seperate relays, the commons of each relay are connected together giving the illusion that there is only a single relay with a change-over contact inside.

 

When you analyse a star deta timer further you will find that there is a delay of approximately 40milli-seconds between one contact (star) opening and the other contact (delta) closing. This delay is necessary to ensure that the arc drawn when the star contactor opens is fully extinguished before the delta contactor closes.

 

If you use a standard timer in place of a star delta timer, the time delay between the contacts changing is close to zero. It is therefore highly likely that the delta contactor will energise before the arc drawn by the star contactor fully extinguishes. This condition is exactly the same as a short circuit and will cause contactors to weld.

 

Hope the above helps.

 

Regards,

GGOSS

[jraef]

Well summarized by GGOSS.

Furthermore, IF you only have a standard timer at your disposal, you then MUST have the mechanical interlock mechanism between the contactors that prevents the Run contactor from physically pulling in until the Star contactor has dropped out. If I'm not mistaken, that is a simple snap-on accessory on the Telemecanique contactor line, so someone got cheap on you and failed to recognize the importance. The Star-Delta timers are designed for electrical interlocking that avoids this issue, but personally, I prefer having both methods, one as a backup to the other.

 

Actually though, I prefer not to ever use Star-Delta in the first place, the number of potential problems is too great. Soft starters are so much better and simpler!

[AB2005]

When you analyse a star deta timer further you will find that there is a delay of approximately 40milli-seconds between one contact (star) opening and the other contact (delta) closing. This delay is necessary to ensure that the arc drawn when the star contactor opens is fully extinguished before the delta contactor closes.

 

If you use a standard timer in place of a star delta timer, the time delay between the contacts changing is close to zero. It is therefore highly likely that the delta contactor will energise before the arc drawn by the star contactor fully extinguishes. This condition is exactly the same as a short circuit and will cause contactors to weld.

 

Hope the above helps.

 

Regards,

GGOSS

Dear Mr. GGOSS & Mr. Jraef

Thanks for your replies. I think, that was the reason which is mentioned above.