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Thermal overload relay selection

Created by: jeet2907 at: 7/3/2013 1:05 PM (22 Replies)

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7/3/2013 1:05 PM	Rate (1)
jeet2907 Posts: 235 Rating: (52)	Dear all Please suggest me how to select thermal overload relay size(Rating) in star delta starter, and actual loaction(position) of OLR in S/D circuit.
jeet2907 Posts: 235 Rating: (52)
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7/3/2013 2:14 PM	Rate (0)
PISCEAN 2010 Diamond Member Joined: 4/23/2010 Last visit: 12/31/2023 Posts: 8092 Rating: (878)	Please follow below links which shall help you to understand functioning of the overload relays. Link1 Link2

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7/3/2013 4:25 PM	Rate (0)
Ste39 Platinum Member Joined: 1/21/2013 Last visit: 10/23/2024 Posts: 4620 Rating: (708)	Hi jeet2907, Can you explain to us a bit more in detail your application?

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7/4/2013 6:51 AM	Rate (0)
Valli Posts: 262 Rating: (45)	jeet2907 Dear all Please suggest me how to select thermal overload relay size(Rating) in star delta starter, and actual loaction(position) of OLR in S/D circuit. Hi, Selection of overload relay in Y-D starter is a bit tricky.. and one needs to understand Y-D starting properly. Anyways, the overload relay should be connected within Delta Circuit only and its setting range must correspond to the load current in the motor windings themselves, i.e. 1/ root 3 times the line current drawn by the delta connected motor. The overload relay is therefore set to 0.58 times the operational full load current of the motor and provides overload protectionin both star and delta modes. Refer the circuit diagram to understand the wiring and position of mounting etc. Hope this willclear allthe doubts Attachment Y_D wiring.jpg (1230 Downloads)
Valli Posts: 262 Rating: (45)
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7/4/2013 7:44 AM	Rate (0)
jeet2907 Posts: 235 Rating: (52)	Thanks but please can you give me some methemetical calculation for current in s/d starter. how it 0.58 times in delta and wht abt star connection? and as per your power wiring relay you put relay below maincontactor its ok but can i put it below s/d contactor? Thanks in advance
jeet2907 Posts: 235 Rating: (52)
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7/4/2013 7:50 AM	Rate (0)
PISCEAN 2010 Diamond Member Joined: 4/23/2010 Last visit: 12/31/2023 Posts: 8092 Rating: (878)	No. It should not be in s/d power circuit. Please note that you are connecting motor windings either in star or delta. Where as main contactor is used to feed power to motor winding. Hence protection must be in the circuit of main contactor.

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7/4/2013 10:47 AM	Rate (0)
@nkur Posts: 202 Rating: (16)	Hi there As the current is divided between the main & delta so it means that we need to select the relay half the rating of motor??? please do reply.....
@nkur Posts: 202 Rating: (16)
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7/4/2013 11:31 AM	Rate (0)
Valli Posts: 262 Rating: (45)	jeet2907 Thanks but please can you give me some methemetical calculation for current in s/d starter. how it 0.58 times in delta and wht abt star connection? and as per your power wiring relay you put relay below maincontactor its ok but can i put it below s/d contactor? Thanks in advance Hi, No complex calculation is involved. Kindly note one thing.... since we dont want motor to draw more starting current we have introduced delta connecion of motor windings and hence star delta starter came into the picture. Since, In delta the current reduces to 1/1.732 = 57.7% that of star contactor we acheive reduction in starting current. Now, Since overload relay will see the full load current of the motor it will be selected as per the rated current of the motor, but its setting would be 0.58times of the operational full load current. To prevent short circuits from occuring during the switch over from star to dela during star delta starting and YD time-delay relay ( timer) should be used. this ensures that the switching arc in the star contactor is completely quenched ipon opening before delta contactor closes. Hope you guys get the clarity on selection and setting of overload relay
Valli Posts: 262 Rating: (45)	Last edited by: Valli at: 7/4/2013 11:32 AM
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This contribution was helpful to 4 thankful Users kinjalpatel1899 DILIP_1111 Jason85 Ma Qingyu

7/4/2013 2:20 PM	Rate (0)
@nkur Posts: 202 Rating: (16)	Hi Valli I have a question that how come one can set OLR to 58% when the olr is connected to the main contactor.?? if we consider 58% for delta than for OLR below main contactor should be 42% of the value???? please clarify the above points i might be wrong so guide me.....
@nkur Posts: 202 Rating: (16)
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7/4/2013 2:57 PM	Rate (0)
Ste39 Platinum Member Joined: 1/21/2013 Last visit: 10/23/2024 Posts: 4620 Rating: (708)	Hi @nkur, In star-delta connection, the current is just below the main contactor and the star contactor is closed, after the time the exchange takes place and is part of the delta contactor and always the main one. Now you have the current divided into two halves under the main contactor and half below the delta contactor.

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