3 Phase Overload Setting

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alan.rotech Participant Join Date: Jul 2010 Posts: 1	3 Phase Overload Setting 07/23/2010 8:46 PM
	We have a 3 phase 3 Kw 415v motor, FLC 5.7 amps, locked rotor 9 amps. When motor is running under load it draws 2,5 amps per phase should the overload be set at 7.5 amps.
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msamad Guru Join Date: Feb 2009 Location: Houston, USA Posts: 946 Good Answers: 244	#1 Re: 3 phase overload setting 07/23/2010 9:52 PM
	If the overload relay is built per (a) IEC standard, the maximum allowable setting is not more than the motor FLC. (b) NEC standard, the maximum allowable setting is not more than the motor 1.15 x FLC. Your motor FLC is 5.7A, but it is drawing only 2.5A. So, it is freely allowed to keep the setting up to 5.7A. Even for NEC, there is no possibility of nuisance trip. 7.5A overload setting is much higher for this motor. - MS __________________ "All my technical advices in this forum must be consulted with and approved by a local registered professional engineer before implementation" - Mohammed Samad (Linkedin Profile: http://www.linkedin.com/in/msamad)
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JRaef Guru Join Date: Sep 2006 Location: California, USA, where the Godless live next door to God. Posts: 4665 Good Answers: 804	#2 Re: 3 Phase Overload Setting 07/24/2010 1:04 AM
	I think the nature of your question is, do you add the currents of the 3 phases? The answer is no. When we speak of current, we are referring to current in each phase only; there is no cumulative issue involved. So as said, you set the OL to the motor nameplate FLC; 5.7A in your case. What it actually draws is irrelevant, the OL is a protection device to prevent damage from excessive current under abnormal conditions. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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PWSlack Guru Join Date: Jan 2007 Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on. Posts: 32175 Good Answers: 839	#3 Re: 3 Phase Overload Setting 12/28/2025 5:51 AM
	<...should...be set at 7.5 amps...> Why? If the <...overload...> were set at <...5.7 amps...> then the motor would be correctly protected. __________________ "Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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