Star-Delta Motor Starters

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Anonymous Poster	Star-Delta Motor Starters 09/13/2010 5:32 AM
	We have recently installed two induction motors,one rated 30HP(22kW)/380V/42.5A,and the second one rated 10HP(7.5kW)/380V/15A.For 30HP motor, the starter recommed by manufactuer(Havels product) is star-delta with all 3 contactors(main contactor,Y-contactor and Delta contactor) rated at 32A.I know from the theory that at starting time the load current will be reduced by a factor of 0.58 and after the motor has attain its rated speed the motor will be switched at its normal rated name plate current(i.e 42.5A).So,why these 2 contactors (main & Delta) are rated different from the motor nameplate current? Best wishes,by Thobias. Dar eS Salaam-Tanzania
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TonyS Guru Join Date: Aug 2010 Location: Resting under the Major Oak Posts: 4347 Good Answers: 181	#1 Re: Star-Delta motor starters 09/13/2010 7:26 AM
	The line contactor has to handle the full current of the machine where as the other two don't. I personally think it's bad practice to use two sizes of contactor. I like standardisation it saves on spares stock. __________________ The reports of my death are greatly exaggerated.
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SHAMITHGAMBHIR1 Power-User Join Date: Feb 2010 Location: dubai,mangalore,bangalore..... Posts: 145 Good Answers: 8	#2 Re: Star-Delta motor starters 09/13/2010 7:46 AM
	The Star/Delta starter consists of three contactors, a timer and a thermal overload. The contactors control winding currents only. The currents through the winding are 1/root 3 (58%) of the current in the line which is around 25A in your case.so select a contactor around 25-32A There are two contactors that are close during run, often referred to as the main contactor and the delta contactor. These are rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star current when the motor is connected in star. The current in star is one third of the current in delta, so this contactor (star contactor) s rated at one third of the motor rating which is around 15A...so select 15-18A contactor..... __________________ shamith gambhir, electrical engineer
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THOBIAS MABUGO Associate Join Date: Sep 2010 Posts: 45	#4 In reply to #2 Re: Star-Delta motor starters 09/13/2010 10:43 AM
	Thank you for you good comment and it is self explanatory. Best wishes. T.mabugo(graduate electrical engineer)
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wasiq_zafar1230 Member Join Date: Sep 2011 Posts: 5	#5 In reply to #2 Re: Star-Delta motor starters 09/15/2011 3:44 AM
	really its a very good comment.
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SHAMITHGAMBHIR1 Power-User Join Date: Feb 2010 Location: dubai,mangalore,bangalore..... Posts: 145 Good Answers: 8	#3 Re: Star-Delta motor starters 09/13/2010 8:15 AM
	i forgot to mention...also one mccb is required for short ckt protection.... __________________ shamith gambhir, electrical engineer
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