Facing Problem in CT Operated Over Load Relay

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mukeshkumar5609 Active Contributor Join Date: Jun 2011 Location: Lucknow , Uttar Pradesh , India Posts: 15	Facing Problem in CT Operated Over Load Relay 12/13/2013 3:57 PM
	We are facing problem in operation of AC Motors. We have many flange mounted AC motors of 315 KW,FLC-531 Amp and RPM 1475, Siemens make . Motors are connected with star-delta starters. We are using CT operated relay of siemens for over load protection in phase . CT range is 250-400 amp . 250 Amp is equal to .625 and 400 equal to 1 amp and mounting relay range is .625-1 Amp. Relay is mounted upon of CT set.Problem is that motor runs normal till many hours but any time it tripped by O/L while current does not exceed till maximum setting. O/L setting is on .75 amp but when we check secondary current of CT than we found 1.1-1.7 amp while primary current is near about 100-120 amp, measured by clamp meters. Than how is possible this . If output is false than why not relay tripped in some time. we have replaced CT and Relay also one by one with new one but problem is same. Secondary current is not proper. Kindly guide us what should we done .... > Mukesh kumar 09897991032 __________________ Mukesh Kumar
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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	#1 Re: facing Problem in CT operated Over Load relay 12/13/2013 5:13 PM
	Is this a packaged OL relay mounted to the CTs provided by Siemens? Or did someone build this on their own? If it was not built by Siemens, someone may have used the wrong CT ratio. Also, I never like using a 1A CT secondary, the range of operation is very small and can be unduly influenced by outside issues. I use 5A secondary ratios, and I try to select the ratio so that the full load current current of the motor lands in the middle of the ratio. So for example on yours, where the motor FLC is 531 and is a Star-Delta controller where I would put the OL in the Delta connection, the motor FLC through that set of conductors will be 58% of the total, so 308A. I would then use a 600:5 CT, and an Over Load Relay that has a range up to 5A, then set it at 2.57A 351A FLC 58% = 308A 600:5 CT ratio = divide by 120 308/120 = 2.57 setting You failed to mention your CT ratio, nor do you explicitly state where in the circuit you placed the OL relay. But I would venture that somewhere in those issues lies your problem. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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mukeshkumar5609 Active Contributor Join Date: Jun 2011 Location: Lucknow , Uttar Pradesh , India Posts: 15	#5 In reply to #1 Re: facing Problem in CT operated Over Load relay 12/15/2013 4:25 PM
	Good Morning Sir and thanks for guide me. Sir,O/L relay is not packaged with CTs but this is also siemens make and same rating. This is also CT mounted but not purchase with CT. Is this can be a reason of this problem. CT primary Range is 250-400 Amp and secondary current .63 Amp=250 Amp & 1 Amp=400 Amp. Location is in North India , State is Uttar Pradesh . Atmosphere temp is 15-20' C at night. kindly guide.... __________________ Mukesh Kumar
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ramvinod Guru Join Date: Jun 2007 Location: chennai,India Posts: 592 Good Answers: 19	#2 Re: facing Problem in CT operated Over Load relay 12/13/2013 10:18 PM
	You should use Siemens make thermal overload relay with matching CTs supplied along with Over load relay. The relay is calibrated with matching CT. In Siemens OLR over load setting markings are indicated in Primary Current __________________ Ramesh,Freelance Electrical/automation Consultant
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raghun Power-User Join Date: Jan 2010 Location: Hyderabad Posts: 343 Good Answers: 22	#3 Re: facing Problem in CT operated Over Load relay 12/14/2013 4:36 AM
	When star-delta starter is used, the CT location is important. the CT will see a primary current of 1/v3 times the motor flc when the CT is inside the delta. Hence, the OLR setting range, CT ratio both need to be decided according to the CT location. Otherwise, it can cause maloperation (as you are probably experiencing). __________________ Raghunath
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mukeshkumar5609 Active Contributor Join Date: Jun 2011 Location: Lucknow , Uttar Pradesh , India Posts: 15	#4 In reply to #3 Re: facing Problem in CT operated Over Load relay 12/15/2013 4:13 PM
	Sir , thanks for guide. We are using CT after Main contactor . CT is 250-400 Amp range and O/L is .63-1 Amp and .63=250 Amp and 1=400 Amp. __________________ Mukesh Kumar
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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	#6 Re: Facing Problem in CT Operated Over Load Relay 12/15/2013 11:37 PM
	"CT primary Range is 250-400 Amp and secondary current .63 Amp=250 Amp & 1 Amp=400 Amp. " Sorry, your information makes no sense whatsoever, even the math statements are not cohesive. This indicates to me that the task at hand may be beyond your capabilities, I suggest that you contract the services of a qualified electrical engineer. __________________ All I every really wanted to be, was... A LUMBERJACK!.
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