Starting of an Induction motor
Posted 02 October 2003 - 07:08 PM
Yes, you are correct that when a motor is connected in star, the current and torque are reduced to 1/3 of the delta values. In star connection, this current is the same whether measured in series with the windings or in the supply phase. In the delta connection, the current in series with the winding is not the same as the line current.
If a star/delta starter achieves it's ideal results, then the start current will be reduced to 1/3 of the DOL current and this will reduce the start current loading on the supply, transformer or generator and reduce the voltage drop during start. In theory, this could result in a smaller supply transformer or a smaller generator set. In reality,because you are not able to control the start torque, many machines do not reach full speed before changing to the delta connection and so the full DOL current is drawn from the supply. Nothing hs been achieved by the star/delta starter!! There also an additional problem due to the open transition switching from star to delta which causes severe transient currents and torques causing more damage than a DOL starter.
There are other posts on this subject, and there is also information on our web pages covering this subject.
I hope that this answers your question.
Mark Empson | administrator
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Posted 04 October 2003 - 09:54 AM
Thankyou for your immediate rply with excellent explination. Would you please give details about open trasition and how close transition is acheived in star/delta starting.
We generally provide timer delayed output NC contact to open in star contactor circuit and timer instant output NO contact to close in delta contactor circuit to achieve close transition.
Are their any superior methods available to ahcieve accurate close trasition. Pls provide details.
Further, any IEC or international stds are available for terminal connectons of star/delta starting motor, to enable connections of 2 Nos. power cables from motor feeder to motor terminals. Because, different practices are being followed, one is changing the RYB bus/wire sequences for delta contactor inside the panel and identify BYR as RYB at the panel terminals or vice a versa and to ensure proper connections at motor terminals.
Posted 07 August 2004 - 04:04 PM
Posted 12 August 2004 - 07:24 AM
Check out the thread "auto-transformer versus star delta" by herselman74.
If the information there does not answer your specific question, let us know.
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